Arghh, Philip put a colon on the [EE] ...

Subject: Re: Extracting the video signal from a 38.9MHz TV IF


>> 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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