On Friday, 23 June 2017 14:55:08 UTC+1, NY wrote:
"Max Demian" wrote in message
o.uk...
There were also controls at the back - I think you had to insert a
screwdriver through holes - one was "Height" which I had to adjust to show
the whole picture from my BBC Micro.

Yes, the PAL (either colour or plain 625/25 monochrome) outputs from a lot
of computers were a bit borderline-compliant. I had a Wren CP/M computer in
the early 80s and that had 625/25 RGB output, rather than the more normal
US-standard 640x480 in 525/30 format. The waveform was evidently a bit
borderline (if you connected one of the three colours to a TV's composite
video input) because the TV picture would roll briefly and suffered a bit of
tearing near the bottom of the picture. The waveform looked OK on an
oscilloscope: it had line sync etc included with each colour, as well as
having a separate sync pin, but evidently the timing was slightly out.

I made a PAL converter using a 4.43 MHz crystal and an RGB-to-PAL IC, and
this produced better results as regards sync - no rolling or tearing - so
maybe the presence of the PAL colour burst and clamping of sync, black and
white levels improved things. But although TVs could display the signal
fine, VHS VCRs couldn't lock and the head servo and tape-transport motor
made some alarming noises as they had a jolly good try!

The Sinclair ZX80 produced a good (though monochrome) signal from its RF
modulator: that would display and record fine.

625 line PAL VCRs recorded 405 line material happily enough. They didn't give a damn what was in the signal as long as the frame pulses were the right time apart.


NT