17-11-2017, 09:46 PM
As far as I know, the UK setmakers continued to use the 39.5 MHz in the single-standard era. For as long as the legacy 405-line transmissions continued, there was an advantage in that the 41.5 MHz trap did double duty as the lower adjacent channel sound trap and the channel B1 sound trap. A full range of SAWFs was developed around the 39.5 MHz IF in the late 1970s, with quasi-split sound variants added a bit later, and the later still, SAWFs that accommodated NICAM sound. So, there was no compelling reason to change away from 39.5 MHz.
I have a vague notion that one or two of the European setmakers may have used the 38.9 MHz IF in the 1980s or so, but right now I cannot find a reference.
Returning to the double-Nyquist notion, I first thought of that when reading this Wireless World (WW) 1961 October item about the Pye prototype dual-standard receiver:
Clearly, the basic IF bandpass curve was of the double-Nyquist type, with -6 dB points at 34.65 and 38.9 MHz. The narrower bandpass required for 405 was obtained simply by means of the 38.15 MHz sound trap. As shown, this gave a 625 video response of 4.25 MHz at the -6 dB point. That would have been poor even for System B, where one would hope for at least 4.5 MHz and a bit beyond. The 405 video bandwidth I’d estimate as having been around 2.75 MHz. It’s hard to tell from the graph, but the lower-side Nyquist slope should have been a bit steeper than the upper-side slope. Given that the lower-side trap was 1.75 MHz was below the lower -6 dB point, and the upper-side trap 2.0 MHz above the upper -6 dB point, it seems likely that the lower-side slope would have been a bit steeper.
Anyway, given that the 405 IF was more-or-less cast in stone, whereas that for 625 was not, a way to improve the 625 video response was by moving the IF above 38.9 MHz. One may imagine that some of the setmakers would have lobbied BREMA to do this, so that at least they had the option of using the double-Nyquist IF curve for their cheaper receivers. And as said previously, 39.5 MHz was probably about as high the IF could be set without running into impossible image rejection requirements and without overlapping the lower edge of Band I. It helped a little but not that much, and I imagine that many dual-standard receivers did not use the dual-Nyquist approach in the interests of obtaining a video bandwidth that was towards 5.5, and at least say above 5 MHz.
Here is the commentary on the image rejection requirements from WW 1962 October:
From the opening statement of that item, I should expect that the BREMA decision in favour of 39.5 MHz to have been announced in a preceding WW issue, perhaps with reasons, but if so, I cannot find it.
Cheers,
Steve
I have a vague notion that one or two of the European setmakers may have used the 38.9 MHz IF in the 1980s or so, but right now I cannot find a reference.
Returning to the double-Nyquist notion, I first thought of that when reading this Wireless World (WW) 1961 October item about the Pye prototype dual-standard receiver:
Clearly, the basic IF bandpass curve was of the double-Nyquist type, with -6 dB points at 34.65 and 38.9 MHz. The narrower bandpass required for 405 was obtained simply by means of the 38.15 MHz sound trap. As shown, this gave a 625 video response of 4.25 MHz at the -6 dB point. That would have been poor even for System B, where one would hope for at least 4.5 MHz and a bit beyond. The 405 video bandwidth I’d estimate as having been around 2.75 MHz. It’s hard to tell from the graph, but the lower-side Nyquist slope should have been a bit steeper than the upper-side slope. Given that the lower-side trap was 1.75 MHz was below the lower -6 dB point, and the upper-side trap 2.0 MHz above the upper -6 dB point, it seems likely that the lower-side slope would have been a bit steeper.
Anyway, given that the 405 IF was more-or-less cast in stone, whereas that for 625 was not, a way to improve the 625 video response was by moving the IF above 38.9 MHz. One may imagine that some of the setmakers would have lobbied BREMA to do this, so that at least they had the option of using the double-Nyquist IF curve for their cheaper receivers. And as said previously, 39.5 MHz was probably about as high the IF could be set without running into impossible image rejection requirements and without overlapping the lower edge of Band I. It helped a little but not that much, and I imagine that many dual-standard receivers did not use the dual-Nyquist approach in the interests of obtaining a video bandwidth that was towards 5.5, and at least say above 5 MHz.
Here is the commentary on the image rejection requirements from WW 1962 October:
From the opening statement of that item, I should expect that the BREMA decision in favour of 39.5 MHz to have been announced in a preceding WW issue, perhaps with reasons, but if so, I cannot find it.
Cheers,
Steve
