Hi Pieter:
Thanks very much for your comprehensive follow-up.
Re the UK UHF situation and its image rejection requirement, the attached Wireless World item provided some background:
Essentially it stated that the decision to move the UK standard VIF from 38.9 to 39.5 MHz significantly increased the image rejection requirement. That VIF change was made after the European UHF channelling plan was developed at the ITU Stockholm 1961 (ST61) meeting. The planning appears to have been based upon the VIFs existing at the time. From this page of the ST61 documentation:
although the actual IFs used in the calculations were not stated, one may deduce certain aspects. For example, the Italian case was calculated for both the Italian standard 45.75 MHz and CCIR standard 38.9 MHz VIFs. The Russian case was evidently calculated for two VIFs. One would have been the then-standard 34.25 MHz, the other might have been the CCIR 38.9 MHz number as a surrogate for a future higher VIF in that vicinity, which later materialized as 38.0 MHz. (Or perhaps 38.0 MHz was in view even then.)
I have not found any information as to why the UK VIF was moved up to 39.5 MHz. There did not seem to be any intrinsic necessity, as 38.9 MHz was used in South Africa for System I with the same UHF channels as Europe and with the addition of Band III channels. In the absence of “hard” information my best guess is that it was done to assist those setmakers who wanted to use a dual-Nyquist IF channel to simplify standards switching. Pye had done that with its prototype dual-standard receiver. But with the -6 dB points on the dual-Nyquist curve at 34.65 MHz (405 VIF) and 38.9 MHz (625 VIF), the 625 vision bandwidth was a paltry 4.25 MHz against the 5.5 MHz transmitted. (Recalling that the 1950 CCIR Gerber decision was a choice between 4.25 MHz vision bandwidth in a 6 MHz channel and 5 MHz vision bandwidth in a 7 MHz channel, and was made in favour of the latter.) Moving the 625 VIF up as far as reasonably possible was desired, and I suspect that 39.5 MHz was about the limit without creating near-impossible conditions in respect of image rejection, etc. This allowed a 4.85 MHz 625 vision bandwidth for the dual-Nyquist case, still on the poor side but perhaps just escaping being risible.
That VIF change did require a rethink in respect of UHF tuners. Cyldon had been the first UK maker to introduce such, and its initial UT model was three-gang, with an aperiodic input. It was reconfigured with a fourth gang and a tuned input. Mullard also had 4-gang valved UHF tuners, such as the AT6380:
Cheers,