Blackmer RMS detector
The Blackmer RMS detector is an electronic true RMS converter invented by David E. Blackmer in 1971. The Blackmer detector, coupled with the Blackmer gain cell, forms the core of the dbx noise reduction system and various professional audio signal processors developed by dbx, Inc.
Unlike earlier RMS detectors that
Operation
Contemporary electronic RMS detectors had "normal", linear outputs, and were built exactly following the definition of RMS. The detector would compute square of the input signal, time-average the square using a
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.[3] Squaring and taking square roots in log domain is very cheap, being simple scaling by a factor of 2 or 1/2.[7] However, simple linear filters do not work in log domain, producing incorrect, irrelevant output. Correct time-averaging required nonlinear filters of yet unknown topology. Blackmer proposed simple replacement of a resistor in RC network with a silicon diode biased with a fixed idle current. Since small-signal impedance of such diode is controlled linearly by current, changing this current controls settling time of the detector.[3] Cutoff frequency of this first order filter equals
- ,[5]
where is
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. According to Robert Adams, it "seemed to behave ideally",[4] and rigorous tests with various waveforms confirmed ideal RMS performance. The circuit was absolutely insensitive to phase shifts in input signal. It was immediately patented and employed in dbx, Inc. professional audio processors. No one in the company, including Blackmer, could explain why it works at all until 1977, when Robert Adams began work on proper mathematical proof of RMS compliance.[4] Adams tried to extend log-domain concept to Sallen–Key topology and failed.[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.[11][2]
References
- ^ a b c d Adams 2006, p. xii.
- ^ a b Roberts & Leung 2006, p. 11.
- ^ a b c Adams 2006, p. xiii.
- ^ a b c d e Adams 2006, p. xiv.
- ^ a b Roberts & Leung 2006, p. 10.
- ^ a b Adams 2006, p. xi.
- ^ a b c Tyler & Kirkwood 2008, p. 346.
- ^ Roberts & Leung 2006, p. xv.
- ^ a b Tyler & Kirkwood 2008, p. 348.
- ^ Roberts & Leung 2006, p. 6.
- ^ Adams 2006, p. xv.
Bibliography
- Adams, Robert (2006). "Foreword". In Gordon W. Roberts, Vincent W. Leung (ed.). Design and Analysis of Integrator-Based Log-Domain Filter Circuits. Springer Science & Business Media. ISBN 9780306470547.
- Roberts, Gordon; Leung, Vincent (2006). Gordon W. Roberts, Vincent W. Leung (ed.). Design and Analysis of Integrator-Based Log-Domain Filter Circuits. Springer Science & Business Media. ISBN 9780306470547.
- Tyler, Les; Kirkwood, Wayne (2008). "12.3.4 Dedicated Analog Integrated Circuits for Audio Applications". In Glen Ballou (ed.). Handbook for Sound Engineers. Fourth Edition. Focal/Elsevier. ISBN 978-0-240-80969-4.