[sdiy] FPGA based synthesizer progress report....

[Eric Brombaugh]

Jim Patchell wrote:
> Tonight...I need to get the Midi Parser working...I wrote one a long 
> time ago that was compiled with Image Craft ICC, and I need to make a 
> few minor changes so it will work with WinAVR gcc and also with the real 
> time operating system I wrote...

Good luck with that - I've been working on something similar lately and 
it's fairly entertaining. You definitely want to do this kind of thing 
in C - using assembly to build a MIDI parser & scheduler is a PITA, and 
most current MCUs are fast enough compared to MIDI data rates that you 
won't notice the difference in processing time.

> One interesting thing I have noted.
> 
> The phase relationship of two sawtooth waveforms (even when they are 
> exactly the same frequency) makes a huge difference in the timbre you 
> get.  I am going to have to add (later) a feature to make sure that the 
> phase of the oscillators is known at the start of a note so that the 
> sound is predictable.  This also means by adding phase modulation, I 
> should be able to get some interesting (but subtle) effects.

I've noticed this too - the effect is similar to PWM. That makes 
intuitive sense when you consider that adding a rising saw to a falling 
saw will give you a pulse wave whose duty cycle changes as the phase of 
the saws changes. Flipping one of the saws so they're both rising or 
falling makes a minor change in the harmonic cancellation, so the end 
result sounds very similar.

Eric