KR810001993B1 - Apparatus for reading a record carrier - Google Patents

Abstract

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Description

Record carrier decoding device

1 shows an optical decoding system.

2 is a schematic diagram showing a known time error measurement system.

3 shows signals related to the present system.

4 is a diagram illustrating a time error measurement system according to the present invention.

5 shows the associated signals.

Figure 6 illustrates an embodiment of a keying signal generator for use with the apparatus according to the present invention.

7 is an extension of the measurement system.

Fig. 8 is a diagram showing an incident burst signal on a line sync pulse.

The present invention relates to an apparatus for decoding a record carrier on which a television signal is recorded.

This television signal is composed of horizontal synchronization pulses and a burst signal coupled to the horizontal synchronization pulses, which are composed of a plurality of periods of a carrier having a frequency that is an integer multiple of the half-line frequency. The decoding device includes a time error correction device for correcting the time error in the decoding television signal and a time error detector for detecting the time error and supplying a corresponding control signal to the time error correction device. The time error detector includes a keying signal generator triggered by a horizontal sync pulse, which supplies a keying signal that is delayed by a first period with respect to the horizontal sync pulse. The first period is set so that each time the beginning of the keying signal is occupied by the burst signal. The time error detector also includes a zero path detector for detecting a first zero path of the burst signal that appears after the beginning of the keying signal.

In this regard, the recording carrier generally includes a television signal in a coded form, that is, a television signal encoded in a special method before being recorded. According to frequently used encoding, a finished television signal, such as a standard PAL or NTSC color television signal, is a modulated frequency on a carrier wave. In other coding systems, chromaticity and chromaticity signals are added to separate carriers. The encoding used while the television signal is being recorded indicates a certain synthesis of the burst signal associated with the horizontal synchronization pulses after the television signal has been compensated for decoding, but is not suitable for the present invention.

The signal decoded from the record carrier generally shows a time error. When decoding the recording carrier in tape form, these time errors are caused by variations in the feed rate and extension of these recording carriers. When decoding disc shaped record carriers, these time errors are caused in part by a change in the feed rate (in this case, the rotational speed) of the record carrier and in part by an eccentricity of the driving point with respect to the center of the record carrier.

These time errors are very disturbing when reproducing the television signal recorded on the recording carrier, especially in the case of a color television signal in which two color components are upper-modulated on the same color carrier wave. In such color television systems the time errors cause unacceptable very annoying color hue shifts.

The apparatus for decoding these record carriers therefore comprises a time error correction system in which these time errors can be corrected as much as possible. In such a decoding device, both the electromechanical auxiliary system and the complete electronic system can be used as time error correction systems. In an apparatus for decoding a record carrier, for example in the form of a tape, it is commonly used to control the speed at which the read head moves along the record carrier by measured time errors and the feed rate of the record carrier. The apparatus for decoding a disc shaped record carrier also has the same effect on the rotational speed of the disc shaped record carrier. Moreover, such reading devices generally include a calibration system that is incidental to the injection unit. In the case of a mechanical scanning system, this minor calibration system controls the positioning of the scanning head in the longitudinal direction of the track, for example, on the record carrier. In the case of the optical decoding system, the information on the recording carrier is decoded by adding the radiation beam, and the position of the scanning spot generated on the recording carrier is controlled by the beam in the longitudinal direction of the track. Electronic time error correction systems may use variable delay networks such as bucket brigdae, charge coupled devices (CCD), and the like.

In all these time error correction systems, it is of great importance that the time error contained in the readout television signal can be measured very precisely. In addition, such a time error correction device preferably has a fairly wide control range for the purpose that the time error should be measured over a wide range.

