16-11-2017, 03:44 AM
I haven’t yet found the definitive reason for the choice of the 39.5 MHz. As far as I can find, the reasoning behind BREMA’s decision were not recorded in the pages of either Wireless World or Practical Television. I can’t say that I’m actively looking these days; rather it is a sleeper that I might just happen across when looking for something else.
My best guess remains that it was because for low cost dual-standard receivers, 39.5 MHz allowed a relatively simple double-Nyquist IF bandpass that did not ridiculously restrict the 625-line video bandwidth. The 4.85 MHz that it allowed was hardly stellar, and perhaps it could be corrected to a bit over 5 MHz by a post-demodulator tilt circuit, although that was hardly likely on a low-cost receiver. But it was probably visibly better than the paltry 4.25 MHz that would be obtained at 38.9 MHz with a double-Nyquist curve. Receiver makers could, of course, for their less cost-constrained models, use largely separate IF-strip input bandpass filters and traps for each standard, in which case it wouldn’t matter much whether the 625 IF was 38.9 or 39.5 MHz.
The price to be paid was a steep increase in the image rejection requirement in order to accommodate the co-siting of transmitters at channels n and (n+10), which was part of the matrix agreed at the Stockholm 1961 ITU meeting, before the 39.5 MHz number was in play. This required the addition of an input tuned circuit to the German-style of valved UHF tuner, which normally had an aperiodic input with a bandpass interstage. That probably cost a dB or three in terms of noise factor, which had little reserve to begin with. (As it happens there is a concurrent discussion about valved UHF tuners in this thread over at UKVRR: http://www.vintage-radio.net/forum/showt...p?t=140359.)
39.5 MHz was probably about as high as the IF could be pushed without getting into impossible image rejection requirements. Evidently it checked the boxes in the self-interference avoidance category. And it placed the 625-line lower adjacent-channel sound at 41.5 MHz, which was the same as the channel B1 sound-trap, also a desirable feature at the IF strip entry given the high channel B1 field strengths found in some parts of the UK. So that was a small economy, and one that persisted into the single-standard receiver era for as long as the 405-networ remained live.
Cheers,
Steve
My best guess remains that it was because for low cost dual-standard receivers, 39.5 MHz allowed a relatively simple double-Nyquist IF bandpass that did not ridiculously restrict the 625-line video bandwidth. The 4.85 MHz that it allowed was hardly stellar, and perhaps it could be corrected to a bit over 5 MHz by a post-demodulator tilt circuit, although that was hardly likely on a low-cost receiver. But it was probably visibly better than the paltry 4.25 MHz that would be obtained at 38.9 MHz with a double-Nyquist curve. Receiver makers could, of course, for their less cost-constrained models, use largely separate IF-strip input bandpass filters and traps for each standard, in which case it wouldn’t matter much whether the 625 IF was 38.9 or 39.5 MHz.
The price to be paid was a steep increase in the image rejection requirement in order to accommodate the co-siting of transmitters at channels n and (n+10), which was part of the matrix agreed at the Stockholm 1961 ITU meeting, before the 39.5 MHz number was in play. This required the addition of an input tuned circuit to the German-style of valved UHF tuner, which normally had an aperiodic input with a bandpass interstage. That probably cost a dB or three in terms of noise factor, which had little reserve to begin with. (As it happens there is a concurrent discussion about valved UHF tuners in this thread over at UKVRR: http://www.vintage-radio.net/forum/showt...p?t=140359.)
39.5 MHz was probably about as high as the IF could be pushed without getting into impossible image rejection requirements. Evidently it checked the boxes in the self-interference avoidance category. And it placed the 625-line lower adjacent-channel sound at 41.5 MHz, which was the same as the channel B1 sound-trap, also a desirable feature at the IF strip entry given the high channel B1 field strengths found in some parts of the UK. So that was a small economy, and one that persisted into the single-standard receiver era for as long as the 405-networ remained live.
Cheers,
Steve
