NL8005624A - IMPROVED ERROR CODING FOR A VIDEO DISC SYSTEM. - Google Patents

Description

. % y TO 1082 * 'Title: Improved error encoding for a video disc system.

The present invention generally relates to video disc systems and more particularly to error codes used in video disc systems to encode and decode digital information in a recorded video signal.

5 Video disc systems can be provided with some more advanced features by recording digital information along with the video signal. Examples of such advanced features are automatic jumping over "locked groove" errors, displaying program play time and automatic detection of the end of 1Q the program. US patent application no. 084Λ65 describes a video disc system with a player, comprising a simple, effective video-to-digital device for separating pre-recorded digital information from the video signal and also how such digital information is used to achieve the aforementioned features. to bring.

The recorded digital format includes a start code, an error code and information bits. During playback, the video disc playback device samples digital data encoded on the video signal until a start code is detected. After the start code 20 is detected, the error code and the information bits are clocked into suitable registers. In a sequential process, the error code and the information bits are decoded to determine whether there is an error. The decoding process results in a predetermined result (referred to herein as the "remainder") when no errors are detected.

In a known system for encoding digital data on a video disc medium, the digital format includes a start bit, followed by information bits, followed by a group error code. The information bits include a groove identification number to indicate the location of a display needle on the video disc. The complete digital message is encoded on the video signal during one line of the vertical blanking interval.

In the known system, in order to decode such recorded digital data in the player, the line in the vertical blanking line containing the data is supplied via gates to a decoder circuit. When scanning the start bit, the decoder 8005624 2 clocks each subsequent bit in a data register and checks the received group error code for received errors, if any. A group error code, after decoding, has a certain error checking result (referred to herein as the "remainder"), which is equal to zero when starting with zero in the decoder, and assuming that no errors are detected.

The data system described above can be disrupted by any of a number of noise-induced errors. These errors include frame failure, in which the received message is shifted one or more bits from its correct position, and error code defects, wherein the error code check indicates validity in the presence of noise-induced errors. It is important that the digital data read by the player be substantially free of undetected errors. These noise-induced errors, as well as other drawbacks of the data system described above, can be reduced by using the improved data coding / decoding device according to the invention.

The digital data encoding apparatus of the present invention comprises generating a start code at the beginning of each digital message, and an associated coset error code and information bits. Preferably, the error code and the infoimitation bits follow the start code successively. Placing the information bits at the end of the message is advantageous in that it allows a simpler decoder in the video disc playback device.

A Barker sequence can be generated as the start code to improve self-synchronization and to reduce raster errors.

A coset error code is equal to a group error code except that either the remainder after decoding, or the initial contents of the remainder register before decoding, or both, are not equal to zero. In other words, by using a coset code, the situation is avoided, with all zeros appearing for a valid error-free message.

Using an error code with a residue not equal to mil, according to the present invention, results in a lower amount of undetected errors than is the case with a group code with a residue equal to zero. It is believed that this result is due to the particular nature of the video signal and. the manner in which the 80 05 62 4 3 ê * * digital information is recorded thereon. The decoder searches for a digital message during the vertical blanking interval, where the transmitted lines are at a black level (logical zero). During this time, logical zeros are more likely to occur than ones. Therefore, since the remainder of zero equals zero (after decoding), noise is more likely to cause a residual equal to zero than any other residual unequal to zero. For example, in the previously described system, when a noise burst equal to the start code followed by a black level (all zero-10. Len) would result in a residue equal to zero. The present data system, in which the decoding process begins with a non-zero number and / or ends with a non-zero number, is not subject to such errors.

Various equipment configurations are possible for decoding a digital message in accordance with the above format. Minimum requirements are data storage means for storing received data, error code checkers including an error code check register for calculating a remainder, means for detecting a start code, means for detecting a valid remainder and re-20 gel means for controlling the entire successive decoding process. The present invention also aims to simplify and reduce the necessary equipment for decoding the digital message.

According to an aspect of the present invention, there is provided a recording device which records an error code such that at least a portion of the recorded digital data word gives an error checking result equal to the start code. According to another aspect of the present invention, there is provided a playback decoder which indicates that a received data word is valid if at least part of the received data word has an error checking result equal to the start code.

According to the embodiment of the present invention, the error code is arranged such that (1) the remainder after decoding a valid message is equal to the start code and (2) such a remainder is calculated over the entire message, including the start code . The error code check register therefore starts with the start code and when no errors are detected, after the complete message has been received, 80 0 5 62 4 k it also ends with the start code in the register.

When in operation, consecutive data hits are clocked into the error code check register until a start code is detected. After that, the error code check register does not need to be cleared, but simply starts calculating the rest, starting with the start code. After the full message has been received, the same start code detecting means are now used to detect a valid remainder. The encoding equipment used in the video disc recording apparatus is arranged to accommodate the described video disc playback decoding apparatus without substantially increasing the recording equipment.

The invention will be described in more detail below with reference to an exemplary embodiment, with reference to the drawing. Herein: 80 05 62 4 -5- Fig. 1 shows a graphical representation of a television signal with the vertical blanking interval between an even and odd frame; FIG. 2 is a graphical representation of the digital data format using the described recording method; Fig. 3 is a block diagram of a video disc encoder; Fig. k is a block diagram of a video disc player; FIG. 5 is a block diagram showing in more detail the digital data generator of the video disc encoding apparatus of FIG. 3; FIG. 6 is a block diagram showing in more detail the information buffer 10 'chain for the video disc player of FIG. FIG. 7 is a schematic diagram of a means for generating an error check code from the information bits for the video disc encoder of FIG. 5; FIG. 8 is a schematic diagram, partly in block form, of the information buffer circuit for the video disc player of FIG. k; FIG. 9 is an embodiment of a receiver check counter for the information buffer circuit of FIG. 8; FIG. 10 is a state transition diagram for the microprocessor controllers of FIG. k; and 2.0 FIG. 11 is a flow chart showing a program algorithm for the microprocessor controllers of FIG. U.

Certain details of an HTSC-type television signal constructed according to the "buried" subcarrier technique as described in U.S. Pat. No. 3,872,998 are shown in FIG.

25 A vertical blanking interval separates the interlaced odd and even grids. Those skilled in the art will recognize the standard vertical blanking interval with a first equalization pulse interval, a vertical synchronization interval, a second equalization pulse interval, followed by a number of horizontal line intervals at the beginning of each new frame. As shown in Fig. 1, the video signal information starts up. line 22 "of raster 1 and on line 18h-" of raster 2.

