[sdiy] Wavetable Design Update

[Matthew Smith]

Quoth Paul Maddox at 03/02/11 13:02...
...
> HUH?
> What compiler are you using, that's wrong, so very wrong.
> you don't have chars promoted to int or something do you?

I thought HUH, too. I'll have another look at the code to make sure that 
it's not anything daft like the DATA being out (as in something that 
can't be represented as an unsigned char in there) and avr-gcc (for such 
it is) trying to be clever and giving me a different type, rather than 
an error.

I just know that when I was looking to save space, I whipped out what I 
thought was 128 bits worth of waveform and, well, it didn't add up.

> ahhh, ok, I forgot that part.
> You don't have enough IO on that to do what I suggested.

Think I found I was tight on pins for other stuff as well, as I am using 
pins to set the I2C ID (probably 3 pins) as well as having pins 
allocated to trigger and gate. (Also keeping the SPI pins free just for 
programming.)

> Use the phase accumulator approach.
> Only don't use 8 bits for address, just use the MSBit as your output
> to your counter circuit.
> All your ATTiny2313 need do then is the following;

Right. Pin count stays the same.

 > if MSBit of accumulator = 1, set output pin high
 > else if MSBit of accumulator = 0, set output pin low

Which looks like an AND and a shift, at first glance, so no conditional 
required. (Unless ANDing with an intermediate value and doing a 
conditional is more efficient than the shift.

...
> Just do that in your main loop and *nothing* else.
> The you have a UART RX interupt, and you have such that you send the following
Yeah - might still use UART, with a device address byte in addition to 
that list.

...
> to get a second oscillator, you could easily use the same MCU, like
> this (excuse bad ASCII)
> 
> MIDI -> MCU(MIDI) -> Tiny2313 (Oscillator 1) -> CPLD -> EPROM -> DAC
>                   -> Tiny2313 (Oscillator 2) -> CPLD -> EPROM -> DAC

That's EXACTLY the layout I was proposing. So the only thing that really 
changes with the phase accumulator approach is the software.

> superb, there's a whole chapter on phase accumulator oscillators.
> It's a great book, wish I had kept my copy now.
Which I will proceed to read, in addition to the Wikipedia reference. 
I'd actually been persevering with the other technique as a means of 
avoiding getting my head around phase accumulators which *looks* hard to 
me at first glance. However, at least I've found that there are some 
interesting issues in my approach, so I can't say I haven't tried that 
way before diving into PAs!

Thanks again.

Cheers

M

-- 
Matthew Smith
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