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).
> 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).
>
I recommend Basic Television by Grob. I have the third edition, came
out during the 60's, and while it deals with a lot of tube circuits,
television never changed until recently with the DTV mess. This book
however, is NTSC/US specific. I understand that the basics of
operation can be applied to PAL signals and maybe even SECAM.
But this book is a great resource on every part of a TV, from the
carrier signal to the tuner, oscillator, amplification, gain control
and FM sound.
When are they planning to turn off the analogue TV in your area? Seems
like a very limited project life.
Best source of SAW filters etc is probably an old TV. Look around for
a faulty one, mostly they're just thrown away around here.
Google books has a lot of examples of TV circuits etc.
>> 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).
>>
>
> I recommend Basic Television by Grob. I have the third edition, came
> out during the 60's, and while it deals with a lot of tube circuits,
> television never changed until recently with the DTV mess. This book
> however, is NTSC/US specific. I understand that the basics of
> operation can be applied to PAL signals and maybe even SECAM.
>
> But this book is a great resource on every part of a TV, from the
> carrier signal to the tuner, oscillator, amplification, gain control
> and FM sound.
Richard Prosser wrote:
> When are they planning to turn off the analogue TV in your area? Seems
> like a very limited project life.
ukfree.tv reckons September 2011 for Emley Moor, so plenty of time yet :)
Truth be told, it just looked like a fun project. I'm also toying with
the idea of hacking together a COFDM/DVB-T receiver if I get a bit of
spare time, and the UV916 looks like it would probably make a nice
starting point.
And it would also make a nice front-end for a VHF/UHF software radio... :)
> Best source of SAW filters etc is probably an old TV. Look around for
> a faulty one, mostly they're just thrown away around here.
Yeah I thought of doing that, but it seems most of the modern TVs are
using tuners with built-in demodulators, or the whole lot is on one PCB.
Most of the folks round here got rid of their CRT TVs ages ago... :(
I do have a video board from some unknown ancient TV which *does* have
one SAW filter on it, but given the scorchmarks on the board, I doubt
it's still in functional condition. Helpfully enough my signal generator
only goes up to 20MHz, so testing it wouldn't be particularly easy either :(
Adam Field wrote:
> I recommend Basic Television by Grob. I have the third edition, came
> out during the 60's, and while it deals with a lot of tube circuits,
Just found a copy on Amazon for £5. It appears to be a later edition,
but essentially the same concept.
> television never changed until recently with the DTV mess. This book
> however, is NTSC/US specific. I understand that the basics of
> operation can be applied to PAL signals and maybe even SECAM.
Yeah, the main differences are the colour encoding, field rate and
number of lines per field. IIRC there are some other subtle differences
(sound subcarrier frequency?).
> But this book is a great resource on every part of a TV, from the
> carrier signal to the tuner, oscillator, amplification, gain control
> and FM sound.
Might be worth picking up Practical Television magazine if it's still
being published.
UK analogue was/is
PAL I = IF 39M5Hz - Sound 33M35Hz - 6MHz below. Modulation is
negative going, transmission horizontal or vertical (Bluebell Hill my
local transmitter was vertical if I recall correctly.) polarisation.
Video bandwidth 5M5Hz
Vision is AM vestigial sideband
Sound FM Nicam same as NZ!
Sources - my memory, RSGB handbook of yonks ago and splurge written by
University of St. Andrews
(www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part21/page2.h
tml)
Amateur TV uses full AM and can be slow or fast scan.
>
> I do have a video board from some unknown ancient TV which *does* have
> one SAW filter on it, but given the scorchmarks on the board, I doubt
> it's still in functional condition. Helpfully enough my signal generator
> only goes up to 20MHz, so testing it wouldn't be particularly easy either
> :(
>
Set the generator to 1/2 or 1/3 the frequency, and look for a harmonic to
come through the filter. If the generator is "too clean" add a diode to
make lots of harmonics.
philpem.me.uk