The digital information representing the frame number appears on line 17 "of frame 1 and line 280" of frame 2. Digital information may also be included in other lines of the vertical blanking interval. In order to show the details of the digital signal type, in Fig. 2 the time scale is stretched along the horizontal line containing the data (line 17 'or line 280').

Q Π / 1 K ft 9 L

-6-

The data are presented in terms of luminance level: 100 ΊΉΕ units is a logic "one" and 0 IRE units (blank) is a logic "zero".

The first data hit follows the standard horizontal sync 5 pulse 140 and the color burst lk2. The frequency of the burst lb2 is un-. . spring 1.53 MHz, the frequency of the "buried" subcarrier. Each data hit is transmitted synchronously with the 1.53 MHz "buried" subcarrier signal. As shown in Fig. 2, the digital message includes a 13-bit start code named B (x), a 13-bit redundant error check code, 10 ms named C (x), and 52 info-imitation bits named 1 (x). The beginning of the next horizontal line is indicated by the next horizontal synchronization pulse 100a and the color burst 125a. Thus, the individual data bits are synchronous with the color subcarrier and the total digital message is synchronous with the vertical synchronization pulse. It is pointed out that the data rate may be a multiple or a portion of any suitable subcarrier frequency. Also, other luminance values can be assigned to the logic one and zero, and more than one bit can also be associated with a given luminance level.

In the described system, a starteode is used to synchronize the data system with the digital message, eliminating the need to detect the edge of the horizontal or vertical synchronization. Synchronization errors in a serial digital data system result in image synchronization errors, i.e., when the received data has shifted one or more bits from its proper location. Known systems for recording digital data on a signal encoded on a video disc have shown that the edges of the synchronization signals are not reliable as a time reference and have resulted in image synchronization errors. It has been found that start codes are more reliable.

The specific chosen starteode 1111100110101 is one of the

Barker codes, which is known in radar and sonar technology. See "Group Synchronization of Binary Digital Systems" by R. H. Barker. Barker codes are designed so that the auto-correlation function of a signal containing a Barker code offset from itself is maximum when a coincidence occurs and elsewhere minimal. That is, if one assigns a value of +1 or -1 to each bit in the start eode and calculates the sum of the respective 80 05 62 4 • t 5 bit products for each shifted position of the start code relative to itself, such an auto-correlation function will give a sharp maximum when a coincidence occurs. More specifically, a Barker code shifted every odd number of places from itself gives an auto-correlation of 0. A Barker code shifted every odd number of places from itself gives an auto-correlation from -1. However, when a coincidence occurs, the auto-correlation is W, where li is the number of bits in the Barker code. In other words, a Barker code, each of which is shifted a number of 10 positions relative to itself, gives a difference with a maximum number of bit positions. In the presence of noise, these properties reduce the probability of an incorrect start code detection, compared to the situation at a randomly chosen start code.

The information bits I (x) include a frame number, a band number 15, and spare information bits that can be used in the future.

The frame numbers identify each frame of the video signal with a unique 1.8-hit binary number. At the beginning of the video disc, the first frame of the video program frame is "zero". Thereafter, each frame is numbered consecutively in ascending order. Band numbers 20 refer to recorded video signal in a group of adjacent revolutions of the spiral grooves, which have a band-like shape.

All material in such a band of grooves is identified by having a common band number. An example of using the tape number is, for example, that the video signal after the end of the video program material is recorded with tape number "sixty three". The video disc player scans tape sixty three as the end of the program and responds by lifting the needle off the record.

The error check code C (x) is calculated from l (x) in the video disc format * 30 device. To do this, l (x) is multiplied by a constant H (x). The resulting product is divided by another constant g (x). After such a division, the remainder (the quotient is not used) is added to a third constant M (x). The result is C (x ').

In the video disc player, the received message is checked for errors by dividing the entire message, including the start code, by the constant g (x) mentioned above. If the rest is equal to the start code B (x), the message is considered error free.

fl η η ς r? a -8-

The constants H (x) and M (x) are chosen so that the rest of the total message would in fact be the start code. The constant g (x), which is used in both the video disc recorder and the video disc player, is called the generator polynocm of the code. A 5 g (x) is chosen to generate a code with error detection properties, which are particularly advantageous when applied to video discs. In the described system, the addition, multiplication and division operations referred to above are performed according to certain rules, which are suitable for the available equipment to perform them. The forut coding will be discussed in more detail below, along with the coding and decoding equipment.

In Fig. 3, a block diagram of a video disc encoder is shown. A composite video signal from source 30 is linearly combined in adder 36 with a series of digital data bits on line 1.5 37, which are supplied by the digital data generator 38. The synchronizing means 32 supplies a color subcarrier and synchronizing pulses so that the data bits generated by the digital data generator 38 are synchronous with the color subcarrier appearing on terminal 31a and so that the digital message is encoded at the correct horizontal line in the vertical blanking interval. Information bits, which appear on data rail 39 and display the video frame number and tape number, are outputted from device 3

The use of the frame number and band number information will be discussed below in conjunction with the microprocessor program (FIGS. 10 and 11). The digital data and the video signal are combined in adder 36. Further signal processing means +0 0 bring the composite video signal into a suitable shape for the recording medium. The composite video signal is of the "buried" subcarrier wave type and is recorded using M modulation techniques.

In the video disc player of Fig. K, the FM signal is detected using a recording transducer and needle build 20 and converted in the video processing circuit 18 to a standard television signal which can be viewed on a conventional television receiver. The video editing circuit 18 includes means which can respond to the color -35 burst signal to phase-lock a local 1.53MHz color oscillator with the color subcarrier. The color oscillator, in addition to its normal use for demodulating the "buried" subcarrier 80 05 62 4 ... -9- * <* · is also used to provide a digital clock signal and this signal appears on conductor 72. The video enhancement circuit 18 further comprises means for demodulating the video carrier and treating the recovered video signal with a comb filter. The comb filter 195 subtracts two adjacent grid lines from each other, the result of which appears on conductor 70 as edited video. Because line 16 *, which is at the black level, is subtracted from line 17 ". which is modulated with digital data, the processed video signal on conductor 70 forms the recovered digital data. Of course, line 16 * can each have constant 10 noise level. It is noted that when line 18 'following the. data line 17 "is also a constant luminance line (also black)" also the next output of the comb filter during line 18 "is the recovered digital data, but the data is then inverted.

Subtracting a line from an adjacent line with a constant 15 "luminance, the recovered digital signal has its own reference, thereby eliminating data errors due to shift in the DC voltage level of the video signal. If it is desired to transfer data cp consecutive lines, compared to placing data next to lines with a constant 20 luminance, means would be needed to reference the video signal at a predetermined luminance level, or a DC reference level would be needed to separate digital data sequence from the video signal.

