08-12-2017, 02:48 AM
(18-11-2017, 01:51 PM)colly0410 Wrote: Was lucky that there were no broadcast stations in the 30 to 40 Mhz spectrum, that would have messed things up big time. When I was a squadie we'd use 36 to 38 Mhz for artillery - inf/armour/cav coms, what effect that had on local TV's I never knew. I got into trouble for tuning a radio to 41.5 Mhz to listen to TOTP's in Aldershot area, was a bit distorted as our sets were FM & TV sound was AM..
In most cases TV IF channels were chosen to be in a relatively quiet part of the spectrum, and once assigned as a national standard, whether de jure or de facto, were usually given some measure of protection against future deleterious occupation.
In the US case, the 40 MHz “high” IF channel was developed by a subcommittee of an industry body, namely the RMA, in 1948, but no doubt watched by the FCC. The FCC then used it as the basis for its UHF channel allocation plan, so had a vested interest in keeping it clear of interference.
In the UK, the standard TV IFs were also developed by an industry body, namely BREMA. Possibly the GPO was represented on the pertinent subcommittee, but would at least have had a watching brief.
Returning to the South African choice of 38.9 MHz for System I, as well as easing the UHF image rejection requirement, it may also have been seen as having a slight advantage in respect of local oscillator placement in Band III.
For example, the local oscillator frequencies for channel ZA4 were 214.15 MHz for the 38.9 MHz case, and 214.75 MHz for the 39.5 MHz case. Channel ZA9 vision was at 215.25 MHz, with a lower vestigial sideband extending down to 214.0 MHz. The 214.75 MHz local oscillator frequency (for 39.5 MHz IF) would be around 8 dB down on the receiver Nyquist slope, the 214.15 MHz local oscillator (for 38.9 MHz IF) around 24 dB down, and so less likely to cause any interference problems. So, absent any compelling reason to choose 39.5 MHz, 38.9 MHz looked to be the logical choice.
Cheers,
Steve
