It is currently Sun Aug 20, 2017 6:56 am

Television Receiver Intermediate Frequencies

Pre War 240/405, Post War 405 Line, B&W Dual Standard, Colour Dual Standard, B&W S/S 625, Colour S/S 625 line, Hybrids. Standards converters & modulators, video recorders.
 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: Television Receiver Intermediate Frequencies

Post by Synchrodyne » Sat Aug 27, 2016 6:51 am

I recently found some information on some French Philips multistandard TV receivers at this website: http://tsf-schoser-2.e-monsite.com/page ... nes-page1/.

The examples shown generally seem to have followed Belgian multistandard practice, covering Systems B, C, E and F, with System E being limited to the Band III channels, F5 and upwards. The earlier VHF-only models had a vision IF of 38.9 MHz for all systems, and sound IFs of 33.4 MHz for Systems B, C and F, and 27.75 MHz for System E. Mostly, but not entirely, the border areas where such receivers would be used were covered by Band III French transmitters, so the lack of the French Band I channels might not have been a major drawback. But on the other hand, they were not suitable for France-wide use.

The VHF-UHF models from 1962 had 39.9 MHz vision IF for French System L, with 33.4 MHz sound IF, this being the same as apparent Belgian practice. But the vision IF for System E was also moved up to 39.9 MHz, with the sound IF moving to 28.75 MHz. Apparent Belgian practice in the VHF-UHF era kept System E at 38.9 and 27.75 MHz.

Given that receiver complexity was about the same either way, one wonders why the change in the French Philips case. Perhaps it was because, assuming correct receiver IF bandpass design, the Nyquist slope over 39.9 MHz was gentler, being intended for a 1.25 MHz vestigial sideband. That made it closer to what was ideally required for System E, with its 2 MHz vestigial sideband. The Nyquist slope over 38.9 MHz would have had to be suitable for the 0.75 MHz vestigial sideband of Systems B, C and F, and I doubt that it would have been switched to something gentler for System E. It is understandable that with French domestic multistandard receivers, more attention to detail would have been observed in respect of System E than was generally the case for Belgian receivers. I imagine that those French multistandard receivers also had a wider vision bandwidth (such as 9 MHz) for System E, whereas Belgian practice was to use the same bandwidth for System E as for System F.

The earlier receivers shown at the above-mentioned site include valve lists, and although the functions are not shown, the EF183/EF184 counts are suggestive of four-stage vision IFs, which in turn suggests that they had wideband IF strips.

I still harbour a suspicion that there might have been some French multistandard receivers that accommodated all of the French VHF channels, Band I and Band III, and so – to accommodate channels F2 and F4 with oscillator high - had standard (or similar) IFs with vision below sound. These might have had a standard CCIR (or similar) IF within the wider IF channel, as IF channel with vision below sound would not have enabled use of the Band I E-series channels with oscillator-high. That is not beyond the bounds of possibility, as after all, British dual-standard receivers had what was a tête-bêche IF channel.

Some French TV receivers (for example from Grammont and Telemaster, I think) were also equipped to receive UK System A broadcasts, as well as Systems B, C , E and F, and here I think the Band I channels would have been necessary. Thus the System A IF channel would necessarily have been inverted with respect to the System B/C/F IF channel.

Regarding oscillator position, one might say that although oscillator-low was used in some cases, oscillator-high was the modal choice for TV receivers, established by American practice in the immediate post-WWII period. For the high band/Band III (and later for UHF), it didn’t matter, as either oscillator position was workable.

It was the low band/Band I that was the determinant. Oscillator-low could be used, but it placed significant restrictions on IF choice, given that the oscillator should always lie outside (and above) the IF channel. So the highest available vision IF would be half of (the lowest channel vision frequency less the channel width). Oscillator-high obviated that problem, and allowed the IF channel to be placed just under the low band/Band I, which was its desirable position.

Thus in general oscillator-high, with resultant inverted IF channels, was adopted worldwide. France though was a different case. The adoption of tête-bêche channelling required the use of both oscillator-low and oscillator-high depending upon channel orientation. That was fine for the Band III channels, but for Band I, where oscillator-low working was difficult, it meant that only the tête or only the bêche channels could be used. The standard IF (28.05 MHz vision, 39.2 MHz sound) was inverted relative to the tête channels, which in Band I were F2 and F4, so these could be and were used. But the sole Band I bêche channel, F3, was effectively lost.

When UHF and System L arrived, the standard IF was chosen as 32.7 MHz vision, 39.2 MHz sound, for ease of dual-standard receiver design. Thus the IF channel was non-inverted with respect to the European UHF channels, but the required oscillator-low working was not a problem at UHF. Nor was it a problem when System L was extended to Band III. But the later extensions to Band I was problematical; to allow oscillator-high working, inverted channels were required, hence System L’.

The above-mentioned Philips multistandard receivers, at least those with UHF System L capability, would also have dealt with Band III System L (with appropriate system switching) but not with Band I System L’.

Cheers,

Steve

Previous

Return to Television



Who is online

Users browsing this forum: No registered users and 4 guests