[sdiy] Wavetables

[Matthew Smith]

Quoth Scott Nordlund at 22/01/11 17:21...
...
> How are you clocking this?  
Microcontroller doing MIDI to square wave at f.n, where n is the samples 
per cycle.

I'm working on a single waveform generator at present. If I were to 
attempt polyphony, I'd have one microcontroller as an "assigner" (to use 
Roland terminology) reading in the MIDI, then passing out to a waveform 
generator using SPI or I2C.

So the clock rate to the system should always be pretty accurate, 
allowing for crystal variation.

>... Another option is to use 
> multisamples.  This can also mean that your clock source doesn't have  
> be adjustable over such a wide range.

I did wonder whether to split into, say, 4 units, each with a different 
number of samples per cycle - but if the hardware can handle the maximum 
clock rate, didn't think it necessary in the end.

> Your waveform can correctly represent n/2 harmonics for n samples.  
> Above this the spectrum will be inverted and repeated.  Historically 128
> or 256 has been reasonable.  Very low resolution waveforms are too buzzy.
> It may be useful to reduce this resolution, which you can do by switching
> off address bits.

Ah, now I have a magic number!

128 samples per cycle x 12544Hz (MIDI note 127) = 1.6MHz (ish.)

Have to check that my old rail of flipflops are OK up to that speed - if 
not, I might just simplify the whole thing, store the wavetables in 
microcontroller FLASH and do it that way. (Might also get me a working 
prototype this year rather than who knows when!)

Certainly no problems getting the clock speed from the microcontroller, 
as the timer only needs to toggle at 3.2MHz - I'd be running at 12MHz or 
even 20MHz.

Many thanks for your input!

Cheers

M

-- 
Matthew Smith
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