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farting compressor problem Dynamics Processors (HW)
Old 10th September 2007
  #1
farting compressor problem

for some reason when I have my compressors at extreme settings like mics turned up really high and compressor output really high, sometimes it will do this clicky farting thing that will become faster if the mic is turned up more and changes speed depending on certain settings. I thought it was caused by a feedback loop between the control room and live room but this only happens with dynamic mics. I also thought it was just my Art Pro VLA but it happens with my FMR RNC too. SM 57s do it the worst but my Beta 52 does it too.

I'm recording a noise artist so it's nice to have compressors set so that when he makes sudden exploding noises they don't clip. That's why i have the mics set really high for the quiet parts, and ride the gain in combination with the compression. It's a problem for the really quiet parts.

Anyone know why this is?
Old 15th September 2007
  #2
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bongomania's Avatar
 

Compressors at extreme settings will have artifacts, end of story. However, some have more (or worse-sounding) artifacts than others. I haven't used the VLA, but the RNC has some of the worst-sounding artifacts at extreme settings that I have ever heard. I would never, ever use the RNC for anything other than gentle smoothing, it's terrible as a limiter.

Try setting your VLA with the slowest release setting possible, that sometimes helps.
Old 16th September 2007
  #3
I think it's more than artifacts... it's like the unit is freaking out. The meters bounce fast with the noise which sounds to me like a feedback loop. Like if you some piece of equipment that says "dont do this it will cause a feedback loop" and then you do it - sorta sounds like that.

I used to be in a noise band... I showed the guy I was recording what it sounds like, added some bass through a 31 band EQ. Of course he thought it was really cool because his music is made by using a bunch of pedals and hi-z mic driving them to make some really horrible sounds. But for me it was a practical problem
Old 16th September 2007
  #4
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Hi
'Feedback' compressors can oscillate if there is a significant amount of feedthrough of control voltage onto the audio path. Some valve types that use a tube as the VCA element suffer from this as their 'balance' drifts over the years, eventually oscillating when set at high ratios.
I don't know if it is the problem you are seeing but just a thought.
Matt S
Old 16th September 2007
  #5
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soundbarnfool's Avatar
 

Light a match.
Old 16th September 2007
  #6
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Jim vanBergen's Avatar
 

truly sounds like you are overdriving the circuit at some point- might be the preamp putting out a square wave, albeit for only a millisecond, for the circuit to erupt before the attack of the comp catches it.

Try double-tracking side-by-side microphones and comps, one set for low gain, one set for high gain, with each one doing more gentle compression and less radical. This works wonders...though requires editing and a very smooth demeanor.

Hope this helps!
Old 16th September 2007
  #7
Matt S: I think that's the problem. Now I need a "brick wall" limiter that's built for this purpose
Old 17th September 2007
  #8
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Hi
Exceptionally fast and deep attack rates can 'sound' like clipping as it is bordering on 'Fourier' clicks experienced when you suddenly cut or uncut a signal. The brain interprets the sudden change in slope as part of a note which is not related to the audio thus giving the impression of a click. This is a 'standard' issue with mute circuits on mixing desks where people perceive 'clicks' when muting / unmuting audio but if the audio is removed prior to the circuit there is no disturbance. Better desks actually implement a fast fade of around a few milliseconds rather than a sharp cut.
You need to find a feedFORWARD compressor and then fiddle with attack rates. Alternatively aim ror a chain with as much headroom as possible and 'post recording' edit that track.
Matt S
Old 19th September 2007
  #9
jrp
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Hi. I like to join into this dicussion, actually wanted to start a new thread, but this absolutley covers it.
I experianced the same thing. Currently i´m designing a compressor using two different vca cores. One being a diode bridge, the other is made of two pentodes, the second grid used to controll the amplification.
Quite unice sound...
Well, i was experimenting with feedforward and feedbackward of the sidechain.
When getting the controll signal from the input it is possible to do some pretty extreme things, like cutting down the volume way below the threshold level. With feedbackward i was expecting the circuit to act as an compressor, and with a certain level of cv as a limiter. Even if you push the cv louder, it will still settle around the threshold level.
This was the case with all chip designs (SSM2018) i tried and with the diode bridge. But the pentodes behave different.
At extreme settings the volume can still be pushed below the threshold, and the kind of oszillation described can occur.
I tried this:
Feed an sinewave to the vca, raise the volume, watch cv and signal on the scope. As soon as i hit the threshold the cv kicks in, attenuates the signal way beond the threshold, cv rises, volume rises, threshold is hit - start from the beginning.
It´s strange to me somehow, not sure why this is happening.
BTW, these were settings that sounded cool on a drumbeat. Extreme, but not useless no oszillation. But with the steadily rising sine i got this oscillation. When i adjust the timing it changes the speed (of course).

