Philips tech. Rev. 33, 181-185, 1973, No. 7 181 Signal processing in the Philips 'VLP' system W. van den Bussche, A. H. Hoogendijk and J. H. Wessels On the 'YLP' record there is a single information track in which all the information is stored for the reproduetion of a colour-television programme with sound. The photographic process used for writing the information on to the master record is highly nonlinear, and can therefore only be used for recording a signal with just two levels. The information can then only be present in variations in the distances between successive transitions from one level to the other. A system of coding will therefore be necessary if four or five signals are to be recorded simultaneously in this way (brightness, two colour signals and one or two sound signals). As is customary in television techniques, the two colour signals are processed as a single signal: a quadrature-modulated subcarrier, as in the NTSC and PAL systems [1]. At playback a reference signal is required for decoding; this must have exactly the same phase as the original sub carrier. Special measures have therefore been taken to ensure that the 'YLP' record player provides a reference signal that meets the specified requirements. In the coding process the brightness signal, which modulates the frequency of a carrier, is the principal signal. The. colour and sound signals, each modulating a carrier of lower frequency than the principal signal, are contained in symmetrical displacements of the zeros of this principal signal. The fully coded signal is recorded on the record as a pattern of short grooves or pits of the same width and depth, but variable length and spacing. On average the total length of the pits occupies exactly half of the total track length. This feature is made use of in the control system [2) that guides the read-out beam along the track. We shall now describe the systems for recording and playback, and explain the operation of the electronic units with the aid of block diagrams. Recording At a speed of 25 rps and with our coding system, the central part of the record will not reproduce correctly the part of the normal video spectrum (fig. 1) lying above 3 MHz. The original colour information present Ir W. van den Bussche, A. H. Hoogendijk and Ir J. H. Wessels are with the Philips Audio Division, Eindhoven. in this part ofthe spectrum thus disappears completely. For the parts of the track that are further out from the centre of the record this cut-off frequency has a higher value, so that colour information can be directly recorded and reproduced' there. However, this directly recorded colour information would cause an annoying interference with the colour information, which as - will be seen later, is recorded in such a way as to enable corrections to be made on playback for frequency variations in the colour carrier caused by variations in the speed of rotation of the record. By using filters to cut off the brightness spectrum at 3 MHz before recording, annoying interference can be avoided. In o Fig. 1. The video spectrum in the PAL colour-television system. The brightness signal occupies the frequency band from 0-5.5 MHz. This band also contains the colour information, which modulates a carrier of frequency Je (4.43 MHz). In the 'VLP' system the video spectrum is cut off above 3 MHz, while the colour carrier is shifted to I MHz. addition, the record can now be used with a minimum track diameter of 10 cm, which gives a considerable increase in playing time. The brightness signal, limited in bandwidth, modulates the frequency of a 4.75 MHz carrier; the frequency swing of this modulation is made less than the bandwidth of the information to be transferred. This gives sidebands that are wider than the frequency sweep. However, the higher-order sidebands of the high-frequency signals can be neglected, because of their small modulation index. It is therefore sufficient to record a frequency band extending 3 MHz on either side of the carrier. This means that there is bandwidth available below 1.75 MHz for colour and sound signals. The frequency of the colour sub carrier is therefore reduced from 4.43 MHz to 1 MHz, in the case of the [1] F. W. de Vrijer, Phiijps tech. Rev. 27, 33, 1966. [2] P. J. M. Janssen and 1;>. E. Day, this issue, p. 190.
