log filtering (was:Re: Identifying tempcos)

jhaible jhaible at debitel.net
Wed Nov 10 11:56:43 CET 1999

> I am guessing the uA726 was used in analog military radar sets
> for logarithmic conversion.

Don't call it out loud - I see it coming: "German synth manufacturer
buys your junk cruise missiles to build keep production of original
Minimoog VCO modules alive" ... only joking.

But more seriously:

> That is H(t) * F(t) = (Exp (log H(t) + log F(t)) which is the basis
> for multipliers like the RC4200. But something funky happens
> mathematically when you filter a log converted time varying
> signal before exponent conversion since you dealing with an
> addition rather than a multiplication of two functions.
> Could this maybe be useful for music? Anybody?

I think this is often done in compressors, logarithmic envelope detector,
filter, exponential VCA. That's the way to keep response times independent
of the input level in a feedback type compressor circuit.

The EMS HiFli has a logarithmic enverlope follower for detecting the start
an stop time of individual notes from the guitar. This is critical because
of the
high dynamic range of a guitar, and the EMS circuit works remarkably well.
The signal chain is
FW Rectifier (with some Treble boost circuitry included) -> log amp
(based around 2SC1583, not temperature compensated) -> 4-pole fliter.
This is the envelope follower, and it produces the same voltage step for a
certain input level change, regardless of absolute input level.
The filtered signal is then fed into a simple HPF to get the d/dt, and
extract the start and stop signals from that.


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