The Studer "90° Filter" Stereo to Mono Summer/Recorrelator

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
Post Reply
User avatar
Posts: 3659
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas

The Studer "90° Filter" Stereo to Mono Summer/Recorrelator

Post by mediatechnology » Fri Nov 16, 2018 5:46 pm

The Studer 90° Degree Filter
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.
In other words L+R for coherent signals results in a +3 dB increase instead of +6 dB.
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 ... 14.533.pdf

Thanks for sending this to me some time ago Doug.

SOCIAL MEDIA PLATFORMS should advance FREEDOM OF SPEECH. Yet too many Americans have seen their accounts suspended, banned, or fraudulently reported for unclear “violations” of user policies. No matter your views, if you suspect political bias caused such an action to be taken against you, share your story with President Trump.
White House Tech Censorship Reporting Link:
The Most Banned Network In The World:
BANNED VIDEO - UK Police Escort A Muslim Mob To ATTACK Tommy Robinson: ... n-england/

Post Reply