[sdiy] PWM VCA was: CMOS Switch as VCA?

[Tom Wiltshire]

The R.M.Marston book (I forget the title - the CMOS one) recommends a  
PWM rate in excess of 100KHz for this technique Preventing the  
aliasing is basically a question of getting the sampling rate *well*  
up above the audio range. Then you can reintegrate the audio on the  
other side with a simple filter at some reanable frequency (20-25KHz)  
and get smooth audio and remove all the clock noise.

You decide how high you need to go for the level of quality you want.  
If 100Khz isn't good enough, try 200.

T.


On 15 Feb 2010, at 12:19, cheater cheater wrote:

> How do they prevent aliasing there?
>
> D.
>
> On Sun, Feb 14, 2010 at 23:30, Magnus Danielson
> <magnus at rubidium.dyndns.org> wrote:
>> Scott Nordlund wrote:
>>>
>>> Forgive the dumbness of the question, but has anyone tried using a
>>> standard CMOS switch (4016/4066) as a "sort of" VCA?  I mean with  
>>> an actual
>>> linear CV input, not using PWM.  I haven't tried it and I've  
>>> never seen any
>>> reference to it, but I can picture it almost working, though  
>>> probably not in
>>> an optimal kind of way.  At least it wouldn't have the CV  
>>> feedthrough
>>> problems of a single transistor VCA....
>>>
>>> Any ideas?
>>
>> The Sennheiser VSM-201 vocoder uses CMOS switches as VCAs through  
>> PWM.
>>
>>> I've also seen a completely bizarre 4049/4069 hex-VCA that uses  
>>> the power
>>> pins as the output, but this has the obvious disadvantage that  
>>> all the
>>> signals are mixed together.
>>
>> The 4069UB is cheap. Use several if it bothers you. Those VCAs  
>> uses the fact
>> that a FET transistor can be made into a linearised voltage- 
>> controlled
>> resistor.
>>
>> The internal logic of 4016 or 4066 does not allow similar tricks.  
>> You are
>> essentially bust on that quest. I will look again, but I am pretty  
>> sure.
>>
>> Cheers,
>> Magnus
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