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Fri Oct 26 22:07:00 CEST 2007

SD-dac

A Sigma-Delta Modulator (SDM) can be thought of as an
error-accumulation DC generator: given a constant input, it will
generate the correct average DC output, with a quantization
error noise spectrum that is high--pass.

A First-order SDM is an extremely simple circuit: it consists of an
accumulator with carry flag output: at each output instance, the
current output value is added to the accumulator, and the resulting
carry bit is taken as the binary output, and discarded.

I had this idea of running an 'inverse interrupt' machine: instead of
loosing time in ISR, just run an infinite loop, but allow at each
instance one primitive to run, which needs to spend an exact amount of
cycles. Probably not worth the hassle, but could be interesting for
really tight budget.

Anyways, this could be an alternative to PWM for kriket sound
generation. It should in theory give better quality. but probably that
also needs a deeper accu. With fast interrupt it's only 3
instructions:

	movf	OUTPUT, 0, 0
	addwf	ACCU, 1, 0
	rlcf	PORTLAT, 1, 0

assuming it's bit number 0 in port, and the rest of the bits we don't
care about (i.e. are inputs)

the problem here of course is that it's not just output that counts:
the output also needs to be computed.

Looks like it's not really worth it. Best to use PWM interrupt with
plugin generator code. At 2Mhz to get the carrier above audible
frequencies would put the divider at 64, and the carrier at 31.25
kHz. (The interesting thing here is that it could also be used for
bit-bang midi output at the same time :)

To get this going: best to add a small modification to sheepsint to
switch it into PWM mode.



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