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Old 21st February 2015
  #852
Gear Maniac
 

Xatthew> Cool, at least it helped finding a true bug

After some research as to why the P600 is said to be harsh sounding, I think I found at least part of the reason!

First, I thought it was the way SCI implemented the CEM3372, ie. the values of the supporting resistors/capacitors and tried to alter their value to see if I could improve it but no luck this way, even if the VCA does seem to play part in it.

I then tried to scope the P600 sawtooth wave with the filter fully open. I found there's something like small wavelets at the top of it, this should be normal if there was some resonance, otherwise not really!
Also, as I closed the filter, they quickly disappeared, even with the filter cutoff still being higher than 20Khz.
Then I used a frequency analyser to see how the harmonics behaved while changing the cutoff, and indeed, when the cutoff frequency goes past 22Khz, the sawtooth higher harmonics get overly emphasized, even in the hearing range!

So, I tried to always limit the cutoff frequency to 22Khz in P600fw, and indeed it sounds much better, mellower, nicer, and yeah, less harsh when the filter is fully open! (either because the cutoff pot is at max or the envelope opens the filter at max)

Now, I certainely don't want to remove this code for my personal use, but it does change the sound of the P600, so I wonder if people would want to keep that harshness or if I can just make the fix permanent...