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

[cheater cheater]

Interesting, how fast can PWM choppers go - cheaply, as in, 'under 30
dollars for the chopper'?

Either way:
wouldn't it be a bit unwise to just assume that the audio is the only
thing that goes into the pwm chopper? I'm fairly sure that the
preceding circuits will add at least *some* sidebands..
I think even some rudimentary band limiting is in order? Any idea how
this is addressed in the Sennheiser?

Thanks

D.

On Mon, Feb 15, 2010 at 14:02, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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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>>
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