The Studer "90°" Dome Filter Stereo to Mono Quadrature Summer

[mediatechnology]

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 https://www.proaudiodesignforum.com/ima ... 14.533.pdf

Thanks for sending this to me some time ago Doug.

[mediatechnology]

I've always wondered if the Studer "90 degree filter" would be useful during tracking or mixdown to solve the problem of synths not properly folding down to mono.

This thread at Gearspace is one example "Mix Elements Half Disappear in Mono": https://gearspace.com/board/so-much-gea ... oting.html

Over at PRW "Improving Mono Fold Down": https://www.tapatalk.com/groups/proreco ... 09763.html

And at GroupDIY: "Allpass for decorrelating L/R?": https://groupdiy.com/threads/allpass-fo ... ost-796469

At GroupDIY I found sims:

"In phase" response:



"Out-of-phase" response.



So it appears that the Studer 90 degree filter makes mono, which would normally sum at +6 dB, fold into the mix at about -3 dB.

Completely "out-of-phase" polarity-inverted mix elements, which would normally disappear in mono, now fold-in to mono at about -9 dB.

The question I had was "Would the Studer 90 degree filter solve the problem of synth's not properly folding down to mono?"

This has been on my "bucket list" so I built one and the answer appears to be a qualified "yes."

Though I wouldn't recommend it to correct a mix I did try it on the Stone's "Sympathy for the Devil" and it restored the piano during the intro which almost completely disappears in mono.

On a problematic synth sample posted at Gearspace it brought forward an upper string part and gave it a little more sheen.
In mono that part was more subdued and dark.

I need more samples and the time to critically listen to them.

One way to use the 90 degree filter would be to insert it between the keyboard and console during tracking.
The outputs could be left in stereo to improve downstream mono compatibility and left wide (wider) during tracking or folded down to mono in the filter, recorded in mono and then panned in the mix.

Another potential application, if you're into it, is mastering "elevator" background music.
Studer designed it for AM broadcasting.

With mono sine wave wave inputs an X-Y goniometer displays an almost perfect circle - mono music appears like a Brillo pad.
The 90 degree filter can also me used in reverse by making mono into "pseudo-stereo."

If the synth part is wide to begin with it will be, if recorded in stereo, even wider.
Folded down to mono the processed version has less tonal changes than the unprocessed version.

If there's interest, or when I get time, I'll record a couple of samples.

If this does prove to be a tool it will be one that might be used infrequently and only in very problematic situations.

[mediatechnology]

I decided to sweep the Studer 90 degree filter.

The sims I posted earlier are not correct.

The input levels in the images below are 0 dBu.
The top red trace is the conventional mono sum which has a +6 dB build-up.
The blue trace is mono fed into both inputs and processed by the 90 degree filter then summed to mono. The build-up is +3 dB.
The green trace is out-of-polarity inputs fed into the 90 degree filter. Out-of-polarity inputs also mono sum to +3 dBu.
The violet trace, for reference, is out-of-polarity inputs summed to mono.
(The cancellation is frequency dependent due to unbuffered inputs driving the allpass filters and is also subject to DAC channel imbalance and 1% resistor error.)



Here's an expanded view of the mono, and processed mono with in and out-of-polarity inputs.



Not shown is the output with a single channel driven.
Under that condition 0 dBu In produces 0 dBu out.

[mediatechnology]

Sample Soundfiles

From the Stone's "Sympathy for the Devil"

Focus on the piano chords in the first 15 seconds.
For whatever reason, after 15 seconds the mono compatibility of the part changes.

Original stereo: https://proaudiodesignforum.com/content ... Stereo.mp3

Original in mono: https://proaudiodesignforum.com/content ... d_Mono.mp3

Stereo folded down to mono using the Studer 90 degree filter: https://proaudiodesignforum.com/content ... d_Mono.mp3

When conventionally folded down to mono the piano in the intro is virtually missing with only vocals, percussion and bass appearing in the mono sum.

How it got this way, and why it changes around 15 seconds is anyone's guess.

[mediatechnology]

At GroupDIY I found sims:

"In phase" response:



"Out-of-phase" response.



So it appears that the Studer 90 degree filter makes mono, which would normally sum at +6 dB, fold into the mix at about -3 dB.

Completely "out-of-phase" polarity-inverted mix elements, which would normally disappear in mono, now fold-in to mono at about -9 dB.

I just discovered the errors in the above sim (from GroupDIY) and why it departs from the actual real circuit.

(1) The mono output in the first example is not actively summed. The resistive summation has a 6 dB loss. Thus, when equal inputs are applied the actual sum should be +3 dB. The 6 dB resistive loss makes it appear to be -3 dB.

(2) The "out-of-phase" example should use two equal but opposite voltage sources to correctly model Left and Right. In normal use left and right are presented in common mode. (Single-ended, unbalanced, ground-referred.) The model treats the filter and the mono summation as a differential device fed by a differential source. The filter, and it's output network are not differential devices. Thus two equal but opposite voltage sources are required to correctly model out-of-polarity Left and Right inputs. The mono sum of the output driven by out-of-polarity inputs should also be +3 dB. In the model it's not: The differential to common mode conversion (where the outputs sum) combined with the loss of the resistively-summed output in the same network reduces the modeled output voltage by 12 dB from +3 to -9 dB.

This is reality:

[FrankLacy]

R. B. Dome "Wideband Phase Shift Networks"

dome.PDF

(906 KiB) Downloaded 3417 times

[FrankLacy]

Donald K. Weaver "Design of RC Wide-band 90 Degree Phase-Difference Network"

weaver_scan.PDF

(1018.53 KiB) Downloaded 3384 times

[FrankLacy]

S. D. Bedrosian "Normalized Design of 90deg Phase-Difference Networks"

bedrosian.PDF

(643.75 KiB) Downloaded 3443 times

[mediatechnology]

Thanks for the citations Frank!
Lots of reading to do...

[mediatechnology]

I had a chance today to listen to a bunch of different songs folded to mono and I have to conclude that Studer was onto something when they took the Dome filter and used it to improve mono fold-down.

I compared the stereo presentation to conventional summed mono and the Studer 90 degree filter. Two songs in particular stand out.

One is The Beatles' "Paperback Writer." The original stereo version is ultra-wide hard-panned. As you may recall the vocals are hard-panned. The "Dome filter-summed" version sounds more like the stereo version with the vocals up front. The mono version sounds darker and somewhat lifeless in comparison.

The Red Hot Chili Peppers' "The Adventure of Rain Dance Maggie" also produced some interesting results. The stereo version is low end heavy in a pleasing way but becomes excessively so when mono'd conventionally. The whole balance of the mix changes in mono and it becomes bass-heavy and a little dark. The 90 degree filtered mono version maintains a more balanced mix. The bass is pulled back a tad, the side information brought forward and its a little brighter having a nice pop the conventional mono version lacks. Overall the 90 degree filtered version sounds more like the stereo version with only the image collapsed.

Not all material benefited from it but what did just seemed to have more of the "life" the stereo version had with conventional mono sounding duller and imbalanced.