[sdiy] Another FPGA synthesis project [resent]

[Scott Gravenhorst]

Eric Brombaugh <ebrombaugh1 at cox.net> wrote:
>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.

I guess my lack of experience with a dsPIC prevents me from understanding how the dsPIC and FPGA
would work together...  Can you give a kind of 50,000 foot level description?  

I'm going to guess - the FPGA does the actual wavetable work and presents the oscillator outputs to
the dsPIC for further and more complex processing - such as filters, waveshapers, etc?

>> 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
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-- ScottG
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-- Scott Gravenhorst
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