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Old 12th Jun 2012, 9:15 pm   #1
colly0410
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Default Video modulation question.

I've been reading up about world TV systems & I've noticed a pattern. Systems with positive video modulation use AM sound & systems with negative video modulation use FM sound, is this coincidence or is there a technical reason for this?
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Old 12th Jun 2012, 10:32 pm   #2
Peter.N.
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Default Re: Video modulation question.

I can't say for sure but I would think its because the use of FM wasn't very widespread in the days of positive modulation so as the newer systems took to using negative modulation FM technology was readily available available and was much less likely to cause the 'sound on vision' effect of AM sound.

Peter
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Old 13th Jun 2012, 12:12 am   #3
Synchrodyne
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Default Re: Video modulation question.

It was mostly by happenstance, I think, that we ended up with positive/AM and negative/FM systems, and no positive/FM or negative/AM systems. (Although some pre-WWII 441-line broadcasts might have been negative/AM?)

The EMI 405-line system pretty much predated FM broadcasting anyway, and no doubt Mr. Blumlein had good reasons for choosing positive vision modulation at the time.

NTSC (I) considered the vision and sound modulation separately and independently, and chose negative for vision and FM for sound. There is no suggestion in the deliberations that the choice of one affected the choice of the other. So positive vision with FM sound and negative vision with AM sound were both technical possibilities.

The NTSC work in turn informed the initial Russian work on the 625 line system, which was thus also negative/FM, as were most of the subsequent 625-line variants, including the Western European Gerber, Argentinean and UK TAC.

France followed the EMI 405-line precedent of positive/AM with its 819 line system, also eschewing the use of equalizing pulses. Perhaps similarity to its 441-line system, also positive/AM, was a factor in its choice. Or dare I say it, not following the American precedent, which thus outruled the use of negative vision modulation and FM sound, whether together or individually.

The Belgian situation seemed to have been determined more by political than technical considerations, but the French choice seems to have been influential as Wallonia was receiving French 819 transmissions from Lille (whose transmitter was allegedly sited to give good coverage into Belgium) before the start of Belgian TV. Thus the Belgian 819-line system was essentially the French positive/AM system shoehorned into a Gerber 7 MHz channel, albeit with the addition of equalizing pulses. And the Belgian 625-line system was a positive/AM version of the Gerber. If these choices were made to allow some simplification of multistandard receivers, then that was negated by the fact that most receivers also covered the Gerber negative/FM system, to allow reception of Dutch broadcasts in Flanders. Still, once it was determined that a less bandwidth-hungry version of the 819-line system would be used in Wallonia, thus making it different to the neighbouring French system, maybe it was thought of as some form of equality that the 625-line system for Flanders had to be somewhat different to the neighbouring Dutch system. Both Belgian positive/AM systems used pre-emphasis in the sound channel, unlike the other positive/AM systems. It is interesting to note that NTSC (I) had envisaged the use of pre-emphasis regardless of whether AM or FM was chosen. Tibbs and Johnstone ascribe some benefit to the use of pre-emphasis with AM, although less than for the FM case, not surprising given its rectangular rather than triangular noise distribution.

Thus largely because of the NTSC (I) choice, negative/FM became the majority type, with positive/AM in the minority. France chose positive/AM for its 625-line system to align with its existing 819-line system and so simplify dual-standard receiver design. But for its outré mer territories, it chose the negative/FM System K’.

Interestingly, and has been mentioned in other threads, Carnt and Townsend, in their seminal work on colour television, came down emphatically in favour of positive/AM over negative/FM for NTSC colour systems (and presumably this applied equally to the SECAM and PAL derivatives). On the other hand the NTSC (I) choices were more in the nature of one being a bit better than the other overall, rather than major differences.

Cheers,
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Old 13th Jun 2012, 10:58 am   #4
colly0410
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Default Re: Video modulation question.

Thanks for a very informative reply Synchrodyne.
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Old 16th Jun 2012, 9:48 pm   #5
Ray Cooper
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Default Re: Video modulation question.

Quote:
Originally Posted by Synchrodyne View Post
...There is no suggestion in the deliberations that the choice of one affected the choice of the other. So positive vision with FM sound and negative vision with AM sound were both technical possibilities....
Negative vision with AM sound is certainly a possibility, but positive vision with FM sound is somewhat of a no-no, for the following reasons:-

Systems using FM sound for TV have mostly (always??) used an intercarrier system for the sound channel, for definite reasons largely associated with local oscillator drift in the receiver. If you don't use intercarrier sound, then the local oscillator drift has to be kept within much tighter limits, which are easy enough in Band I but get increasingly problematic for Band III, becoming impossible for Bands IV/V. Hence the use of intercarrier sound, which largely eliminated the problem because the intercarrier interval (6MHz for UK) was constant and was set at the transmitter end, and was independent of receiver local oscillator frequency. A big plus, then , but there was a drawback.

