Audio Gain Control Question

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Most of the work that I do is on set and in a controiled environment. Ocasionally I do event video work where the audio levels can clip and distort quite easily. In addition to focusing, controlling the iris, and the various other tasks involved with shooting I find it very difficult to monitor the audio as well.

I have tried using the AGC feature, but it does not appear to control the levels. What exactly does the AGC feature do? When enabled, my audio still moves into the red. How do you all monitor, control and keep levels contant when in a "run and gun" situation?

Thanks in advance for the advice.
 
Re: Audio Gain Control Question

Do you have to have stereo sound for your 'run and gun' situations? You could set up your camera to record from an external mic set to the level you think will capture 90% of what you want and leave one side of the internal set low. This way, you can take the internal if the audio gets too crazy from the external.

Automatic Gain is a blessing and a curse. The AGC on the panasonic seems to be really better than most. In a quite room you might hear the AGC woosh up as it tries to find some sound and then compress itself when it hears talking (or noise).

Best option... Turn it off and hire a sound guy who can monitor it on the fly.

Might not be best for YOU!!! Good luck,

CaptM
 
Re: Audio Gain Control Question

The AGC is basically a limiter. once your sound reaches a set decible level it triggers the automatic gain control circuit to reduce the audio by a certain number of decibels.

I don't know what the threshold is set for in the camera but just for giggles.If the threshold is -2 db and the signal reaches the threshold, The auto gain control would brick wall limiter the signal to go no further.

So if you hit it hard you will hear it working, which isn't good. If you set your level a bit lower and let the AGC just catch the un expected levels, then it works great.

hope this helps.
 
Re: Audio Gain Control Question

There is no AGC on the DVX100.

There's an ALC, which is an automatic limiter control.

AGC controls gain, which means it monitors levels and boosts the gain during quiet periods, and lowers the gain during loud segments. But the DVX doesn't have that.

It has a limiter, which tries to clamp down the volume if it would mean overdriving the signal and peaking. The limiter tries to put a "soft ceiling" at -4.5 decibels, however it cannot always be 100% effective, so a very loud spike in sound might cause it to peak and distort a bit.
 
Re: Audio Gain Control Question

Oops..
AGC= compression
ALC= limiter. I guess thats what I was tring to say :-[
 
Re: Audio Gain Control Question

Unfortunately, as has been discussed elsewhere, going "into the red" on the DVX100 is not a problem. Red starts at -12dB on the camera, where on every other system I've seen red indicates clipping, or very close to clipping (say, -2 or -3). Most of the DVX's red should be yellow.

If I'm not mistaken, there are 6 red bars, the final one being tall and narrow. Unless you're hitting that final one, you're most likely OK. Try to set gain so you only tickle the one before it once in a while.

-Barry
 
Re: Audio Gain Control Question

Ah -- so it is. All these japanese translated names for things on this camera, I guess I just thought they got it wrong. (still don't know what SW in Auto SW stands for.)

However, I guess I still stand firm that this camera's AGC is much better than most!! Being that NO auto gain is much better than ANY auto gain.

For that I STAND TALL!

Thanks for clearing that up...

CaptM
 
Re: Audio Gain Control Question

Thanks for the replies. It is my understanding that
-20db digital = 0db Analog. That said, I am noticing that anything in the red on the DVX shows over -20db on my digital VU meter at my editing suite and all the audio is distored. I tried using the ALC function on the camera to keep levels at an acceptable level, but it doesn't seem to do anything at all. I am not just talking about occasional peaks either. For example, I was at a red carpet event the other day. I used an external mic on one channel to capture audio from a band and used a handheld wireless on another channel to capture audio from interviews. The idea was to capture the live audio on a different track and use it later. The band audio is all in the red and distorted. The ALC function appeared to do nothing at all. It was difficult to try to keep all the levels in check and shoot the event. Of course if I notice the levels peaking I adjust them, but sound fluctuates, and it's difficult to monitor, adjust and shoot at the same time. Am I not using the feature correctly? How can I test it to make sure that it is working?

Thanks again.
 
Re: Audio Gain Control Question

It probably was working. If you hit the limiter hard it will sound terrible. I think in a situation like that I would ether set my levels on the really safe side or have someone else doing the audio. There is just to much stuff going on to catch everything.

As far matching digital and analog dbu's. because there is no set standard between manufactures for digital zero, they are all different to some degree. But this is not a big deal anyway. just set you levels on camera to get most signal as practical to the conditions. Then when you fly it in to your NLE you can make any adjustments from there.