As shown in FIG. 1, the information buffer circuit 16 responds to the processed video signal on conductor 70 and the 1.53 MHz clock signal on conductor J2 to extract the digital data from the video signal.

The buffer circuit 16 is controlled by a digital, binary control signal on conductor 71 from the microprocessor 10. In a binary state, the control signal on conductor 71 causes the information buffer circuit 16 to collect data 30. In the other binary state, the control signal on conductor 71 causes the information buffer circuit 16 to transfer the received data to the microprocessor 10. In particular, when the control signal on conductor 71 is high, the information buffer circuit 16 opens to receive data on conductor 70 for the processed sample video signal using the 1.53 MHz signal on conductor 70 as a clock signal.

Once a full message has been received, the state signal on conductor 75 provides an indication that the message is complete. Because of the message? When a -10 is transferred to the antoprocessor memory, the control signal on conductor 71 is made low. This closes the information buffer circuits 16, resets the internal control circuits, and transfers the results of the error detection in the message to state conductor 75.

5 If the state signal indicates that the message is valid (ie that the error code check indicates validity), the microprocessor 10 is programmed to transfer the data in the data buffer buffer 16 to the microprocessor 10. The microprocessor provides an external clock signal on conductor 73 cm data from the information buffer chain 16 over 10 '. to carry. For each clock pulse, a data bit on conductor Tk is shifted out of the information buffer circuit into the microprocessor 10. When all data has been transferred to the microprocessor 10 and the program is ready for another digital message, the control conductor 71 is returned to a high state and the process is repeated.

1J The microprocessor 10 controls the passage of line 17 "(or line 280") of the video signal via the information buffer circuit 16. The first digital message is obtained by continuously searching the video signal for a start code. Then, the information buffer chain 16 is closed. Then the information buffer chain, based on 20. arrival time of the first digital message, approximately six lines before the next digital message is expected. If no valid message is found, the information buffer chain 16 is closed approximately six lines after such expected arrival time. When a valid digital message is found, the information buffer chain 16. is closed and a new arrival time for the next digital message is calculated based on the arrival time of the current digital message. In such a manner, the microprocessor 10 opens a gate, or "data window", which is approximately twelve lines wide and centered around the expected data. The time interval 30 from the center of one data window to the next is approximately the time interval for a video frame. The width of the data window is chosen so that, at worst conditions, the expected data will fall within the data window. Sources of time errors are, as will be explained below, a finite resolution 35 of the digital timing device; the drift rate of the timing device; the uncertainty in the program in determining the arrival time of the current data; and time differences between odd 80 05 62 4 i '3 -11- and. honor interlaced grids. The use of another microprocessor and / or timing device is possible by adjusting the width of the data window accordingly. The mieroprocessor program that controls the logic for searching data and centering the data window is in. The following is discussed with reference to FIGS. 10 and 11.

The microprocessor 10 also responds to the player controls 1¼ (load, pause and scan) to operate the player mechanism 12 and the player display panel 22 in accordance with a pre-determined program, which will be discussed in the following. control. The playback mechanism is further provided with at least one needle displacement device which can be operated by the microprocessor 10. Such a displacement device is a piezoelectric, electromagnetic, or other impulse moving means 15 ”from the signal pickup to adjacent grooves or signal tracks on the video scene. The use of the displacement device to emerge from "Locked" grooves will be discussed hereinafter with reference to the flow charts of FIGS. 10 and 11.

As mentioned above, the video recorder uses 20 the information bits l (x) to calculate C (x). Because of the large number of possible combinations - I (x) and C (x) together are 6b bits long - and the desire to determine the error detection and correction properties of a given code without going into addition, error codes are treated mathematically. A general mathematical development of ring-25 theory and Galois Fields GF (2D1), which are generally applicable to error codes, are described in "Error Correcting Codes" by W. Wesley Peterson. "For the time being, the error coding in the video scene can best be understood by reference to a number of simple definitions.

A digital message containing ones and zeros can be considered to represent an algebraic polynoam containing powers of x '. The coefficients of the respective powers of x are the individual bits of the message. For example, the four-bit message 1011 can be represented by the polynomial P (x), where P- (xl * 1 .x3 + 0.x2 + 1 .x + 1 .X ° 35 = x3 + x + 1

By applying this notation method to the start code 1111100110101 one gets «η ηl -12- B (x) = 212 + X11

The highest power of x is called the degree of the polynomial.

In the example above, B (x) is a twelfth degree polynomial.

Polyncms can be added, subtracted, multiplied, and divided using the common algebraic rules except that the coefficients are expressed in modulo 2 terms. A short notation of the remainder of a polynocm after division by another polynomial is indicated by square brackets, ie when 10 ffff + til} in which the remainder r (x) has a degree lower than that of the divisor g (x ), then [F (x) l - r (x) '·

In the video disc recorder, the total message recorded on the video disc is represented by a polynocm T (x). Fig. 2 shows that T (x) «B (x) x ^ + C (x) x51 + ï ( x) (1) 6k

The term x shifts B (x) by 6b bits, because B (x) is at the beginning of the data format. Likewise, the term x ^ 1 C (x) 20 shifts 51 bits to indicate that C (x) is recorded for 1 (x). In accordance with the described device, the recording device calculates a value for C (x), so that the total message T (x) has a remainder equal to B (x) after being divided by g (x). I.e. if one assumes that C (x) has the form C (x) * [r (x). H (x)] + M (x), (2) then H (x) and M (x ) are constant polynomials chosen so that [Τ (χ)] - B (x) * (3)

It can be proved that the expressions (1), (2) and (3), when solved for the constant polyncms H (x) and M (x), return 30 H (x) = [x12T3

M (x) a £ B (x) x1 ^ + B (x) x12 ^ J

Pig. 7 contains a table listing the selected values for B (x) and g (x) as well as the derived values for H (x) and M (x). -It's pointed out. that the table in Figure 7 shows higher order bits on the right side 35 so that they are in the same order as the flip-flop storage elements shown in the logic diagram in the same figure.

80 0 5 62 4 -13-

In the video disc player, the recorded digital "message is read by the player's electronics.» The recorded data on the video disc is T (x). The data read by the player is R (x). When no errors have occurred between recording and display 5 is T (x) * R (x). The received message R (x) is checked for errors by dividing R (x) by g (x). When the remainder equals B (x), the start code, the message is considered error free »However, if the remainder is not B (x), an error is indicated.

