From: Richard Atkinson (rga24_at_cantab.net)
Date: 2002-04-07 19:01:53
On Sun, 7 Apr 2002, Marko Mäkelä wrote:
> Yep, the video chip definitely makes the Oric the Spectrum of the 6502
> world. Without hardware modification, it's almost impossible to write a
> raster interrupt routine. (I say almost, since one probably could
> observe the memory access patterns of the video chip by reading from
> unconnected address space (if the Oric has anything like that).)
The "hardware modification" you speak of can be done entirely externally -
it's just a lead going between the monitor connector (and SYNC pin
thereof) to the cassette connector (IIRC). And the VIA's own timers can
then be used once synchronisation is achieved.
> > It uses a MOS 6522 VIA to control the PSG data, address and control
> > busses with some very interesting hacks resulting from this method
> > of controlling the PSG.
>
> Could the slightly audible distortions from the internal speaker while
> trying to load from tape be caused by this?
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.
Richard
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