182 w. VAN DEN BUSSCHE et al. Philips tech. Rev. 33, No. 7 PAL system, while the total bandwidth ofthe encoded colour signal is limited to ± 0.5 MHz. The sound information modulates the frequency of a 250 khz carrier, with a frequency sweep of 75 khz. If desired a second sound channel can be included at the lowfrequency end of the spectrum for stereo purposes or for a spoken text in another language. The three signals processed as described above for brightness, colour and sound are combined in an amplitude ratio of 10 : 2 : 1. The amplitudes of the signals for colour and sound 'can be smaller since the noise in these signals is less because of the smaller bandwidth. (Apart from spectral-distribution effects, the noise in a particular band is proportional to the bandwidth.) In the resulting signal, whose frequency spectrum is shown in fig. 2, colour and sound can be considered as artificial lower sidebands in a single-sideband modulation ofthe brightness signal as the carrier. Symmetrical amplitude limiting of such a single-sidebandmodulated signal results in the synthesis of the missing upper sidebands at the expense of the power in the lower sidebands. The amplitude ratio therefore becomes 20 : 2 : 1 after limiting. Looking at it in another way, the symmetricallimiting gives a signalof rectangular pulses in which the brightness information is contained as frequency modulation, while colour and sound information give a symmetrical modulation of the width of the pulses (duty-cycle modulation, fig. 3). This rectangular-pulse signal is suitable for recording in. a pattern of pits on the 'VLP' record. It now only remains to say something about the way in which a stable frequency is obtained for the colour subcarrier in spite of speed variations that may occur when the record is played. The frequency used for the colour subcarrier on the record is 1 MHz. This is exactly 64 times the line frequency in the video signal. By locking the two frequencies together, the colour information can be recreated at playback on a stable 4.43 MHz carrier obtained from the line frequency and the frequency of a stable 4.43 MHz oscillator. We shall now discuss, with the aid of a block diagram, the electronic equipment used in recording. Fig.2. The frequency spectrum recorded on a 'YLP' record. Solid lines: the spectrum after summing the brightness, colour and sound information; dashed lines: the spectrum after symmetrical limiting. The relative amplitude ratios of the various components are not shown to scale. fm centre frequency of the brightness-signal spectrum. fw frequency corresponding to the white parts of the picture. Ï«frequency corresponding to the peaks of the synchronizing pulses. fs> fs' the I MHz colour subcarrier.and the corresponding frequency arising in the upper side band on symmetrical limiting. fg, fg' the 250 khz sound carrier and its counterpart in the upper sideband. Fig. 3. Schematic diagram of the waveforms in the 'YLP' system.. H brightness signal, K colour and sound signal; the three signals each modulate the frequency of a carrier. S superposition of brightness, colour and sound signals. C the rectangular-pulse signal produced on symmetricallimiting; the brightness information is still present here as a frequency modulation. Because of the finite rise time of H the colour and sound signals give a dutycycle modulation of the rectangular-pulse signal. filter 3. The FM modulator 4 is a multivibrator driven by the modulation; it gives a rectangular-pulse signal modulated in frequency. However, if the duty-cycle modulation described earlier and shown in fig. 3 is to function properly, then the modulated brightness signal must not have too short a rise time. To ensure this the lowpass filter 5 is included, which makes the signal almost sinusoidal. The filter 6 separates the colour information from the video signal. The variable-gain amplifier 7 ensures a constant level for the colour signal, with the reference level derived from the 'bursts', constant-amplitude signals consisting of a number of oscillations at the colour-carrier frequency, which follow each linesynchronizing pulse. These bursts are added to a colour-television signal as a phase reference for decodt' 9 l Block diagram Fig. 4 gives a block diagram of the electronic circuit used in recording a 'VLP' record. After what we have said above, no further explanation is required of the processing of the sound signal by the audio amplifier 1 and an FM modulator 2. The part of the brightness signal to be recorded is separated out from the video signal by a lowpass ing the colour signal. The bursts are observed by the burst detector 11 at a command from the synchronization separator 10. If there is no colour signal and hence no bursts, the gain is reduced to zero, so that interfering signals in the colour circuit cannot cause coloured spots in the picture. oo_o 1
, Philips tech. Rev. 33, No. 7.. 'VLP' SIGNAL PROCESSING 183 The phase discriminator 12 synchronizes the 4.43 MHz crystal oscillator 13 with the subcarrier frequency given bythe bursts. This combination forms the 'electronic flywheel' normally used in television receivers to make sure that the subcarrier is available with the correct frequency and phase between bursts. A second oscillator 17 provides the 64th harmonic of the line frequency. This oscillator is locked to the line frequency by dividing the frequency of its output by 64 (I6), and comparing the result with the line frequency (I5). The mixer 14 gives the sum frequency of the two oscillators, 5.43 MHz. This sum-frequency signal is mixed in the mixer stage 8 with the colour signa I on the 4.43 MHz carrier, after which the lowpass filter 9. separates out the difference-frequency component. Playback The" output signal from the photodetector of the 'VLP' record player must be translated back again so that it can be played back via a monitor or television set. Apart from demodulating the brightness and sound signals the most important operations here are the transformation and correction of the subcarrier for the colour signal, and correcting the 'drop-outs' on the record as well as possible - 'drop-outs' are places where the signal is missing because of damage to the record. For playback on a television receiver the signal also has to modulate a suitable UHF or VHF carrier so that the aerial socket can be used as the input. We shall now look at the electronic circuit of the record player with the aid of the block diagram shown in jig.5. AUDIO VIDEO 4 5 Ps (1MHz) Fig. 4. Block diagram of the electronic circuit used in recording a 'VLP' record. 1 Audio amplifier 2 FM modulator 3 Lowpass filter 4 FM modulator 5 Lowpass filter 6 Bandpass filter 7 Variable-gain amplifier 8 Mixer stage 15 Phase discriminator 9 Lowpass filter 16 Divider circuit 10 Synchronization separator 17 Oscillator 11 Burst detector 18 Summation circuit (the ra- 12 Phase discriminator tios in which the compo- 13 Crystal oscillator nents are added is shown) 14 Mixer stage 19 Symmetrical limiter This is the colour information, modulating a 1 MHz carrier. After the three signals have been added in the correct ratios (18), they go through the symmetricallimiter 19. The signal is then suitable for supply to the light modulator used for recording on the 'VLP' record. Sound, colour and brightness signals are separated from each other by filters (2 to 7). The brightness signal (frequency between 1.5 and 6 MHz) then goes through the correction amplifier 8, which gives a falling linear characteristic. In the symmetricallimiter, which follows next, a purely frequency-modulated signal is produced.
