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.

Previous circuits 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 (mute).

The ASM303 takes this design and adds a GATE/TRIGGER detector circuit to increase the connectivity of the module in a modular system.

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