In I.E.E.E Broadcast Report, Vol. 17, No. 1, March 1971, page 35, it is stated that the time error detector for NTSC color television signals meets these two requirements. With this device, a color burst signal is produced on the backporch of the horizontal sync pulse in the color television signal. This color burst signal is composed of a plurality of periods of a carrier wave at a frequency that is an odd integer multiple of a half-line frequency, and the burst signal has its position relative to the corresponding horizontal synchronizing pulse. The keying signal generator triggered by this horizontal synchronization pulse generates a keying signal lying within a period occupied by the color burst signal. By detecting the first color burst zero path that appears after the beginning of the keying signal, a preferred control signal for the time error correcting apparatus is derived in comparison with the reference signal of the line frequency to obtain a pilot signal of the line frequency. Since the zero path of the color burst signal can be detected in a very precise way, the time error measurement is very accurate. Furthermore, since this actually produces a pilot tone of line frequency, the time error measurement range and time error correction range correspond to one cycle of the television signal.

In order to detect the time error described above, the start of the keying signal within the time interval of the burst signal is essentially determined so that the same zero path of the burst signal always acts as a pilot signal. However, the relative position of the horizontal synchronizing pulse, in particular its tip and the burst signal, is not accurately determined. As a result, the beginning of the keying signal derived from this horizontal synchronization pulse is not accurately determined. This leads to an incorrect time error measurement in which the time error is displayed without generating a time error in the readout signal due to the change in the zero path of the burst signal acting as a pilot signal.

SUMMARY OF THE INVENTION An object of the present invention is to avoid this problem, and the apparatus according to the present invention provides a measuring circuit for measuring the period between the beginning of the keying signal and the next zero path of the detected burst signal and supplying a corresponding calibration signal. And a keying signal generator suitable for supplying a keying signal associated with a horizontal synchronizing pulse having a delay equal to a first period which can vary according to a calibration signal.

The means according to the invention ensures that the beginning of the keying signal always lies in the middle between two successive control passages of the burst signal irrespective of changing the position of the horizontal synchronization pulse in relation to the burst signal. Ensure that the passage always acts as a pilot signal for time error detection.

The present invention is in no way limited to using color burst signals as described above in connection with NTSC color television signals. Optionally, an additional burst signal can be added to the television signal to be recorded within a suitable period for each line period. One of these is to insert a burst signal during the period of horizontal sync pulses so that the burst signal is superimposed on the horizontal sync pulses, the possibility of which is described in Consumer Electronics, pages 5-1-76, page 54 and the like. have. The use of these additional burst signals is very important when using PAL-standard color television signals. This is because it cannot be used for color burst detection in this case.

Although the present invention is not limited to this system, the principle of the optical decoding system for a disc shaped record carrier will now be described.

The decryption apparatus shown in FIG. 1 is suitable for decoding the disc shaped record carrier 1, in which information is recorded in a stable pattern (e.g. in a spiral line) on the upper surface tracks on the carrier. Such a track forming and coding method on a recording carrier is irrelevant to the principles of the present invention. The disc shaped record carrier 1 is rotated in the direction V ₁ by a motor 2 having disc support spindles 3 protruding through the center opening 4 of the record carrier 1.

An optical system for decoding record carrier 1 is contained in housing 5. First of all, this optical system includes radiation source 6. This radiation source impinges on the reflector 8 through the translucent reflector 7 and emits a radiation beam a ₁ which is reflected by the reflector 8 as the radiation beam a ₂ . The radiation beam a ₂ is reflected by the reflector 9 to the radiation beam d ₃ , which is focused by the lens 10 to the scanning spot PLH on the upper surface of the recording carrier 1. The upper surface of this recording carrier 1 is reflected so that radiation beam a ₄ (modulated by information) is reflected and impinges on reflector 9 through lens 10 and is reflected as radiation beam a ₅ and then radiation beam a by reflector 8 Reflected as ₆ . This radiation beam a ₆ impinges on the translucent reflector 7 such that a portion of the radiation beam is reflected as radiation beam a ₆ incident on the decryption detector 11. This schematic decryption detector 11 detects the information contained in the radiation beam a ₇ and supplies the corresponding electrical signal to the output terminal 12.

