Did you consider the easy way out? http://www.ceefax.tv/ , type in page 199 and go from there Works fine I use http://teletekst.nos.nl/ for the dutch version to read up on my (old) home countrys news Peter van Hoof ----- Original Message ---- > From: Philip Pemberton > To: Microcontroller discussion list - Public. > Sent: Wednesday, July 29, 2009 2:18:17 PM > Subject: [EE]: Extracting the video signal from a 38.9MHz TV IF > > 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 -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist