[sdiy] Wavetable Design Update

[Matthew Smith]

Quoth Dave Manley at 02/02/11 17:38...
...
> roughly:
> 
> note number mod 12 -> note
> note number / 12 -> octave
> 
> 12 entry look up table followed by divide/mult by power of 2 for octaves

Thanks for that. Unfortunately I've already cracked the MIDI note number 
issue (got a bit of Perl to do the sums for me.)

My problem is that given a frequency f, I have to work out timer 
configuration values which are:

* The number to count up to
* The prescaler value, if required.

...for a given CPU clock frequency.  The frequency range I need to be 
able to calculate is pretty massive - from 2093.00 Hz for MIDI note 0 up 
to  3,211,226.61 Hz for MIDI note 127. Note a) this is to clock 256 
samples per cycle, b) I'm working to 2 decimal places, so really working 
in cHz.

So, whether I calculate on-the-fly or use a lookup table, I've somehow 
got to find an algorithm to give me the OCR1A (timer count) and TCCR1B 
(prescaler) values needed to set up the counter/timer to give me my 
desired output frequency. Higher frequencies will need little/no 
prescaling, whilst lower frequencies might require different prescale 
values to get as close as possible to the target frequency.

I *could* just spend half a day working the whole lot out by hand (trial 
an error on the prescale values) but it would be nice to have a means 
whereby I can throw an arbitrary frequency at the device and have it all 
calculated.

NOTE: The calculation needs to be done ONCE per note request, so I'm not 
terribly worried about timing. However many clock cycles it takes to do 
the math, I can't believe it will cause a sufficient delay before the 
timer is set and the note starts to sound that anyone would notice.

Cheers

M

-- 
Matthew Smith
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