[sdiy] dsPIC Stuff

[Scott Gravenhorst]

Robin Whittle <rw at firstpr.com.au> wrote:
>Hi Scott,
>
>Thanks for these swinging, delicate little electronic music-box pieces!
> I assume these are straight, without reverb.  The lower notes have a
>somewhat resonant quality so it sounds rather physical.

These are, in fact, completely bone dry.  Karplus-Strong is a digital waveguide model of
a tight string.

>Its great to hear what you have been able to do with three very small
>and inexpensive pieces of *digital* silicon, DIP packages, protoboard,
>hand-soldering and some serious firmware.  Did you write in C or Assembler?

100% Assembly language for both the voice engines and the MIDI controller.

>What do you think about the audio quality of the DACs?  

Qualifier: I'm no audiophile - I think it sounds pretty good.

>Did you go to any great trouble with power supplies, sheilding or op-amps to get the
>desired audio quality?

That's an interesting question - The power supplies - 9VAC 10000 mA wallwart powers the
whole thing.  3 half wave rectifier power supplies with 3.3v 5A reg, 8v 1A reg and -8 1A
reg.  Raw DC feeds to the regs: 3.3V has 3300uF, +8 has 470 uF and -8 has 470 uF.  No
shielding at all, bypass caps are what is recommended by Microchip.  Very short wires. 
Mixer/buffer opamp is a TL074.

>Can you tell us the sample rate you chose?

The DAC sample rate is 56.818 kHz.  The voice engines process the strings at 4 times
oversampling.

>For those not up to speed on the dsPICs with dual audio quality DACs,
>they are in various packages, including 28 pin DIP:
>
>  http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=8192&mid=14
>
>This table used to list the dual channel 16 bit audio DAC as something
>like "1-D/A 2x16-bit @ 100(ksps)", but I can't see this information now.
> An example of the DIP devices with this DAC is the dsPIC33FJ128GP802:
>
>  http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en532298
>
>  - Robin

Yeah, the part I used is the dsPIC33FJ128GP802, 28 pin DIP.  DAC is capable of up to 100
ksps.  Maximum rate depends on how the DAC is clocked and if clocked from the system
clock, what that clock speed is.  With 40 MIPS from a 20 MHz xtal using the system clock
as the source, 89 kHz or so is possible.  I chose 56.818 because the math worked out to
more in-tune strings tuned to concert pitch.  The design emulates a 50 string harp in range.

-- ScottG
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-- Scott Gravenhorst
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