All posts by Briantist
Below are all of Briantist's postings, with the most recent are at the bottom of the page.
frank watkins: Hmmm... Ofcom says ArqA is C52 and ArqB is C56 ( http://stakeholders.ofcom….pdf top of page 4) but Digital UK says the other way around ... Postcode Checker - Detailed View ...
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Mark A.: Actually, you're not quite right about BBC Mode 7.
In teletext mode, each character is a character not a pixel. The pixels were generated from the ROM, so each of the 40x24 lines used a byte.
In teletext mode, the colour was changed using a character, and there were codes such as "switch to graphics colour", where each of the following characters represented a six bit graphic (2 horizontal, 3 vertical) with some characters remaining normal.
The top bit wasn't used in normal teletext systems (as it was used for "parity").
As the colour and other codes were bytes, they occupied a space on the screen (unlike the way VT-100 works, for example).
One difference between Mode 7 and normal teletext was that you had to repeat the characters of double height text.
The ZX Spectrum had a mono-bitmap screen, by comparison, but with foreground and background colours set for every 8x8 pixels.
The BBC other modes were bitmapped and palette mapped.
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... you tended to use the CHR$ values above 128 to set the teletext colours in mode 7 because the OS would interpret the values 0-31 as "ascii" such as 13 being CR and 10 being LF.
In other modes, the 128-255 characters were "user defined characters" which were set using VDU ..
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Did I mention I wrote a BBC ROM to do teletext better than the one that came with the hardware?
I love 6502!
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.michael: Yes, it wasn't that long since the invention of the transistor - Moore's Law was in it's infancy.
Out of interest, I also wrote a networked Ceefax system when I was at BT Broadcast Services back in 1994
Here's an index page from 4th July:
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At switchover the Whitehead transmitter will be off
air from midnight until 11am on 24th October 2012 - see UK Digital switchover ends - Northern Ireland completes on 24th October 2012 | ukfree.tv - 10 years of independent, free digital TV advice for full details.
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Monday 22 October 2012 4:54PM
Richard Davis: To be fair, I did say "most equipment", not "all"!