One thing you don't want to do is set your levels way to low on camera and then try to make up for it in post because you will be bringing up the noise floor right along with it.
 
Re: Audio Gain Control Question

[quote author=Dave Craig link=board=Events;num=1084791122;start=0#7 date=05/18/04 at 03:14:03] It is my understanding that
-20db digital = 0db Analog. That said, I am noticing that anything in the red on the DVX shows over -20db on my digital VU meter at my editing suite and all the audio is distored.[/quote]

Be careful not to confuse these... what you're talking about is "-20dbFS = 0dbu Analog"; which is a convention, not a fixed relation.  there is no real relation between digital dbFS and analog dbu. Many factors in your system determine the relation...

However, you shouldn't hear distortion anywhere near -20 dbFS; only on peaks around 0 dbFS...

I tried using the ALC function on the camera to keep levels at an acceptable level, but it doesn't seem to do anything at all. I am not just talking about occasional peaks either. For example, I was at a red carpet event the other day. I used an external mic on one channel to capture audio from a band and used a handheld wireless on another channel to capture audio from interviews. The idea was to capture the live audio on a different track and use it later. The band audio is all in the red and distorted. The ALC function appeared to do nothing at all.
Click to expand...

I'm not sure if the ALC handles channels separately, and what it would do in the situation you describe... I doubt you'd want to use ALC if you're not recording either stereo or a single mic.

Again, "in the red" is meaningless; is it hitting the last bar or two of red? And what does your signal look like in the NLE?

And, did the interview sound come out OK?  What kind of mics are you using, with what types of output?  It's possible that your "band" mic signal is just too hot, in which case you may need a pad, or to change the camera settings top the lower mic sensitivity level.

You shouldn't be having these problems... if you set your audio level for the band reasonably, and then lower it a bit to be conservative, you shouldn't be getting constant distortion and peaks.  If you CAN'T lower the level enough to avoid distortion, the mic signal is too hot.

One thing you don't want to do is set your levels way to low on camera and then try to make up for it in post because you will be bringing up the noise floor  right along with it.
Click to expand...

This is true, but I'd just make extremely clear: in DIGITAL audio, too low is always better than too high. Clipping is much uglier than  noise. So, make levels as high as you can and still come close to guaranteeing that it won't clip.  In real world settings, this often means setting them fairly low... but that's OK.
 
Re: Audio Gain Control Question

Ullanta, Thanks for your reply. I have an Analog VU meter and Digital peak level meter attached to my editing suite. 0db Analog = -20db digital. Using a 1khz tone rasied to 0db shows up as -20 on the digital meter. Everything that goes past the first few red blocks on the DVX ends up being somwhat distored and will clip on the digital meter. The sound seems to be somewhat distorted past three or four blocks. Anyway, that said I have noticed that when mixing audio on my suite anything past -20 or -18 (digital db) will be distorted. Here is a website that may explain a bit more http://www.creativemac.com/2003/02_feb/tutorials/analog_dv_levels.htm. Tell me what you make of this. I have found most of it to be true. Audio can be so bothersome that at times I wish we could go back to silent films... lol

My main question is how does the ALC work, and what does it do? i think you may be right. Maybe it only works on stero mics. Although I have noticed that it doesn't appear to do much when I am just using the built in mic.
 
Re: Audio Gain Control Question

Let me clarify: I'm NOT saying the ALC only works with stereo mics. I'm saying that if you have two mics attached, it probably treates them as stereo... that is, it modifies the gain of both the same amount, rather than tracking and modifying each track individually. Now, the question is, does it adjust for the peak on either track? Does it watch only one track and adjust based on that? There are certainly systems that take both of these approaches, but I don't know about the DVX. Anyone?

You say you have an analog VU and digital peak meter attached to your editing suite... can you explain exactly how? That is, what is your signal chain, and where are you metering it? Tell us about your setup.

In that article, Douglas advocates using 0dbu = -20dbFS in order to protect yourself from OCCASIONAL TRANSIENT PEAKS. What you're telling us is that you have constant distortion. So, if your digital signal indeed stays below -20dbFS (and below the top red bar in the camera), then your distortion is being added BEFORE the signal is digitized or AFTER it's analogued and amplified.

I wouldn't be surprised about weird results from your two-mic setup, as discussed before. But have you tried recording a single source, and has it been problematic?