The properties of a code generated in the above manner on-10 depend on the choice of g (x), which is called the generator polynomial. The particular g (x) chosen for the video disc is one of the computer generated codes described by Tadao Kasami in Optimum Schortened Cyclic Codes for Burst Error 4 v

Correction "in IEEE transactions on Information Theory 1963» A salvo-15 error in a digital system "is a type of error in which adjacent bits in the digital message are lost. Volvo errors are considered a probable type of transmission error on video discs.

As shown by Kasami in the aforementioned article, a code, which can correct some burst errors of 6 bits or less, can be generated using a generator polynomial given by g (x) * X1 V2 + X11 + X1 ° + Χ7 + Χ ^ + χ5 + Χ ^ + χ2 + 1

Furthermore, it can be proved that for the above given g (x) all single burst errors of 13 bits or less will be detected. 25 and that 99,988 $ of all single burst errors longer than 13 bits will also be detected. The video disc player described herein uses only the error detect capabilities of the chosen code.

As a specific example of generating the error code 30, the case is considered where the frame number is 25,000, the band number 17 'and the spare bits are equal to 0. Because 25,000 in binary representation is equal to 000 110 000 110 101 000 and 17 in binary representation is equal to 010 001 (high order bits are on the left), the 51 data bits are 000 000 000 000 000 000 000 000 000 000 110 000 35 110 '101 000 010' 001. The transmission sequence is first spare bits, followed by frame number and then band number, with the most significant bit being transferred first. The error red for the specific top -line * (l), calculated as the remainder of l (x) times H (x), plus M (x), - is represented by 0111100100010. The next video frame is 25 * 001 or in binary representation 000 110 000 110 101 001. For the corresponding information bits 000 000 000 000 000 000 000 000 000 000 110 000 110 101 001 5, the correct error code is. 1000101101110. The complete digital message for raster 25.001, including the start code, is therefore 1111100110101 1000101101110 000 000 000 000 000 000 000 000 000 000 110 000 110 101 001/010 001, shown in transmission order .. The start code is preformed by the first 13 bits , the error code through the next 13 bits and the 52 information bits come last. In the video disc player, the above. digital message checked for errors by dividing the received message by g (x). If no errors are detected, the rest is equal to 1111100110101, which is exactly the same as the start code.

A block diagram of means for generating T (x) is shown in Figure 5. Controlled by the transmitter control device 50, 2b information bits are fed via data rail 39 and 27 spare information bits are fed via data rail 39 into a 51 hit shift register W. I (x) comprising 51 bits is then shifted into another 51 bit shift register 52.

At the same time, during the 51 shift pulses, an encoder U5 calculates C (x) in the following manner. The polynoc divide and multiply device b responds to the 51 bit serial transmission of l (x) to calculate the remainder of l (x) times H (x) divided by g (x).

25 M (x) is then added in parallel in polynomial adder H8. The resulting code, C (x), is fed into a 13-bit shift register 5 ^ and B (x), the start code, is fed through the data rail b9 into another 13-bit shift register bj. Since the start code is a constant digital value, the feed is preferably performed with fixed connections to the parallel feed inputs of shift register bj as opposed to a software version. In positive logic notation, the corresponding parallel inputs for shift register bj are connected to ground potential when the start code has zero and to a positive potential when the start code has one. The transmission control device 50 controls the total message T (x), which is contained in the three shift registers 52, 5 ^ and 1j7, being sent out in series in synchronism with the color subcarrier on conductor 31a 80 05 62 4 -15-> · · <* · Slid. A video synchronization pulse applied to conductor 33 provides the timing control device 50 with a time reference so that the digital message is transmitted at the correct time relative to the video signal.

A specific embodiment of the encoder (45 from FIG. 5) is shown in FIG. Clocked flip-flops with output terminals Qq - form a residual register. Multiplication by ïï (x) and division. . . g (x) simultaneously performs hit series mode.

The rest is then retained in the residual register from output terminals Qq -10. For a general treatment of such chains, reference is made to chapter 7 ». 107-114 of the above-mentioned book by Peterson.

To endorse the simplicity of the polynomial multiplication and division circuit of FIG. 7, it is noted that both the addition and subtraction (of coefficients of terms of equal power) are performed by an exclusive OR gate. Multiplication of l (x) by H (x) is performed by suitable connections with one or more exclusive OR gates 80 -91 · In particular, whenever a coefficient of H (x) but not of g (x) equals 1 (bit positions 1, 3 and 8) input signal l (x) connected to an input of an exclusive 20 OR gate, 80, 82 and 87 respectively. The division of l (x) by g (x) is performed by multiplying the output of Q ^ ^ by g (x) and subtracting the resulting product from the contents of register Qq-Q ^. In particular, whenever a coefficient of g (x), but not of H (x) equals 1 (bit positions 4, 7 and 11), the output of is connected to an input of exclusive OR gate, respectively 83, 86 and 89. When H (x) and g (x) are both equal to 1 (bit positions 0, 2, 5S 6f 10 and 12), the output of the exclusive OR gate 91 is connected to an input of the exclusive 0F gates 81, 84, '85, 88 and 90. Ha 51 clock pulses, one for each bit of 30 l (x), the content of register Qq - Q ^ is equal to the rest of l (x). x) after division by g (x).

It is pointed out how M (x) is added to the contents of the residual register. The addition of coefficients takes place in a modulo 2 calculation, performed as the exclusive OR function. Whenever M (x) has 35 coefficients of +1, the complementary output Q of the corresponding flip-flop is used; when M (x) has coefficients 0, the non-complementary output Q is used.

80 05 62 4 -16-

A block diagram of an apparatus for decoding the received message, R (x), is given in Figure 6, and is an embodiment of the information buffer circuit 16 of Figure 1, discussed above. Control signal on conductor 71, an input, places the receive decoder of FIG. 6 either in a state for receiving data from the video signal or in a state for transferring data to the microprocessor.

In the receive state, each bit is shifted into two separate registers simultaneously. One such register 60 is for data and the other 62 is for error checking. The error check register 62 is a polynomial divider. However, when new data is received, the divider feedback path is disabled so that it functions as a normal shift register. The operation of sub-register 62 will be discussed in more detail below with reference to Fig. 8. For the time being, suffice it to say that register 62 responds to receiver control means 61 to either shift successive bits of R (cc) or divide consecutive bits of R (x) by g (x). In either case, the contents of register 62 are available on data rail 78 and are issued to the start code and valid data detecting device 20 66.

