GB1565233A - Device for recording or reproducing a video signal - Google Patents

Description

(54) A DEVICE FOR RECORDING OR REPRODUCING A VIDEO SIGNAL (71) We, TEO BILDPLATTEN AKTIENGESELLSCHAFT AEG TELEFUNKEN-TELDEC, of Bahnhofstrasse 28, CH-6301 Zug, Switzerland, a Body Corporate organised under the laws of Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- It is known to record a video signal, e.g. a television signal, along a spiral track of a round recording carrier, a so-called video disc. Recording and reproduction thus take place at a constant rotational speed, for example 1500 rpm. At such a constant rotational speed the relative speed between the disc and the recording or reproducing element is not constant. This speed becomes smaller as the diameter of the track decreases, thus towards the inside of the disc. The recorded wavelength thus decreases towards the centre of the disc for a predetermined recording frequency. This would not be any disadvantage in itself if the transducers operated independently of the respective wavelength.

In practice however the transducer has a characteristic curve according to which its output amplitude decreases as the wavelength drops. When recording, this means that the mechanical movement amplitude for a cutting process with a dropping wavelength is smaller with a constant control voltage for the transducer.

It is also possible that, in fact, the mechanical deflection amplitude remains the same yet the amplitude of the signal cut into the disc decreases as the wavelength drops, because there is a lack of sharpness of the cutting stylus.

During reproduction, the output amplitude of the electrical signal supplied by the, for example mechanical-electrical, transducer drops accordingly as the wavelength drops.

Recording of the video signal takes place generally in the form of a frequency modulated carrier. As fairly large wavelengths, i.e. low frequencies, have a higher output amplitude according to the stated characteristic curve of the transducer than small wavelengths, i.e. high frequencies, then there is a rise in amplitude of the lower side band of the frequency modulated carrier. Therefore it is necessary to lower the amplitude of the lower side band of the frequency modulated carrier with a filter according to the characteristic curve. The upper side band will not be considered in greater detail below because in general only the lower side band is utilized during reproduction and the upper side band is omitted.

In practice, it has been proved that when there is this type of correction, given a large track diameter, i.e. at the outer edge of the video disc where the average wavelengths are large, the sharpness of the image reproduced is poor and the signal to noise ratio is relatively small. On the other hand with a small track diameter, i.e. at the inner edge of the disc the sharpness is relatively good while the signal-to-noise ratio is small.

Thus sharpness and signal-to-noise ratio are not constant over the entire length of the recording track.

The invention seeks to provide means by which the sharpness and signal to noise ratio may be made as independent as possible of the respective track diameter so that they remain substantially unchanged during playing of a disc.

According to the invention, there is provided a device for recording or reproducing a video signal in the form of a carrier frequency modulated with the video signal on a spiral track of a video disc, comprising a transducer used during recording or reproduction and having an output whose amplitude decreases as the recorded wavelength drops and a frequency selective filter connected to the transducer for automatically lowering the amplitude of the lower side band of the carrier at the output of the transducer during recording or reproduction to an increasing extent as the diameter of the track decreases.

A video disc player is known (German Offenlegungsschrift No. 2,503,944) in which, when there is spiral scanning, the change in the transmission function (dependence of the amplitude of the scanned signal on the frequency) is taken into account by means of a filter as the diameter of the track decreases, the scanned frequency modulated carrier passing through the filter.

With this circuit, however, the amplitude of a pilot signal recorded at constant amplitude is evaluated and the pass region of the filter is changed in dependence thereon. Direct control of the decrease in amplitude of the lower side band of the scanned frequency modulated carrier is not provided in dependence on the respective scanning radius. the filter does not cause any variable lowering in amplitude of the lower side band and the respective position of the scanner radially to the video disc is also not directly evaluated. This arrangement does not deal with holding the sharpness and signal-to-noise ratio as constant as possible over the length of the track.

However in embodiments of the present invention, the varying gradient of the stated characteristic curve of the transducer is taken into consideration with large and small wavelengths, so that correction is matched in each case to the requirements of a certain track diameter. The correction which takes place and is dependent on the radius, may be used basically when recording or reproducing or both.

Correction during recording may also take the distortion brought about by the transducer characteristic curve during reproduction into consideration either completely or partially. Correction during reproduction may also either completely or partially balance out distortion which has occurred as a result of the transducer characteristic curve during recording.

