Hi guys,
Here's an interesting question for a Wednesday evening...

I'm hacking together a VBI inserter/extractor. Basically, it's a little 
box with an FPGA and some analogue support parts inside it, which 
receives a TV RF signal, strips the Teletext data off of it, and spits 
said data out of a parallel port. On top of that, it reads Teletext page 
data out of a RAM, and can either inject it into the output video 
(composite, CVBS, FBAS, whatever you want to call it) or leave the 
original data intact.

"It slices, it dices, it juliennes"

The problem -- and, as always, there's a problem -- is with the RF input 
side of things. While I can live with CVBS in/out, I'd rather like to be 
able to feed it a signal from my TV aerial, and tune in (say) BBC1 and 
browse Ceefax from the comfort of my computer desk.

Now obviously I need a tuner -- that's not a problem. I have a box full 
of Philips UV916MD tuners (think: UV916M with a booster amplifier tied 
to the front, and a LO/DX input). I give the tuner a frequency, and half 
a second or so later either the LOCK output goes active, or it stays low 
and I notice (after lots of debugging) that the aerial cable is 
unplugged... :)

The tuner spits out a 38.9MHz IF signal. That is to say, the signal I 
want is centred at 38.9MHz, with the sound carrier at (38.9+6)=44.6MHz. 
Frankly I couldn't care less about the sound, as long as the video is in 
good enough condition that I can extract it and pull the data out of the 
VBI.

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?

Now as I understand it, if it's AM I can just filter off the sound 
carrier, then use a synchrodyne-type receiver to get the video (which 
would probably involve designing a PLL to recover the carrier, and 
finding a suitable <100MHz mixer in my junkbox). Alternatively I could 
take the cheapskate's way out, and use a germanium or Schottky diode and 
a capacitor to demodulate it. I'd be a little worried about HF response 
with this option, though...

If it's SSB, then my only real option is to lock a PLL off the carrier 
(from experience, it's hard enough to get the things to lock to a pure 
50% duty 5V/0V signal) then use a mixer to pull the signal from IF to 
baseband.

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?

   * There's no way a 4046 PLL is going to run at nearly 40MHz; the 
datasheet for the HC4046 says 12MHz max. So if I go down the synchrodyne 
route, I need something that can handle a higher input frequency. Does 
anyone have any suggestions?


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...

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).

Thanks,
-- 
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist