HELP! Need a UHF-SECAM modulator

[Synchrodyne]

As I understand it – and I could well be wrong:

Infradyne conversion = oscillator on low side of signal

Supradyne conversion = oscillator on high side of signal.

As you say, with Band I signals and IFs in the 30 MHz range, infradyne conversion would be difficult as for some channels, the oscillator would be within Band I, or worse, within the IF channel. Also, the oscillator frequency range ratio required to cover the whole of Band I would be rather high, maybe around 15:1.

One may speculate that in the L’ case, backward compatibility with existing receivers might have been an issue. Recent system E/L receivers would have tuned the system E Band I channels (F2 and F4, supradyne conversion), and with appropriate fine tuning adjustment, might have been able to tune the new system L’ channels. The tuner output would have needed to match the existing system L IF channel, for which the channels needed to be inverted.

Also, the existing system L channel allocations (certainly UHF and presumably Band III) had been assigned on the assumption that receivers would use the standard IF, with “taboos” developed accordingly. (That’s in the ITU ST61 UHF allocation meeting notes.) Even it had had been decided that the 625 Band I channels would be receivable only by new receivers with a new IF channel, the latter might have carried some risk for the authorities in the event of any interference problems. On the other hand, receiver makers who departed from the standard IF for multistandard applications or other reasons did so at their own risk. At the time that system L’ was introduced (1985, I think), cross-border multistandard receivers were probably still very much a minority, so making things easier for that species might not have been a high priority.


Cheers,

[marceljack]

In fact most late french analog TVs were multistandard (B/G - L/L' -I and sometimes D/K), therefore since the use of SAW filters they used in practice a SAW filter designed for B/G (38.9 MHz with a video bandwidth of 5 MHz).
It was not possible to cover band I channels in a conventional manner with these IF values, so there were two solutions:
-using a converter in front of the tuner for band I channels,
-using a SAW filter with a second Nyquist slope around 33 MHz and sound 6,5 MHz higher.

Since band I was only used by two or three Canal+ transmitters (pay TV) the converter was sometimes supplied only upon request (free of charge), despite the fact that it was mandatory for a TV set sold in France to cover all the L / L' channels.

[Synchrodyne]

Thanks for that.

Approximately when did the double-Nyquist SAW filter IF strips become commonplace or in the majority in French receivers? My impression that the double-Nyquist SAWFs arrived circa 1990, but that is based upon just one or two datapoints.


Cheers,

[winston_1]

Originally Posted by inaxeon

You sure about that? I have one similar which claims to support L but doesn't actually do positive modulation and AM sound. Nothing in the specs for that positively state that one does?

Are you sure it is the MT-47?

There are modulators on eBay that claim to do system L that don't however.

[marceljack]

Originally Posted by Synchrodyne

Thanks for that.

Approximately when did the double-Nyquist SAW filter IF strips become commonplace or in the majority in French receivers? My impression that the double-Nyquist SAWFs arrived circa 1990, but that is based upon just one or two datapoints.


Cheers,

Yes, you are right, as far as I remember it's at the beginning of the 90s that these unconventional methods of covering band I L' channels appeared.

[patrickgnl]

Originally Posted by winston_1

Are you sure it is the MT-47?

There are modulators on eBay that claim to do system L that don't however.

The MT47 does support L as does the MT57, I have both…

I found the manual online. Attached…

Patrick