Studer 1 194 533. 90 degree shifter. LTspice file

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Martin Griffith said:
Just trying out dropbox.
A  ecent topic about 90 phase shift. so did a LTspice asc file

https://www.dropbox.com/s/k3dzlzu9ixsuola/Studer1_194_533.asc?dl=0
Is it the actual values in the Studer unit? because the phase shift is not very consistent; I'm used to better performance from the Studer engineers. I had in my old LTspice files an experiment and I had very consistent 90° shift over a 6 octave range with only two stages. With 4 stages I would expect perfect linearity across the whole audio spectrum.
 
Yep, original values, from the Studer datasheet, I've put the Studer doc on there as well. I left out the 6k8 summing resisitors on the output in the LTspice file.  the studer circuit has 2 signal paths with different filters which are summed together. I think this is the pdf link https://www.dropbox.com/s/qzdxh0x4zslakhv/1.914.533.pdf?dl=0 (Still trying to get the hang of dropbox)
 
Martin Griffith said:
Yep, original values, from the Studer datasheet,
That's weird; as I wrote before, I think it is possible to improve significantly the accuracy and variance of phase-shift; I just wonder why the Studer designers have chosen to settle for 80° shift maximum. I know it doesn't make much practical difference, though.
 
Also keep in mind that this kind of tool is originally used as mono sum for AM radio, one octave is enough  :)
I don't have LTspice to look at file and see what you see,
but if both side have same inconsistency in phase shift at same place, the sum will be ok ? or maybe I'm wrong.
What this do exactly? +45 at one side and -45 at the other ?

Best
Zam
 
Hi

Thanks to share picture ! now I see

Curve show a pb at second filter, and I check your schemo, capacitor of first channel second filter  should be 22n not 10n !!!
(C2 at your drawing, C5 at studer drawing)
I don't check all other component value, but maybe a double check at your side will be a good thing to do :)

Let us see how it work with corrected value !

Best
Zam
 
Thanks for pointing out my  error , updated, looks much better
https://www.dropbox.com/s/sj3taps77hj231v/Studer_inphase_new.jpg?dl=0
https://www.dropbox.com/s/8sbzx0yguqa4vzr/Studer_outphase_new.jpg?dl=0
 
Martin Griffith said:
Thanks for pointing out my  error , updated, looks much better
https://www.dropbox.com/s/sj3taps77hj231v/Studer_inphase_new.jpg?dl=0
https://www.dropbox.com/s/8sbzx0yguqa4vzr/Studer_outphase_new.jpg?dl=0

Yes it is !
Best
Zam
 
I've taken an interest in the Studer 90° Filter and found both an error in the sims and, with the aid of a forum member, an error in the Studer implementation.

I built one of the 90° quadrature summing filters and think it has two limited, but useful, applications.

The first is making useful mono from out-of-polarity synth patches that don't fold down properly. The second application is mono playback for ambient sound and AM broadcasting.

My overall listening impression is that mono created this way more closely resembles its stereo counterpart than L+R/2.

The original images in this thread do not render inline due to the forum upgrade and Dropbox.

Here is the "in polarity" response posted previously.

Studer_90_Degree_Filter_Inphase.jpg


Out-of-polarity response:

Studer_90_Degree_Filter_Outphase.jpg


There are a couple of errors in the above sims:

(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 electrically 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.

The bottom line is that the 90° filter actually produces identical level outputs, (at +3 dB) with both in-polarity and out-of-polarity inputs.

The prototype looks like this:

Studer_90_Degree_Filter_Swept_Response_Expanded.jpg


The red trace is conventional L+R. Green and Blue are both in and out-of polarity.

A forum reader found the Weaver optimization for the Dome filter which corrects errors in the Studer response.

The Weaver optimized 90° Dome Filter looks like this:

Recorrelator_90_Degree_Filter_Summed_Freq_Response_Weaver_Alignment_Blue_In-Polarity_Green_Out-Polarity.jpg


There are some sample sound files here. The first demonstrates "disappearing" L-R when mono'd - in a following post there are two mono versions of the Beatles' Paperback Writer. The Studer "90° Filter" Stereo to Mono Summer/Recorrelator - Page 3 - Pro Audio Design Forum

When the "lost" Side information is preserved the result provides both higher peak and RMS levels. Hard-panned mix elements are brought more to the forefront when mono'd as "I+Q."

