Compressor: Auto Gain

[El Versus]

Hello guys. Until now I don't understand what happens with the compressor when I leave it at 0dB or -12 dB. With "Off" there is no problem. My question is, if I leave it at -12dB, does it mean that the output signal is normalized to -12dB? Although, when I look at my peak meters, the signal is over -12 dB. I do not understand the truth. I don't know what it means and I don't know how many dBs the compressor compensates me when I leave it at -12dbs and 0dbs? Someone explain to me please. Thanks in advance.

[David Nahmani]

10 hours ago, El Versus said:

if I leave it at -12dB, does it mean that the output signal is normalized to -12dB?

No. 

Those settings attempt to automatically compensate for the loss of volume when compressing. Some people prefer to have a compressor that yields a signal that stays around the same perceived volume while they're increasing the ratio or decreasing the threshold, and may want to use one or the other. 

For me, it's easier (and more predictable) to leave the auto gain set to Off, and manually adjust the Make Up knob as needed. 

[Atlas007]

I understand that OFF means that one has to resort to the MAKE UP setting to compensate the loss/gain level. But that would be enlightening to know about what those settings (-12dB vs 0dB) actually mean...

[des99]

Just try it and see. I loaded a drummer playing a pattern, stuck a compressor on with no make up gain, compressing 5dB or so. When you auto gain to 0dB, it pushes the signal up towards 0dBFS. -12dB auto gain pushes the signal up to around -12dBFS.

(I never use auto make up gain personally.)

Edited July 21, 2022 by des99

[Atlas007]

Thanx @des99! That makes sense!

[des99]

What auto gain *should* do imo is average out the gain reduction, and bump up the signal by the average gain loss (although this is tricky to do in practice). I've never found Logic's implementation of auto gain particularly useful...

[David Nahmani]

I suppose the behavior depends on the levels and settings you're using: 

[des99]

1 hour ago, David Nahmani said:

I suppose the behavior depends on the levels and settings you're using: 

Indeed, I played around a bit more with some different settings, gained up and down. It's very unpredictable as to what it's going to do, and honestly I have no idea what it's trying to do algorithmically speaking.

I've had times where the 0dB and -12dB don't result in much level change switching between them, other times where it's wildly different. I know auto-compensating for level differences due to compression is tricky, but the only thing it consistently seems to do is make the end result louder - but by how much it seems difficult to predict, unless there is some secret behaviour pattern I'm missing.

In any case, for these and other reasons, it's firmly set to off in my default settings and I don't use it...

Edited July 21, 2022 by des99

[des99]

Also, if the current manual contains very little detail, sometimes it's useful to go back to older manuals. Here's what the Logic Pro 7 manual says about the compressor auto gain function:

Quote

Auto Gain automatically sets the level of amplification to a value equivalent to the “sum of the threshold value minus the threshold value divided by the ratio” or put less confusingly T—( T/R). This function ensures that a normalized input signal is amplified so that the output signal is also normalized, regardless of the values set for Threshold and Ratio—provided you are dealing with relatively static signals. 

...so that's apparently what it does.

(This is before the 0dB and -12dB options existed, though.)

The Logic 8 manual includes those options:

Quote

You can use the Auto Gain parameter to compensate for the reduction in gain produced by compression, referenced to either –12 dB or 0 dB. Auto Gain sets the level of gain (amplification) to a value of T—(T/R), where T = the Threshold and R = the Ratio.

 

Edited July 21, 2022 by des99

[analogika]

Ah! That makes sense. 

Does anybody work with normalised audio files, though? I seem to recall that that was a thing back when everything was 16 bits. But today, where we have at least 24 bits of range (and much more than that internally)?

That would explain why results are unpredictable. 

[David Nahmani]

2 hours ago, analogika said:

Does anybody work with normalised audio files, though? I seem to recall that that was a thing back when everything was 16 bits. But today, where we have at least 24 bits of range (and much more than that internally)?

That would explain why results are unpredictable. 

You can work with a signal that has a more or less normalized level (peaking close to 0 dBFS) without using normalized files, just using gain staging to get your level where you want it. 

[David Nahmani]

In the final analysis, while the behavior of the 0 dB and 12 dB auto gain options have now been defined mathematically and we've agreed that the result can be unpredictable, what matters is why anyone would want to use these options. 

When compressing a signal, you're lowering its gain based on a user adjustable threshold and ratio. The ultimate goal is often to get a more consistent signal, or sculpt the transients and releases of each note to give them a different shape. And because compression decreases the perceived volume, you end up having to make up for it by applying positive gain at the output of the compressor. 

The make up can be done manually however while you're adjusting the threshold and ratio, you hear the volume go down, then you use the make up gain knob and hear the volume go back up. With auto gain, you can adjust the threshold and ratio settings while directly hearing the end goals without having to manually adjust the make up gain with every threshold or ratio adjustment. This can be very useful especially when compressing an individual track in the context of a full mix. 

Try this: import a drum loop to a track, press U to get a cycle area around it and press play. 

1. Turn auto gain off, turn the ratio up to 20:1 and turn down the threshold to -50. The volume goes so far down that you can barely hear your drums, and you have to use the make up gain knob to compensate. This can make it challenging to hear the results of threshold/ratio adjustments in real time. 
2. Option-click the Threshold and Ratio knobs to reset them.
3. Click the 0 dB auto gain button and repeat the process of adjusting the ratio up and the threshold down: this time you can hear the effects of compression without having to manually adjust the make up gain knob. 

Imagine trying to adjust the compression of a snare in the context of a mix, this can be quite useful. 

There are alternative ways to achieve a similar effect such as leaving the threshold alone and using the input gain knob instead to feed the compressor a more or less powerful signal that will trigger the compressor more or less, while not affecting the perceived volume as much. Obviously, all depending on the compressor settings, what you're trying to do with it etc. 

The keyword here is: experiment. 

[analogika]

11 minutes ago, David Nahmani said:

You can work with a signal that has a more or less normalized level (peaking close to 0 dBFS) without using normalized files, just using gain staging to get your level where you want it. 

Ah, I took "normalised" to mean actual normalised audio files, not signals gain-staged to hit around 0 dB. 

You're right, of course. 

[Atlas007]

1 hour ago, David Nahmani said:

There are alternative ways to achieve a similar effect such as leaving the threshold alone and using the input gain knob

Wouldn't that overdrive the plugin? At a certain point causing distortion?

Edited July 22, 2022 by Atlas007

[analogika]

5 minutes ago, Atlas007 said:

Wouldn't that overdrive the plugin? At a certain point causing distortion?

Eventually, yes. Until then (and of course, even then), the effect may be desirable. 

Different (analog) compressor circuits respond very differently when you drive them hard, compared to when you drive them lightly and lower the threshold accordingly. 

[David Nahmani]

14 minutes ago, Atlas007 said:

Wouldn't that overdrive the plugin? At a certain point causing distortion?

No: in itself, the technique where you adjust the compression using the input gain rather than the threshold knob doesn't cause distortion. 

What would overdrive the plugin (which may be desirable) is the specific level you're feeding the plug-in - whether or not you use that technique.