A Discussion About True Power Summing for Stereo Compressors

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JR.
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by JR. » Sun Mar 29, 2009 12:11 pm

Crusty wrote:Ah, so I was comparing the JR var-cap to the non-linear cap circuit and stumbled upon where I must have gotten that timing cap/ratio idea:
http://www.thatcorp.com/datashts/600034-1.pdf

The above calculations assume a stand-alone RMS
level-detector. When the detector is placed in a feedback
compressor topology, the effective time constant
that results is calculated by taking the level detector’s
stand-alone time constant and dividing it by the compression
ratio. Therefore, if we plan to operate with
a compression ratio of, say, 20:1, we will need to increase
the timing capacitor by a factor of 20. So, for
our design the timing capacitor, C2, becomes 220 uF,
the nearest standard value.
Well that's what they say... My gut says no, but you can confirm or deny whether there is merit in linking these two variable with some bench testing. If desirable the variable gain in the cap modulation circuit could perhaps use another VCA to perhaps track a little better (?). But again I'm not sure I would want to do this so tell us how it works out.

RE: the non-linear cap, I hadn't seen that schematic before but yes some similarities. I probably need to scan in a copy of my old tape NR schematic if I can find it (early '80s). They are basically switching between one fast and one slow time constant based on a threshold step size.

I designed a somewhat more involved variant of that. Mine in addition to switching between slow and "fastest" time constants for severe transient overloads, also provided a level dependent "faster" attack but with same slow release. I was operating in the linear amplitude domain so this may not translate to RMS comps for several reasons.

JR
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by Crusty » Mon Mar 30, 2009 2:01 am

This stuff's really interesting and I will definitely experiment with this when I have some time. Thanks John for your thoughts and explanations. Wayne too!

Wayne, do you have access to a copy of Attack and Release Time Constants in RMS-Based Compressors and Limiters, by Fred Floru? I hear it's pretty math-intensive, and I don't want to pop for it if I can't understand it. LOL Can you look at it and see if there's anything about the feedback vs forward time constants?

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Re: A Discussion About True Power Summing for Stereo Compressors

Post by mediatechnology » Sat May 02, 2009 2:07 pm

Wayne, do you have access to a copy of Attack and Release Time Constants in RMS-Based Compressors and Limiters, by Fred Floru?
Crusty - Just saw this. Check your-email.
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by emrr » Tue Jun 11, 2019 4:03 pm

Stupid question:
How would one vary the amount of interaction between, say, a pair of 4301 detectors in a stereo comp with a single side chain? I envision leaving pins 15 EC+ connected to each other. 0% and 100% are obvious (pin 5 connected, or not), but 50% is not. Resistive isolation? Other? No good way?

Real world example:
Acoustic bass DI and microphone, with substantial drum transient bleed in the microphone. Microphone affected by room modes far more than DI, so the bass envelopes are related, but different in places. You don't want the drums contributing substantially to the side chain, but some lesser contribution that takes a bit of the excited room modes into account, if there's a sweet spot to be found between room mode and drum contribution.

It's possible the best answer is to run the slave channel level lower enough than the master channel so it doesn't become part of the side chain calculations, or somewhere in between for lesser contribution.
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mediatechnology
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by mediatechnology » Tue Jun 11, 2019 5:11 pm

Good question.
Since they're summing currents I wonder if a scaling current mirror to change the weight(s) would work.
Or a current-in current out VCA (218X) to vary the weight of one or more detectors before summing their currents.
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by JR. » Tue Jun 11, 2019 5:53 pm

I am not sure I follow what you are trying to accomplish? Stereo linking is about keeping the image locked hard center so it doesn't move around. Both channels get same control voltage. Mostly about same peak control voltages as low level relative average movement is not as problematic for localization.

JR
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by mediatechnology » Tue Jun 11, 2019 6:56 pm

I think he's talking about splitting, for example, a Bass DI and Bass Mic into a two-channel comp.
The source is not a stereo pair but a split instrument feed where one channel's sidechain keys the other based on a variable weight.

@doug
I think you could run one channel with lower input level so it has less sidechain contribution then, if necessary, increase the makeup gain.
Are the outputs summed to ultimately come out in mono?
It's almost like a keyed parallel compression trick.
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by emrr » Wed Jun 12, 2019 9:32 am

Yeah Wayne, you got it. Ends up in mono in a mix. Both sources need compression that track each other to avoid changes in sonic texture, yet bleed in the mic is always a problem. Letting the DI be all of the side chain source is the easiest path, but some weighting control might really be the most ideal.

Running lower in practice works fine too, after all, this is tracking stage, with mix levels set later.
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by JR. » Wed Jun 12, 2019 9:40 am

I still don't understand the question/application. Is this for a new design, or to modify and existing design to overcome a problem?

Does a single side chain mean single set of controls for both channels or single control voltage for both channels.

It is not unusual to have insert jacks for side chains so summing and weighting might be done externally as needed.

JR
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by emrr » Wed Jun 12, 2019 10:06 am

This thought directly modifies the existing DIY product that started this forum, which had a single side chain with one set of controls, RMS detector pin 5 joined and VCA pin 15 joined.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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