Bandlimited waveforms algorithm?

The easiest way I've found to downsample is simple:

Accumulate the output of each channel every cycle and when you output a sample, divide that value by the number of cycles passed. It's not dreadfully slow, and produces a mighty-fine quality sound (although there are better methods)

For an example... every cycle the channel is clocked (this can be optimized by mutliplying Channel_Output by a value... I'll show sample code later):

OutputBuffer += Channel_Output;


When you output a sample:

SampleOut = OutputBuffer / CyclesPastSinceLastSample;
fTicksUntilNextSample += fTicksPerSample;

fTicksPerSample should be floating point and set to CPU_CLOCK_RATE / SAMPLE_RATE. On an NTSC system outputting at 44KHz, this will be roughly 40.58 (don't round too much). This means CyclesPastSinceLastSample will alternate between 40 and 41 as samples are generated.

This is the method I use in NotSo Fatso and I'd say it works quite well.

In case it's still unclear... here's some demo code to further clarify:

void DoSquareTicks(int ticks)
{
int mn;

while(ticks)
{
mn = min(nFreqCount, ticks);

ticks -= mn;
nFreqCount -= mn;

if(nDutyCount < nDutyCycle)
nSquareOutputBuffer += nVolume * mn; /*output volume if duty cycle is in positive section*/

if(nFreqCount <= 0)
{
nFreqCount = nFreqTimer.W + 1;
nDutyCount = (nDutyCount + 1) & 0x0F;
}
}
}


void RunAPU(int tick)
{
int mn;

while(tick)
{
mn = min( tick, ceil(fTicksUntilNextSample) );

fTicksUntilNextSample -= mn;
tick -= mn;

DoSquareTicks(mn);

nCyclesPassed += mn;

if(fTicksUntilNextSample <= 0)
{
fTicksUntilNextSample += fTicksPerSample;
OutputSample( nSquareOutputBuffer / nCyclesPassed );
nCyclesPassed = 0;
}
}
}



That code wont' work... but it'll give the general idea.

The method Blargg discusses in his doc (the one where you put transitions in a buffer, then run through the buffer later to generate samples) is likely faster and higher quality. It's kind of hard to understand though. I have yet to actually finish a player which impliments it.