The invention is preferably used with so called pressure scanning, as is described in U.K. Patent Specifications Nos. 1,266,202 and 1,266 203. It is not however restricted to a certain recording method but may be used generally for a spiral recording having different relative speeds between the scanner and the disc. The lowering of the amplitude of the lower side band is generally less during recording than during reproduction.

The invention will now be described in greater detail by way of example with reference to the drawings in which: Figure 1 shows a general circuit diagram of an arrangement embodying the invention; Figure 2 shows a characteristic curve of the scanner for the purpose of illustrating the mode of operation of the arrangement of Figure 1; Figures 3 and 4 show embodiments for the filters used in the arrangement of Figure 1; Figure 5 shows a group of curves for the decrease of the lower side band in accordance with the invention, which decrease is dependent on radius of the track; Figure 6 shows the dependence of the decrease of the lower side band on the radius of the track at a frequency of 1 MHz; Figure 7 shows a further group of curves for reproduction; and Figure 8 shows a curve for recording for the lowermost side band frequency of approximately 1 MHz.

In Figure 1 a video disc having a drive dog 2 is coupled to a shaft 3 of a drive motor 4.

The video disc I rotates above a fixed support 5 on an air cushion 6. Scanning takes place by means of a scanner 7 which is fixed to a carriage 8. The carriage 8 is passed over the video disc 1 in a radial direction by means of a drive 10 through a cord 9, according to the gradient of the spiral recording track. The exact guidance of the scanner takes place by means of the mechanically cut record groove in the disc and by means of a resilient mounting of the scanner 7 on the carriage 8. The signals are recorded mechanically on the video disc 1 in the form of a frequency modulated carrier for scanning during reproduction according to ' the pressure scanning principle. The drive 10 is also controlled by the motor 4 as is indicated by the broken line 11. The signal removed by the scanner 7 in the form of a carrier 17 frequency modulated with the video signal is supplied via a line 12 to a filter 13 acting as a correction network. In an FM demodulator 14 a video signal 16 is obtained at a terminal 15. The filter 13 serves to decrease the amplitude of the lower side band of the carrier 17 in order to correct the frequency response of the scanner.

The frequency curve of the filter 13, i.e.

the lowering of the amplitude of the lower side band is changed in dependence on the respective diameter of the track i.e. the respective position of the scanner 7 on a radius of the video disc 1, as indicated by the variable resistor 21 in the filter 13. this change is caused by the drive 10 via a mechanism indicated by the broken line 18.

In addition the variable resistor 21 of the filter 13 for example is mechanically coupled to the radial feed of the carriage 8.

The resistor 21 may be a potentiometer the axle of which is coupled to the drive of the cord 9. The change in correction may also take place via an element other than a variable ohmic resistor. Basically the use of a variable inductance or capacitance (Varicap diode) is possible in an appropriate network.

Figure 2 shows the characteristic curve of the scanner 7 during reproduction i.e. the amplitude of the output signal S in dependence on the recorded wavelength.

For clarity S/So, where So is the maximum value, is plotted instead of S. When there is a constant frequency deviation in the recorded carrier, a frequency deviation range 19, shown as a hatched area, occurs at the outer end of the track as a result of the lowering of the actual speed of the scanner relative to the track as the scanner moves inwards and a further frequency deviation range 20, shown as a hatched area, occurs at the inner end of the track, for the same reasons. These ranges are equal to one another. Starting from the deviation range 20, if a spectral line of the frequency modulated carrier is considered at a spacing A from the centre frequency towards the lower side band then there is a rise in the amplitude of the lower side band by the amount B. Thus the filter 13 would have to lower the amplitude of the lower side band by this amount at this spectral line. At the frequency deviation 19, thus at the outer diameter of the track when there is an equal spacing A of the spectral line from the centre frequency, there is only a rise in the amplitude of the lower side band by the amount B'. It can be seen that B' < B. Thus, at the outer diameter of the track a smaller lowering of the amplitude of the lower side band is necessary. This matching of the lowering of the amplitude of the lower side band in accordance with the respective diameter of the track during scanning is carried out by the filter 13 in Figure 1 controlled by the drive 10.

For example, during reproduction, the filter 13 may lower the amplitude of the lower side band by approximately 12 to 20 db as the track radius reduces from 105 to 45 mm respectively. A rise in amplitude of the lower side band means a rise in high frequency emphasis. This emphasis differs between the outside and inside of the video disc track. While a constant emphasis can be completely desirable, variations in the emphasis should be compensated for.