This signal at output terminal 12 is applied to a decoding device 18 which converts an applied television signal (encoded in a special way) at terminal 19 into a standard television signal which can be used. The design of this decryption apparatus is determined by the encoding of the television signal used during recording on the record carrier. For example, if a complex system is used in which a complete standard television signal (i.e. complete synthesis of brightness and chroma signals is added to the carrier as frequency modulation), the decoding device uses a frequency demodulator. have no relation.

In order for the scanning spot P to always be incident on the information track on the recording carrier, the radial tracking control system is equipped to control the radial position of the scanning spot. The control system firstly comprises a measuring detector for measuring the radial position of the scanning spot P. For simplicity, it is assumed that this measuring detector is included in the decoding detector 11 and this supplies a control signal to the terminal 13. Examples of systems in which this radial position of the injection spot can be measured are described in US Patent No. 3,381,086, US Patent No. 3,876,842 (PHN 6292), and Canadian Patent Application No. 957,067 (PHN 5503). This control signal at terminal 13 is applied to a drive device 15 capable of pivoting the reflector 8 around the axis 14 in the direction V ₂ . By turning this reflector 8, the direction of the radiation beam a ₁ changes, so that the radial position of the scanning spot P also changes. From the average angular position of the reflector 8, a second control signal for the drive device 16 capable of moving the housing 5 in the radial direction V ₃ is derived.

Due to variations in the speed of the drive motor 2 or the eccentricity of the opening 4 with respect to the center of the recording carrier 1, the decryption television signal exhibits a time error which is particularly disturbing for color reproduction. To correct these time errors, a time error correction system is included in the optical decoding system. This time error correction is achieved by the reflector 9 which can be rotated by the drive element 17 in the direction V ₄ . By turning the reflector 9 in this direction, the scanning spot P is moved in the longitudinal direction of the information track on the recording carrier 1, so that the time error can be corrected.

The control signal required for the drive element 17 is obtained by the time error detector 25. The decrypted television signal is applied to this time error detector. First, the time error detector 25 includes an apparatus 20 for extracting a pilot signal suitable for time error measurement from the decoded television signal. The circuit arrangement of this apparatus 20 will be described later. A pilot signal extracted by the device 20 and whose frequency and phase exhibit time errors is applied to the phase comparator circuit 21 which receives a reference signal from the oscillator 22. At this time, the phase difference measured between the two signals is a measurement of the time error and is used to add the control amplifier 23 to obtain the desired control signal for the driving element 17. This figure also shows the possibility of deriving a control signal for the drive motor 2 for the recording carrier 1 from the time error detector 25 via a control amplifier 24. Since the two control systems do not have to meet the same requirements, the pilot signal contained in the television signal can also be moved for this purpose.

The above known form of the apparatus 20 for extracting a suitable pilot signal is schematically shown in FIG. 2 and the signals related to it are shown in FIG. 3. To clarify the operation of the FIG. 2 apparatus, reference is first made to FIG. 3A. Figure 3a shows an enlarged horizontal sync pulse S followed by a color burst signal B in accordance with the NTSC standard. For the sake of simplicity, the time scale is stopped within the horizontal sync pulse. The frequency of the color burst signal B is an odd integer multiple of the half-line frequency. This means that one line period after a particular zero path of this color burst signal accurately represents the other zero path of the color burst signal corresponding to the next line period. This is used for the apparatus of FIG.

The device is suitable for always detecting the same zero path of color burst signals, so that a pre-sequential pilot signal can be obtained, allowing a wide range of control of the time error correction device and accurate time error measurement. This accuracy is due to the fact that the edges of the color burst signal are very steep, so that the correct zero path can be detected in an accurate manner, and the burst signal is filtered through a narrowband pass filter to suppress the interference effects of noise superimposed on the decryption television signal. By passing through in advance, this zero path detection can be reduced by a simple method.

