Hi Ray, thanks for that.
For the longest time I had also thought that the positive vision-with-FM combination was outruled for the reasons you outlined. That was until I learned that the French System L had been amended to accommodate NICAM stereo sound with a 5.85 MHz carrier. Thus video bandwidth was restricted to 5.1 MHz and residual carrier level was pegged at 5 +/-2%, thus allowing the intercarrier technique to be used to recover the NICAM carrier. Details are available at:
http://www.pembers.freeserve.co.uk/W...tems.html#CCIR, footnote (6).
So that suggests that the vision carrier level adjustments of the kind that you suggested were needed to accommodate intercarrier sound were in fact implemented for one of the P (positive/AM) systems. 5% seems quite low for residual carrier, but evidently it was enough for the NICAM case. I think that the original NTSC value was 10%.
Perhaps a digression, but still I think germane to the issue: re the use of split sound for N (negative/FM) systems, it is interesting to note that the D.C. Read/Wireless World high quality TV Tuner design (October 1975 ff) used this technique. The oscillator drift problem was addressed by afc derived from the sound channel discriminator. I have a vague notion that some early American TV receivers used the same approach in the days before intercarrier sound became close to universal. In the UK, Dynatron also used sound channel-derived afc on some of its late 405-line only receivers. The original US version of the Sony Profeel TV tuner (VTX-1000R) in 1981 used split sound. In this case, there was a second conversion in the sound channel, from 41.25 to 10.7 MHz, with afc from the FM demodulator back to the second conversion VCO to deal with drift. But there was also an alternative intercarrier sound channel that could be used for incoming signals (cable channels, etc.) that had suffered from ICPM and so were not suitable for split sound treatment. Luxman did the same with its T407 and T107 TV tuners. I think though that the European versions of the Sony Profeel TV tuner used quasi-split sound from the start.
And still in the realm of intercarrier sound, quite recently I stumbled across the data for the Motorola MC44302A multistandard video/sound IF IC. It includes a PLL fully synchronous vision demodulator, but the big surprise was that it uses the intercarrier technique for AM as well as for FM sound. The fact that the regenerated reference carrier is continuous, not modulated, would facilitate this, but even so, there must have been a high level of linearity overall. This is an IC that seems to challenge the conventional wisdom. Also it includes a sync separator and horizontal PLL to provide timing for the AGC gating. The agc system is quite sophisticated, and at risk of wandering further off-topic I’ll quote the piece relating to P systems:
“With SECAM mode selected, a black level reference is established by AGC keying during the back porch. In order to correct for the inconsistent back porch level that is common between SECAM transmitters, a long time constant non–keyed peak white reference level is also established, and is used in conjunction with the black level reference to control the video output level. The peak white level is used in effect to slowly readjust the black level reference threshold over a limited range of ±10%. With this dual reference approach, the accuracy associated with a typical peak white detecting system is maintained without the usual sacrifice of speed, thus allowing a quick AGC response to airplane flutter and channel changes.” Motorola has used “SECAM” as a proxy for System L in its write-up.
Cheers,