The receive operation begins when register 62 is set to operate as a shift register. After 3 (x) has been detected by detection device 66, controller 61 causes register 62 to operate as a polynomial divider. The polynomial division by g {x) thus starts when B (x) is in the sub register 62. The receive controller 61 may further respond to detecting B (x) to measure a period of time equal to the remaining bits of the message (61 clock pulses). After the annealing period, the divider 62 contains the remainder of R (x) modulo g (x), which must be equal to B (x) if the message is valid. During the error checking process, the data register has pushed 6θ bits into data. At the end of the timing period, register 60 contains only the last 2 1 bits. However, since the 2l bits are placed at the end of the message, register 60 will contain the information bits. If it is desired to use spare information bits, additional 35 shift register stages can be added.

The interpretation of the output status signal on conductor 75 depends on the state of control signal on conductor 71. When it is 80 0 5 52 4 ƒ.

-17-. control signal on conductor 71 puts the receiver in a state for receiving data (receive state), the status signal on conductor 75 is determined by "message received". When the control signal on conductor 71 puts the receiver in a state for transmitting data 5 (transfer state), the status signal on conductor 75 indicates "data valid". The control signal on conductor 71 also sets. The receiver controller 6b returns, and outputs the results of the check the rest to the status signal on conductor 75

The received information is transferred from shift register 60 in response to external clock pulses supplied by the microprocessor on conductor 73. After the data has been shifted out, the control signal on conductor 71 can be returned to the previous state, which the receiver decoder will return to a state to continuously search for another start code.

FIG. 8 shows a logic diagram, partially in block form, of the receiver decoder of FIG. 6. The flip-flops with output terminals - Q12? form a residual register. Polynomial division by g (x) is performed by multiplying consecutive register output terms of Q12 by g (x) and subtracting the product (via exclusive OR gates 100-108) from the contents of the residual register. From Q12, a feedback connection is made to an exclusive 0F gate (via IIT-OR gate 109), always where g (x) has coefficients 1, with the exception of bit 13. Because the coefficients of g (x) are 1 for the bit positions 0, 2, -U, 5, 6, 7, 10, 11, 12 is an exclusive OR gate placed at the data input 25 of each respective flip-flop of the remainder register, as shown. NAND gate 118 detects B (x), which is both the start code and the valid error check code. The receiver control counter 117 starts counting in response to a start signal from EET gate 120, counts 63 clock periods and outputs a stop signal used by NON EET gate 111 to stop the clock to all decode flip-flops. A representative embodiment of the receiver control counter 117 is shown in FIG. 9, with seven flip-flops 130-136.

The sequence of operations when receiving data is as follows. When the control signal on conductor 71 is arc, data is supplied to divider 62 via AND gate 110. Flip-flop 119 has been previously set, which disables the feedback signals in divider 62 by blocking HOT-OR gate 109 Register 62 now functions as a shift register. When detecting B (x), the output of HIET AND gate 118 goes low, and the Q output of flip-flop 119 goes one clock period later.

Therefore, the feedback is energized for polynomial division 5 by the output of AND gate 120 through HITE-OR gate 109 when B (x) is detected in the residual register. After 63 clock periods, the receiver control counter 117 stops and the status signal on conductor 75 goes high, indicating "message received". Shift register 60 keeps the last 2b bits of 1 (x). To transfer data, the control signal 10 on conductor 71 is made low. The inverted output signal of ΝΙΞΤ-AND gate 118, which is low when the remainder after division is equal to B (x), is supplied through gates to the status signal on conductor 75 · External clock pulses on conductor 73 cause successive shifts of data in register 60 to the output data signal on conductor 15 7 ^ The external clock pulses also clear the residual register by inserting zeros. -

The device described above shows a residual register starting and ending with the same constant, which is not equal to zero. However, it will be appreciated that other devices are possible using a "coset" code. For example, after detecting B (x), the residual register can be set to a first arbitrary constant. After division, the residual register is checked for the presence of a correct second constant. The first constant, or the second constant, can be zero; neither can both constants be equal to zero.

Due to the error cc type described above, the equipment is simplified. By ending with the start code, B (x) as a valid remainder, the start code detector (ΗΙΕΤ-ΞΗ port 118) also serves as a valid code detector. By starting the division with the start code in the divider, a control step is eliminated, because the residual register does not have to be deleted.

Pout codes are in particular placed at the end of a message. However, by placing the error code in front of the information bits, the receiver control device is further simplified in that it does not need to distinguish the 35 information bits from error code bits with Revocation to the data storage register 6θ. In addition, as shown in Fig. 8, the receiver control device is a simple counter 117 with a start 80 0 5 62 4 '/. "*" * -19- terminal, a stop klea, and provides a timing signal for a single time interval.

Digital information, including ban & number and frame number, is recorded on the video signal and used by the player to obtain a number of properties. The tape number information is used by the player to detect the end of playback (tape sixty-three) Rasöanunmer information is used in ascending order to calculate the program play time and display it on the LED display 22 in Fig. 1. Once the length of the program number is known 10, the grid number information can be used to calculate the remaining program play time. For NTSC type signals, the elapsed program time in minutes can be obtained by calculating the frame number divided by 36ΟΟ »If desired, the remaining program time *« * * »· ·· can be derived from the previous calculation. This property is useful for the viewer as they search for a desired point in the program. A particularly useful property, which is derived from the grid number information, is the locked groove correction, which is described below in conjunction with a more general case, track error correction, will be discussed ».

20 The grid numbers represent the actual needle position.

Thus, when the needle re-enters a groove, either after the change. jumping over tracks or after the scanning mechanism has been actuated, the actual needle position can be determined from the first valid read frame number. Both the track error correction system and the program play time display means use frame number data and therefore both use the decoding portion of the digital data system of the video disc. The particular embodiment of the track error correction system, which will be discussed below, uses raster number data. (needle position) to hold the needle 3Ό at or ahead of its expected position, assuming a predetermined relative velocity of the needle relative to the plate. The program play time display uses the frame number a to indicate play time, which is basically another representation of the needle position.

The microprocessor controller has a number of internal modes. .

Fig. 10 shows a state transition diagram indicating the mode logic,

Which is executed by the microprocessor program. Each of the an η ς a? & -20- circles represents a machine mode: LOAD, RINSE. ON, AfflLQCP, PLAY, PAUSE, PAUSE LOCKED and END. For each mode, the position of the needle and the state of the display device are indicated within each respective circle. The arrows between the modes indicate the logical combination of signals supplied by the controls (charge, pause, scan), which cause a transition from one mode to another. The charging signal indicates that the playing mechanism. is in a state for receiving a video disc. The pause signal is derived from a corresponding operating switch and the scanning signal indicates the operation of the scanning mechanism. After the power is turned on, the system enters CHARGE mode. A video disc can be placed on the turntable in this mode. After loading, the player will switch to RUNNING mode in a few seconds, allowing the turntable to be brought to full speed of 450 revolutions per minute. At the end of the ramp-up mode, ρ- the SEARCH mode is entered.