As i said, i don´t understand why this is happening. Normaly the feedback should stop the attenuation as soon as the threshold is undergone. Settling everything at that level. Seems like the vca is too fast for the sidechain, but after all it´s feedback, so this is impossible....

Any comments welcome, and glad to hear i´m not the only one :-)
Old 19th September 2007
  #10
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a compressor shouldn't be pushing the signal below the threshold, unless there is a sharp transient. What I mean, is that the compressor should be attentuating by the ratio between the signal and the threshold. so, with a 2:1 ratio, if a signal goes over the threshold by 2 db, it should attentuate 1 db, leaving the signal above the threshold. if the signal goes over the threshold by 10 db, it should attentuate 5 db, and still be 5 db over threshold. now, if there is a sharp spike 10 db, and the compressor attentuates 5 db, but by the time the attentuation kicks in the signal is already back down to say 2db above the threshold, then the compressor will push down 5 db and end up 3 db below threshold. But this should not be happening with a sine wave.
Old 19th September 2007
  #11
jrp
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exactly.
With feedforward it´s easily possible though, but i suppose most (all) "normal" compressors use feedback.
Ok, in my case, the unit was never meant to only act like a compressor "should".
It´s meant for sounddesign rather than protecting speakers. So if extreme settings a possible i like that.
Still i would like to know what´s going on.
I actually thought that the compressor with feedback not only "shouldn´t", but also technically "cannot" push the level below the threshold.
Old 20th September 2007
  #12
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Hi
If there is sufficient 'gain' in the sidechain and it ever becomes 'inphase' then it can oscillate.
It can become 'inphase' when overloaded or the 'VCA' element admits more than it's original amount of CV onto the audio path either through saturation / clipping in the sidechain or drift (usually with valves).
Look out tech descriptions for things like Alison's Gain Brain, and consider Peak or RMS sensing (or both) and to an extent possibly feedforward AND backwards with thresholds that track so that normally it is feedback but on excessive inputs will actrually respond with a feedforward chain.
See also THAT corp data sheets.
Matt S
Old 21st September 2007
  #13
jrp
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That sounds very interesting!
Butz how can the sidechain be inphase? after all, when it hits the vca it´s only a cv, no ac signal.
Old 21st September 2007
  #14
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Tim Farrant's Avatar
 

Quote:
Originally Posted by 6dyslexicelephnt View Post
for some reason when I have my compressors at extreme settings like mics turned up really high and compressor output really high, sometimes it will do this clicky farting thing that will become faster if the mic is turned up more and changes speed depending on certain settings. I thought it was caused by a feedback loop between the control room and live room but this only happens with dynamic mics. I also thought it was just my Art Pro VLA but it happens with my FMR RNC too. SM 57s do it the worst but my Beta 52 does it too.

I'm recording a noise artist so it's nice to have compressors set so that when he makes sudden exploding noises they don't clip. That's why i have the mics set really high for the quiet parts, and ride the gain in combination with the compression. It's a problem for the really quiet parts.

Anyone know why this is?
What preamp are you using?
Old 21st September 2007
  #15
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Hi
The CV is still feedback, admittedly with a very low maximum frequency range compared to audio but still feedback non the less. Amplifiers when driven to clipping can 'latch up' or output reverse if you are unlucky (bad design).
Matt S
Old 21st September 2007
  #16
jrp
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I´ll post a shematic in a while...
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