Compressor Feedback and Feed-forward switch - developers ?

[kylen]

Hi Folks - after a hearty discussion with citizen chunk the other day I went off and looked more into the compressor side-chain design he was telling me about known as feedback.

I'm still in the process of experimenting with my feed-forward compressors using very low threshold (VLT) and very low ratio (VLR) to get more of a smoothing sound but it's just not the same - is it?

Then I naturally began to wonder why a feedback/feed-forward switch just couldn't be put on the compressor and what serious issues that might raise - besides possibly affecting the controls differently.

Then I stumbled on this article which shows an API compressor that has just that - feedback or feed-forward switch. So this idea is not without precedent and I stumbed on this naturally in my own thinking before seeing any article on it - I'm thinking some of you developers thought about this too already!

Here's the article:
http://www.tangible-technology.com/dyna ... h_pw2.html

So before I go around to each compressor company I thought I'd just post here to see if there's anything obvious I'm missing - quite likely!

Also I wanted to establish that the idea is one that anyone would stumble on naturally and could not be regulated by any patents that API may have anymore than the idea of feedback or feed-forward could be patented itself in this day and age.

Any thoughts developers? Voxengo, Kjaerhus, and chunk are the folks I'm thinking about - possibly PSP. Those folks are pretty responsive and I thought might be a good place to start.

Thanx

[ED...PS I'm asking for developers comments but obviously everyone is welcome to engage in the discussion!]

[citizenchunk]

hi Kylen. check out Digitalfishphones' Blockfish. it's got an "Opto/VCA" switch, which i believe is exactly what you need. just FYI: opto compressors are feedback, and VCA comps are (generally) feedforward. the Waves Ren Comp has a similar switch, but i don't know if it is actually employing both topologies or just "fudging" it.

btw, Blockfish is free.

actually, Sascha (of digitalfishphones) recommended this functionality to me when he was testing Vanilla, so you're in good company.

== chunk

[Lunch Money]

Could've sworn on another thread that someone said Voxengo Voxformer's compressor uses feed-forward?

Might have been daydreaming.

Greg

[citizenchunk]

Could've sworn on another thread that someone said Voxengo Voxformer's compressor uses feed-forward?

Might have been daydreaming.

Greg

i think Aleksey said that Voxformer is feedback and Crunchessor is feedforward.

== chunk

[kylen]

actually, Sascha (of digitalfishphones) recommended this functionality to me when he was testing Vanilla, so you're in good company.
== chunk

Thanks chunk ! I see it now in the Blockfish manual - I guess I didn't appreciate the difference till lately after you mentioned it, Blockfish does have both feedback (opto) and feed-forward (vca). Now I'm starting to get that sound in my head!

Does this mean Vanilla might get that feature ?

[kylen]

chunk and Greg - I think Aleksey 'implied' that Voxformer used feedback and said that Crunchessor didn't.

So far it looks like these are actual feedback compressors:
Voxformer
Blockfish

The Sonitus Ultrafunk Compressor has a 'Vintage' selector that emulates opto designs but I believe is still feed-forward. The difference is obvious (to my ears anyway), regardless how the compression curve looks - which is what you were saying the other day chunk. Here's what Sonitus says in the compressor manual:

"The Vintage compression type emulates the compression curve found on some analog compressors, such as the classic Teletronix LA2A. The major difference is that the compression ratio is gradually reduced at a distance above threshold, slowly allowing the level to go back to a ratio of 1:1. This allows the loudest parts of the signal, such as drum beats and other peaks, to pass without being compressed as much as the rest of the signal. In this way, the Vintage compression type emulates electro-optical analog designs and can greatly enhance warmth and 'punch'."

As far as using other feed-forward compressors like Polysquasher to get a super-smooth vocal sound it can be done by digging real deep. The sound is flatter I think because of the transient compression but thanks to Polysquasher extreme controls I can set a threshold of -70dB and a ratio of 1.05:1. I'd love to hear what a feedback switch could do in there!

I'm still working with the PSP Mixpressor (emulation - right PSP?) and Sonitus compressor which use the 'emulation' modes - so far I'm hearing that the emulation just isn't as pleasing as the actual feedback design for vocal tracks.

[Rune L-H]

So far it looks like these are actual feedback compressors:
Voxformer
Blockfish

Endorphin in "Opto"-mode also uses the feedback approach. But note that the VCA/Opto mode switch propably does much more than simply switch between feedforward and feedback designs, so you won't be able to audition exactly what a feedback design isolated does to a compressor sound.

In general I would say, be carefull not to overrate the importance of this feature in software compressors. As far as I know, feedback designs were originally designed to provide a more smooth and stable response, but in the digital age there are other flexible methods to achieve similar results. And as far as emulating the sound of those old devices I would also be carefull not to consider feedback approaches as always more true to the original opto sound. IMHO many(!!) other and equally important factors are in play when emulating that response.

Best regards,
Rune

[multree]

only when you stand on stage with your guitar around your shoulders turning twoards your half stack you will realize that feedforward doesn't make a sqeeeeky sound

[kylen]

only when you stand on stage with your guitar around your shoulders turning twoards your half stack you will realize that feedforward doesn't make a sqeeeeky sound

hehe - true enough. That's why I'm focusing on somewhat erratic pre-recorded vocal tracks as my test case for the smoothing features of feedback compression.

[kylen]

Endorphin in "Opto"-mode also uses the feedback approach. But note that the VCA/Opto mode switch propably does much more than simply switch between feedforward and feedback designs, so you won't be able to audition exactly what a feedback design isolated does to a compressor sound.

Ok thanks for the Endorphin tip!

In general I would say, be carefull not to overrate the importance of this feature in software compressors. As far as I know, feedback designs were originally designed to provide a more smooth and stable response, but in the digital age there are other flexible methods to achieve similar results. And as far as emulating the sound of those old devices I would also be carefull not to consider feedback approaches as always more true to the original opto sound. IMHO many(!!) other and equally important factors are in play when emulating that response.

OK - I'm just learning the sound so I guess I'm wearing it on my sleeve right now.

Also part of the reason why the UAD-1 plugins are popular - the set includes 2 feedback type compressors - LA-2 and 1176. I suppose those software emulations included the feedback type side-chain in their design (?).

[kylen]

I just goes to show ya...

I put Kjaerhus GCO-1 (feed-forward I think - set to fast attack, fast release, low threshold, low ratio) in front of Voxengo Voxformer (feedback) and dialed the Xover to the sweet spot and it all sounds like a million bucks to me.

...some of us are just born to be knob twisters no matter how much theory we can try and apply!

[citizenchunk]

Rune is very right when he says that there is probably a lot going on under the hood when you flip the opto/vca switch. opto emulations can be very complex; much more than simply where you "tap" the sidechain signal. it would be very easy to implement such a simple switch in Vanilla, but it probably wouldn't make it sound like a vintage opto design.

FWIW, the nonlinearity that Kylen mentioned (the ratio returning to 1:1) is a property of opto cells--originally a problem that was eventually embraced for its character. (some people say it adds punch!) i believe that's what GCO-1 is emulating with its "Linearity" control. but GCO-1 is definitely feed-forward.

i've thought about it in the past and figured that one could adequately approximate a true opto design with a completely soft-knee, feedforward design with the aforementioned nonlinearity. IMO, using such a feedforward design allows one more accurate control over the envelope, since a true feedback design is very unpredictable.

sorry, Vanilla probably won't get an opto mode. but there was an opto/FET compressor project in the works a while back. if i ever get around to finishing it, it will be very cool. then again, these days it feels like the LAST thing that the world needs now is another compressor plugin!

== chunk