[sdiy] Lin/Log VCAs and envelopes - compensating for log VCAs

[Neil Johnson]

The traditional capacitor-charging-and-discharging ADSR is exponential
since during the A,D and R phases the output voltage is described by
an exponential expression.

The logarithmic term comes from audio pots that try to give a constant
dBs-per-degree as our ears sensitivity is somewhat logarithmic,
although they are not directly logarithmic - we don't hear in dBs, and
the response is highly frequency-dependent.

Neil