[sdiy] Wavetables

[Tom Wiltshire]

On 24 Jan 2011, at 03:13, Matthew Smith wrote:

> Quoth Scott Gravenhorst at 24/01/11 12:26...
>> I apologize, but I can't help myself...  FPGA.
> 
> Were it not another thing to learn, I might even consider it AFTER I've got something working from bare(ish) basics, although it's a bit too close to "fix it all in software" for me. (Which I could do with an AVR XMega - onboard DAC.)

This isn't as easy as it sounds for variable-rate systems. I don't know specifically about the XMega, but the dsPIC I use is a similar thing. The limitation is that the DAC has a maximum update rate of 100KHz. Whilst this is great for fixed-rate systems, since you can do 96KHz sample rate at 16-bit and have double-rate quality, it isn't so good for variable-rate systems, where you really need the update rate plenty faster than that.
The PPG used a 250KHz rate, I believe, and your calculations gave you a 1.5MHz rate, which is what PPG would have used if the chips could have coped with it! If the rate is too slow, you finish up skipping samples in the wavetable at higher output frequencies. This makes the whole thing more like a fixed-rate system and causes aliasing.

As to the FPGAs, think of it as "custom hardware" rather than "all in software". But I hear you about "another thing to learn". It's something I'd like to look into, but I'm not going to get to it for a while yet.

Regards,
Tom