Extracting the video signal from a 38.9MHz TV IF

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

Reply to
Philip Pemberton
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On a sunny day (29 Jul 2009 23:53:01 GMT) it happened Philip Pemberton wrote in :

Analog TV was PAL, and the modulation in UK was AM wit one sideband partly suppressed. Did it not all go digital there already? In that case you are wasting your time. Anyways BBC Ceefax on digital is a weird standard (non-standard), and the normal BBC Ceefax on satellite only shows a message that it is not there.. (The Ceefax data is part of the digital transport stream).

As to decoding the analog teletext I do it (or rather did it) with a Philips chip:

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Simple, output on the PC, interface via parport, no FPGA needed. But now all is digital here in the Netherlands so it does not work anymore :-) You can still use this chip to add text and simple graphics to analog video, but analog video is not used anymore, ALTHOUGH I MUST SAY THAT GOOD PAL IS MUCH BETTER THEN THAT DIGITAL STUFF. Especially better then the very low bandwidth encoded mpeg2 crap that is transmitted these days by some stations. Some audiophiles will also claim the old FM modulation had better sound then the mp2 encoding now used, and I will not challenge that. hehe

The digital way to do it, teletext / ceefax, you can find here, I adapted my videotext xkrs program for digital TV, scroll down a bit to jpvtx (the decoder), and xvtx-p (the display).

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All Linux and open source, so if you use, it respect the GPL license, or else be careful next time you meet a penguin, or the FSF even.

WTF is juliennes????

You need one of the IF strips, or a tuner IF module from an old BW portable, that is, again, if there is still analog in the UK. Else you can very simple make a tuned transistor amp with about 38MHz centre frequency, say 3 stages, and a diode detector, for IF amplifier. Use LCs, make the bandwidth a bit bigger by putting a resistor in parallel, use common base, coupling to next base with a few turns. Was it 10 turns, 220pF, 5k6, on 8mm former with adjustable core? More then 40 years ago I did it like that :-) But, again, hey, analog is dead, digital works different, find an other project that will at least give a few years of usability, like growing plants or flowers... I have an avocado plant (planted the pit), and it is not in its fifth year or so, getting really big, a tree, dunno where to put it :-)

Reply to
Jan Panteltje

It's definitely VSB that carries PAL System I and this must be stated in an ITU-R report somewhere although, oddly, it (VSB) doesn't appear in BT804 ('Characteristics of TV receivers essential for frequency planning with PAL/SECAM/NTSC television systems'

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However, you'd better be quick because analogue television will have been phased out from the UK by the end of 2012, or thereabouts - the London area will be the last to switch over, after the Olympics.

Chris

Reply to
christofire

And then you'll need to build a time machine to go to a time when this was relevant. Parallel port? VBI? WTH?

Reply to
a7yvm109gf5d1

On a sunny day (Thu, 30 Jul 2009 23:16:02 -0700 (PDT)) it happened snipped-for-privacy@netzero.com wrote in :

Mant cheap mobos have a par port these days. I did a search for those boards a while ago. I mean something like this:

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1 par 1 serial onboard graphics VGA out. etc etc.

The par port is very nice if you want to make a small embedded system. Look at it as an universal I/O port.

As to VBI, well, large parts of the world still have good quality analog PAL. For those, if they want to view Ceefax / Teletext/ Videotext NOW, you need to know about what is in the VBI. US never had any videtext to speak of, so for them the whole issue is like talking about going beyond LEO.

Reply to
Jan Panteltje

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UK vision IF is 39.5MHz, sound IF is 6MHz LOWER at 33.5MHz.

Some UK receivers do use the european 38.9MHz but the sound will be @=20

32.9MHz. (Not really any difference, as far as the tuner is concerned.)

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Lower vestigial sideband - only the upper sideband is tranmitted in full=20 but be carefull - the IF is inverted in the tuner (which is wht you got=20 the sound IF wrong above.)

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Can't help but bear in mind that the SAW replaces a complex filter which=20 ensures the correct vestigial sideband response, including co-channel=20 and adjacent channel sound traps and adjacent channel sound and vision=20 traps.