Figure 3 shows a practical development of the filter 13. The resistor 21 which is varied in dependence on the radius causes a lowering of the lower side band amplitude dependent on radius. The frequency modulated carrier 17 is supplied to the terminal 22 by the scanner 7 while the output terminal 23 in Figure 3 is connected to the demodulator 14 in Figure 1.

Figure 4 shows another development of the filter 13.

The group of curves shown in Figure 5 show the drop in the lower side band for the purpose of balancing the rise in the side band as shown in Figure 2. The lower side band which is approximately in the range of between 0.5 and 3 MHz is therefore lowered by the filter 13 as the curves in Figure 5 show. It can be seen that this drop depends on the respective track radius therefore on the position of the scanner radially of the video disc 5. It can be seen that the drop in the lower side band thus in the lower of these high frequencies of approximately 0.5 to 2 MHz is greater the smaller the track radius, i.e. the farther the scanner 7 has moved towards the centre of the video disc 5.

Figure 6 now shows the drop in the lower side band in accordance with Figure 5 only for the frequency 1 MHz in dependance on the track radius. It can be seen that the signals are lowered to a greater extent at the frequency of 1 MHz at the inner edge of the track than at the outer edge of the track. As a result the larger rise B at the inner edge of the track in accordance with Figure 2 is lowered to a greater extent than the smaller rise B' at the outer edge of the track. A signal 1 MHz in the lower side band of the frequency modulated carrier would, without the present invention have the rise B' at the outer edge of the track and the substantially greater rise B in accordance with Figure 2 at the inner edge of the track for example. By means of the filter whose effect is shown in Figures 5, 6 the larger rise B at the inner edge of the track is balanced out by the larger drop in accordance with Figure 6 at the inner edge of the track. The signal at the outer edge of the track having the smaller rise B' is lowered to a lesser degree accordingly in accordance with Figure 6 because the rise B' in accordance with Figure 2 is also smaller than the rise B.

The result shown may be achieved by special construction of the filter 13, e.g. by using a logarithmic potentiometer 21.

Figure 7 shows a further group of curves for the amplitude lowering during reproduction achieved by similar apparatus to that which achieves the result in Figure 5.

Figure 8 shows a curve for recording and in fact shows the lowering of the amplitude of the lowermost side band frequency (0.3- I MHz) in dependence on the respective radius of the recording track. It may be seen that there the lowering is smaller than during reproduction, amounting to 6 db from the outer to the inner track diameter.

In principle the filters according to Figures 3 and 4 may also be used for the lowering during recording.

Claims (11)

WHAT WE CLAIM IS:

1. A device for recording or reproducing a video signal in the form of a carrier frequency modulated with the video signal on a spiral track of a video disc, comprising a transducer used during recording or reproduction and having an output whose amplitude decreases as the recorded wavelength drops and a frequency selective filter connected to the transducer for automatically lowering the amplitude of the lower side band of the carrier at the output of the transducer during recording or reproduction to an increasing extent as the diameter of the track decreases.

2. A device as claimed in Claim 1, wherein an adjustable component is provided in the filter having a displaceable part coupled to a drive for a carriage for moving the transducer radially with respect to said video disc.

3. A device as claimed in Claim 2, wherein the adjustable component comprises an adjustable resistor with its displaceable part coupled to the drive of the carriage.

4. A device as claimed in Claim I, wherein the filter includes an inductance connected in series with a resistance element and a capacitance element connected in parallel across the resistance element.

5. A device as claimed in Claim 1, wherein the filter includes a resistance and a capacitance connected in parallel with one another, and a series circuit of an inductance and a resistor connected between earth and the output end of the parallel connected resistance and capacitance.

6. A device as claimed in Claim 4 or 5, wherein said resistance of the parallel connected resistance and capacitance is adjustable automatically in dependence on the radius of the track.

7. A device as claimed in Claim I, wherein the frequency selective filter lowers the amplitude of the lower side band, during reproduction by the transducer, by approximately 12 to 20 db as the track radius reduces from 105 to 45 mm respectively.

8. A device as claimed in Claim 1, wherein the filter has a characteristic curve the gradient of which increases as the diameter of the track decreases.

9. A device as claimed in Claim 1, for use in recording wherein the lowering by the filter of the amplitude of the lowermost side band frequency changes from the outer to the inner diameter of the track by approximately 6 db.

10. A device as claimed in Claim 9, wherein the lowering of the amplitude of the lower side band changes in linear manner in proportion to the radius of said track.

11. A device for recording or reproducing a video signal in the form of a carrier frequency modulated with the video signal, the device being substantially as described herein with reference to the accompanying drawings.