[sdiy] Blackmer VCA cell

[Neil Johnson]

The Blackmer cell, as well as the current THAT, the Frey and the 2164
(Bowers) cells are designed very much for audio quality.  The
fundamental mode of operation is to compute the log of the audio
signal, add the control signal, and then perform the anti-log to
return the signal to the linear domain. In this way they do the
amplitude multiplication by addition in the log domain.

Their control function is exponential, linear-in-dB which is great for
audio fading (you can use a linear fader rather than an expensive log
curve pot, and not the cheap log pots you buy from the usual online
suppliers, if you want an audio-quality log taper you need to look to
Penny&Giles and not complain about the price...) so they are not
suitable for the traditional envelope generator that needs a linear
control VCA.

As an aside, expo VCAs are also great for audio filters where you
naturally want the control of cut off frequency to be in the log
domain (octaves), not linear Hz.

Irwin's ingenious linearisation scheme only works in practice because
of the 2164 having multiple expo VCAs on the same silicon giving
excellent process and thermal tracking.  Various other folks (Dixon,
etc) have used this thermal tracking to build very stable VCOs using
the 2164 as well.  You wouldn't get this from separate expo VCAs as
they just wouldn't track as well.

Of course price is always a key factor.  The original DBX modules were
trimmed in the factory, which costs (there are four SELECT resistors).
The THAT VCAs use their special isolating bipolar process to give them
fully isolated NPN and PNP transistors, and they laser trim.  Again,
expensive.

Compare that with the cheap-as-chips cost of the CA3080 and LM13x00
linear VCAs - lower audio quality but significantly cheaper.
You can read about the history of the LM13x00 from one of the original
engineers: http://www.idea2ic.com/LM13600/LM13700.html

> I'm a bit surprised no synth
> maker used these, as they're worth the extra price in some
> applications, such as setting the "final output" volume control for
> each patch.

Products are engineered (features for a given price point), and for
the price point the CA3080 is/was good enough.  The SNR of a synth is
usually well-defined, and the signal paths are driven quite high so
the noise floor is less of a concern.

> I've not heard of Blackmer cell VCAs ever being used
> in effects such as guitar pedals

You likely won't - that market is incredibly price-sensitive.
Spending several bucks on a single VCA is just not on the radar.

Neil