Hi Pieter:
Thanks for the detailed analysis, which is quite compelling. I have added a few comments, as follows, but nothing material in respect of the numbers themselves.
Quote:
Originally Posted by Pieter H
our discussion on where the UK IF choices, or standard-related IF choices in general, came from kept haunting me. Putting it all together I've come to the following observations.[LIST][*]It seems that in the ideal case one would like to position the LO frequency in an "empty" frequency space between channels.[*]This is most consistently done with the French system-E 819-line standard, where most (Philips) receivers had the LO exactly on the band edge ow the active pair/impair band, while additionally it co-incided with the sound carrier of the opposite band.[*]This is exactly what is being done for UK CCIR-I. The need to change from the CCIR-G/H 38,9MHz IF was that the sound carrier of N+4 was in comparison 0,5MHz higher. For reasons as yet unclear to me there was apparently a preference to put the LO nearer the PC than the SC.[*]So for CCIR-I the centre between N+4 SC and N+5 PC would have been 39,0MHz, but it was likely moved to 39,5 to have the N+1 IF sound trap be on the channel A1 sound carrier at 41,5MHz, as discussed earlier.
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That all makes sense. As it was advantageous to move the IF upwards a little, then moving it to have coincidence between A1 SC and N+1 SC was even more advantageous.
Quote:
Originally Posted by Pieter H
Hi Synchrodyne,
[*]The interesting thing is that the G/H 38,9MHz is similarly perfect for the standard with 5,5MHz picture-sound distance AT UHF! At the time the 38,9MHz was standardized when only VHF off-air was being transmitted. However, this suggests the 38,9MHz choice was mainly determined by the future UHF use. I don't have the 1954 article by Holm and Werner on this, so can't check whether this assumption is true.
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It does seem possible that Holm and Werner were looking at a future that included UHF. In the USA, the 45.75 MHz IF was derived from consideration of the VHF requirements, and then the FCC used it to plan UHF channel assignments whilst avoiding interference problems. This resulted in a list of co-located and adjacent service area assignments to be avoided, known as the “UHF taboos”. Holm and Werner were no doubt aware of this history, and perhaps wanted to consider the UHF case in their work. I am not sure when the idea of standard pan-European 8 MHz channels for UHF was first mooted, but I think it was quite early on. A general consensus appears to have been reached at a 1958 CCIR Moscow study group meeting, reported in the CCIR 1959 Los Angeles documents, which suggests that the question had been under study for a while before then.
Quote:
Originally Posted by Pieter H
[*] As to the Italian (Philips) IF of 45,9, it is noteworthy that this is exactly one channel width (7MHz) up from 38,9, and would thus from an interference rejection perspective give the same performance as the standard 38,9. Might have been the reason, but just a theory.
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Well spotted! I’d missed that connection.
Cheers,