In order to ensure that the same zero path of the color burst signal is always detected, the apparatus of FIG. 2 comprises a pulse generator 38. This pulse generator 38 can be triggered by its leading end between the horizontal synchronous pulses, in particular between the horizontal erase level V _B and the peak level V _T , which are extracted from the decoded television signal. For this purpose, the device 20 comprises a low pass filter 36 connected to a synchronous separator 37 connected to an input terminal 31 and a low pass filter 36. This known type of separator detects the horizontal synchronizing pulse S as, for example, a threshold detector having a threshold VD, and supplies the keying signal generator 38 with pulses (Fig. 3C) generated simultaneously with the synchronizing pulses. The keying signal generator 38 includes a first single-stable multitude vibrator 39 which is triggered by the horizontal synchronizing pulses and in response thereto supplies a pulse T (FIG. 3d). The interval of these pulses T is selected such that the terminal edge of these pulses T appears within a period occupied by the burst signal. This pulse T is applied to the second V monostable multivibrator 40 to be triggered by the terminal edge of the pulse T and in response to supply the pulse W (Fig. 3e). This pulse W has, for example, a pulse width of 140 x 10 ^-9 seconds, that is, a half period of a color burst signal.

The output of the pulse generator 38 which supplies this pulse W is connected to the first input 45 of the gate circuit 41. A short pulse (FIG. 3B) follows the second input 44, which represents the zero path of the burst signal B. For this purpose, the device 20 comprises a low pass filter 32 which allows the frequency band in which the frequency of the burst signal is located to be extracted from the television signal applied to the input terminal 31. The extracted signal is amplified and limited by the limiter circuit 33 to obtain a square wave signal. The zero path of the square wave signal is detected by the detector 34. Detector 34 operates in the absolute manner in the case of NTSC color television signals. That is, detector 34 supplies an output pulse according to each zero path of the burst signal irrespective of the tilt indication. This is necessary in the case of NTSC color television signals because the burst signals B and B 'in two consecutive line periods are in opposite phase (Figure 3a) because the chromatic carrier frequency is fixed at line frequency 1/2.

The output pulse of the gate circuit 41 (FIG. 3B) which performs an AND function on the signals at inputs 44 and 45 corresponds to the pulse from the zero path detector 34 which always appears within the period of the pulse W from the pulse generator 38. As a result, a pulse is generated in a straight line of FIG. 3f which can be used as a pilot signal for time error measurement. If desired, this pulse can be applied to a monostable multivibrator 42 which derives a pulse with a particular pulse width, which is then useful for output terminal 43 and applied to comparator circuit 21 in FIG. 1 as a pilot signal. Can be. If the pulse width selected by the pulse supplied by the monostable multivibrator 42 is greater than the pulse width of the pulse W from the pulse generator, only the first pulse from the zero path detector 34 becomes the output pulse of the multivibrator 42 during pulse W. Because of this, different pulse widths can be used for these pulses W.

In order to increase the reliability of the operation of the device, one comprising a detector 35 connected to the low pass filter 32 and detecting the presence of a color burst signal can be used. The detector supplies the output pulse at the moment and during the presence of this color burst signal. Since this output pulse of the color burst detector is applied to the gate circuit 41 (input 46), the gate circuit can supply the output pulse only when the color burst signal is present. This additional circuit ensures that many video lines do not contain color burst signals during the vertical retrace period of the television signal (e.g. by noise generation). If the zero path detector 34 supplies a pulse within pulse W despite the absence of the color burst signal, no (error) pulse is generated at the output of the gate circuit.

The effect of this ancillary circuit becomes even more apparent when one considers that when using a modified comparator circuit 21, the "holding effect" occurs when the pulse is not supplied by the pilot tone separator. This comparator circuit 21 operates with a sawtooth reference signal from the oscillator 22, which is sampled at the moment determined by the pulses of the pilot tone separator. This sampled value is retained until the next sampling takes place. As long as the pilot tone separator 20 does not supply a pulse, that is, for a portion of the vertical period, it is held as a control signal for the last sample value time error correction system.