In SEARCH mode, the digital subsystem lowers the needle and continuously searches for a "good reading". In the SEARCH mode, a "good reading" is defined as a valid start code and a valid error check residual. After finding a "good readout", the system switches to PLAY mode.

In the PLAY mode, the microprocessor in memory determines an expected, or predicted, next frame number. The expected grid number is increased or adjusted to the correct value for each grid. For all subsequent readings, the microprocessor uses the predicted frame number to perform two additional checks to further improve the accuracy of the data,

The first additional check is a sector check. In the present embodiment, the video disc contains eight fields at each amount, dividing the disc into eight sectors. Because the relative physical position of the sectors is fixed, the sectors follow a periodic recurring order as the disc rotates, even when the needle jumps over a number of grooves. Although the digital information cannot be read for one or more frames (sectors) when the needle jumps to a new groove, the microprocessor keeps time and increases the predicted frame number accordingly. When the needle enters a new groove and records an 80 05 62 4 -21- - new digital message, the new grid number is checked by comparing it to the predicted grid number. If the sector is wrong, the data. considered a "bad reading".

The grid number is represented by an 18 bit binary number.

5 The sect or information can be obtained from the grid number by finding the remainder after dividing the grid number by eight. It is noted, however, that the three. count the least significant bits of a binary number modulo eight. Therefore, the three least significant bits of each new frame number must be equal to the three least significant bits of the predicted frame number to pass the sector check.

. A second check of the accuracy of the data is an area check, a test of the maximum area of needle movement along the radius of the disc. It is expected that no more than 63 grooves will ever be jumped when the worst conditions occur in any mode. The slot numbers are represented by the most significant 15 "bits of the frame number. The microprocessor subtracts the current groove number from the predicted group number. When the difference is greater than the acceptable range of 63 grooves, the data present is considered a "bad reading". All other readings are considered good readings and are used to correct the predicted frame number. After fifteen consecutive bad readings, the system switches back to SEARCH mode.

The presence of a scan signal in certain modes will also cause a transition to the SEARCH mode, as shown in Fig. 10.

When changing from SEARCH mode to PLAY mode, the microprocessor sets the bad read count to thirteen. This means that when switching to PLAY mode from SEARCH mode, one of. the next two frames should provide a good readout, otherwise the count of bad readings will reach fifteen 30 which will change to the SEARCH mode.

When the PAUSE key is pressed during PLAY mode, the system enters PAUSE mode. In this mode, the needle is off the plate and held in its radial position above the plate. When the pause button is released, the PAUSE-35 LOCKED mode is entered and held. Pressing the pause button again releases PAUSE LOCK mode, which switches to SEARCH mode. The PLAY mode changes to the ft A A 5 A 2- 4 -22- EMD mode when band number sixty three is detected.

Fig. 11 shows a flow chart of the program executed by the microprocessor. The microprocessor equipment includes an intruder line and a programmable timer. A commercially available microprocessor suitable for the present system is Model F8 from Fairchild Semiconductor.

The microprocessor uses the timer to determine the window of time during which the information buffer device searches for data. This "data window" is approximately twelve horizontal 10 lines wide and is centered around the expected data. When no data is found, the timer maintains the internal program synchronization at a frame time interval.

The microprocessor interrupt line is coupled to the status signal on conductor 75 (fig. M · Interruptions are only possible in the SEARCH mode when the system is continuously searching for data. The program is interrupted when a digital message is received. The interrupt operation routine (which not shown) set an interrupt flag when the error code check indicates validity, then in PLAY mode the programmable timer is used to indicate the estimated time of arrival of the next digital message.

Switch inputs (load, scan and pause) are conditioned to prevent switch clicks from causing unwanted responses from the player. The microprocessor program includes logic 25 to free the switch input signals from switch clicks (debounce).

. Debounced switch values are stored in memory. A separate debounce count is maintained for each switch. To check the debounce, the switches are sampled and compared to the stored switching value. If the sampled state and the stored state are the same, the debounce count for that switch is set to zero. The switch states are sampled as often as possible. Each grid (every millβ milliseconds for UTSC) increases all debounce counts unconditionally. When the resulting debounce count is equal to or greater than 2, the stored 35 value is replaced with the new (debounced) value. Then work is carried out on the new switch state.

The first programmed step (fig. 11) after the power has been switched on is to start all of the programmer ameters. The timer is set to measure one video frame. The mode is set to CHARGE.

The next step 152 is a program to execute the state transition logic shown in Fig. 10. At this time, the flow counts are normally increased and tested to determine if a. new switch state is fully debounced. _. . After the select logi ca T52 mode, the program enters a closed loop 153 to sample (1) switches, resetting the 10 bounce counts to 15 ^ .and. (2) if necessary to verify that the timer near the end of the time size is 155 and (3) check whether the interrupt flag is set 156.

When the interrupt flag is set 156, the program transfers data, 157s, from the information buffer chain and sets the timer, 15Tb, to measure a new frame interval. When the interrupt service routine sets the interrupt flag, the contents of the timer are stored in a memory. The program now uses the previously stored timer contents to set the timer, 157b, with a corrected value, which approximately predicts the time at which the next digital message will occur. As noted earlier, although the data shows the first good reading in the SEARCH mode, the count of the bad readings is set to 13, 157c,

When the interrupt flag is not set, the program 25 branches when the timer approaches the end of the time size. 155 · When the machine is not in PLAY mode 159, the timer is set to measure another raster interval, 158 When the machine is in the PLAY mode, 159s a number of tasks, which are critical in time, are performed, 160. The data window is opened, 160a (by putting the control signal on conductor logically one 71 in FIGS. 1 and 8) approximately six horizontal lines for the expected data. The received data is read and checked, as described above. After the data has been received, or if no data has been received, the data window is closed. The timer content, which represents the actual arrival time of the digital message, is used as a correction factor to display the timer, 160b. To this end, the tempering sn ης r? δ

-2U

device so that the next data window centers around the expected arrival time of the next digital message, based on the actual arrival time of the current digital message.

