Compression in the mains??

[exit143]

One of my sound techs compresses the mains. I've asked him why he did that, and he said that it's how he does it in his studio... He figures that he can reproduce his studio sound and it will sound good live. I think it loses the dynamic range (obviously)... and it sounds......... not great.

What are your opinions? Do you compress live audio (not individual instruments, but the mains themselves)... Why or why not?

[cmchamp]

No, do not compress the full mix, you are correct, it reduces dynamic range and increases susceptibility to feedback. It also doesn't sound that great.
You can and should put limiting on mains, preferably after crossover, before amplifier, to protect from clipping and over excursion. However, constantly hitting the limiters may be worse than allowing clipping and causes heat buildup in amplifiers as well.
C.

[2cool]

C. you are right about that. I have been dealing with this for a while as our mains blew out and the Pastor insisted that it was our Youth Band at fault, which is the group I do the sound for. Anyway when we got the new horns he insisted that we connect a compressor and limiter which has KILLED the dynamics and is a constant struggle with feedback. BTW I know it wasnt us. We share the system with 3 other groups.

[pdc]

I use multi-band compression on channels and groups. I use limiting as protection on the main sometimes.

Why someone would compress the whole mix is beyond me. The band's components have differing dynamics. Vocals have differing dynamics. The room's sound is going to come into play even more. Compress hard enough and you will have nasty room roar equaling the source and then a feedback fiesta. The operator will then start hacking away with the EQ. Nah. Forget it.

If your guy doesn't know the difference between live sound and recording, he should not be behind your desk. The two are not the same thing.

[2cool]

PDC that is exactly what I am dealing with right now. HELP. I am going to connect an outside fx unit in hopes it will solve the clip level on the fx's needed. I sorry to ask again but can you give me the exact wiring setup for this. I have the 32unit mixer, fx unit, 2 power amps and a limiter unit.

[pdc]

1. What are the components of your system (Make and model)?
2. If you have that many people using your system, I can almost guarantee that you have operator error as the biggest concern. No piece of gear is going to replace education by an experienced person on-site. Your gain structure could be completely wrong, you could have a lack of headroom or both.
3. If you do use a limiter, it should be the last thing in the chain before your crossovers and amplifiers. FX processors will not help you.
4. If you have internal FX units in your mixer and those are clipping, it is most certainly your gain structure on the mixer that is the problem.

[Brad Weber]

Going back to the point of reducing dynamic range, the related issue is that the compression can raise the average level. If you compress the signal and then raise the overall level the same amount as the compression you maintain the same peak levels but increase the average level. This is a common, and increasingly discouraged, studio and broadcasting trick to make things louder without increasing the peak levels.

In a sound system, this practice means the system works harder and makes thermal failures more likely. And unless you have very fast or look ahead peak limiting, it also means any unexpected peaks now hit the system that much harder.

[exit143]

Since I was receiving all these replies, I asked him to clarify his reasons for compressing the mains. He replied:

Let me explain why I use the compressor in the overall mix.
First of all it is used in moderation, more like a gentle limiter. meaning it does
not have a flat ceiling like a limiter. When I apply around 2:1 compression to
the signal the compressor is barely active, threshold is near 0db. The
compressor is not being used like on a channel instrument.
This keeps me from riding the mains fader. The compressor helps prevent
sudden volume peaks, crescendos. If the volume jumps to an uncomfortable
level, by the time one react to fix it it's already too late. Everyone felt the
discomfort of the volume and then heard you fix it.
I am not compressing to raise the overall signal like in studio recording or like
in a tv commercial.

Thoughts?

[pdc]

If you do not have compression for your channels and or groups and this is his only option for dynamics control, then I see nothing wrong with this. 2:1 compression on a system with proper gain structure is not going to do drastic damage to the sound. 2:1 compression is not going to protect your system from damage either.

The ratio is only part of it however. The attack and the release is what makes the sound pump.

If there are specific bands of frequencies that are needing compression more than others, take a graphic or parametric EQ and insert it into the side chain of the compressor.

