Hi guys, I'm hacking together a VBI inserter/extractor. Basically, it's a little black 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. Or just leave the thing idling on BBC1 and dump the entire of Ceefax to my PC's hard drive.
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