Compressor questions

[PRR]

With perfect implementation and obvious computation, FF and FB are equivalent. Blesser said:

"If each of these functions exists and contains no singularities, then a compression system could be built using either the input, output, or both for controlling the gain. In practice, however, it is much easier to implement a feed-forward system for compression ratios less than one, and a feedback system for ratios greater than one."
Barry A. Blesser, IEEE Transactions On Audio And Electroacoustics Vol. AU-17, NO. 1 March 1969

Barry says your strum flattener is "easier" as FB. FWIW, nearly all our small limiters are FB.

When Barry says "easier", listen. I have been poking-at and building compressors for many decades, and I still don't have 5% the insight he had as a youngster. It is far "easier" to build an awful compressor than anything nice. So don't run down the path that a guru has marked "less easy".

[rankot]

...but, TBH, I find it hard to see what the clipping limits would be of a raw OTA "current" output and where that leaves the bias, and clip limit, of the following transistor. So it may be difficult to set up another transistor that's any better off than Q2.

Datasheet says that maximum current output from CA3080 is 0.5mA. Would this work:

[PRR]

Rectifier loading on Q2....

Remember that we normally use short-attack long-decay. This means the rectifier "hits" only a small percent of the time, and only on sudden attacks. I do not believe this plan needs separate buffers for output and rectifier.

[Rob Strand]

With perfect implementation and obvious computation, FF and FB are equivalent.

If you stick with circuits you might find that hard to believe.   However, I've done my own studies and it is in fact true.  The result of wasting many hours of my life!  It's just tricky to find the "correct" non-linearity functions.  There's also some non-obvious things which have to be matched as well, like the effect of ripple from the rectifier - you have to put the non-linearity in the right places otherwise the ripple gets shaped incorrectly.     Some other trickier problems, which IIRC make it them different in practice, is the effect of linearizing networks around a JFET.  If you use a  transductance amps you don't have the issue.

[BTW: thanks for the link.]

[EBK]

Random thought:  could one make a tremolo out of two aggressively fighting sluggish compressors (not a serious design proposal -- just an academic lark)?

[PRR]

> hard to believe.

The corner cases can be tough. Consider a Noise Gate (Blesser's framework is general) which is feed-back. It watches its own output and turns-on if signal happens. Well, if it goes to infinite attenuation, it can never turn-on again. In the math you have a similar problem with feed-forward limiter trying to reach infinite attenuation (in practice you can't or never-need-to go there).

The other, and once killer, advantage of feedback is you can use any crappy thing for the control element. As long as the audio passes cleanly, the control law can be dog-legged real bad, and the NFB will mask it. With FF you need to know your control law quite well, and be able to pre-correct its kinks. The arrival of current-splitter or proportional-to-current VCAs made this much less messy. (There is a notorious FF tube limiter; it starts well, over-corrects, then runs out of correction by 13dB over.)

[samhay]

>Rectifier loading on Q2....

I also vote for don't bother.

The phase splitter has unequal output impedance (emitter << collector), so you don't have a perfectly symmetrical full wave rectifier anyway. If anything, the loading might help even things out.

[electrosonic]

So with feedback you can use a poorly characterized control element, which is probably why I don't know of any examples of a feed forward compressor that uses a vactrol. Unlike,  the Engineer's Thumb which uses a LM13700 (a proportional-to-current VCA), which allows a feed forward design.


Andrew.

[Mark Hammer]

Nothing to do with the direction of feedback, but I recently noted a big improvement in audible envelope ripple when I replaced the silicon diode used for rectifying the envelope in the Univox compressor clone I made for myself, with a schottky diode.  The diode in that circuit is also doing its rectifying by shunting a half-cycle to ground just as we see in the Dynacomp circuit above.  If the forward voltage is too high, however, more of the unwanted half-cycle gets to pass through unscathed, resulting in the audible ripple.  I think it is worth trying an experiment in which the silicon diodes used in that form of full-wave rectifying are replaced with schottky types, or even germaniums.  In theory, it ought to work.

[ElectricDruid]

+1 agree with Mark. I've improved an envelope detector by replacing silicon diodes with germaniums. Don't expect miracles, but you'll see an improvement.

I'd read about precision rectifiers, and even for those, germaniums are supposed to give the best results. Based on that information, I thought it was worth a try in what we can charitably call a "non-precision" rectifier circuit. It does help.

T.

[PRR]

> feed forward compressor that uses a vactrol.