The expected frame number is shown from the latest information for -5, 160c, the tape number is checked before starting (tape 0) and ending playback (tape. 63), and the count of bad readings is increased, 160g, at. a bad reading. For valid raster data in the material of the material to be viewed, the time is calculated and displayed, léOf. If the valid frame data indicates that IQ the needle has jumped backward, the needle displacement device is activated, 160e, and the SEARCH mode is entered. Also, when the bad reading count reaches 15, the SEARCH mode is immediately switched to. During the time used for critical tasks I60, the switch-debounce control routine is repeated periodically, so that the switches are tested as often as possible. The program unconditionally goes back to closed loop 153 via mode selection logic 152 and waits for the tempering test 155 or the interrupt check 156 to indicate the arrival of a subsequent digital message.

The timer can be adjusted by charging the timer 20 directly through programmed instructions. However, instead of using a series of instructions, it is best to "set" the timer by determining a location in the memory (a mark) corresponding to a past timing of the timer. The timer is running then free The past 25 time state or being close to this state is detected by comparing the contents of the timer with the mark set in the memory The next desired past time state is set by adding the next desired time interval to the previous contents of the timer and by storing the result in memory. Thus, the timer is "set" at any time when valid data has been received, or when no data has been received within the data window, by a new mark in set the memory corresponding to the next past time state.

The microprocessor programmable timer used in the described arrangement is set by the program to share cycles of the 1.53 MHz clock input by a factor of 200.

80 05 62 4> & -25-

The timer thus counts once for every 200 cycles of the 1.53 Miïz clock. One vertical grid (1/60 sec for HTSC) is then about 128 counts from the timer. On the other hand, one can use a timer which counts another multiple of the 1.53 MHz clock, or one that uses a timer source. . which is independent of. the video signal.

The data window is made wide enough to account for different timing error sources. The timing uncertainty due to its finite resolution equals a least significant bit, which corresponds to two horizontal lines.

The drift error collected is because 128 counts from the timer are not exactly equal to one vertical frame, less than a line after 16 'consecutive frames in which no valid message was found. It is noted that because the 1.53 MHz color subcarrier clock is an odd multiple of the half line frequency, a counter that counts a corresponding multiple of the color carrier clock would have a drift rate equal to zero. In the embodiment described herein, the. uncertainty in the program in determining the arrival time of the data is about 97 microseconds, or about 2Q 1.5 lines. Finally, because the alternating fields are interlaced, the time from one digital message to the next is either 262 lines or "2.63 lines, depending on whether the current field is odd or even. Although the program could track with even and odd grids, it is easier to widen only the data window with one additional line 25. By combining the factors described above, it can be indicated that a data window, which extends over three counts of the timer (about 6 lines), before and after starting the expected data is sufficient to take into account timing conditions in the worst case.

As mentioned previously, the grid nano information can be used to detect Locked grooves. If the new frame number (after sector and area check) is less than the expected frame number, the needle has jumped backward and repeats the scan of a (previously played) revolution (s), i.e. encountered a locked groove. If the new frame number is greater than the expected frame number, the needle has moved forward, i.e. to the center of the record. In the present application 80 05 62 4 -26- no account is taken of skipped grooves, if the new raster number is larger (but the sheet complies with the sector and area control), the expected raster is adapted to the new raster. In certain other applications, such as when the video disc is used to record digital information on many horizontal lines, it may be necessary to detect and correct skipped grooves as well. However, in the present video application, a locked groove is corrected by operating a needle "mover" until the needle has returned to the expected track.

Finally, the needle will move forward past the locked groove.

More generally, the use of the frame provides emummer information of the present application in an accurate means for detecting general track errors. In any video disc system with spiral or circular tracks, including optical and groove-free systems, track errors due to defects and contamination are always possible. The present system provides means for detecting and correcting such track errors in a video disc player. For positive traces, there is provided a directional displacer for moving the sensor backward and forward in the program material. Thus, when a track error is detected, either a skipped track or a closed track, the sensor is moved in such a direction that it corrects the track error. Although the conventional sensor servo could be used for track error correction purposes, a separate displacement device or sensor positioner is preferred. The conventional servo is generally adapted to a stable scan of the helical spiral track and may not have proper properties to respond to abrupt tracking errors. A separate displacement device, on the other hand, can be specifically adapted to provide the rapid response required to correct track errors.

A specific example of a displacement device suitable for use with the described device is described in U.S. Patent Application No. 39 * 358.

Several control algorithms are possible. The pick-up device can be returned directly to the correct track by providing a movement of the needle which is proportional to the magnitude of the track error detected. Also. A displacement can be operated in response to a series of pulses, the number of pulses being proportional to the magnitude of the track error detected. The opener is moved a given number of tracks per pulse until the needle is back on the expected 5 oor track. For certain applications (for example, recovery of digital data stored on a video disc medium), it may be desirable to return the sensor to the starting point and attempt a second reading, rather than returning the sensor to the expected In any case, it is clear that by using a repositioning device and appropriate control logic, proper tracking can be obtained even if the video disc contains errors or contamination which would otherwise cause unacceptable track errors.

In a digital track correction system, protection against undetected data errors is particularly important to prevent noisy signals from advancing or delaying the recorder without need. The present data system reduces the probability of an undetected reading error to a negligible level.

In a rough approximation, it is possible to estimate the probability that a random digital input for the data system will reach a valid message containing a non-sequential frame number by which the needle displacement device is operated. The random guess hero of a correct start code is 1 in 2. The random value. The probability of a good error code is also 1 in 2. The random probability of a good grid number is calculated as follows. Grid numbers contain 18 bits. Because there are eight sectors on the disk. in the system under consideration, the three least significant hits of each frame number indicate the sector number, which must correspond to the expected sector number. The remaining fifteen bits, which represent the groove number, may vary over the allowable range (plus or minus 1863 grooves). Therefore, only 126 of 2 0 random grid numbers will pass sector and area checks. Combining all protections shows that the probability of an undetected shk 35 error is 126 out of 2.

The above estimate is based on the assumption of a true random input signal and does not take into account several factors that further reduce the probability of an undetected error.

On a video disc track, burst noise, where the erroneous bits are adjacent to each other, is more likely than other types of noise. As -5 as previously mentioned, the particular error code chosen detects all single burst errors up to 13 bits and also a high percentage of longer bursts. Thus, as previously explained, the choice of a non-zero remainder for the error check code (a coset code) further reduces the probability of an undetected error. Furthermore, the particular selected start code, a Barker code, reduces the probability that noise will cause an erroneous start code detection.

