[sdiy] Wavetables

[Tom Wiltshire]

I absolutely agree with all Olivier's points here. This is a very concise summary of the pros/cons of computing a single sample against computing blocks of samples, and the problems you'll likely encounter.

T.

On 24 Jan 2011, at 21:43, Olivier Gillet wrote:

>> Which reveals a flaw in my current code - I'm checking to see if my pointer
>> value has reached maximum (and duly resetting if it has) *before* copying
>> the value to the port. Need to shift that down to the end of the function,
>> after the pointer increment. That way, the copy from the table to the port
>> is executed FIRST in the ISR. (Current code is looking for an external
>> interrupt, but same would apply if I had the code space to do this with an
>> on-board counter.)
> 
> I don't adhere to the "generate one sample per ISR call" philosophy,
> but if you ever do so, the right workflow is:
> 
> static sample_t sample;
> ISR() {
>  OUTPUT = sample;
>  // Code to recompute sample
> }
> 
> but not:
> 
> ISR() {
>  sample_t sample;
>  // Code to compute sample
>  OUTPUT = sample;
> }
> 
> Because in the second case, if the code has a variable runtime (due to
> conditional branches) you'll see jitter. I think this is the problem
> you describe.
> 
> 
> Personally, in absence of DMA, I favor:
> 
> ISR() {
>  OUTPUT = ring_buffer_read(buffer);
> }
> 
> +
> 
> while(1) {
>  if (ring_buffer_readable(buffer) < N) {
>    for (i = 0; i < N; ++i) {
>      // Compute sample.
>      ring_buffer_write(buffer, sample);
>    }
>  }
> }
> 
> The benefit is the following: if the synthesis code is very
> complicated, you don't have to push/pull from the stack the entire set
> of registers everytime there's a sample to render. A ringbuffer
> read/write per sample is way cheaper than pushing/popping 20 registers
> per sample. And this allows the computational time to be "smoothed",
> so you can use algorithms that do very fancy things once in a while
> (eg: sync).
> 
> Olivier
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