Limiting vs. Compression vs. AGC

[David Ennis]

Could someone "compare and contrast" these three, as we say in the high school teaching business?

I'm particulary interested in the idea that limiting is good but AGC is always bad ( I often see advice here to run the audio in manual, or to turn off AGC). I have the notion that limiting is one-sided AGC.

I understand that you loose dynamic range with AGC on, but it seems that sometimes you may need to do that to be able to both hear the quiet sounds and avoid clipping the loud ones

Thanks.

[Jay Massengill]

AGC does automatically adjust the gain, both up and down, depending on the sound level it's receiving. Generally the worst artifact of this operation is a substantial rise in hiss during quiet passages followed by clipping when a sharp sound occurs suddenly. Unless you have no choice and can't avoid problems otherwise, it is best not to use most AGC circuits. This is something that has to be judged on your own.

Limiting is useful as long as the circuit is performing properly and you don't have the input set high enough that the limiter is on all the time. Some limiting circuits perform better than others. Again it can be a judgement call for your equipment and situation.

Compression lowers sound levels that go higher than a selected threshold. It lowers the sound by a ratio. For example if you've selected a 4:1 ratio, then the sound is lowered by 4 db for every 1 db that it went over the selected threshold. There are usually selections for attack and release times, as well as hard or soft threshold points.
A whole book can be written on compression techniques. Generally compression is something best left for post production, although I will use mild compression for events like stage plays and other predictable but difficult to manually control events. Mild compression is also helpful when recording voice-overs.

[Mike Rehmus]

There can be a big difference between AGC systems. Pro AGC systems (including those on the PD150/170 but especially those on the pro cameras, tend to be pretty darned good. They don't overboost during quiet moments so you don't get a lot (if any) hiss.

I don't know why AGC would reduce your dynamic range. Compared to a fixed manual setting, it functions to extend dynamic range.

I regularly run one channel of my cameras in AGC with the other channel kept in manual mode and at fairly low levels (with a single input sent to both channels). That way I get almost everything I want and if someone screams or drops the odd pipe, I have good sound.

[David Ennis]

<<<-- Originally posted by Mike Rehmus :

I don't know why AGC would reduce your dynamic range. Compared to a fixed manual setting, it functions to extend dynamic range.-->>>

How so, Mike?

I thought that AGC reduces amplifier gain for loud signals and increases gain for soft signals. If so, then a smaller difference between softest and loudest would be recorded than existed live. So, on playback wouldn't I hear less dynamic range compared to what I heard live?

[Mike Rehmus]

Yes and no. It all depends on how the AGC is set up.

Functionally, the gain is not doing much more than one does with a manual gain control, it just happens to be somewhat automated. Most of the gain control circuits I've seen are just voltage controlled amplifiers. Either the AGC signal or the gain pot is adjusted to vary the gain signal on the same input to the amplifier.

IIRC, this holds true even for fairly sophisticated audio boards even though they don't have the AGC function.

[Troy Tiscareno]

The problem with AGC is that you never really know what you're going to get, because you no longer have control over the circuit. Sure, there may be times when that's a good thing, because it keeps the average cam user from screwing up the audio all the time, but for folks doing more professional work, especially with external equipment like XLR adapters (BeachTek) or field mixers where you are setting levels on a second piece of equipment, it is important that you know FOR SURE what is going to tape.

Monitoring via headphones is manditory, of course, but there's still the issue of the AGC taking control and doing unexpected things, often right at the wrong moment. No thanks; I much prefer to turn it off unless I'm using the on-board camera mic (rarely). I use mics I'm familiar with, set the levels appropriately, and don't run into problems. Then, if necessary, I can use compression/limiting/gates in post on the PC, again, allowing me to retain control of the settings and selectively use the necessary tools only where and when they are needed.

It's more work this way (not too much, really, but more), but there's rarely a risk of the camera doing the unexpected and getting irreversable bad results.

To be fair, I come from an audio background, so I'm comfortable in manual and used to "having it my way." YMMV.

-Troy

[Mike Rehmus]

Troy, there are lots of run and gun situations where the pro AGC is very useful and produces acceptable results. Lots of incidents where manual control would give you worse results. That's why I like to run one channel in AGC and one in manual when I'm on my own. You cannot do a good job as both a camera operator and audio operator. It is job overload and one or both aspects of the video will usually suffer.

When you have time to use a field mixer and have an audio person running that side of the show, then yes, there is no place for AGC. But a lot of us ocassionally have to work faster and more mobile than that and without a crew.

I don't agree with the 'never know what you are going to get,' statement. I do know what my AGC is going to do and can adequately compensate for its deficiencies when I have to.

To set the record straight, Pro AGC is not at all like the AGC built into a consumer camera or your favorite consumer tape deck. It works very well most of the time.

[Troy Tiscareno]

Point taken, and yes, I also use the AGC when doing Run & Gun. I'm usually just using the on-board mic when I do. When using a wireless handheld, I usually have it off, though, because the SM58 that I typically use is so good at rejecting distant sounds that I can set levels that aren't likely to overdrive the audio circuit.

Also, I rarely get to play with cameras beyond PD-170 level, so I'll conceed that Pro AGC is likely much better.

-Troy

[Marco Leavitt]

The AGC on my camcorder never surprises me. I can always count on it to screw me over. :) We shot a live performance over the weekend in which the AGC COMPLETELY botched the sound. Gotta get a camera with meters.

[Mike Rehmus]

<<<-- Originally posted by Troy Tiscareno : Point taken, and yes, I also use the AGC when doing Run & Gun. I'm usually just using the on-board mic when I do. When using a wireless handheld, I usually have it off, though, because the SM58 that I typically use is so good at rejecting distant sounds that I can set levels that aren't likely to overdrive the audio circuit.

Yes, the SM58 is my choice in loud environments like receptions and it works very nicely with manual level control.

Also, I rarely get to play with cameras beyond PD-170 level, so I'll conceed that Pro AGC is likely much better.

The DSR-300, for example is as much better than the 170's as the 170 is above a consumer camcorder. Maybe much better.

-Troy -->>>

[Mike Rehmus]

<<<-- Originally posted by Marco Leavitt : The AGC on my camcorder never surprises me. I can always count on it to screw me over. :) We shot a live performance over the weekend in which the AGC COMPLETELY botched the sound. Gotta get a camera with meters. -->>>

You can get an outboard set of meters that plug into the headphone output jack. I think they can be calibrated but you do have to have manual control over the audio gain of your camera.