LA2a is primarily FB. But at high levels the sidechain tap ahead of the 2.7K resistor adds a bit of FF action as the FB action weakens.

Another problem is the slow action of an LDR. FB tends to quicken this. FF would want a matched LDR to linearize the action (essentially inside a FB loop) and more trickery (possibly FB) to speed the response.

But all this is mostly simple engineering and possible to "work the same" as FB or FF. The more interesting details start with timing, and go on and on and on from there.

[Rob Strand]

LA2a is primarily FB.

I saw an interview with David Hungate saying the LA2a  is the only piece of equipment he would regret losing.

Another problem is the slow action of an LDR. FB tends to quicken this. FF would want a matched LDR to linearize the action (essentially inside a FB loop) and more trickery (possibly FB) to speed the response.

I recommend reading this article.   (Keep in mind it doesn't addressing adding non-linearities to make FB = FF).

The article does a fairly good job of showing the differences between FB and FF.  It shows how the time constants are affected differently.  They are also linked to the compression ratio, so for a fixed set of time constants you will get a different perceived behaviour.  I've had the AES paper version for 25 to 30 years (which is formatted more nicely).

http://ajoliveira.com/ajoliveira/gen/pdf/preprints/paris88.pdf

One thing I find interesting is for guitar the dynacomp type FB is very popular. As you crank up the gain the threshold  decreases and it all feels kind of natural.  All the time constants seems to work.  Where as for bass and studio stuff both FF or FB seem to work well.

[Transmogrifox]

Here's the result of some work I did in DSP land, which shows up the theory about a "feedback compressor can be made mathematically equivalent to a feed-forward compressor."

https://forum.bela.io/d/307-dynamic-range-compressor

In an analog circuit, getting things like an ideal e^x function and infinite gain make all of this stuff tricky to get better than the ears.  Every analog circuit has its own set of unique nonlinearities, and it's different with every OTA or every JFET.  As can be seen in the digital world, a linear feedback compressor has a continuously increasing ratio.  Something with an e^x gain function (like an OTA) approximates that of a soft-knee feed-forward compressor.

If you put the attack time into the feedback loop, it gets swamped by the gain, and doesn't really change in a material way.  This is typical of an analog circuit, so attack time isn't really something you can adjust in a feedback compressor unless you do something like my software equivalent, where you apply a feedback detector to a feed-forward gain stage -- really difficult to do accurately in an analog circuit.

Just some more food for thought...

[Rob Strand]

In an analog circuit, getting things like an ideal e^x function and infinite gain make all of this stuff tricky to get better than the ears.  Every analog circuit has its own set of unique nonlinearities, and it's different with every OTA or every JFET.  As can be seen in the digital world, a linear feedback compressor has a continuously increasing ratio.  Something with an e^x gain function (like an OTA) approximates that of a soft-knee feed-forward compressor.

Interestingly, I also used the exp(x) function to make FB = FF.   IMHO, the dynacomp detector is more or less an exp() function built by virtue of the exponential behaviour of the transistors.

One thing about FF is a look ahead detector is pretty darn easy.   For FB the idea of a look ahead is a bit twisted.

[Mark Hammer]

Requires a flux capacitor.  I think Steve Daniels is trying to source them, but no luck so far.

[Rob Strand]

Requires a flux capacitor.  I think Steve Daniels is trying to source them, but no luck so far.

The Materials:


The Theory:


Prototypes:


Commercial Products:

[Eb7+9]

oddly enough, of everything written in this thread so far I would say this picture approximates closest to the truth of what a "correct" AGC topology should be doing to dynamically varying signals ... in the end, yielding a system with NO attack/release artifacts ... and, it can be implemented in either FF or FB "mode" // forest for the trees stuff ... TI already has a bunch of IC's with that stuff in it

[Rob Strand]

forest for the trees stuff

forest for the trees cheese stuff

[EBK]

Cheese can promote feedback compression, depending on the flora in your sidechain.

[Kipper4]

Nice. Some good insight there..


However there are always more questions.

Re the rolls ducker.

http://www.rolls.com/pdf/M_DU30b.pdf

How does it attenuate? Which parts. The J113s?

Would removing the mic pre have a significant effect? Or is it kinda tied to the other inputs?


Or is it a similar method to what PAIA did with theirs?

http://www.freeinfosociety.com/electronics/schemview.php?id=1092

I'm a long way from done wasting my time with some mods and stuff but I have much to learn.

Cheers
Rich