So, tell us about your signal chain...

-Barry
 
Re: Audio Gain Control Question

Hey Barry,

A few things;

1. I understand what you were saying about the ALC adjusting for both channels An additional question here would be what does the ALC balance to? -12? -6? In my case even if I use the on board mic and set the levels at the mid point, the ALC does not seem to adjust the levels at all. Also, I may have mis communicated something, I am not getting constant distortion. Anything below the first couple of red bars sounds great. My issue is that the ALC is either not working, or it is set too high because the audio was going a bit too far into the red and causing occasional distortion.

3. I did some testing sending a 1khz tone to the deck and the camera via firewire -12 on the camera appears to be the last white bar before the red. My question here is how far do you let your audio go before you get distortion or clipping occurs? Where are the safe areas? What are the limitations for broadcast?

PS. Please disregard what I said earlier about 1khz showing -0 analog and -20 digital. My VU was not getting an accurate signal. It does appear that there is a 6db difference between dig and analog. Hope I didn't create too much confusion.
 
Re: Audio Gain Control Question

Well, check page 48 of the DVX user manual (maybe different for the "A"):

AUDIO METERS:

-----------+------|
-----------+------|

Everything up to the '+' is white; after that, red;
'+' is -12dB, '|' (or maybe the one before) is 0dB.

In my experience, ALC prevents the signal from ever going more than 4 bars above the '+'.

You should get undistorted sound at least that far; in fact, it should be fine as long as the '|' is never hit. My experience confirms this.

For broadcast - I'd still keep levels as high as possible in the digital domain without clipping, then calibrate my 0dbFS to whatever the broadcaster wants in dBu after A/D conversion. Most (I'd say all) broadcast facilities will have their own equipment to regulate the signal as they see fit... so you're best off delivering them the best audio you can, and let them worry (and compress, etc, as they see fit).

So, again... tell us more about your whole signal chain, because it really seems that your distortion is not in the camera, but somewhere else in the chain. Tell us everything from the mic to any mixer you may have to the camera audio settings to the camera to the NLE to the output mixer if any to the speakers/headphones - including levels of everything (+4 pro or -10 consumer, etc).

-Barry
 
Re: Audio Gain Control Question

Hey Barry, the signal chain varries depending on the production, but the results are always the same. Anything above the first three or four red bars begins to distort and gets worse if the signal gets any higher. This is with the built in mic, or any mics that I use.

It's evident if I play back from the camera onto any TV set, or if I playback using the AGdv1000 pana deck. Basically I am finding that if the audio signal goes over -10db it begins to sound muddy and distorted, again this is with the built in mic or any other mics I may use. Are you saying that you can hit the last red bar and stlll get clean audio?
 
Re: Audio Gain Control Question

(I'm not the Barry you're talking to, but...)

I know I certainly can. As long as it doesn't peak out, the audio is clean and clear.
 
Re: Audio Gain Control Question

Well, I'll vouch for the one before the last... I try nevber to hit the last ('|') bar...

I've done this with internal and external mics, internal and external preamps, line and mic level signals...

Hmmm...

if you're using the internal mics, are not hitting the last few bars, and hear distortion at a level where other signals (i.e., from a CD) at the same digital level DON'T produce distortion... I'd have to swing around and suspect a defect in your camera's metering.

UNLESS... you're hitting the limiter real hard, and you're getting distortion from the limiter... but with the levels you're discussing, that shouldn't be the case. Have you tried lowering your levels during recording, to see if the peaks stay just as high? Set the levels to the point where any lower, the peaks move below middle of the red...?
 
Re: Audio Gain Control Question

I have a shoot this weekend, I'll report back and tell you what I find. Thanks for the time you all have dedicated to this thread. One last question, what is the peak point or area where audio clips, on the meter in this camera? Is it the last red ---! area on the meter. On the meter where do you guys try to balance your audio? Where do you allow your audio to peak?

Sound has always been a questionable aspect of production for me. I understand the limits, IRE's, setup levels for picture, but what are the broadcast limitations for audio? Where is dialogue balanced at? What is the maximum DB levels that are FCC legal? Is the level meter in Premiere Pro accurate for digital and analog audio output?

I am sorry for all the questions, but I really want to get this straight. I am slightly confused by all of the material that I have been reading. Some folks do everything by ear, others reference db levels.

Thanks guys! I appreciate the time.
 
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