[stella] New idea for 6 channel music on the 2600

[Kevin Horton <khorton@xxxxxxxxxxx>]

I have an interesting idea for generating 6 channel music using nothing but 
a stock 2600.  The basic idea is to use the two volume registers on the 
chip to generate 3 channels each.

It appears that I can generate square waves, ramps, and triangle waves in 
this fashion, mix them in software, and then output their values to the 
volume registers with just enough time to spare.  The only downside is 
there can be no video while this is occurring (except for maybe flashing 
the screen or some other relatively static display).

The basic idea for this is to increment a phase accumulator for each 
channel every scanline and use the upper bits of it to select bytes from a 
waveform table; then add them to an accumulator, and finally output the 
results to the sound volume registers.

I already have some basic code written for this, and should post more about 
it in a couple days.  Unfortunately, due to cycle constraints, this thing 
will only work for linear note/length/instrument data, and cannot use a 
track/pattern/note setup (which I originally wanted to use).  There's just 
not enough cycles otherwise.

Would such a piece of code be at all useful?  I was thinking maybe a 
power-on tune, in between level tunes, etc.  It may be possible to 
"multiplex" the sound updating every couple scanlines with some TIA 
updating, but I dunno yet if this will be possible to do.  This code as it 
stands is very very cycle-intensive.

Thoughts? Comments?


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