Date   : Fri, 27 Nov 2009 00:45:16 -0000
From   : mfirth@... (Michael Firth)
Subject: Teletext receiver revisited

----- Original Message ----- 
From: "Philip Pemberton" <philpem@...>
To: "Rick Murray" <rick@...>
Cc: <bbc-micro@...>
Sent: Thursday, November 26, 2009 9:56 PM
Subject: Re: [BBC-Micro] 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.
>
You could very well be right, and that was my first thought when you 
described the stream.

About once a second is probably right for subtitles.

The subtitles generally have to be inserted seperately from the rest of the 
TTXT because:
1) They need to be synchronous to the program, so I believe are usually 
broadcast with it.
2) They come directly from the broadcaster, unlike the ITV / Ch4 / Ch5 case 
for the rest of analogue teletext, where it is sub-contracted to another 
company.

Regards

Michael