> What I haven't been able to find out for definite is what modulation is
> used for UK-standard (which if memory serves is PAL I) RF video. About
> half of the references I've looked at say it's ordinary double-sideband
> AM, the other half say it's vestigial sideband modulation (aka VSB).
> Does anyone know for definite?

The signal is normally generated in the transmitter as a full DSB signal, 
and then filtered to drop one sideband - at least that is how it was done 
back when I was an apprentice. Now it probably is generated as an SSB and 
the carrier re-inserted, to get best transmitter efficiency.

> Now as I understand it, if it's AM I can just filter off the sound
> carrier,

The 'usual' method was to use a standard diode detector after the video IF. 
This then provides the baseband video plus 6MHz audio carrier. The standard 
trick was to use a parallel tuned circuit as a stop filter in series with 
the video siganl, and have a secondary winding on this to feed the audio IF. 
It is normal to put a stop filter in the video signal at the audio IF to 
stop what were known as 'sound bars' in the picture with audio changes.


> And now for the problems:
>   * I don't have any 38.9MHz TV IF (SAW) filters, nor can I find them
> in any of the catalogues I've looked in. I know Maplin used to sell them
> *years* ago. Does anyone know of a UK supplier of small quantities of 
> these?

I wouldn't use a SAW filter. You are not wanting the proper video, so I 
beleive that a bandpass tuned circuit ith a few stages (probably a single IC 
worth) of gain should get you enough. Essentially you are looking for the 
sync pulses and the data stream in the horizontal retrace periods. I cannot 
remember the data rate, but it is fairly slow, and has lots of ECC info as 
well.

> AIUI, the signal that's modulated onto the video carrier is the
> composite video signal, so all I'd have to do at the end is a bit of
> amplification and offset-correction (probably including AGC) to bring
> the sync, black and white levels back to where they should be...

You are interested only in the luminance part of the signal. The chrominance 
(colour) portion is on another subcarrier at 4.433MHz, and carries no Ceefax 
info. The Ceefax info is all during the 'visible' signal time in the 
vertical retrace blanking period of the transmission. About 4 lines carry 
the signal.

See http://pdc.ro.nu/teletext.html for some technical details.

> Lastly, does anyone know of a reliable reference (or indeed more than
> one reference) for this type of technology? As in, TV, video and so
> forth (maybe radio as well, though I'm already aware of the existence of
> the ARRL Handbook).

<VBG> See if your local library has issues of Wireless World. They had a 
good technical article or two when the service first started. I have a 
feeling they also had a project to build a decoder from discrete chips. 

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