[sdiy] Interesting f vs. t graphs of pitch instability

[Scott Gravenhorst]

Tom Wiltshire <tom at electricdruid.net> wrote:
>Andre,
>
>These are indeed very interesting graphs.
>
>I've been thinking lately about digital modelling of the drift and  
>noise in analogue oscillators, since I'm working on a digital  
>oscillator design on dsPIC. And then you come along with some  
>beautiful images of exactly the sort of noise and variation that I'm  
>thinking about. Synchronicity!
>Having a knob on an oscillator that goes from cool digital sterility  
>at one end, via CEM3340 to drifty moog, and then finishing at Andre's  
>wavering MS20 sounds great to me. The question is what signals do you  
>need to feed into the oscillator and how to generate them efficiently.
>Your graphs represent some actual data, which is very helpful indeed.
>
>Regards,
>Tom

Interesting you mention this, My GateMan FPGA synths all have noise modulated pitch,
it is something I really like.  Mine go from subtle and slow drift up through tonal
noise to almost just plain noise.

>
>
>
>On 6 Oct 2008, at 14:56, Andre Majorel wrote:
>
>> The wavering on my MS-20 has become worse lately so I've started
>> to look into it. Some interesting graphs have been made in the
>> process :
>>
>>   http://www.teaser.fr/~amajorel/ms20/files/waver1.png
>>
>> Those are four frequency vs. time plots. On the X axis, the frame
>> number. On the Y axis, the frequency in Hz. Each dot represents
>> one cycle. Each plot is one minute long so should have about 5400
>> points.
>>
>>> From top left to bottom right :
>> - VCO2 driven by the CV from by the keyboard,
>> - the same with preset VR4 shunted,
>> - VCO2 driven by an external fixed voltage (no wavering !),
>> - mathematically generated sawtooth of the same approximate
>>   frequency.
>>
>> Note how much frequency noise there is. It probably isn't an
>> artefact of the frequency counter as the graph for the
>> mathematically generated sawtooth is clean.
>>
>> Also note how the dot density is higher at the extreme
>> frequencies. I have no idea why. It might be related to the fact
>> that most of the time-domain noise is 50 Hz hum...
>>
>> -- 
>> André Majorel <URL:http://www.teaser.fr/~amajorel/>
>> Do not use this account for regular correspondence.
>> See the URL above for contact information.
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>
>
>
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-- ScottG
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-- Scott Gravenhorst
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