[sdiy] Imperfect VCO

[ASSI]

On Friday 01 August 2003 16:35, Oakley Sound wrote:
> I am not too sure why this should
> cause aliasing since the effective BW should be limited to 20KHz (or
> whatever) directly in the waveform generating software.

Dig up "Alias-Free Digital Synthesis of Classic Analog Waveforms" by 
Tim Stilson and Julius Smith to get an impression of how hard it really 
is to produce a sawtooth or square wave in the digital domain with 
proper bandlimiting.

> Maybe its not
> though... do they really try to produce instantaneous rise/fall times
> with these things?

> I don't think it does, but I need to do some maths before I can work
> out what effect even a slight slewing on the flayback will have on
> the harmonic structure of the sound. But it does seem likely that it
> should not have a significant audible effect, and that there is
> indeed another source for any sonic difference in VCOs.

A sloped flyback weeds out certain harmonics (much like a comb filter) 
and in the extreme of triangle every second one is missing. Given a 
somewhat fixed slope of flyback, modulating the oscillator will mask 
different harmonics for each frequency. I would not think it 
unreasonable that this is one factor that (together with many others) 
makes some saw oscillators sound more "lively" than others. What's the 
Roland SuperSaw again? Right, a mix of several slightly out of tune 
saws, which is just another method of getting some movement to the 
harmonic structure. The human ear is very discernible when it comes to 
dynamic harmonic content, to the point that you'd be hard pressed to 
measure a change in the waveform that you are clearly able to hear.

> Yes, that's right. But I was quite suprised that I could hear the
> difference before I could see the difference on the scope. Even on a
> harmonically rich sawtooth.

Just what I said. :-)

I know a concert flutist who tried to characterise the tone of various 
concert flute constructions (as a final thesis, she is now with a 
renowned philharmonic orchestra). While the difference in sonic 
character could be clearly heard and described by almost all people in 
a double blind test, there was no significant parameter to correlate ta 
a series of measurements that were undertaken. It wasn't the fault of 
the measurement equipment, the lab of the PTB where these have been 
performed would make everyone envious.


Achim.
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