In other words L+R for coherent signals results in a +3 dB increase instead of +6 dB.This active 90° filter is used to form a monophonic signal from the left and right channel of stereo signals. Simple mixing of the left and right channel will not produce a mono signal of satisfactory quality, but results in an emphasis of the center information. By summing the stereo signals in a 90° phase-shifted manner, this undesirable effect can be avoided.
This is done by phase shifting L and R so that they are 90° apart.
L+R then becomes decorrelated.
Studer 90° Allpass FIlter Block Diagram
The 90° filter consists of two all-pass filter chains, producing a uniform 90° phase difference across the whole audio range. The left and the right stereo signals are each passed through one of these filters and added at the filter’s output. Doubling of equally-phased signal components as well as canceling of opposite-phased components is thus avoided.
Studer 90° Allpass FIlters Schematic
Studer Stereo to Mono 90° Allpass Filter https://www.proaudiodesignforum.com/ima ... 14.533.pdf
Thanks for sending this to me some time ago Doug.