[sdiy] Synth applications of CMOS inverter as SPDT switch?

Mattias Rickardsson mr at analogue.org
Wed Oct 31 21:16:57 CET 2012


On 31 October 2012 01:03, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> Roland's SVC-350 vocoder used a whole bunch of CMOS analogue switches as
> cheap VCAs back in the 70's.  A quad analogue switch and quad comparator (to
> generate the PWM control signals) must have been a lot cheaper than four
> BA662s!  The filters surrounding the "VCA" inherent to the vocoder
> application automatically attenuate the PWM carrier as well.

IIRC the Sennheiser Vocoder VSM-201 uses a similar PWM technique.

> The main problem with this type of VCA is that its dynamic range is limited.
> The carrier frequency must necessarily be high so that it can easily be
> filtered out, but then this requires incredibly small pulse widths to be
> generated reliably when you want large amounts of attenuation like 60dB.
> Once the pulse width gets too small the switch will stop responding and just
> remain in the off state.

Perhaps a greater dynamic range could be reached by using some kind of
leaky sampling of the signal... if the signals go through a leaky S/H
that outputs a sampled value with an exponentially decaying tail, and
a switch passes a well-chosen well-timed slice of this tail, then it
might be possible to reach both high and low amplitudes without
decreasing the switch times as much. (Or am I just insane?) The start
of the tail is much much bigger than a later part of it, since it
decays exponentially. Compared to the switching times discussed above,
which just scale linearly with the desired amplitude.

/mr



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