However, the error pulses from the pilot tone separator during the vertical track period control this time error correction system with a complete comparison method. After some image line with color burst signal is generated, this time error correction system is used again, but it is obvious that confusion occurs in the delayed image because it involves a delayed response.

It is also possible to add an additional burst signal to the television signal to be recorded during this vertical period in order to minimize the time when no measurement signal is available during the vertical period.

The problem associated with the above described method of generating a suitable pilot signal for time error measurement is caused by the fact that the position of the horizontal sync pulse relative to the burst signal is not accurately determined. This has several causes. First, the accuracy with which horizontal sync pulses are generated before the television signal is recorded is limited. Further, as described above, the accuracy with which the horizontal synchronizing pulses can be detected during reproduction is limited and in this regard, the sensitivity of the noise operates in part. Also, the phase relationship between the horizontal synchronization pulse and the color burst signal is confused because this burst signal passes through the band pass filter.

Since the keying signal W is derived directly from the detected horizontal synchronizing pulse, the position of this keying signal within the period of the burst signal is not precisely defined. In an ideal case, the moment at which the keying signal W starts, i.e., the position of the leading end is exactly positioned in the middle between two successive zero paths of the burst signal. If this keying signal W is shifted by an incorrect position of the horizontal synchronizing pulse, for example, the leading end of the keying signal may not appear until after the desired zero path of the burst signal as shown in 3h. In this case, the next zero path of the burst signal is used to generate a pilot signal (FIG. 3i). This change from one zero path to the next zero path for generating a pilot signal represents a time error of 140 × 10 ^-9 seconds, that is, a time error detection as a period between two zero paths, thus measuring the relative constant time error. This corrects the uncorrected time.

This problem is overcome by the means according to the invention. The means according to the invention mainly affect the circuit arrangement of the keying signal generator 38. To illustrate the means according to the invention, FIG. 4 shows the circuit arrangement of the gate circuit 41 and the pulse generator 38 forming part of the arrangement of FIG.

The pulse generator 38 shown in FIG. 4 includes a monostable multivibrator 51, to which an extracted horizontal synchronous pulse supplied by the synchronous separator 37 is applied (FIG. 2). This monostable multivibrator 51 supplies a pulse T applied to the set input 55 of the multivibrator 52 whose output is connected to the input 45 of the gate circuit 41. The pulse from the zero path detector 34 is applied to the input 44 of the gate circuit 41, which is keyed out by the gate circuit, which is then applied to the reset input 56 of the multivibrator 52, in which case Occurs after being processed by the monostable multivibrator 42. However, the pulse width of the pulse T supplied by the monostable multivibrator 51 is not constant and is converted in accordance with the calibration signal supplied to the control input 57 of the monostable multivibrator 51.

The operation of this circuit will be described with reference to FIG.

Figure 5a shows a number of pulses supplied by a zero detector 34 representing zero paths of burst signals.

FIG. 5B shows the pulse T supplied by the monostable multivibrator 51, the tip of which is again coincident with the tip of the horizontal synchronous pulse, and its tip is in the period occupied by the burst signal.

Ideally, the end of this pulse T should occur at exactly instantaneous to midway between two consecutive zero paths of the burst signal.

However, due to the inaccuracy of the horizontal synchronizing pulses, it is assumed that this end appears at the instant t ₁ . This end of pulse T triggers multivibrator 5c, so that the multivibrator is converted (Fig. 5c). When pulse Q from the zero path detector at time t ₂ is applied to gate circuit 41, the pulse of FIG. 5d is generated at the output of monostable multivibrator 42, which is long to reset input 56 of multivibrator 52. Since this pulse resets the multivibrator 52 to its original state at the instant t ₂ , the output signal of this multivibrator becomes the pulse w shown in FIG. 5C.

Since the pulse width tw of this pulse w corresponds to the period t ₂ -t ₁ in the figure, it is clear that this is a measurement of the position of the end of the pulse T between two consecutive zero paths of the burst signal. This data is used in the present invention to automatically correct the position of the distal end of pulse T.

