This information can be found at https://archive.org/details/bbc-micro-advance … ge/360/mode/2up pages 361-366. For modes 0-2:
R0 = 127
R1 = 80
R2 = 98
R3 = 8
R4 = 38
R5 = 0
R6 = 32
R7 = 34
R8 = 0
R9 = 7

reenigne wrote on 2026-01-19, 10:34:

This information can be found at https://archive.org/details/bbc-micro-advance … ge/360/mode/2up pages 361-366. For modes 0-2: R […]
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This information can be found at https://archive.org/details/bbc-micro-advance … ge/360/mode/2up pages 361-366. For modes 0-2:
R0 = 127
R1 = 80
R2 = 98
R3 = 8
R4 = 38
R5 = 0
R6 = 32
R7 = 34
R8 = 0
R9 = 7

Thank you very much!
It turns out that the BBC Micro video signal complies with the PAL/SECAM standards.
The total line duration is: (R0 + 1) * 8/16 = 64 µs.
The total frame duration is: (R4 + 1) * (R9 + 1) / 15.625 = 19.968 ms.

Indeed it does, being a British machine! There was also an NTSC version I think, but I don't know what the CRTC parameters are for that.

CGA also had PAL/SECAM versions, also running at 16 MHz. But without the 6845 and with RGB and monochrome composite outputs.

I've never come across a PAL version of CGA before! Can you point me to an example? The Amstrad PC1512 is another British computer that was (mostly) CGA compatible but it uses NTSC timings, same as IBM CGA. That one also didn't use a 6845 but didn't have any kind of composite output.

reenigne wrote on 2026-01-20, 08:23:

I've never come across a PAL version of CGA before! Can you point me to an example? The Amstrad PC1512 is another British computer that was (mostly) CGA compatible but it uses NTSC timings, same as IBM CGA. That one also didn't use a 6845 but didn't have any kind of composite output.

The Russian "Электроника МС 1502" is IBM PC compatible and has a CGA-like video adapter with PAL/SECAM timings:
https://ru.wikipedia.org/wiki/%D0%AD%D0%BB%D0 … 0%9C%D0%A1_1502
Even the original Digger worked correctly on it. That is, for the programmer, it emulated the original CGA. I even found a video online. Here Digger is translated into Russian, but the original also works.
https://www.youtube.com/watch?v=2fFnCFx0HuU
And the 8-bit Russian "Ириша" has a CGA-like video adapter with PAL/SECAM timings:
https://ru.wikipedia.org/wiki/%D0%98%D1%80%D0%B8%D1%88%D0%B0