From a 'design idea' by Harry Bissel in EDN December 26,2002
Envelope followers extract amplitude
information from complex
audio waveforms. The resulting dc
voltage often drives nonlinear stages,
such as voltage-controlled amplifiers or
filters.You must make a careful trade-off
between the speed of response to a rapidly
changing input signal and the
amount of ripple in the dc output that
you can tolerate. If the system is too slow,
the output has low ripple but
badly distorts the envelope
shape. If it’s too fast, ripple can
modulate the nonlinear stages,
causing audible distortion products.
Audio sources, such as a guitar, pose special problems.
The instrument has an
attack of a few milliseconds
and a long decay time. The musician
may “mute” the strings at
any time, causing the normal
exponential decay to terminate
abruptly. The waveform
is sometimes unsymmetrical
and may have multiple
zero crossings. The fundamental frequency
range is typically from approximately
80Hz to 1.5kHz.
have used a full-wave bridge and a large
averaging filter. A filter time constant sufficient
to reduce ripple makes the circuit
unable to follow rapid changes in amplitude.
Peak-detecting circuits can follow
the rapid attack and provide low ripple
during the exponential decay but cannot
follow the rapid decay of a muted string.
The design here features fast attack
and low ripple with minimal filtering,
and it can follow a rapid decrease in signal
The ASM303 takes this design and adds a GATE/TRIGGER detector circuit to increase the connectivity of the module in a modular system.