[sdiy] Another FPGA synthesis project [resent]

[Eric Brombaugh]

On 11/19/2009 09:35 AM, Scott Gravenhorst wrote:
> Eric Brombaugh<ebrombaugh1 at cox.net>  wrote:
>>
>> I've started another FPGA synthesis project. Details here:
>>
>> http://members.cox.net/ebrombaugh1/synth/dsPIC_fpga/index.html
>
> Very cool.  How would the FPGA be used?  I would think a dsPIC could do wavetable
> synthesis by itself, so I imagine that you have something special in mind for the FPGA
> to do?.

The dsPIC can indeed to a certain amount of wavetable synthesis, but the 
processor bandwidth imposes a fairly serious limitation on the sample 
rate vs computational complexity. For this board the FPGA will have 
access to a huge wavetable space stored in the 32Mbit SPI Flash memory 
and sufficient bandwidth to run multiple voices at high sample rates 
with some fancy interpolation.

> Also, that's the same DAC as on the little boards you designed.  If you use 192 as a
> divider and 38.4 MHz as MCLK, you get 200 KHz as a DAC sample rate.  I've tested that
> on the little Avnet board because you can actually get 38.4 MHz with it's xtal and a DCM.

Yep - I'm going with something I already know works. This board has a 
24.576 MHz oscillator, so 96kHz is probably the top rate I'll run the 
DAC. I'm not so much interested in running the DAC at high rates because 
with the FPGA I can clock the oscillators at much higher sample rates 
internally and decimate down to the DAC rate without losing audio 
quality. That said, using a DCM to multiply the clock up to 49.152MHz is 
possible and then the DAC could be operated at 192kHz.

Eric