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MIDIbox SID errata/planned changes


TK.
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Since I'm now able to test the synthesizer implementation on a much easier way (thanks of the control surface) and since the upload of code changes takes significantly less time than with the PIC16F (thanks to the bootstrap loader), I'm now able to fix some bugs/imperfections which are annoying me since months :-)

Unfortunately these changes will make the sound engine incompatible with V1.4, but I think that they should be done ASAP before the first sound patches are provided by users... (of course, I will also try to integrate these changes 1:1 into the PIC16F firmware)

  • When a LFO is assigned to OSC pitch, the intensity depends directly on the "PitchBend range" value. Higher ranges are leading to erroneous waveforms, since the modulation handler used for Finetune and the Pitchwheel is working with a logarithmic scale.
    In an excelent book about modular synthesizers ("Synthesizer" by Florian Anwander, I can recomment it) I read that linear frequency modulation should be the prefered method, so I will change the algorithms in this way.
  • the LFO depth->Pitch path will get a logarithmic scale so that it is possible to control the whole frequency range of an oscillator when "depth" is set to the maximum value
  • currently the envelope depth controls only the half range of the modulation target (e.g. of the filter CutOff frequency). It should control the whole range
  • Filter: I noticed that the quality of the 8580 filters is much better when expected! I've programmed some kind of calibration routine which scales the filter value before it will be written into the SID register. The result sounds already better than with a 6581 - I don't want to know how this routine is working with a 6581 (haven't found the time to try this out yet). However, the "calibration" will make the sound patches compatible to both SID derivatives, and it leads to an improved dynamic range :)

More about this topic in the next days/weeks ;-)

Best Regards, Thorsten.

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