Blackmer RMS detector

[1] The circuit, created by trial and error, computes root mean squared of various waveforms with high precision, although exact nature of its operation was not known to the inventor.

In audio applications, RMS is the only metric directly related to perceived loudness, being insensitive to the phase of harmonics in complex waveforms.

[1] Thermal RMS conversion was too slow for audio purposes; electronic RMS detectors worked fine in measurement instruments, but their dynamic range was too narrow for professional audio - precisely because they operated on squares of input signal, taking up twice its dynamic range.

[1][7] Blackmer reasoned that the log-antilog detector may be simplified by taking up processing to log domain, omitting physical squaring of input signals and thus retaining its full dynamic range.

Blackmer proposed simple replacement of a resistor in RC network with a silicon diode biased with a fixed idle current.

[9] When the crude test circuit was built, Blackmer and his associates did not expect it to work as a true RMS detector, but it did.

[4] He published his thesis in 1979, and was later credited as the inventor of log-domain filter concept,[10] but the idea remained unknown to general public until the 1993 pioneering work by Douglas Frey.