[sdiy] Korg DSS1 DMA oscillators, the old beast!

[karl dalen]

But how did Korg, Emu ,Fairlight do it then?
The timers run at max 64khz so the DMA must run at....???
Some how the gate arrays must be synced to this clock?

KD

--- Den lör 2010-01-23 skrev Eric Brombaugh <ebrombaugh1 at cox.net>:

> Från: Eric Brombaugh <ebrombaugh1 at cox.net>
> Ämne: Re: [sdiy] Korg DSS1 DMA oscillators, the old beast!
> Till: "karl dalen" <dalenkarl at yahoo.se>
> Kopia: "synthdiy DIY" <synth-diy at dropmix.xs4all.nl>
> Datum: lördag 23 januari 2010 20.46
> Probably wouldn't work: The dsPIC
> audio DAC runs at a constant rate controlled by the main
> clock oscillator and a set of dividers. If you don't keep
> its input FIFO full then it substitutes a default value
> specified by a register in the DAC control area. If you ran
> the DMA from a timer rather than from the DAC interrupt then
> the FIFO will be constantly out of sync and you'll get very
> glitchy data. Since it's a sigma-delta architecture, it
> oversamples by 256x so you don't get a lot of resolution on
> the output rate setting either.
> 
> If you want a DAC loaded at a variable rate controlled by a
> timer then you'd be better off using an external SPI DAC,
> driving the SPI TX from a DMA channel that's timer
> triggered.
> 
> Eric
> 
> On Jan 23, 2010, at 8:59 AM, karl dalen wrote:
> 
> > ->There seams to be a *interesting app* for the
> dspic
> > ->DMA, a transfer can be trigged by a Timer event!
> :-)
> > 
> > Hint, hint, nudge, nudge?!:)
> > 
> > KD
> > 
> >>> 2010-01-23 skrev Tom Wiltshire <tom at electricdruid.net>:
> > 
> >> Från: Tom Wiltshire <tom at electricdruid.net>
> >> Ämne: Re: [sdiy] Korg DSS1 DMA oscillators, the
> old beast!
> >> Till: "karl dalen" <dalenkarl at yahoo.se>
> >> Kopia: synth-diy at dropmix.xs4all.nl
> >> Datum: lördag 23 januari 2010 20.23
> >> 
> >> On 23 Jan 2010, at 15:46, karl dalen wrote:
> >>> 
> >>> It seams your trying to implement such a DMA
> sample
> >>> oscillator mechanism, the very same i was
> asking
> >> about?
> >>> 
> >>> So by fiddling with the DMA transfer interrupt
> rate
> >> one gets
> >>> a DMA driven oscillator? (yes we already know
> DMA was
> >> intended
> >>> to be used for fast data transfer).
> >>> 
> >>> Ai'nt that a very inefficient implementation?
> >>> 
> >>> There seams to be a *interesting app* for the
> dspic
> >> DMA, a transfer can
> >>> be trigged by a Timer event! :-)
> >> 
> >> No, sorry. I'm being much more conventional than
> that.
> >> I'm generating audio samples in blocks of sixteen.
> This is
> >> more efficient because I only have to do one set
> up for
> >> sixteen samples, and I can make better use of
> pointers and
> >> DSP instructions.
> >> I then need the DMA to move those samples from the
> DPSRAM
> >> to the DAC in a timely manner (e.g. at the fixed
> sample
> >> rate). The DMA is operating in "ping-pong" mode,
> which means
> >> it uses two buffers. It reads one, generates an
> interrupt
> >> when it's finished, then reads the other,
> generates an
> >> interrupt, etc etc.
> >> I use the DMA interrupts to set a flag telling the
> main
> >> code that one of the buffers is empty. The main
> code then
> >> runs its sample generation routine and refills
> it.
> >> At least, that's the theory. I haven't got it
> working yet,
> >> as I said.
> >> 
> >> T.
> >> 
> >> 
> > 
> > 
> >     
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