Inspired by Blackmer's figure 4 in his RMS detector patent I'm working on a stereo compressor sidechain that uses a sin/cos Dome filter along with "True" Power Summing.
I call the sidechain topology "Quadrature Power Summation" due to the 90°quadrature relationship between the signals being power-summed.
The "Unocomp" or "Uno Compressor" is named for the Pythagorean identity sin²Φ + cos²Φ = "uno."
Blackmer's original detector RMS detector patent is here: https://proaudiodesignforum.com/images/ ... 681618.pdf
In figure 4 Blackmer describes a method of summing 90 degree phase-shifted inputs to summed detectors. Block 72 in the drawing is an identical detector to the one detailed above it.
Note: The Blackmer detector is often called an "RMS" detector. A free-standing Blackmer detector performs about half the computations of RMS conversion. The final computations, taking the square root and exponentiating back into the linear domain, are performed by the VCA control scale and law. The output of the RMS detector is RMS, but it's in the log domain and, at 6 mV/dB scaling, is the square of RMS relative to a 3 mV/dB scaling.Blackmer writes:
In FIG. 4 there is shown an embodiment of the present invention employing two of the structures of FIG. 1 to achieve a substantially ripple-free output related to the logarithm of the instantaneous rms value of an input signal. In the embodiment of FIG. 4 there is included means for providing a constant phase difference such as a 90° phase network...
Preferably, each of circuits 26 and 72 provides an output current which is proportional to the square of its input current in any quasi steady-state interval of the input function. Hence, the outputs of circuits 26 and 72 are connected to junction 40 so that the current from the two circuits can be summed. Because of the 90 phase network formed, inter alia of amplifier 60, these output currents sum to meet the condition that Sin²Φ + cos²Φ = 1 or will thus provide a substantially ripple-free output logarithmically related to E (or to the input current as the case may be).
Having built a detector using this technique I can assure the readership that even with the timing capacitor removed the output is essentially ripple-free. Reduced ripple allows super-fast response times with less distortion and audible artifacts.
Details and a schematic for an experimental sidechain will follow...