184 W. VAN DEN BUSSCHE et al. Philips tech. Rev. 33, No. 7 Here signals are generated at the high-frequencyend of the spectrum at the expense of the signals at the low-frequency end, which were emphasized in the preceding amplification on account of this process. As well as this main channel, the brightness signal also goes through an identical subsidiary channel (11, 12, 13), which gives a signal delayed with respect to the main channel by exactly the time of one line scan. The signal is produced whose frequency is exactly half that of the frequency-modulated brightness signal. The drop-out detector 18 reacts to this and the switch 14 is operated during a period of 3 {J-s,so that the brightness of the preceding line is employed instead. This 3 {J-Sis found to be a sufficiently long time in practice since very few drop-outs of longer duration are encountered: The transit time of the signal through the Fig. 5. Block diagram of the electronic circuit of the 'VLP' playback unit. 1 Photodetector and preamplifier 2-7 Filters 8 Correction amplifier 9 Symmetricallimiter 10 FM demodulator 11 Delay line 12 Symmetricallimiter 13 FM demodulator 14 Electronic switch 15 Delay line 25 Electronic switch 16 Summation circuit 26 Crystaloscillator 17 Lowpass filter 27 Mixer stage 18 'Drop-out' detector 28 Oscillator 19 MonostabIe circuit 29 Frequency-divider circuit 20 Synchronization separator 30 Phase discriminator 21 Variable-gain amplifier 31 Symmetricallimiter 22 Mixer stage 32 FM demodulator 23 'Burst' detector 33 Correction circuit for 24 Switching amplifier 'drop-out' transmission characteristic of the delay line is corrected in such a way that a falling linear characteristic is again produced. In this way if a drop-out should be encountered the signal from the previous picture line is available as a replacement signal. The filter 17 separates out the part of the signal for which the frequency is lower than 2.5 MHz. This is used to detect drop-outs. If the detector misses a pit a demodulation circuit is longer than the time required by the drop-out detector, so that the switch 14 is already changed over before the drop-out in the signal arrives there. The most complicated processing is that undergone by the colour signal. The line amplifier 21 gives a constant level for the signal that has been separated out by the filters 3 and 6 (frequency between 0.5 and \ h
Philips tech. Rev. 33, No. 7 'VLP' SIGNAL PROCESSING 185 1.5 MHz). As in recording, the blocks 28, 29 and 30 again form an electronic flywheel, now driven by the line frequency ofthe playback signal, which reproduces the 1 MHz carrier, including the frequency deviations that have arisen during playback. The free-running crystal oscillator 26 gives a 4.43 MHz signal that will now have to function as the new colour carrier. The mixer stage 27-gives a 5.43 MHz signal, containing any frequency errors that may be present. The difference between this frequency and the frequency of the colour signal reproduced from the record is formed in the mixer stage 22. Since frequency deviations of the two signals cancel out, the desired stable 4.43 MHz carrier is recreated. The burst detector 23 provides the reference signal for the variable-gain amplifier 21 and at the same time ensures that in the absence of a colour signal the colour channel is inoperative ('colour killer', 24). If there is a drop-out the switch 25 short-circuits the colour circuit. Because of the averaging with the signal of the preceding line in PAL receivers, the missing colour fragments are now filled in at half saturation. 'Spikes' are thus prevented. The sound signal is separated out by the filters 2 and 5 (frequency between 150 and 350 khz) and is demodulated (32) after symmetrical amplitude limiting (31). This signal then also passes through the circuit 33, which counteracts spikes in the signallevel at drop-out. Summing the colour and brightness signals gives the complete video signal. A delay line 15 is used here to correct for differences in transit time in the two circuits. As was stated earlier, audio and video signals then have to be used to modulate a carrier for playback on a nórmal çolour television receiver. Summary. To be able to record all the video and audio information in a single track on the 'VLP' record a number of signal processings are required, particularly since the recording has to be done with only two signalieveis. The brightness signal, whose bandwidth is limited to 3 MHz, is used to modulate a 4.75 MHz carrier, the carrier of the quadrature-modulated colour signal is shifted to 1 MHz and the sound information is supplied as frequency modulation of a 250 khz carrier. Summation of the signals processed in this way in the ratios 10 : 2 : 1 and symmetricallimiting of the sum signal give a rectangular-pulse signal. This contains the brightness signal as a pulse-frequency modulation and the colour and sound as 'duty-cycle' modulation. The new colour-carrier frequency of 1 MHz is exactly 64 times the line frequency. By linking these two frequencies together a stable 4.43 MHz colour carrier can be restored at playback, independently of variations in the speed of rotation of the record.