Heterodyne LD players? I've worked on a Pioneer LD-660 tube player, & LD-700 & CLD-V710 solid-state machine (based, if I recall, on the CLD-1010), & reviewed the service manuals for other players, & never seen a heterocolor circuit. Up until the introduction of sampling (CCD or digital) TBC, the players never had anything more complicated in the video circuitry than a 1H delay line for DOC. How did you determine that SCH phase was unlocked? I wonder if you might not have been fooled by timebase jitter somehow.IEEE Transactions/BTR
for 1974 November contains the paper "A Review of a Video-Disc System" by Kent Broadbent of MCA Labs (beginning on p. 338), based on the initial DiscoVision 20-minute CAV disc, which recorded a whole broadcast-standard NTSC signal, audio subcarrier & all. He gives a very clear description of the TBC method implemented in, to my knowledge, all LD players without a sampling TBC. As the servo loop for the spindle motor is controlled by a standard 3.58 MHz crystal oscillator, time-base deviations are due mostly to disc eccentricity & out-of-roundness, & mechanical vibrations, most of which are at frequencies substantially less than 15.75 kHz, so that line-by-line variation is rather small (about 20 nanoseconds in the worst case allowed by the specification for eccentricity, which is about 10 microseconds over the whole frame). The detected color burst is compared to the reference oscillator, & the error is used to drive a tangent mirror to vary the velocity of the read beam, keeping the line time & thus the SC frequency reasonably constant, although some variations still occur. Apparently a total variation of 5 ns was originally considered acceptable for reproduction by domestic receivers. See also Adler, "An Optical Video Disc Player for NTSC Receivers", Trans BTR
, 1974 August, pp 230 et seq, which describes the matter in terms of the Zenith thin flexible disc system. After discussing their initial use of a heteorcolor system, with downconverted subcarrier & pilot tone, which made for a complex & expensive signal path, he discusses the attempt to eliminate this complication using a tangent servo system.
Finally, when the phase detector output was added to the error signal, the horizontal stripes disappeared. Clearly, then, it is possible to let the tangential servo take care of correcting the chroma phase and thereby simplify the decoder considerably.
In the same paper, Broadbent gives the FM signal-to-noise ratio at 40 dB, equivalent to a 58 dB demodulated SNR, admitting that at that point the video SNR of actual pressed discs was limited but still "better than 40 dB". Winslow, "Mastering & Replication of Reflective Videodiscs" (IEEE Transactions on Consumer Electronics, 1976 November, pp318 et seq), gives CNR values from 57 to 66 dB for metal-film cutting, & 54 to 63 with photoresist mastering. This is for the condition with 3 unmodulated carriers, namely, video at 7.5 MHz, & 2.3 & 2.8 MHz for audio each 26 dB down from video. He does, however, report significant levels of intermodulation products, as high as -30 dB in one case, again with photoresist mastering. For injection molded replica discs, the figures are as much as 5 dB worse for IM, with little change in CNR.
My impression is that the actual discs were limited partly by pressing & mastering quality, as defects such as pit asymmetry or size variation would impair the SNR, & that considerable improvements were effected in these areas over the life of the format. I have a number of LDs, mostly later pressings, which strike me as being of excellent quality, & fully comparable with my experience of 1", although it has to be admitted that the heads on the C-Format machines in my living room are far from new. I also have some which are of mediocre quality at best, & the observation that some of these were pressed on the same equipment in the same years as the excellent ones reinforces my opinion that the source material is a strong influence.
Obviously, if the cutter is being fed from even a first-generation 1" tape, as is usually the case, the replica quality is going to be degraded from that to some degree, more so if the master is a tape-to-tape dub, which again was (to my knowledge) more the rule than the exception. More than that, if the mastering practices involved, or the actual masters themselves, were originally intended for VHS tape, there may well be substantial impairments. I have seen on disc what really looked like chroma noise from a colour-under tape, & wondered if it had been produced from a U-Matic source. This was from a company which did most of its business in the VHS market, & most of its LD releases had just the sort of muddy audio & video one would associate with that format.