The pulse width tw of the pulse w is measured by the measuring circuit 53 and converted into a calibration signal for the monostable multivibrator 51.

In the example of FIG. 5, this calibration signal reduces the pulse width of pulse T in such a way that the distal end of pulse T moves toward instant t ₀ , resulting in pulse w from para multivibrator 52 in FIG.

Therefore, the leading end of the keying signal w is always controlled toward the intermediate position between two consecutive zero paths of the burst signal, so that the symmetric maximum allowance at this position can avoid false time error measurements.

The multivibrator 52 has a monostable type that is automatically reset after 140 × 10 ^-9 seconds, for example, when there is no reset signal at the input 56.

An embodiment of the monostable multivibrator 51 associated with the measurement circuit 53 is shown in FIG. Measuring circuit 53 includes a capacitance C ₂ (4.7 μF) connected to voltage source V ₂ and charged through resistor R ₄ (220 kΩ). The charging of this capacitance C ₂ , ie the voltage across it, changes in accordance with the signal at the base of the transistor T ₁ . Since the base of the transistor T ₁ receives the output pulse W of the multivibrator 52 and this transistor conducts during the period of this pulse, the voltage across the capacitance C ₂ is determined by the pulse period of the output pulse W of the multivibrator 52. The voltage across capacitance C ₂ is applied to emitter follower T ₂ and converted into a control current applied to control terminal 57 of monostable multivibrator 51 by resistor R ₃ (150 kΩ).

This monostable multivibrator 51 includes an SN 74123 (signal) type integrated multivibrator circuit 58, whose time constant, i.e., the pulse duration, is defined by the capacitance C ₁ (3300 pF), resistance R ₁ (2.2 kΩ) and R ₂ ( 6.2 kΩ). Because the control input 57 is connected to the capacitance C _2, it can be charged on a time constant and the capacitance C ₂ is changed according to the correction current. By specifying the magnitude of the resistance and capacitance as specified, the function of the correction current in the pulse period of the output pulse of this monostable multivibrator circuit 58 is at least about 140 x 10 ^-9 seconds, i.e., up to 1/2 of the NTSC color burst signal. To change. This circuit can be modified in many ways. For example, a variable delay device may be used instead of a monostable multivibrator.

FIG. 7 shows a variation of the circuit arrangement of FIG. 4, in which the corresponding elements are designated by the same reference numerals. The output signal of the limiter circuit 33 in this circuit device is directly connected to the input 64 of the continuous line, which is applied directly to the zero path detector 34, and is not directly connected to each other. It is applied to the input 63 of the two-position switch 62 with the main contact 65 connected to the detector 34 '. This switch 62 is controlled by control circuit 66 which receives a series of horizontal synchronizing pulses supplied by the synchronous separator 37 at input 67.

At this time, the control circuit 66 supplies the switch 62 with a symmetrical control signal of 1/2 line frequency so that the switch is switched between the lines.

Therefore, the color assist signals of the continuous lines applied to the zero path detector 34 'always have the same phase with respect to each other.

This means that the zero path detector 34 'does not require an absolute form, such as the variant of FIG. 4, but must respond to the zero path occurring in the case of one particular indication of the slope of the color burst signal. This means that this zero-path detector 34 'takes the form of a simple monostable multivibrator.

As described above, the present invention is not limited to the optical decoding device, but can also be applied to the decoding device for magnetic recording carriers in the form of a tape. In addition, the circuit arrangement of the time error correction system is not limited to the above-described apparatus.

In principle, suitable time error correction devices can be used electro-mechanically and electronically, the ultimate choice usually depends on the type of decoding device.

