OTB Mixer Using Current Summation

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
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JR.
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Re: OTB Mixer Using Current Summation

Post by JR. »

thanx and very cool.. let me know if any questions. or further discussion.

There are several interrelated dependancies. While I'm not sure how to test completely in isolation.

I have two questions about pan law..

A) is there a group consensus on nominal pot impedance?
B) is my first try approach reasonable or what direction does it need to go for user friendly pan.

To test pan law control V can be fed to a pair of stock VCA circuits, that you compressor guys may have laying around. :D

I'd maybe look at B first using A convenient for you. If different that my guess easy to scale. I don't think we can confirm how well low noise CV approach really works till later. At this point just confirm it works at all is doesn't oscillate or do something stupid.

When happy with A and B

perhaps dummy up one channel to summer with some added Cs to model a bunch of vca's.

JR

PS: Perhaps a propitious sign, I hit the winning basket in one of my pickup games Thurs night.
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mediatechnology
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Re: OTB Mixer Using Current Summation

Post by mediatechnology »

Forgot to mention that I have a pair of 2162 demo boards. Each demo board represents a channel. With those two boards I can make a 2 input mixer as a current summation test bed.
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Re: OTB Mixer Using Current Summation

Post by emrr »

What I recall all seems reasonable, and any common pot value is fine. You guys have a better feel than I about specific value.
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mediatechnology
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Re: OTB Mixer Using Current Summation

Post by mediatechnology »

I just found the PAR article "Digital vs. Analog Summing" I promised earlier. I couldn't find it anywhere on their website so here's the scan. It makes a nice case for OTB.

http://www.ka-electronics.com/Images/pd ... y_2007.pdf
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JR.
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Re: OTB Mixer Using Current Summation

Post by JR. »

Thanks.. I really am curious to know if there is any there there... I scanned through the article briefly and didn't see anything that drew me in... I'll try to give it a more leisurely perusal.

I still expect a well executed Digital summer to beat (or at least equal) a similarly well executed analog summer,,, Apparently there's still less than well executed gear out there.

JR

---------
OK, I re-read that article without my beer goggles, and still didn't find any meat that I could sink my teeth into... He suggests a harshness in digital and less headroom than analog, but it seems both of these could be objectively proved, but AFAIK have not.

JR
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Re: OTB Mixer Using Current Summation

Post by juniorhifikit »

Fascinating thread! I've really learned a lot. Looking forward to hearing about the bread board results and knowing whether it's worth the trouble/extra parts to do symmetrical CV drive, or if simply grounding one port the conventional way will be adequate. That is to say if it makes an audible difference.

I was discussing something similar with a plugin-making friend of mine. We had discussed using two VCA's in parallel for level and panning, and he thought the pan law would be cake to program in C thus letting the microprocessor take care of it. My programming skills are laughable, or I'd offer to jump in and help. At this point I'm just armchair quarterbacking.

Not to make things more complicated, but in regards to keeping the parts-count low, has anyone given any thought to doing the same thing with an MDAC? Something like a Maxim Max532, or perhaps an Analog Devices SSM2018 VCA which already has differential inputs?

**EDIT** I now see that these devices are "pre-buffered" with opamps and are not current-in/current-out, so of no use in a current summing design... duh.
Last edited by juniorhifikit on Tue Oct 13, 2009 12:15 pm, edited 1 time in total.
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Re: OTB Mixer Using Current Summation

Post by juniorhifikit »

JR. wrote:Thanks.. I really am curious to know if there is any there there... I scanned through the article briefly and didn't see anything that drew me in... I'll try to give it a more leisurely perusal.

I still expect a well executed Digital summer to beat (or at least equal) a similarly well executed analog summer,,, Apparently there's still less than well executed gear out there.

JR

---------
OK, I re-read that article without my beer goggles, and still didn't find any meat that I could sink my teeth into... He suggests a harshness in digital and less headroom than analog, but it seems both of these could be objectively proved, but AFAIK have not.

JR
As a mixer for a living, one of the main things that OTB summing gives me is parallel processing without comb filter-inducing converter latency. Even with to-the-sample compensation between things that stay ITB and things that go OTB, the inter-sample rounding causes comb filtering between the two. For parallel drum processing for example, I end up sending my unprocessed signals on a round trip through the converter just so they line up with the analog-processed signals.

The other benefit to OTB summing for me is that I like the distortion "signature" that certain gain stages gives me when transients shoot beyond the means of the supply. No one has yet modeled this well and I think we're a long way off. Personally I don't have a huge problem with the "sound" of ITB summing. I don't think it has one. Maybe that's the problem. :D

Of course, all this can be ignored if one mixes the way nature intended - on a console! Unfortunately, current budgets dictate other (more "creative") means.


which is why I'm looking at building a widget like this
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JR.
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Re: OTB Mixer Using Current Summation

Post by JR. »

juniorhifikit wrote:
As a mixer for a living, one of the main things that OTB summing gives me is parallel processing without comb filter-inducing converter latency. Even with to-the-sample compensation between things that stay ITB and things that go OTB, the inter-sample rounding causes comb filtering between the two. For parallel drum processing for example, I end up sending my unprocessed signals on a round trip through the converter just so they line up with the analog-processed signals.
Do I gather you are talking about dynamics? Any time based effects will have and use delay, so parallel processing can diminish any such effect.

I have looked into parallel compression some and don't understand the attraction, it seems the same transfer function could be generated by side chain manipulations unless there is significant EQ applied to only one path.
The other benefit to OTB summing for me is that I like the distortion "signature" that certain gain stages gives me when transients shoot beyond the means of the supply. No one has yet modeled this well and I think we're a long way off.
?? Long ago and far away digital saturation was nasty as the codes would just rollover through zero, but modern digital paths pretty much saturate like a well behaved analog path. I ASSume you are talking about a well behaved analog path, while most consoles provide more than enough headroom on the master bus to avoid clipping when used as intended.

