A comparison of conventional "True Power Summing" to "Quadrature Power Summing" with a variable timing capacitor.
The test is 10 cycles of 50 Hz with about 10 dB of compression.
The first sequence is conventional True Power Summing (TPS) with the decay rate swept from 60 dB/s to 600 dB/sec and back again.
The second sequence is Quadrature Power Summing (QPS).
The sound file is here: https://proaudiodesignforum.com/content ... vs_QPS.wav
In an editor it looks like this:
You can see in the first segment, which is TPS, that the rate affects overshoot at the shortest settings. As we will soon see this is not necessarily a good thing.
(The overshoot is on the positive half cycle because the burst starts on the positive half-cycle.)
QPS in the second segment has slightly more overshoot - more on that later - but it's overshoot is unaffected by the rate control.
Zooming in to the TPS overshoot we see this:
The first cycle attack is distorted due to super-fast attack time.
Zooming in to the QPS overshoot we see greater waveform fidelity:
The increase in waveform fidelity on the first half-cycle isn't solely due to QPS - it's due to delay in the sidechain allpass filter delaying the first cycle relative to the input.
This causes a bit more overshoot but with less distortion on the attack and succeeding waves.
An FFT of the last 9 cycles (without the first cycle overshoot) of TPS shows that it has a significant (-24dB) amount of HD-3:
The FFT of the last 9 cycles of QPS shows no detectable HD-3:
