A Discussion About True Power Summing for Stereo Compressors

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

Post by Crusty » Wed Mar 25, 2009 2:44 am

JR. wrote:
PS: Some more food for thought, I suspect similar to the way opamps combine with small caps to make them look bigger in DC servos, a clever combination of small caps and opamps (and a pot) could synthesize a variable capacitor at that integration node, so you would have effectively an adjustable speed RMS. It would retain the att/release characteristic of RMS but be variable generally faster and slower. Of course this assumes this is even useful or not, which could be proved with a mutli-position switch.

PPS: I think I could do it in two opamps, but it's is not worth drawing up, unless of determined to be of value or interest.
Yeah, that's cool. I'd have to hack it for separate Attack and Release though... LOL. Changing the value is more obvious sounding on individual tracks, especially third harmonic dist (not always a bad thing!)

The hold function almost sounds like an ADSR - at least the 'S' part... What about a variable delay before the attack, like a scope sweep delay?

As someone who's made a living involved in the recording of music, I think there's room (and need) for both schools of thought regarding gear - fidelity and vibe.
Some broad (very broad!) generalizations: As far as uber-controls on a compressor, rock/pop engineers fall into two basic categories - there are the tracking and mixing engineers who generally want as few controls as possible. They want vibe, it needs to be pretty much instant or they'll move on to some other piece. More so during tracking and overbubs. They're quite happy with "one-trick ponies" that work well on only a few sources. The bigger selection of different units, the happier and more comfortable they are. These are chiefly effect units and not always predictable results when patching them in.

Mastering engineers are different. They want as few pieces as possible, and will gladly accept control over as many parameters as possible. The Mastering guy's buzz words are accuracy, transparency, repeatability, and predictability. Reasons you don't often see the same comps/eqs in tracking rooms and mastering rooms...

Both very valid approaches to compression.

As a wise man says often here, YMMV...

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

Post by JR. » Wed Mar 25, 2009 9:54 am

I first came up with the concept for the uber compressor back in the '80s, before I had the technology to do it. Back then my concept was to have the degree of control optional for the user. Basically the unit comes with presets, presumably dialed in to mimic classic compressors, and users could just use those presets and ignore all the knobs. More sophisticated users could explore what it is about the classic XYZ comp sound that they like and use the preset as a starting point to to play with the variables from there.

Only now, can I almost make one with technology in my grasp. I still would need to figure out how to write the user interface for a PC/MAC and then communicate via USB or whatever to the simpler looking hardware. So comp without the computer is a basic unit with presets, hook up a computer and you can drill down to tweak almost anything. Back in my earlier vision I was going to save the compressor setting in FSK to the leader of the tape track. :roll:

Sorry.. I don't mean to hijack this thread.. I already have another thread where I talked about this concept (shiny hammer), and like several of my ideas, this is not a very high priority on my list...after decades of puttering around with it, the technology is catching up.

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

Post by JR. » Wed Mar 25, 2009 10:18 am

For any interested here is my first pass at a variable C.

Simply the minimum C value is .1uF. The bottom of the cap is connected to a virtual ground, and the output of the inverting amp, is applied through a variable gain stage/buffer to a capacitor in parallel with the first.

At minimum C (fastest response), the parallel C is inverted twice so effectively tracks the first cap. As the gain of the buffer stage increase from -1x to +Nx the effective capacitance value gets multiplied several times. Making Rx smaller makes max C larger.

Note: You can eliminate the -1x from the buffer and the beginning C starts at .2uF instead of .1uF.

I have no idea what is a pleasant range of adjustment, or the nominal C for your application, but values should be easy to scale.

CAVEAT: this has not been bread boarded so may not work.. :lol: Note: The DC feedback R across the inverter makes response fast (2xC) for DC. Cap coupling the ground leg of the pot could make the C max (slow) for DC. I don't expect either to be audibly significant.

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

Post by mediatechnology » Wed Mar 25, 2009 11:04 am

OK, I get it. Very nice!
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by Gold » Wed Mar 25, 2009 6:26 pm

Crusty wrote:
As far as uber-controls on a compressor, rock/pop engineers fall into two basic categories - there are the tracking and mixing engineers who generally want as few controls as possible.

Mastering engineers are different. They want as few pieces as possible, and will gladly accept control over as many parameters as possible.
The great thing about DIY is that I can have it both ways (as an M.E.). Since I know how I like to work and what I need out of my tools I can have a minimum of controls that do what I want. No more, no less. That's why it's taking forever for me to do a final build. I haven't sorted it all out yet. The DAOC optical comp I built was easy from that standpoint. All I need it to do is be a "sloppy limiter". It's great at that and I don't need it to do anything else.

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

Post by Crusty » Wed Mar 25, 2009 11:03 pm

Always good to know what you want. Do you use software comps too? (like this thread needs another direction...). I think it's cool that you build gear that you use professionally - old, OLD, school...

Thanks John for drawing that up, I'll give it a try when I have a moment. Edit: It works in the simulator at least with ideal opamps. About 1.3uf down to 100nf. Neat!

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

Post by JR. » Thu Mar 26, 2009 10:31 am

Crusty wrote:
Thanks John for drawing that up, I'll give it a try when I have a moment. Edit: It works in the simulator at least with ideal opamps. About 1.3uf down to 100nf. Neat!
Yup it worked in my cranium simulator too.. 8-)

I don't know a good range or nominal value to use. Some gain can be moved from second stage to first by making feedback cap smaller, but for now I'd KISS.

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

Post by mediatechnology » Thu Mar 26, 2009 11:12 am

John - I took the liberty of re-sizing your drawing so it would fit without requiring a frame. phpBB3 is supposed to scale image attachments to fit but it doesn't always work.

Your circuit seems to simply and elegantly scale C and maintain RMS response.

The auto circuit using the non-linear C would seem to maintain RMS for small step sizes or slowly changing envelopes but not be fully RMS for large envelope transients. I think it's worth building soon. Roger? Crusty?

Yours truly has problems keeping his bench available 'cause it's always piled up with other people's stuff waiting decisions. ;)
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Gold
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Re: A Discussion About True Power Summing for Stereo Compressors

Post by Gold » Thu Mar 26, 2009 11:23 am

Crusty wrote: Do you use software comps too? (like this thread needs another direction...).
Not usually. Occasionally I'll use the built in one in Samplitude if it's an emergency. The other day I had to piece together something that had bands playing live in a radio studio along with interviews. for the music stuff I used my one and only Maselec compressor. For the voice stuff I used the software comp because sound quality didn't really matter. I'm not big into compressors. It will be a luxury to have the DAOC and the Pico. I just hope I don't get into the habit of using them ;)

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Re: A Discussion About True Power Summing for Stereo Compressors

Post by JR. » Thu Mar 26, 2009 11:38 am

Simple and elegant would be one opamp, :oops: but the only one opamp approach I come up with, adds an RxC load to that node that probably wouldn't sound bad but deviates from the pure RMS attack/release characteristic which seems to be the raison d'etre for this pursuit.

I guess two opamps are not a big deal nowadays, especially when I'm not paying for them, or the one squeezing them into a layout. :D

JR

thanx for the resizing... I seem to be a little graphically challenged. I had to drag it into paint to rotate it to be pointing in the right direction after i scanned it..
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