Perhaps some extreme pursuit of loudness leads to clipped audio path, in which case some different other than solid state circuitry could deliver an audibly different overload characteristic.
Personally I don't have a huge problem with the "sound" of ITB summing. I don't think it has one. Maybe that's the problem. :D
Thank you... This is consistent with my understanding of the the technology.
Of course, all this can be ignored if one mixes the way nature intended - on a console! Unfortunately, current budgets dictate other (more "creative") means.


which is why I'm looking at building a widget like this
I have friends still making analog consoles (not for the recording market). If there was a real need for this, they could step up, but I still have the uneasy feeling this is a fad that will go away as low cost digital systems get more powerful.

I know I could design a better analog summer, just do see the need.. since they are already better than one microphone preamp at nominal gain.

JR
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Re: OTB Mixer Using Current Summation

Post by juniorhifikit »

JR. wrote:Do I gather you are talking about dynamics? Any time based effects will have and use delay, so parallel processing can diminish any such effect.
Yes, dynamics.
JR. wrote:I have looked into parallel compression some and don't understand the attraction, it seems the same transfer function could be generated by side chain manipulations unless there is significant EQ applied to only one path.
On paper it does seem a bit redundant, but in practice it's quite effective. It's become a staple technique for mixers, and the sound of rock and pop mixing for the last 20 years.
JR. wrote:?? Long ago and far away digital saturation was nasty as the codes would just rollover through zero, but modern digital paths pretty much saturate like a well behaved analog path. I ASSume you are talking about a well behaved analog path, while most consoles provide more than enough headroom on the master bus to avoid clipping when used as intended.
"Well behaved" and "as intended" are key phrases here. We're not necessarily try to avoid clipping, and no digital simulation has yet captured the detailed signature of some of our most prized distortion devices (compressors). I believe that the human ear is very sensitive to what's happening to leading edge transients (and their abuse), and that plugin designers haven't "gotten it" yet. Either they don't have the necessary horsepower, or they have no idea how the tools are being used/abused by our industry's finest.
JR. wrote:Perhaps some extreme pursuit of loudness leads to clipped audio path, in which case some different other than solid state circuitry could deliver an audibly different overload characteristic.
It's not about loudness. It's about color
JR. wrote:I have friends still making analog consoles (not for the recording market). If there was a real need for this, they could step up, but I still have the uneasy feeling this is a fad that will go away as low cost digital systems get more powerful.

I know I could design a better analog summer, just do see the need.. since they are already better than one microphone preamp at nominal gain.
My needs are well defined, for me. Not sure how they reflect the rest of the industry:
-Analog summing to avoid comb filters induced by converter latency
-Avoiding the multiple round trips through AD/DA conversion when inserting analog gear into my DAW (sometimes more than 3 trips)
-Automation AFTER my compression, which most of the current sum gizmos don't address.
-Automation that reads from the DAW (HUI, Mackie Control...)
-A topology I'm familiar with and is not too expensive (a la SSL 4K - 5534's and VCA's)

I could buy a used SSL, but that doesn't make financial sense these day. Tonelux has a great solution, but again it's very expensive.
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JR.
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Re: OTB Mixer Using Current Summation

Post by JR. »

juniorhifikit wrote:

"Well behaved" and "as intended" are key phrases here. We're not necessarily try to avoid clipping, and no digital simulation has yet captured the detailed signature of some of our most prized distortion devices (compressors). I believe that the human ear is very sensitive to what's happening to leading edge transients (and their abuse), and that plugin designers haven't "gotten it" yet. Either they don't have the necessary horsepower, or they have no idea how the tools are being used/abused by our industry's finest.
The human ear is constantly bombarded with TMI. The brain is constantly changing what it focuses on and interprets.

The dynamic response of a compressor is a completely different animal than a linear audio path, which IMO should have a well defined and relatively artifact free overload character.


It's not about loudness. It's about color
"Color" generally translates to frequency response and perhaps added distortion products, generally harmonically related so perhaps musical. Again solid state clipping will have a well defined overload characteristic, tube or other obscure variant paths may overload with different harmonic structure... VCA overload depends on which vca and how applied, with a gradual increase in distortion, then a harder standard clipping distortion.


My needs are well defined, for me. Not sure how they reflect the rest of the industry:
-Analog summing to avoid comb filters induced by converter latency
-Avoiding the multiple round trips through AD/DA conversion when inserting analog gear into my DAW (sometimes more than 3 trips)
-Automation AFTER my compression, which most of the current sum gizmos don't address.
-Automation that reads from the DAW (HUI, Mackie Control...)
-A topology I'm familiar with and is not too expensive (a la SSL 4K - 5534's and VCA's)

I could buy a used SSL, but that doesn't make financial sense these day. Tonelux has a great solution, but again it's very expensive.

I have given some thought to this and using a simple microprocessor interface, something like my VCA summer could receive control from protools or DAW? could be cheaper than knobs and controls. I wonder if this wouldn't offend the sensibility of OTB analog mixer proponents? A hybrid analog/digital box, to allow an analog sum under digital control.

My sense is we want to use our old toys, and digital plug-ins don't yet accurately mimic how they overload? If when the plug-ins get it right, OTB becomes moot. Note: this includes a plug-in for parallel processing.

JR

PS: Yes I know Paul Wolff from waaay back.. He needs to charge enough that people value his product, and being a small volume mfr he needs to charge more than a larger company for similar BOM.
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