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

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mediatechnology
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90 Degree Filter Sound File Comparison

Post by mediatechnology » Wed Aug 04, 2021 2:25 pm

I ran a comparison of the Quadrature 90 Degree Filter versus Stereo and conventional L+R Mono.

The waveforms tell a large part of the story.

The first cut is conventional stereo, the second is L+R and the third the "quadrature" sum. (Right-click to enlarge)

Image
Supertramp Breakfast In America Stereo, L+R Mono, I+Q Mono

The levels were adjusted for 0 dB in 0 dB out using tone in all three cases.
The L+R level is attenuated by 6 dB; the I+Q attenuated by 3 dB so that they're all the same.
The level differences shown above are how they appear and sum naturally.
The I+Q sum has greater peak level and RMS power than the L+R because it has Side information folded in.

Here is the soundfile with all three cuts in series.

Supertramp "Breakfast in America," Stereo, L+R Mono, I+Q Mono: https://proaudiodesignforum.com/content ... Q_Mono.mp3

The statistics show both a power increase and a peak level increase when the "lost" Side information is recovered.

L+R Mono statistics:
Image

I+Q Mono statistics:
Image

The I+Q sum has about 3 dB more peak level and is about 1 dB hotter in perceived loudness.
The I+Q sum has the balance and "life" of the stereo original - the L+R mono is more restrained because it lacks all of the L-R information.

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

Post by mediatechnology » Thu Aug 05, 2021 2:15 pm

I decided to run a fun little experiment with tone to demonstrate how elements can disappear when mono'd.

I used Cool Edit to lay down 440 Hz tone at -6 dBFS in mono.
I created a second file using 220 Hz tone, also at -6 dBFS and feeding both channels, but with the relative polarity inverted in the right channel.
I then summed them.

The resulting file was processed by the Quadrature summing filter.
The first section is the unaltered stereo file followed by the I+Q sum, the L+R sum, etc.
The L+R sum can be easily identified - it's missing the 220 Hz tone to to phase cancellation.

Disappearing "out-of-polarity" 220 Hz tone: https://proaudiodesignforum.com/content ... _IQ_LR.mp3

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

Post by FrankLacy » Thu Aug 05, 2021 6:44 pm

Would you consider processing some song, recording the normal and quadrature mono sums, but then only publicly naming the files A and B (randomly selected) ?

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

Post by mediatechnology » Thu Aug 05, 2021 7:10 pm

Sounds like a plan Frank.

I have a transfer of Paperback Writer I recorded today...

File A: https://proaudiodesignforum.com/content ... File_A.mp3

File B: https://proaudiodesignforum.com/content ... File_B.mp3

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