any love for a new THAT comp discussion?

[gemini86]

Luckily the 500 standard includes a link pin, making dual mono to stereo just a push of a button. The app note includes a link switch for stereo also and there's a whole other design note for stereo linking. All you mastering guys can just build two units into one case using only one timing control board for both.

 

[JohnRoberts]

John,

The burst gate looks to be a very useful tool for this kind of development work, I recall you mentioning it before and I hope to get time to construct one.  I love the idea of the cap multiplier circuit to vary the apparent cap size and RMS timings.  One other notion I like is the Massenburg/MacNeill concept of multiple RMS timing paths combined with a peak sensing path and all combining to provide a more accurate read of the dynamic information in the signal.  I need to get back to the 4301 datasheets and get a refresh but I'm trying desperately to stay focused on one project at a time, for a beginner like me it takes a long time to absorb stuff..

Cheers,
Ruairi

I don't think I'll build another one of these burst gates. I've build a few over the decades, and all seem to have grown legs, but I'm not the one here interested in making yet another compressor.  

IMO the side chain is 95% or more of any compressors sound, so worth (you guys) investing some time into. I'm mainly an informed kibitzer here. 

JR

PS: I have thought a lot about the RMS "magic"  I think some of the benefit of the THAT detector chip set is the particular fixed relationship of attack and release from the one cap as loaded by the detector IC. Once you start messing with peak/ave time constants separately, you have a different animal and the THAT (RMS?) magic, is no longer dominant. 

 

[buschfsu]

i like the one knob variable cap idea.  ill try that.  any ideas for a conventional attack release circuits?  the data sheet is anything but simple in this regard.

 

[JohnRoberts]

For attack/release I would suggest asking yourself what you are trying to accomplish. I did a great deal of work with companders, symmetrical compress/expand NR circuits.  In these cases the compression needed to be reasonably fast attack and fast release, but also, slow enough to be easy to track for accurate and transparent playback.  We can make a comp/limiter arbitrarily fast, but all we are doing is distorting the leading edge of the transient attack.

====

Using the THAT chipset the general approach to make variable attack/release is to terminate the RMS detector with a very small capacitor effectively establishing the fastest attack time, buffer that, and then apply variable up and down time constants after that buffer. I have also used a static hold time, for noise gate applications, but find the "ripple filter" approach (slow for small changes, fast for large changes) a lot nicer for compressor/limiters, and it's easier to design with analog circuitry. Using a digital side chain, infinite hold is certainly possible. IIRC I probably used a very slow time constant even with my hold designs, just to insure that during hold, it wouldn't drift away from where it should be, but toward the slow average of where it should be. 

JR

 

[emrr]

http://www.thatcorp.com/datashts/THAT_4301-Demo_Datasheet.pdf


Addressing the OP, and anyone else interested to know:

I have seen several failing 124x chips that passed DC on to following stages.  The only apparent indication was odd meter response due to connector switching transients making the RMS detector flip out.  Meter slammed when a patchbay connector engaged, long slow release.   

I have seen 4301's that were outside of prescribed calibration range.  Switch to another 4301, cal problems gone. 

It is my understanding that the published app note for variable attack and release is tough to build with stability in practice, and that the commercial approaches have altered the approach.  I'd suggest getting the basic RMS design to do what you want first, before going into bells and whistles land.  Look at the GML approach to timing, and look at what Safesound has done with their approach. 

 

[e.oelberg]

here is a white paper on a that based compressor...the part about the side chain might be interesting.
http://www.safesoundaudio.co.uk/downloads/dtwhitepaper.pdf