[bbrunskill]

First post!

I don't currently use mains compression, but I always, always do when mixing in the studio. I've tried it live, and would use it more often, but our church only has 8 comps, and I need them all on channels.

Exit143's last post summed up what it's for nicely. And also, to my ears, it provides some sonic 'glue', and helps the mix sound more polished. I mixed once on a Soundcraft vi-6 digital console with a stereo 1176 on the output, and it was just a beautiful thing.

[Brad Weber]

Originally Posted by exit143

When I apply around 2:1 compression to the signal the compressor is barely active, threshold is near 0db.

Just for some comparison, the maximum output level of most analog mixers is typically +20dBu to +30dBu and the nominal output level (0 on the meters) is commonly +4dBu. So with good gain structure a 0dBu threshold on the compressor means some compression being applied even to nominal signals. And despite just a 2:1 ratio, it could still mean 10dB to 15dB of gain reduction being applied to signals that are likely within the capabilities of the console outputs and compressor inputs.

Originally Posted by exit143

This keeps me from riding the mains fader.

Everyone has their own approach but when I mix I rarely ride the master faders, once a basic mix is established then most of the fader riding relates to individual inputs and groups.

Also remember that if someone plays or sings louder that is increasing not just their level to the audio system but also their natural level in the space. When you adjust the master levels on the console you varying the balance between the natural and reinforced levels for all sources. For example, lowering the master faders may make an audible difference in the keyboard levels but almost no difference in the drum levels as one source relies solely on the sound system while the other may have a significant natural sound component. So in live sound the master fader may not be the 'turn everything up or down' control that some may think it is.

Originally Posted by exit143

The compressor helps prevent sudden volume peaks, crescendos. If the volume jumps to an uncomfortable level, by the time one react to fix it it's already too late. Everyone felt the discomfort of the volume and then heard you fix it.

The compressor tracking transient changes would typically mean fast attack and release times, which also typically relates to audible operation. You might often not want the compressor to react to every little peak but rather to react, and especially release, more slowly in order to be less noticeable.

Another related consideration is that when you have simple compressor on the mains then any compression affects everything. Someone gets close to a mic and if the compressor reacts to that then that same gain reduction is applied to the entire signal. That is where the band limited or multi-band compression that pdc noted may be more effective as something like a big bass note or kick drum hit might then apply some compression on the lower frequency content without affecting vocals or some other sources.

Similarly, a compressor on the outputs can make adjustments to the overall level interesting. As you raise the master fader you get a nice 1:1 relationship and until some point where you start getting into the compressor and getting a 2:1 ratio between the fader changes and the output level changes. So you don't really know what difference an XdB change in the fader makes as it is dependent upon the signal level. A 10dB movement in the fader could result in a 10dB change, a 5dB change or anywhere in between depending on whether and when the compressor started reducing the gain. Probably not a big deal for someone that understands what they are doing and thinks to see when the compressor is applying gain reduction but potentially a little disconcerting to a less experienced operator.

Finally, what do you do if you want the overall mix or a source to get louder? You can raise a fader but will you just end up fighting the compressor? It seems like you'd have to setup the system such that the compressor did not really do anything until you exceeded the maximum desired level or else you could end up fighting it during normal operation.


Perhaps I just have too many experiences with mixes that lose the impact of being a live performance, however I generally avoid compression on the mains and have also found that it is generally agreed among the more experienced professionals that I have communicated with that compression should not be used as a volume control or as a replacement for human intervention. However, every situation is unique and if you have someone who simply is not capable of riding the faders very well or that is too busy playing with effects and such to also pay attention to riding the faders then compression on the mains to minimize volume increases could be beneficial.

[joshuacrispin]

Acrossthe board, live-sound pro mixers like myself either use gentle compression on the subgroups, OR the Master Out for polish...IF needed.

One trick is to slap a class-A analog compressor on the output of a digital console to warm up the final mix output. (If that's your thing)