21-02-2018, 03:09 AM
Wireless World 1961 October included a description of a Pye dual-standard receiver that may have been a stepping stone between the 1960 prototype at interest and the initial production model:
The receiver described was intended for System I (initially known as the TAC system) in 625-line mode, but had a 38.9 MHz vision IF.
A close look at the vision IF passband suggests that it might have been derived from a previous System B model. The basic passband was of double-Nyquist form, with the -6 dB points at 34.65 (405 vision) and 38.9 MHz (625 vision). But the left-hand (34.65 MHz) flank looks a bit less steep than the right-hand (38.9 MHz) flank, which is really somewhat contrary. If anything, the 38.9 MHz flank should have been lesser-sloped. However, one may also imagine that if the 625 sound trap were moved from 32.9 MHz to 33.4 MHz, as would have been the case for System B, then the flanks would have been quite similar. So maybe Pye got from System B to System I simply by moving the 625 sound trap?
The double Nyquist IF was no doubt a low-cost way to build a dual standard IF, in that it could be used “as was” for 625, with a 38.15 MHz sound trap switched in for 405, certainly simpler than separate bandshaping filters for each standard. But there was definitely a penalty in terms of 625 vision bandwidth. 4.25 MHz (against the nominal 5.0 MHz) for System B was not very good, but 4.25 MHz (against the nominal 5.5 MHz) for System I surely fell into the grim category. Considering that Pye was a strong proponent of 625, one would have expected it to have designed prototype receivers that extracted the maximum from the system, including close to full vision bandwidth. (Not to mention the use of gated black level AGC, and black level retention/restoration.) One may hope that the production version got close to 5.5 MHz vision bandwidth.
Cheers,
Steve
The receiver described was intended for System I (initially known as the TAC system) in 625-line mode, but had a 38.9 MHz vision IF.
A close look at the vision IF passband suggests that it might have been derived from a previous System B model. The basic passband was of double-Nyquist form, with the -6 dB points at 34.65 (405 vision) and 38.9 MHz (625 vision). But the left-hand (34.65 MHz) flank looks a bit less steep than the right-hand (38.9 MHz) flank, which is really somewhat contrary. If anything, the 38.9 MHz flank should have been lesser-sloped. However, one may also imagine that if the 625 sound trap were moved from 32.9 MHz to 33.4 MHz, as would have been the case for System B, then the flanks would have been quite similar. So maybe Pye got from System B to System I simply by moving the 625 sound trap?
The double Nyquist IF was no doubt a low-cost way to build a dual standard IF, in that it could be used “as was” for 625, with a 38.15 MHz sound trap switched in for 405, certainly simpler than separate bandshaping filters for each standard. But there was definitely a penalty in terms of 625 vision bandwidth. 4.25 MHz (against the nominal 5.0 MHz) for System B was not very good, but 4.25 MHz (against the nominal 5.5 MHz) for System I surely fell into the grim category. Considering that Pye was a strong proponent of 625, one would have expected it to have designed prototype receivers that extracted the maximum from the system, including close to full vision bandwidth. (Not to mention the use of gated black level AGC, and black level retention/restoration.) One may hope that the production version got close to 5.5 MHz vision bandwidth.
Cheers,
Steve
