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Feedback quandary

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Harpman:
Yeah, RTFM  :lol:  So, Greg chimed in with his recommendation for bus compression.  Anyone else care to share in regards to bus compression.  Noticed the defaults are Hard Knee, 0dB Threshold, 2:1 Ratio, Fast Attack, Release @ 250ms and 0dB Makeup Gain.

RoadRanger:
Basically anytime there is gain reduction you need more gain to get it as loud as it was without the gain reduction. That's fine until the gain reduction goes away when the signal is reduced or stops, then it's feedback city :( .

dpdan:
Exactly RR :)
I had a gig tonight that got cancelled, so I decided to spend some positive time on this forum that I enjoy from time to time.
Here's what goes on when more than one vocal or instrument is routed (assigned) to a group with a compressor on that group....

We have Mary singing soprano, Jill singing alto, Tom singing tenor and Jack singing bass. 
 
Even if all four vocals have their own compressor, (ideal) since all four vocals are routed to a group with additional compression we probably never end up with good sound and here's why...
If all the singers were singing sensibly and with reasonable dynamics that's great, but what happens when the soprano Mary hits a note that takes everyone's head off....

Instead of just Mary's vocal being lowered by "Her" compressor, she has hit the threshold of the group's compressor, and all four voices are lowered. Compressors are dumb, they have no clue about high notes or low notes, they are completely unbiased to frequency and therefore can only reduce volume by shear increase of level (voltage). Using some compression on the master fader of the 1608 can help to minimize potential dangerous levels to your speakers. It is always a tradeoff.

When Mary sang that obnoxiously loud note, we did not want her bad singing to effect the volume of the other three voices in the group, but that's what you get when you think it's good to put a compressor on all the vocals, or all the drums, or whatever. It's not usually a good idea. When things like this happen, the unexperienced sound operator has no clue what is really going on.

Compressors are typically used to ensure that a reasonable dynamic range is heard by the listener without having to worry about wigs flying off now and then or straining to hear soft passages.
A compressor (when set properly) can catch a nasty blast of volume far quicker than any sound person could ever anticipate.

A properly set compressor on a vocal minimizes, to a degree, the dynamic range of the vocalist. Mary is all over the place, one minute she's fine then she's whispering, then screaming, and all the while never uses any amount of microphone control.

When we compress Mary's vocal channel, the first thing that most engineers grab is the threshold knob (Mackie's slider), then the ratio, then attack and release times if available, and finally the makeup gain sometimes labeled output. Think of threshold as "threshold of pain". Usually when we lower the threshold knob (counter-clockwise) (Mackie's slider to the left) we are lowering the threshold of pain meaning.... we are not going to tolerate even the slightest abuse of loud screaming from Mary, if we use a very low threshold, virtually everything she sings will be above the threshold, so if we have a reasonable setting for the ratio knob, somewhere between 3:1, and 6:1, the compressor will minimize her volume when she goes nuts on us. Of course if we have a significant amount of compression on Mary, guess what???? now we can't hear her, sooooooo, we use the makeup gain (output) on the compressor to put her volume back up to where we can hear her. So when Mary is singing, we look at the compressor levels. The 1608's compressor has a red level between the input level and output level. The red level works backwards, showing decibels of attenuation. We can clearly see the input level as it came into the compressor, we also see the amount of gain reduction (red) and then to the right of that we see the resulting output level. When Mary is singing a loud passage the compressor will do it's job and take the volume down, but when she sings softer, the compressor releases (lets go) and allows the mic level to be uncompressed, kinda like riding her fader if you knew when she was going to blast off. Since we increased the output (makeup gain) when we were setting the knobs during soundcheck, this is when feedback could rear it's ugly head. YES, too much compression (with makeup gain added) (DOH!)  ::) will more than likely cause feedback. In these situations, when she is not singing, and her mic is on her mic stand pointing at the drums, we will get a very ugly sound from her mic picking up the drums that she is standing in front of or was.