In addition, the present invention is not limited to the above-mentioned coding apparatus, for example, in which a complete standard NTSC color television signal is frequency modulated on a carrier wave. The present invention can be equally used in an encoding system in which, for example, a chroma signal is converted to a low frequency band during recording, i.e., recorded as a modulation of a separate chromatic carrier, while the brightness signal is frequency modulated on a relatively high frequency carrier. In such an encoding device, the frequency of the separated chroma carrier is generally an integer multiple of the 1/2 line frequency. During reproduction, this decoded chromaticity signal is reconverted to the standard frequency band by combining with an appropriate synthesized frequency, so that a standard NTSC or PAL color television signal is obtained die. Since the synthesized frequency has the same time error as the readout chromaticity signal, the influence of these time errors on the finally obtained standard color television signal is reduced. This can be achieved by the directed selection of the frequency of the separated chromatic carrier, for example, a pilot tone generating this synthesized frequency as a deciphered horizontal synchronization pulse, as described, for example, in US Pat. No. 3,803,347 (PHN 4978). . Instead of a series of these horizontal synchronization pulses, the zero path of the color burst signal can be made by the device according to the invention.

In this case, a color burst signal shown in the color signal can be used, or an additional burst signal on the trailing end of the horizontal sync pulse added to the brightness signal during recording can be used.

Also, there is no need to separate the pilot tone from the complete readout television signal (decoder 18 output). Depending on the encoding device, this pilot tone can be separated anywhere in the decoding sequence.

Although the present invention is mainly advantageously used to decode color television signals, the present invention can be used to decode black and white television signals.

For this purpose a burst signal must be added to the tail of the horizontal sync pulse during recording.

Finally, the possibility of adding an additional burst signal to the television signal during recording will be explained.

This burst signal can be used to accurately measure time error during playback. This possibility is shown in FIG.

This figure shows the horizontal synchronizing pulse S of the television signal having the horizontal clearing level V _B and the peak level V _T.

However, prior to recording, the burst signal E is superimposed on the horizontal sync pulse that is the peak level on the S V _T. The burst signal E is an integer multiple of the half line frequency and preferably has a frequency that is an integer multiple of all the line frequencies.

During playback of the recorded television signal, this additional burst signal is extracted, and then in a similar manner as described above with respect to the NTSC color burst signal, the specific zero path position of this additional burst signal is detected during each line period. It can be used to measure time errors.

Using this additional burst signal is very important when recording and playing back color television signals according to the PAL standard. In this case, the color burst signal cannot be used in the time error measuring apparatus described above because the frequency of the PAL color burst signal is 1/4 fH odd multiple and represents a 25 Hz offset.

Also, if an additional burst signal with a frequency equal to a multiple of the line frequency is used, a non-absolute zero path detector can be used to detect the zero path. In other words, the detector only detects zero paths corresponding to legitimate or subordinate groups.

Therefore, since the period in which the coining signal is generated coincides within the entire period of the burst signal, it is compared twice with the use of the NTSC color burst signal. Finally, using this additional burst signal superimposed on the horizontal synchronizing pulse has the advantage that the first period, i.e., the time between the front end of the horizontal synchronizing pulse and the keying signal is shorter than when the color burst signal is used. This shorter period can be realized more accurately by the multivibrator circuit, which makes the device more reliable.

Claims (1)

As described in the text and shown in the figure, a burst consisting of a plurality of carrier periods having horizontal synchronization pulses and a frequency that is an integer multiple of the half-line frequency and having a predetermined phase relationship with respect to these horizontal synchronization pulses A time error correction device for correcting a time error in the decryption television signal, for decoding a record carrier on which the television signal including the signals is recorded; In order to detect the time error and to supply a control signal corresponding to the time error correcting apparatus, triggered by horizontal synchronization pulses, and in relation to the horizontal synchronization pulses so that a keying signal is generated within a period occupied by a burst signal each time. A time error detector including a keying signal generator for supplying a delayed keying signal; A decryption apparatus having a zero path detector for detecting a first zero path of a burst signal that appears after the keying signal is generated; In particular, a time error detector comprising a measurement circuit for measuring a period between the beginning of the keying signal and the next zero path of the burst signal and for supplying a corresponding calibration signal; And a keying signal generator controlled by the calibration signal to supply a keying signal associated with a horizontal synchronous pulse having a variable delay varying with the calibration signal.

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