From: Richard Atkinson (rga24_at_cantab.net)
Date: 2002-04-08 15:48:46
On Mon, 8 Apr 2002, Marko Mäkelä wrote:
> Yes, similar to the raster interrupts on the VIC-20. You only need to
> read the raster beam's position in order to determine when to start the
> timer. I wonder if the position could be determined by reading open
> address space (like $de00-$dfff on a C64). I wonder if the same could be
> done on the ZX Spectrum. That'd probably break all Spectrum emulators. :-)
I think you can take advantage of the fact that the SYNC signal is
Composite Sync, and play around with the VIA's edge triggerring on that
pin to work out when you receive a vertical sync pulse rather than the
much shorter horizontal ones.
> > I haven't got a reference handy of which lines (if any) the cassette
> > shares with the PSG, but it sounds like simple power supply noise
> > issues to me.
>
> Or adjacent signal lines interfering each other. I remember that the
> Soundblaster card installed to a 286 PC of my friend made quite some noise
> when the hard disk was accessed.
I would suspect power supply noise in this case too - maybe the supply
isolation in that computer was not so good, and the hard drive caused
voltage spikes in the same 12V or 5V line that was powering the SB
op-amps.
By the way, speaking of SoundBlaster cards, I received a very curious 8
bit ISA card from Japan a few days ago. It has one YM3812 OPL2 chip and
one Y8950 MSX-AUDIO chip, but no PCM wave support at all. This is made
even more interesting by the fact that the Y8950 has a 4 bit ADPCM
capability, and the buffer on this card is 64K (two 41256 DRAMs) not the
32K usually found on MSX expansions. It can even be upgraded to 128K. I'm
quite keen to modify some ALSA drivers to work with this card, but don't
quite know where to begin. Any Linux driver hackers on this list? :)
Richard
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