Date   : Thu, 26 Nov 2009 21:56:24 +0000
From   : philpem@... (Philip Pemberton)
Subject: Teletext receiver revisited

Rick Murray wrote:
> You'd need to talk to a BBC bloke, and even then you might not get the 
> full answer if it is test signals or something regional.

Well, the stairstep is blatantly obvious. That's a transmitter 
performance / receiver linearity test of some description.

> The once-a-second could be the TSDP?

Maybe. PDC data?
What gets me is that it's black-level, then chirps once a second or so 
when it wants to send something. Strange.

Here's another idea: is it the Subtitles stream?
That would be vaguely plausible -- if the subtitle stream is coming from 
a separate VBI inserter. Effectively it becomes a backup system -- you 
ALWAYS want the subs, so if your TTX VBI inserter jacks it, you swap the 
cables (or tell the crosspoint switch to do a switchover) and your 
subtitles carry on until you fix the busted TTX inserter.

In the case of the BBC you then put out an alert that Ceefax is bust and 
will continue to be so until /X/. In the case of ITV you ignore the 
problem and hope nobody notices :)

Whilst we're on the subject of ITV, in their case this is even more 
plausible -- Teletext Ltd. provide the data, so ITV deal with subtitling 
and then just pass the video through the Teletext inserter on its way to 
the transmitter.

 > well... assuming your FreeSat receiver
> survived the recent balls-up of an upgrade [and here we can thank 
> FreeSat for *not* supporting DiSEqC because I use my Goodmans box... 
> hardly ever, it's the SL65 24/7!]

Hah, no Freesat receiver here, just a MythTV box with a copy of 
tv_grab_radiotimes for the EPG, a DVB-S tuner to get BBC HD, and a dual 
DVB-T tuner to receive two Freeview muxes at a time. Beats the crap out 
of any commercial DVB box (when it works).

High-gain wideband (yes, a grouped would probably do a better job, but 
all I could get my hands on was a wideband) aerial in the loft, with 
direct line of sight to Emley Moor. I seem to recall the roof of the 
house is within a few dozen feet altitude of the antennae on the top of 
the "big concrete tube o' the North"...

The stock Sky minidish+LNB is a piss-poor combination, IMO. I wanted to 
put a bigger dish, a triple LNB mount (Eutelsat and the two Astra 
constellations) and a DiSEqC switch up, but the neighbourhood 
pen-pushers and busybodies decided to make a big deal out of it (despite 
the fact it's only slightly bigger than the Sky dish, but round).

They make a Russian protection racket look positively welcoming.

-- 
Phil.
philpem@...          
http://www.philpem.me.uk/