[sdiy] Digital ADSR - Converting Digital into Piecewise Linear

[Julien Delgoulet]

Sorry to jump in this ADSR DAC conversation but I'm been looking for VC ADSR using micro controller and I notice that the one from the Roland Jupiter 4 is
actually an "analog asdr" drive by a micro controller.
May be worth having a deeper look into it ...


Le 19 févr. 2011 à 22:45, Grant Richter a écrit :

> If you have a clock that is controlling output data rate that is based on a sawtooth. Then you have a synchronous and continuous analog function that can be used to control an analog cross fader.
> 
> If you take two DACs and output the current sample in the first one and the next sample in the second DAC. Using the sample clock sawtooth wave to cross fade to the next sample instead of stepping gives you a linear line segment rather than a staircase. I believe this would be true linear interpolation and covert the digitally stored samples back into a continuous analog function.
> 
> Two linear wired 2164 VCAs with a precision control voltage inversion and offset op-amp is all that is required to make an analog cross fader. Why John Blacet didn't prewire his dual linear VCA module this way is probably because his envelope generators already output the offset and inverted signal needed to make a cross fader out of two linear VCAs. But the single quibble I can come up with for the whole Blacet module line is that the Dual Linear VCA module wasn't prewired as a cross fader.
> 
> The line generated will have angular steps rather than a continuous curve. But the angular direction changes in the line should be much less audible than a discrete step. In this case I'm guessing that a reconstruction filter at half the sample rate might eliminate the "corners" on the line and output a smooth curve.
> 
> If I recall correctly Don Buchla used this technique in the model 248 Multiple Arbitrary Function Generator. In the model 257 he generates a linear interpolation of two functions by controlling two 4066 switches into a common node with a 31.2 Khz clock and 0 to 100%  pulse width modulation then reconstructing the analog function by filtering the pulse train with a 15 Khz Lowpass Sallen-Key filter.
> 
> There was also an article on piece wise linear synthesis in Electronotes where the author introduced the concept of a "ramp shot" circuit which could be chained to generate piece wise linear functions.
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