Dynamic Equalizer

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mediatechnology
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Dynamic Equalizer

Post by mediatechnology » Wed Sep 05, 2007 9:47 pm

There was a post today over at Prodigy-Pro that reminded me of this:

Larry Blakely while at dbx presented an AES paper "A Variable Frequency Limiter/Compressor/Expander" which was given the working model number dbx169. I don't think it was ever built but it was a frequency-selective dynamics processor. He and I discussed doing it at MicMix because dbx didn't seem interested in doing it.

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Crusty
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Re: Dynamic Equalizer

Post by Crusty » Fri Sep 07, 2007 10:05 pm

Well, this is a multi-band dynamics processor, not a dynamic equalizer, right?
I'm not sure of the band-splitter here; why do the 12 dB per octave upper and lower filters feed back into the 6db per octave middle filter before the processing? I need to mull that...Is this a trick I've been missing?
My first product was supposed to be a multi-band compressor, so I have a bit of experience with it; got really complicated quickly with the feature-set I wanted. So I started with the "easy" one first. Two years later... I need to work faster...

The analog multi-band flavor seems to have cooled off over the last couple of years. M/S seems the hot ticket now. Or is it parallel compression? LOL Someone told me recently that Maselec is working on a multiband compressor to release soon. I'd like to finish mine, it's a different beast from what's out there. I need to finish the mic pre rack and the 5-band eq first... :D

As SSLtech said, the real trick is getting the user interface right and that's tough to do. Especially on something that's messing with eq and dynamics at the same time, yikes!

As far as experimenting, this is the kind of stuff the Picocomp is suited to; very predictable behavior of the detector/vca as a jumping off point. Hint Hint...

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Re: Dynamic Equalizer

Post by mediatechnology » Sat Sep 08, 2007 6:16 am

crusty - thanks for the reply.

I suppose it is a multi-band dynamics processor with only one band processed and I briefly considered renaming the thread. But after thinking about it maybe not. I picked up the thread title from an existing post over at Prodigy where I contributed this. The distinction between compression and equalization gets a little blurred here when one thinks about it terms of it being a three band EQ with an automated "hand" cranking up the boost/cut level. So as an "equalizer" with variable frequency, level and possibly Q only one parameter is dynamic. Is it a multi-band compressor? Yes - in one band. Is it a dynamic equalizer? Yes - in one parameter: level. It's hard to say that this is either. But it doesn't have to be that way...*
I'm not sure of the band-splitter here; why do the 12 dB per octave upper and lower filters feed back into the 6db per octave middle filter before the processing? I need to mull that...Is this a trick I've been missing?
What's going on with that is that the mid band is a "derived" filter. Although not clearly shown in the block diagram, the outputs of the 12db/octave high and low pass filters are fed back into a summing network where they are subtracted from the input to derive the middle band. This is an old active cross-over trick that provides the advantage that when the outputs are all put back together phase cancellation is avoided. There's no notch formed at the frequency transition between bands and the derived band will always be 6 dB/octave. I think the first discussion I can recall about this was in National Semiconductor's Audio Handbook.

* With VCAs in this gizmo being used in a state-variable design to set filter frequency, is there any reason why they too couldn't be made dynamic? Now it moves from being an "almost" dynamic equalizer to having two filter parameters controlled by the "automated hand."
The analog multi-band flavor seems to have cooled off over the last couple of years.
There's a guy by the name of Glen Clark who designed an FM broadcast processor called the "Texar Audio Prisim." It was four band unit and one of the best processors I've ever heard. Glen designed it back in the late 80s and later sold the design to Gentner. The loudness density increase was amazing and it had a gated AGC that prevented AGC action over small input level changes. With the amount of modern POP CDs I'm seeing that use hard clipping to achieve density we really should examine using broadcast-style processing.

As to M/S processing I'm really curious as to what people are doing once the signal is broken down into M/S. You may have seen my precision encoder/decoder thread and a lot of people have become interested in that. I'd like to continue work on a variation of Rick Chen's width control. I think I may have found a way to eliminate the desirable ground tap on the dual pots that when centered don't reduce separation and precision of the matrix. It looks like the matrix panner contraption might play a role here.

The Pico Compressor would make a good multi-band unit: There is one broadcast multi-band using 4301s but I don't remember who it is.
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Crusty
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Re: Dynamic Equalizer

Post by Crusty » Sat Sep 08, 2007 4:42 pm

Yeah, I see what your saying about the semantics. And thanks for the filter details, the one I worked out in my idiot-sevant style indeed works that way. Anything past first order does tend to comb when recombined, and adjustments are extremely interactive. I experimented a bit with a Linkquist-Riley (sp.) because I read that it recombined flat. It is interesting; controllable, but not in ways you'd assume when twisting the knobs.

The multi-band approach as I call it (split the audio into bands, process, sum) is nice sounding, controllable and useful, but not surgical because of the filter limitations. The dynamic eq approach could be a better all-arounder, and I'll study it when I'm back on that project....

Oh. and it's nice to be able to discuss such things without someone feeling I'm taking advantage of them for financial gains or whatever....

The M/S stuff is definitely interesting. Maybe in the audio and the sidechain paths...

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Re: Dynamic Equalizer

Post by mediatechnology » Sat Sep 08, 2007 5:11 pm

Oh. and it's nice to be able to discuss such things without someone feeling I'm taking advantage of them for financial gains or whatever....
That's our genesis here at the Pico Compressor Forum.
The M/S stuff is definitely interesting. Maybe in the audio and the sidechain paths...
Hmm... M/S Sidechain....Out of the box thinking IYAM (if you ask me). I like that. What we do best here.
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