[Coco] 1bit hsync nasties....

[Steve]

Simon ,

Well, you are really down two octaves then.  One from changing the code 
to use to use every other H-sync and the other from an error thinking 
that Octave 5 Note B was  493.883 Hz, when it's really octave 4 Note B.  
So the top end is only Octave 3 Note B.  Not much of an octave range for 
playing music.

If the code is still taking 75 out of 114 clock cycles (two H-syncs), 
then the leftover CPU time for real work is only 34.2% or only about 1/3 
of the CPU.  Still too costly in a Video Game and why it was rarely used 
(if ever) back in the day.

Since you are taking more then one H-sync, why does you try the 
summation trick to put the notes in perfect key?  It code something like 
this...

Sum            ldd #000    ;Sum store in this code
freq            addd #0    ;store the sub added for the freq here
                    std >Sum+1    ;save sum back for then pass
                    bnc  Skipflip   ;skip if over carry over
                    lda $ff20         ;flip the sound DAC
                    xor #$80
                    sta $ff20
SkipFlip

For the Note C in octave 3 (130.813 Hz), you use the following math for 
the number to add to the sum...

65536 / (7875 / note / 2)    for this note, 65536 / (7875/130.83 /2) for 
a freq(adder)=2177.

What does all this do?

This lets our timing loop be 30.10 units long.  About 10% of the time, 
the normal delay of 30 units will be extended by one to make up for the 
off timing.   The best part is there no reloading of the count down 
timer.  You just keep adding till you get an carry to show its time to 
flip the Audio DAC.

But my real problem is how it sounds. It's just a bunch low beeps or 
playing the scale.  How about a real song?  Maybe it will sound better 
to this old Sound engineer if it plays a familiar tune.

The other problem with the sound is all distortion in the waveform.
See: http://www.coco4.com/images/7875waveform.jpg

You will note of the waveform distortions at each transition and how it 
create harmonics and a real pain to these ears.  Try fliping the DCA 
with $80 or $90 in place of $F0.  This should remove some of the 
problems in the transitions.  Remember there is capacitor tied to the 
output of the DAC and it does not take Kindly to big swings on the out.

It good to see you trying to push things along.  But remember, us old 
time CoCo programmers have tried many of these roads before and found 
that some of them were dead ends.  So, if one does not work out then 
there are other roads to travel.

Steve


On 1/23/2014 1:58 PM, Simon Jonassen wrote:
> Hey guys !
>
> was contemplating if i should share todays 8 bit madness or not....
>
> well, here goes:
>
> the hsync music players irq has been refactored to intentionally use 58+
> cycles every pass (no best/worst case scenario anymore), so that means it
> skips an hsync so the irq fequency is now 15750/2 (7875Hz)
>
> yes the music player loses 1 octave, but we now have about 40% free cpu time
> to do other stuff.....
>
> this is a playback of the new irq routine:
>
> http://www.roust-it.dk/coco/sock/7875hz.wav
>
> if you have trouble loading this, it's because my DSL line is sick at the
> moment.... getting it serviced tomorrow.....
>
> /Simon :-)
>
>
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