One got one's intercarrier signal by mixing the sound and vision IFs in the vision detector, thus producing the constant intercarrier frequency. But to make this mixing work, there always had to be some residual vision signal present at all times, 'cos if it dropped below a certain level the intercarrier mixing failed and the result was colossal vision buzz on the sound channel. So, with negative modulation systems, the vision carrier never dropped below 20% of peak signal for white, rising to 76% or so for black and 100% for sync tips.(I'm neglecting the effects of colour subcarrier signals in this description, but in practice they made little difference to the operation).

Try to work the intercarrier stunt on positive modulation systems, and you start running into problems. Here, sync tips are represented by 0% modulation depth, so an intercarrier system would be in a continuous state of failure. True, you could make the system work by tweaks: you could put your sync tips at 20% carrier, black level would need to rise to 41% of peak carrier to preserve the 7:3 picture:sync ratio of the standard signal. You'd be throwing away almost half of your transmitter's dynamic range, pushing up the mean transmitter power requirements thereby increasing your electricity bills, and pushing the video part of the signal further towards that region of the transmitter's characteristic which is traditionally the most non-linear, thereby increasing the amounts of linearity pre-correction required.

I suspect that most TX engineers thought that sticking with AM sound on a positive-modulation system was far the easiest option....
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Old 17th Jun 2012, 4:12 am   #6
Synchrodyne
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Default Re: Video modulation question.

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,
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Old 17th Jun 2012, 9:36 am   #7
Ray Cooper
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Default Re: Video modulation question.

Quote:
Originally Posted by Synchrodyne View Post
... I learned that the French System L had been amended to accommodate NICAM stereo sound with a 5.85 MHz carrier. ... allowing the intercarrier technique to be used to recover the NICAM carrier.
That's very interesting, even though NICAM is not an FM system. Presumably a much larger level of 'vision-on-sound' would be acceptable for a NICAM carrier than for an FM one.

Anyway, we're discussing the early history here, are we not? Things like NICAM, and the complex integrated demodulator systems that you describe, would have had the 1930s pioneers reeling in disbelief.... they had to make it all work using a couple of glowing bottles...
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Old 23rd Jun 2012, 11:41 pm   #8
Synchrodyne
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Default Re: Video modulation question.

Yes, the more modern references were by way of a digression, and to some extent expressing my own surprise at finding situations where the intercarrier technique was used with positive vision modulation.

Anyway, in the light of those comments I have looked back over the NTSC materials to see if there was any suggestion that positive/FM systems were somehow excluded from the matrix of possibilities. But it was definitely the case that the vision and sound modulation questions were considered entirely separately. In the vision case, the deciding factor in favour of negative modulation was the ease with it enabled simple receiver AGC. Black level was set at 75% +/-2.5%, and white level was set at 15% maximum, with no minimum. Concern was expressed that getting down to 15% could be difficult for some early transmitters, particularly at the higher channel frequencies, but that design progress would allow this target to be met. The choice of FM sound was seen to be forward-looking. The difficulty of maintaining correct tuning, particularly at the higher frequencies, was seen as a challenge to receiver manufacturers, but one that would yield to further development.

Nowhere in the NTSC proceedings is mention of the intercarrier sound technique, and as far as I know it was not thought of until a few years later, in which case it might be seen as the modal solution to the FM tuning problem that the NTSC had mentioned. The lack of a minimum white level in the original specifications tends to confirm that intercarrier suitability was not an issue that was considered. Later the white level was specified to be within the range 10 to 15%. As far as I know both CCIR and OIRT originally chose 10% minimum as the carrier white level for their respective 625-line systems. I am not sure why 20% was chosen for System I; it does seem to be an outlier in this regard. I wonder if it was to better allow for colour subcarrier downward excursions?

Possibly then the NTSC unwittingly might have boxed itself into a corner had it chosen the positive/FM combination with sync at nominal zero carrier level, as then the intercarrier system – which it evidently did not foresee - could not have worked without change, and was the least workable of the four combinations given the technology of the time.

Cheers,
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