Finally, attack and release.
The attack knob is used to adjust exactly what it says "attack" how quick do we want the compressor to begin compressing when Mary hits the threshold with a nasty loud note. Let's say she sings a line that goes like this....Happy birthday to you,... happy birthday to you,... happy BIRTHday dear Johnny,

if we had the attack set to a half second, (500 ms).... BIRTH would blast out uncontrollably, if she held the word BIRRRRRRTH for at least 500 ms. then the compressor would lower her volume.
If we had the compressor's attack time set as fast as it could be, the compressor would have taken care of the loud BIRTH regardless of soft lyrics before the nasty loud note.

Now let's talk about release time.
The release knob adjusts how quickly or slowly the compressor will let go once Mary has blasted a note. If we have the release time set too long like one second, we won't hear the rest of her words for that legato line. If we have the release time set too short, the compressor will let go super fast between words and will cause a nasty sound that engineers refer to as "pumping".

Listen to one of your favorite pop recordings with a vocal, all too often, the compressor on the vocal has a release time that has been set way too fast by the "seasoned mix engineer" and every time the singer takes a breath, the breath is actually louder than the singing. There was so much compression on the vocal that when she stopped singing, the compressor let go, then the singer took a breath, but the breath was not loud enough to meet the threshold, so it wasn't lowered in volume like the singing was,.... I hear this all the time.   :facepalm:

For most vocals that are not out of control, I recommend not using any compression, I did't say don't use compression on vocals, it completely depends on the vocalist. Mixing music in a studio is completely different, especially when the vocal track has no other instruments leaking into her mic, and feedback is not an issue since most are wearing headphones. In a studio environment, you can use a compressor to make her sound any way you want... punchy, smooth etc. For vocalists that are not so good with mic control or vocals who weave in and out of the mic some compression will help, this way we don't loose too much as they back away from the mic, and smooth the sound out when they suddenly get too close or loud. 

4:1 is a pretty common "Ratio" setting. Watch the display at the top of the window showing exact numbers as you move the sliders for the various settings. You can also click in the display window and enter exact amounts via the iPad's keyboard. The threshold setting is going to completely rely on your input gain setting,.... (gain knobs on the 1608).
I suggest a very fast attack time and a release time of at least 200 milliseconds, this will virtually eliminate any chance of pumping, and we won't have to worry about missing sibilance and diction.

Now on a completely different note  8)  compression on bass is commonly used to help increase the sustain especially upright acoustic basses. Since the bass note immediately starts to decay after plucking the string, the compressor would gradually release the compression which would make the bass notes appear to sustain longer, and notes on different strings would sound more consistent. This is where a compressor is used to change the way a particular sound sounds. Good engineers also use compression on snare and kick, as well as toms sometimes to increase punch. This can be accomplished by using a slow attack time then adding the sound of the compressed drum to the uncompressed track or channel. This is sometimes referred to as parallel compression, an old studio trick but one that its not known by many. This can be achieved with the 1608 if you run the kick into an XLR Y cord and feed the kick mic into two different channels, one channel would be named kickcomp and the compression settings would be set very radical, but the volume of this kickcomp channel would be added judiciously when needed for extra punch. Same thing goes for snare, brass, anything that you want to be more aggressive sounding.

Compression, (when set properly) is our best friend
when set poorly, it is our worst enemy.
Final conclusion to this advice which you can take or leave,..... if you don't completely understand all the settings of a compressor, you're better off not using one.


Dan
 

WK154:
RR: That's fine until the gain reduction goes away when the signal is reduced or stops, then it's feedback city :( .
Am I reading this wrong? Gain reduction goes away by what means, the signal (vocal or instrument)? I interpret this as the loss of sound will cause feedback! The gain reduction in the compressor is a setting you need to change manually and certainly not during performance. Please explain.

RoadRanger:
Like wow, I don't know how to respond to that. Your compressors don't have a gain reduction meter so's you can see how that varies dynamically with the "key" (usually the input) signal's strength?

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