LASERDISC SIGNAL PROCESSING
The CVBS signal, in standard PAL form, is frequency modulated onto a carrier signal (see Fig. 20.5), such that peak white cor- responds to 7.9 MHz, black level to 7.1 MHz and sync tip to 6.76 MHz.
Very little bandwidth-limiting is applied to the signal, so that its lower sideband extends down to below 2 MHz, and a replay response approaching 5 MHz is possible; the major area in the sideband for chroma signals is centred on about 2.5 MHz. On early discs audio- left and -right signals are frequency-modulated onto carriers at 683 kHz and 1066 kHz respectively, each with a maximum deviation of ±100 kHz. The amplitude of the sound carriers is about 25 dB down on that of the vision carrier. Later discs do not have this form of analogue sound track: their audio signals are recorded in digital form using the same EFM coded data as audio CDs. Fig. 20.5 shows that NTSC discs can carry both analogue and digital sound signals (a), while PAL types can carry one or the other but not both. In practice, late player designs have no provision for f.m. sound.
In the record system the signals may be represented as in Fig. 20.6, where waveform 1 corresponds to the frequency-modulated vision carrier based on 7.1 MHz, and waveform 2 the sound carrier. Add- ing the two renders waveform 3, which is now clipped or amplitude limited at fixed points equidistant from the zero line. The result is a PWM (Pulse Width Modulated) signal whose positive period determines the spacing between disc-pits, and whose negative period determines the length of each pit. This and the pit configuration are depicted in line 4 of Fig. 20.6.
During replay, the PWM signal from the disc surface appears as intensity-modulation of the spot of reflected light on the quad- formation pick-up photodiode array of Fig. 20.3. The outputs of all four diodes are summed to form the h.f. signal output, which is processed to recover CVBS vision, digital sound data, and (where applicable) separate L and R sound signals.
The complexity and high precision of the servo circuits and opti- cal components, and the great bandwidth of the system considerably simplify the signal circuits compared to those of a videorecorder; in fact the only complication in the disc player is a need for phase- correction of the chroma signal in track-hopping modes with CAV discs. At each outward-hop (cue mode) the phase of the subcarrier jumps forward by 90° per track, and at each inward-hop the phase of the subcarrier jumps backward by 90°. To maintain correct burst phase sequence (Bruch blanking) in the field blanking period, and to thus avoid upsetting the subcarrier regenerator in the monitor TV, these phase errors are corrected by a phase-shifter in the chroma processing section of the player.
Replay processing
The carrier signal is preamplified and a.g.c.-controlled, then split off in several directions. One path feeds the servo circuits (see Fig. 20.4) and a second the sound circuits. Each sound carrier frequency is selected by a bandpass filter 100 kHz wide then amplified, limited and f.m.-demodulated to baseband after which de-emphasis takes place. The EFM digital audio signal is processed by circuits virtually identical to those in an audio CD player.
The vision signal process starts with an h.f. compensation stage, which is mainly concerned with equalising the h.f. signal throughout
the playing time of a CAV disc, whose inner tracks are more densely packed with pits than the outer ones, leading to an initial shortfall in replay h.f. response. The ‘lift’ given to high frequencies is governed by the mean level of colour burst gated out of the demodulated CVBS signal. This process is called Motional Transfer Function (MTF).
Next comes a limiter to prepare the f.m. signal for demodulation. In fact there are two demodulators; the secondary one is fed by a 64 μs delay line so that it is working on a ‘1-line old’ signal. Incoming f.m. carrier level is monitored by a dropout detector which, when dropout is detected, throws the switch to select a good signal from the previous TV line, in just the same way as was described for video- recorders in Chapter 14. Because a conventional glass delay line does not have sufficient bandwidth to handle the full spectrum of f.m. signal there is a risk of spurious colour effects during the patch-job, and since no-colour is more acceptable than wrong-colour, a fast- acting colour killer is switched into the chroma channel for the dura- tion of the dropout.
At the demodulator output the video signal splits three ways: to the luminance amplifier/process block; to the frame/chapter code detector (a simplified version of the text decoder described in Chapter 8); and via a 4.43 MHz bandpass filter to the chroma processing stage. The luminance signal passes through the timebase corrector (in CCD or digital fieldstore form) on its way to an edge-enhancer and then to an adder block wherein it is reunited with the chroma signal, which has now undergone the phase-correction process necessary to restore proper PAL order to trick-replay signals.
The third signal to the video adder block is the output of a character generator controlled by the off-disc track-code signals; this inserts the digits (corresponding to the picture number, chapter code or time-counter) in the output video waveform. Its output is enabled by a user ‘display data’ command. Data also passes, via a parallel data bus, from the decoder to the syscon microprocessor, conveying the essential control information for transport management.
Multi-purpose and multistandard disc players
Many Laserdisc players are designed for use not only with full-size Laservision discs but also with smaller ones using the same system, often including NTSC types. The output from the latter may be presented in ‘pure’ NTSC; NTSC-4.43; ‘quasi’- or true-PAL, depending on the design of the player and whether it incorporates a complete digital field store. All LDV players offer composite video output;
some go further with S-video and/or RGB output ports. Also provided is a facility for playing standard audio CDs, in which every part of the machine, save the video processors, is used.