The disclosed data stone, when applied to a video disc system, results in a number of undetected errors, which are relatively low and erroneous alarm signals, which would otherwise cause unnecessary needle movement, have been greatly reduced. The certainty of the data provided by the described system improves the stability of a number of playback functions, such as displaying the program play time, which depends on the recorded digital data for proper operation.

80 05 62 4

Claims (10)

1. Video disc recording device for recording an information for a video signal, characterized by means (.30) for recording a video signal; means (^ 9) for displaying a first data order corresponding to a start code; means (b5) 5 for generating a second data order corresponding to an error code or at least a portion of the information word and means (36, kO) for modulating the video signal in accordance with a recorded data word that the start code, the error code and the information word, the error code comprising a coset code over at least 10 one. part of the recorded data word. is. A video disc recording device according to claim 1 for encoding an information word 1 (x) on a horizontal line of a composite video signal during its yertical blanking interval, characterized in that the means for generating the error code has an error 15 generate code C (xj using a gener at or polymoma g (x), that the error code is in the form c (x) = £ ï (x). H (x) J + K (x), where H (x) and M (x) are each constant polynomials and that the modulation means modulates the video signal during a horizontal line during its vertical blanking interval in accordance with a recorded word T (x), comprising the error code and the information word, wherein E ( x) and M (x) are chosen such that [t (x) J φ 0. Device according to claim 1 or 2, characterized in that the means for generating the error code can respond to the information word; that means are provided for combining the information. .... word and the error code and xn means included in the error code generating means for controlling the error code generation so that at least a portion of the data word has an error checking result equal to the start code. K Video disc recording apparatus according to conelusion 1, characterized in that the means for controlling the generation of the error code are of such a type that the entire data word has an error checking result equal to the start code. Device according to claim 1 for encoding an information value 1 (x) on the video signal, characterized in that the start code B (x) 35 corresponds to a start sequence; that the means for generating the start code C (x) use generator polynoam g (x), the error code being in the form C (x) = [1 (x). H (x) J + M (x), • wherein H (x) and M (x) are also constant polynomials and the modulation means modulate the video signal according to the data word T (x), which is given by T ( x) = B (x) xm + n + C (x) xm + l (x) - where n equals the number of "bits in the error code and m equals the number of hits in the information word and where H (x ) and M (x) are chosen so that [t (x)] = B (x). 6. Decoder for decoding an information word from a video signal, the video signal being modulated during a horizontal line during its vertical blanking interval thereof in accordance with a recorded data word, the data word comprising an error code and an information word, the decoder being characterized by receiving means, which can respond to the modulated video signal to detect individual bits of the received data word; by polynomial sub-members coupled to the receiving members for dividing at least a portion of the received data word by a constant polynomial g (x), the polynomial sub-members having a residual output and by controllers responsive to the residual output to indicate that the received data word is valid if the residual output of the polynomial divider is equal to a predetermined value, wherein the predetermined value is not equal to zero. Device as claimed in claim 6 for playing a video disc, characterized in that the receiving means have a received data word output corresponding to the hits of the recorded data word 30 and have a first decoder for decoding the start code; the polynomial divisions including an error checking register for dividing at least a portion of the received data word by a constant polynomial g (x) and the controllers being able to respond to the first decoders and the error checking register, the controllers comprising means for setting the error check register at a first predetermined constant for division and means to indicate that the data word received is valid as the error register one. second predetermined constant after division, at least one Iran having the first and second predetermined constant not equal to zero. 8. Decoder according to claim 7, characterized in that the 5 starteode is output in "binary format" by 1111100110101. Decoder according to claim 6 or 7, characterized in that the constant, polynomial g (x) is given by x J2 'x 11' 10, 7. .6 A 5. b- 2 M. g {x) = x + x + x + x + x '+ x + x + x + x + 1. 10. Decoder according to claim 6 or 7, characterized in that the polynomial divisions can respond to the regulators for dividing the entire data word received. Decoder according to claim 6 or 7, characterized in that the polynomial sub-members can respond to the control members for sharing the combined information and error code for the received data word. J 12 ". Device according to claim 6 for playing a video disc, wherein the data word contains a start eode and a combined information word and an error check code, characterized in that the polynomial sub-members comprise an error check member coupled to the receiving means for generating an error checking result and the control means being able to respond to the error checking means to indicate that the received data word is valid when at least part of the received data word has an error checking result equal to the starteode . 13. Device as claimed in claim 12, characterized in that the control element. go indicates that the received data word is valid when the fully received data word has an error checking result equal to the starteode. 1U. The device of claim 12, wherein the data word is a start code and a combined information word and an error check code word 30. characterized in that the controller can respond to the residual output to indicate that the received data word is valid if the residual output after division by g (x) is equal to the starteode. 15. Device as claimed in claim 12 for playing a video disc, wherein the data word has a start code and a combined information. word and error check code including detecting means coupled to the receiving means for detecting the start eode, characterized in that the polynomial divider in response to a partial command -32 signal contains at least a portion of the received data word by the constant polynomial g (x), wherein the polynomial divider includes a remainder and the controller reacts lean to the detecting member to supply the subcommand signal after detecting the start code,. 5 wherein the controller further comprises means responsive to the residual register contents for one. provide an indication that the received data word is valid when the inbound of the remainder register after the polysecond division is equal to the start code. 16. Device as claimed in claim 12 for playing a video disc, wherein the data word comprises a start code and a combined information word and error decode code, characterized in that the sub-element comprises shift register means, which are connected to the reception means, which shift register means comprise feedback means. which can respond to an activation signal for multiplying the output of the shift register members by the constant polynomial g (x) and subtracting the result thereof from the contents of the shift register; detecting means, which are connected to the shift register for detecting the start code and which the control means comprises means which can respond to the detecting means to supply the energizing signal to the feedback means after the detecting means. has detected a start code, and further comprising means, which may respond to the detector means to detect the start code in the shift register after the data word has been received to indicate that the received data word is valid. Video disc playback device for playing a video disc having a substantially spiral information track, the information track displaying a recorded carrier signal modulated with a video signal, which video signal comprises information signals. displaying digitized digital numbers, a predetermined value of the digital numbers corresponding to a band of revolutions of the spiral information track after the end of the recorded video program, the video disc playback device including a signal recording apparatus for scanning the recorded video signal and a program end detecting device, characterized by detection means coupled to the signal recording means to decode the recorded digital numbers and by control means responsive to the decoded digital numbers for an end 80 05 62 4 -33- of the program indication signal when the detecting means decodes the predetermined value of the recorded digital numbers, the video disc playback device further comprising means which may respond at the end of the program indication signal. 5 sew to control the operation of the pick-up and prevent. 80 0 5 62 4