Most important part of the design of this filter, as far as teletext is=20 concerned, is the group delay response. SAWs were first used in early=20 teletext receivers because of the difficulty of eliminating group delay =20 using discrete LC filters. You really need to keep it withing 50nS=20 across the band and it probably rather late in the day to start your own=20 learning curve!=20

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There are lots of ICs specifically designed for the purpose. The Philips=20 TDA3853T is one example picked purely at random

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Note that the (test) application circuit specifically omits IF filters=20 and rejectors (see SAW above)

Your best bet will be to get a SAW IF strip from a scap TV.

Hurry up though, otherwise you won't get your project finished before=20 the analogue transmissions are switched off!

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None I can recall although I'm sure there are lots (I never read one=20 because I learn't my TV and electronics stuff the 'drip-feed' way from=20 the age of about 12 - nearly 20 years before teletext and SAWs=20 appeared!)

Try you local library. You never know what you might find on the shelves=20

- if it's a decent library, of course! Try both lending and reference=20 sections. Dewey references are 621.384 (Radio) & 621.388 (TV), IIRC.

--=20

Terry

Reply to
Terry Casey

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drive.

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(38.9+6)=3D44.6MHz.=20

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Geez, i may have started both earlier and younger, under ten and about

50 years ago. Damn, half a century in electronics already.
Reply to
JosephKK

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At that age my interests were magnets/solenoids/electromagnets and=20 motors - electrical, rather than what I would call electronics.

Oh, and that was over 55 years ago ...

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Terry

Reply to
Terry Casey

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Yes. I did say may have.

Oh, and i loved rebuilding clocks and other complex mechanicals at the time as well. I had a big advantage as my dad was an EE and brought home such toys as transistors and even early monolithic opamps since the middle 1950s. Plus we had really good surplus houses in the area at the time, not to mention Lafayette and Olson's many other electronics stores within bicycle distance by the time i was 12. I was buying parts "will call" at Newark by the time turned 16. Heady days. I claim that i understood color TV before my dad did (though not the same kind of understanding).

--=20 Transmitted with recycled bits. Damnly my frank, I don't give a dear

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Reply to
JosephKK

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Unfortunately, my circumstances were a bit different to yours as my dad=20 died when I was 3? and my mum was bringing me and my brother up on a=20 widowed mother's pension. So, no money and no 'state of the art' goodies=20 as presents ...

... however, when news of my interests got around, I became the=20 recipient of a variety of discarded radio receivers of various vintages!

Funny how nobody gave a second thought the safety issues involved when a=20 youngster took thse things apart and plugged them into the 240V mains=20 supply ...!

Somebody presented me with a large chassis for some completely anonymous=20 piece of equipment - no valves or anything obvious to identify it and=20 partly incomplete but it was obviously (fairly) new and became the donor=20 of a great many resistors and caps for a multitude of projects. I also=20 had a couple of useful shops near my secondary school for most of the=20 bits I couldn't otherwise scavenge ...

... Oh! And like you, I was also interested in clocks and other=20 mechanical devices ...

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Terry

Reply to
Terry Casey

than=20

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About as kindred spirits as timing, location, and circumstances would allow. I grew up in silicon valley. Currently in California. You?

--=20 Transmitted with recycled bits. Damnly my frank, I don't give a dear

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Reply to
JosephKK

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England, on the north bank of the River Thames east of London. Moved,=20 more by accident than design, after I got married, closer in to the edge=20 of Greater London. As I worked for 20 years in the City of London=20 itself, it made for an easy commute. To start with, my next/last job was=20 based fairly locally, though I spent the last 10 years of it travelling=20 over a large part of the UK. My position relative to the major road=20 network was as reasonable as one can get, so we stayed put.

My wife is now pressing for a move ... as far away as possible!

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Terry

Reply to
Terry Casey

dad=20

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vintages!

when a=20

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You might try Arizona, i suspect neither of you would like India much.

Reply to
JosephKK

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