09-08-2015, 05:02 PM
Murphyv310 Wrote:... This unit though is pure system A it's certainly not designed around negative modulation ...
I wouldn't expect anything different, Trevor, as that is the system it was obviously designed for!
As for modifying a System I modulator for System A, this should be reasonably easy - and much easier than than an A to I conversion, because of the FM sound.
In fact, the sound modulator in the System I modulator is no longer required.
Modifying the Vision modulator for positive mod shouldn't pose many problems and an external AM sound modulator working on 3.5MHz should be reasonably easy to build.
Assuming the IF SAW in the modulator uses the TAC frequencies of 39.5MHz and 33.5MHz for sound and vision respectively, the ideal response of the SAW should be as shown.
(Note that some System I SAWs follow European practice, which would mean the frequencies shown will be 600kHz high, but it doesn't affect the basic argument.)
If we move the Vision modulator to 34.75MHz and feed it with both the System A video and the AM sound mixed together* the output will be a composite System A VSB IF, for translation to any required output channel. It will have the correct 0.75MHz vestigial sideband and, provided there are not unduly high levels of video components above 4.5MHz, would be highly suited to systems requiring adjacent channel operation.
* It may be necessary to inject the 3.5MHz sound signal directly into the modulator section to avoid any problems with video clamping circuitry.
