Hey Curt, seems like you are in a similar situation to me. Want to do loads of stuff with PG, no time to do it :0
Basically there are several different pieces of information sent to you on an analogue satellite signal. The whole signal takes up a chunk of bandwidth (range of frequencies) lets say for example 8MHz wide. The frequency you actually tune into is the carrier (usually the centre of the frequency range).
Inside that chunk, are several smaller chunks. The luminance (B&W picture just as it would have been in the old days): this might take up to the first 6MHz of the 8MHz chunk depending on the standard used. The chrominance (colour). This is cleverly hidden inside part of the range that the luminance is in, to provide backwards compatibility with B&W TV. and then the audio. In satellite I think the difference to broadcast terrestrial TV standards is that you can have a lot of audio subcarriers.
These separate signals are again each modulated around their own frequency, and it's this carrier frequency that you talk about when saying e.g. 7.38MHz. It's like a reference to the little chunk of bandwidth used by a particular audio signal within the bigger chunk of bandwidth used to get the whole signal across to you.
These separate signals are then mashed together (very imprecise technical explanation there), and then sent over to you on a very specific labelled carrier frequency so you can tune into it. That is the frequency you program into the receiver. However the exact frequency doesn't really matter to us. The LNB on the end of the dish already changes the frequency to send all the signals to the receiver, then the receiver tunes into the particular one frequency you wanted and demodulates the signal off the carrier. What you end up with inside the receiver is the mashed up signal of luminance, chrominance and all the audio channels all together, but these have been demodulated from/off the main carrier - essentially you have the 8MHz (or whatever) chunk that now starts at zero up to 8MHz. The 7.38MHz signal is the little chunk for one of the audio carriers, that is centred around 7.38MHz above zero.
Now what happens inside your standard receiver I can't quite work out. I assume there's more tuners to pull off the separate signals, and pump them into some other format your TV understands (remodulating them for example for RF connection, separating them for S-video output etc). However some receivers, particularly the ones for Weatherstar and PG, have a "baseband" output which is just this mashed up signal, as the receiver has it immediately after taking it off the main carrier, and therefore you'll need to connect something that can pull off the separate parts yourself. I think the video parts inside this signal are already at the same frequencies that a composite monitor will expect to see, so plugging it into a composite input will render the picture, but you'll need something special to get at the different sound subcarriers, as they are modulated way up there in the 6,7,8 or so MHz range. Not what your standard set of speakers/amp etc are expecting to see.
Of course, in PG, we actually WANT all the separate audio channels at their respective frequencies, because we have our own tuners in the sound ISA card that tune into the different sound channels, and the one in the data card that pulls off the data subcarrier (which itself is just again a sound channel with modem-like data broadcast on it), so the input to the Amiga (or Atari of course) comes from a baseband socket on a suitable receiver.
Regarding specifically the weatherstar, and the description you gave, I can't quite work out what they mean by the data being on 7.38MHz but in UK teletext format. I have briefly skimmed over patent 4,725,886 A and actually it seems to me that the data for the weatherstar is actually sent as teletext would be. They use the teletext prior art to solve their problem of data transmission which seems a fair approach to take! This is encoded in the VBI, and therefore I'd wonder what the 7.38MHz is all about, as that would seem to me to be an audio subcarrier.
As for doing it yourself, well for PG, I would wonder if we could find the equipment to take an audio feed and modulate it at 7.38MHz or whatever, and even better, find 4 of these and something that will mix them all together. For Weatherstar, on the face of it, it would seem to need something that takes data and encodes it in VBI.
In the UK, ceefax, the last remaining analogue text system is finally being retired with nearly the whole country being switched over to digital TV. I wonder if there may be some VBI encoders coming up on ebay?