There are also other soundfiles scattered through the thread as well as a working schematic.

I like what this gizmo does despite its limited applications.
 
It's interesting, but it seems to apply to mixes with blatant errors. I mean the example of Sympathy for the devil shows that the piano is totally out-of-phase for a few seconds, as the mono mix demonstrates.
I chalk it to the lack of understanding of stereo at the time by pop-trained engineers.
Today, this kind of mistake is inadmissible IMO.
I know that many synths and FX use out-of-phase crossfeed for "soundstage enhancement", but the foldability of a mix should be controlled before printing.
A mix that doesn't fold correctly is the result of negligence or carelessness, again IMO.
I see this 90° shifter as a useful tool, but it should be used only on sources that show this problem, not an the entire mix, since it results in a destruction of the M/S ratio, which is probably less objectionable, but still is not welcome.
Too often I see SE's using all kinds of tricks to create pseudo-stereo tracke, particularly on acoustic rhythm guitars, with unpredicatble results. I never do that; I run another take. Even if it takes a littl emore time, it's time well spent, instead of time wasted at fixing impossible things.
 
I see this mainly as a tool for out of control keyboard patches to fix them during tracking or before mixing if a mono compatability problem is found.

If this were to be used as a tool to repair a mix I agree it was done too late in the production. I used some full mixes because that's what I had.

Its suitability to an entire mix after the fact might be appropriate if the end product were mono like AM on an oldies format or for ambient background music. That's about it.

I agree you would never want to process with this and leave it in stereo unless you wanted an effect.

Studer and LAWO both designed these to be used in broadcast airchains to improve mono fold-down of everything. I don't suggest that.
 
I think Studer used a TL072 on the first stage for reduced bias current. The resistor values are quite large to use a bipolar input op amp on the first stage. (R6+R7 and R8+R9.)

On mine I scaled the values lower.

Recorrelator_Stereo_To_Mono_Quadrature_Summing_Filter_080221.jpg
 
Last edited:
It's hard to see any of the values.

I wrote: "I agree you would never want to process with this and leave it in stereo unless you wanted an effect."

I'll add that I did listen to it after processing in "stereo" and the imaging, as one would expect, was bizarre and pulled hard to one side owing to the 90° phase difference between outputs. (On my schematic I have a mode where I Q forward to the outputs. I put that there for use as a keyboard stereo DI for "monofication" downstream. It's likely a mode that would never be used.)

Having said the above pretty much everything monitored in mono sounded more like it's stereo counterpart, in terms of spectral and instrumental balance, when summed as I+Q instead of conventional L+R. As many of you know conventional Mono, by definition is "lossy" because L-R is eliminated. When summed as I+Q, L+R and L-R have equal weight.

It will destroy a stereo mix if presented in stereo.

Monitoring in mono on some mixes I+Q summation made modest improvement on others it was quite large. I can't honestly say I+Q made anything sound worse when monitored in mono.

I stumbled into a thread over at Steve Hoffman's about people monitoring early stereo recordings (mostly Beatles) in mono. I've thought about doing a listening test there. It's been awhile since I read it but I got the impression that some people felt the mono button made the unlistenable listenable.

If I were producing background music that I know was going to be played at Target on a mono distributed 25 or 70V system I could see this being in the chain. I'm assuming Studer and LAWO designed these things to live full-time in a mono AM airchain.

There are two niche apps that might benefit us here:

Jakob, in an earlier thread, looked at using the 90° filter in a single-detector sidechain to provide equal weighting of Mid and Side. At the time the sims showed they did not have equal weighting so I'm not sure what came of that. I think it might make a good addition to the GSSL sidechain.

I really want to see how this works with problematic stereo keyboards. For this reason it may some day move from Protoboard to PC board.

In any case the 90° filter is a very niche thing.
 

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