[sdiy] DsPIC timer issue when used as DCO!

[karl dalen]

--- Den tors 2009-12-10 skrev Dave Manley <dlmanley at sonic.net>:

> Från: Dave Manley <dlmanley at sonic.net>
> Ämne: Re: [sdiy] DsPIC timer issue when used as DCO!
> Till: 
> Kopia: synth-diy at dropmix.xs4all.nl
> Datum: torsdag 10 december 2009 10.32
> karl dalen wrote:
> > Tim, Eric, Dave, etc!
> > 
> >   
> >> I wasn't aware that there was an issue with dsPIC
> timers
> >> used to drive the P08/Mopho/(Tetra?) DCOs. Do you
> have any pointers to
> >> this elsewhere online?
> >>     
> > 
> > This are in fact old news and has been discussed in
> several places!
> > I thought the issue had been resolved by now? People
> describe it as
> > poping/click sound between high and low notes.
> > Reminds me of the OSCar rubber band oscillators! :) 
> > anywho, heres one spot its discussed!
> > 
> > http://www.vintagesynth.com/forum/viewtopic.php?f=1&t=49602
> > http://www.vintagesynth.com/forum/viewtopic.php?f=1&t=49602&start=60#p528540
> > 
> > This picture suppose to be an actuall measurement from
> low freq triangle
> > wave and an instant jump to higher frequency, it do
> look a bit odd!?
> > Kind of  counting, counting!  oh! a new
> value! reset! deeeelaaaay!
> > And then load new buffer count? 
> > http://www.mediafire.com/imageview.php?quickkey=yijnnyyy2o5&thumb=6
> >   
> How about another wild speculation: the PA397 is apparently
> a 3396 variant. 

They are basically the same, with the addition of a ext input VCA and insted of two serialized output VCAs there are a L and R channel,
there might be a velo VCA infront of these two as well.
otherwise it's just the same chip Tom oberheim used for
the Marinon box, well RHOS certification and different packing.

 The frequency is set not only by the
> dsPIC sourced timer that resets the timing cap, but also the
> input voltage (Vwf in the 3396 datasheet) that controls the
> current source that charges the cap.  Maybe the dsPIC
> is working perfectly but the control voltage for the current
> source is sometimes messed up after a pitch change.

This current source just changes volume so all there is are
waveshape change not a time change wich can oly be manufactured
by the NCO. but amplitude click would be if so.

> would be easy to look at the relationship between the output
> frequency, the dsPIC output, and the Vwf control voltage
> with a scope.  I remember looking at an image of the
> evolver when it first came out, and it is easy to find the
> timing cap, etc.  If I had one of the synths in this
> family I'd do the measurements.

DSI entire product range are based on this PA397.

> Looking at the linked waveform there's a couple of
> interesting things - the change from low to high frequency
> doesn't happen at the min/max value of the triangle but at
> about -25 (using the scale in the image).  Is this
> because the multiplexed DAC outputs the new Vwf, the current
> source charges the cap at the new rate, the waveshaper
> quickly sweeps thru at the new frequency, and then the
> timing cap doesn't get reset by the dsPIC.  It just
> sits there for ~13ms.  Perhaps the dsPIC is taking an
> interrupt between setting the new Vwf and updating the dsPIC
> timer. The 'deeeeeeeeelaaaaay' is the length of the
> ISR?  I can't imagine any ISR would take that huge
> amount of time though. Hmmm.

Interesting! Yes 35ms appears to be a very long time for an ISR!
What if the code is not correct, lets asume what Dave has coded
for the portamento to be 0 are instead e.g 1,5ms? that would get
stretched over the note range. 


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