Date : Wed, 25 Nov 2009 17:14:35 +0000
From : philpem@... (Philip Pemberton)
Subject: Teletext receiver revisited
Hi guys,
Anyone remember the big teletext receiver thread, wherein it was
suggested that a Teletext grabber might not be too difficult?
I've just got my sticky mitts on a couple of Philips "FI1200 Mk2"
series 'all in one' TV tuners -- specifically, a single FI1246mk2
(CCIR-I) and about half a dozen FI1236mk2 (US NTSC). These are little
(relatively speaking) tin cans that take +5V and UHF (and have an I2C
bus connection), and spit out CVBS video, baseband audio, and the second
audio subcarrier.
It seems the 1236Mk2 has no problem receiving UK TV, despite being
designed for the US "NTSC" system. The first IF is higher, but this
doesn't matter because the tin-can does the detection (IF-to-baseband
conversion) itself. All you really have to be mindful of is that you use
the right IF value when calculating the tuning factors for the PLL.
So anyway, I've been having a quick scrabble around the Vertical
Blanking Interval on BBC One, and it looks like:
- lines 7 thru 19 inclusive are Teletext all the time
- line 20 occasionally carries something that looks like a Teletext
burst (but may not be). Seems to be roughly once every second or so.
Maybe PDC data?
- line 21 is a test pattern. Peak white for 10us, black for a few
usecs, slightly-less-than-peak-white spike, black for a few usecs,
mid-grey spike, then a short break followed by a 6-level greyscale test.
- line 22 contains more Teletext data that looks like an incrementing
binary clock of some kind. The clock in the corner of a TTX page?
It looks like a data slicer shouldn't be too hard to hack together (I'm
thinking a couple of opamps, an LM1881 syncsep, maybe a black/white
clamp, and a 4046 PLL). But just out of curiosity... does anyone know of
a publicly-available spec for the usage of the VBI lines in BBC, ITV,
Ch4 or Ch5 transmissions?
Cheers,
--
Phil.
philpem@...
http://www.philpem.me.uk/