Audio: Automatic Gain Control a.k.a. The AGC Conspiracy


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Hal Robertson, July 2009, Videomaker



In the early '80s, engineers, designers and marketing people discuss an evil plot to control consumer media with a hidden camera feature called Automatic Gain Control.

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The year is 1983. A small group of corporate types huddle in a dimly-lit, smoked-filled board room. Engineers, designers, marketing people and managers discuss their evil plot to take over consumer media with a small device they're calling a camcorder. Speaking in hushed tones, an engineer announces they've developed a new audio circuit that automatically adjusts recording volume, completely removing control from the user. The designers love the idea, as this will eliminate several knobs and buttons from the case. The marketing people can't wait to print new stickers for the box hailing this revolutionary new feature. Little do they know, this circuit will burrow its way into every camcorder ever made, ruining the lives of countless videographers.

Reality Check

At least that's how I see it play out in my head. While the actual events may be much different, the facts remain: Automatic Gain Control - or AGC - takes control away from the user, automatically tracking and adjusting the audio volume of recordings. This makes it easy for a manufacturer to remove manual audio level adjustments from the camcorder body, and it has also been trumpeted as a desirable feature. ACG has been around for a long time, and you'll find it included in virtually every camera - from the $200 models all the way up to six-digit territory.

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So what's wrong with that, you ask? While AGC is fine for average consumer use, it makes professional users crazy. Let's compare applications. It's little Johnny's birthday, and the house is filled with third-graders. You want to capture the events on video, but you have to move fast and don't have time to mess with setting audio levels, white balance or focus. Fine. Set the camera for full automatic control and hit the Record button. When you play the video, you discover that the camera wasn't always in focus, the white balance shifts quite a bit and the audio is very inconsistent. Who cares? It's a birthday party, right? However, you wouldn't tolerate those problems on your favorite TV show or movie, would you? Of course not, and neither would the producer. They use manual settings for visuals and audio. This makes editing simpler and more consistent.

AGC is the auto-focus of audio: sometimes right, sometimes wrong and always changing, either a little or a lot. It constantly monitors your incoming audio, pushing down loud sounds and pulling up softer ones. This gives your soundtrack an unprofessional pumping sound that is difficult to edit. The problem is, unless you paid a fair amount for your camera, you can't turn the AGC off. That means we have to find a way to thwart the evil conspiracy.

Fooling the Foolish

You can sum up the best way to fight AGC during the shoot in one phrase: minimize the variables. This takes many forms and requires some creativity on your part. The first and most important tool is an external microphone. Using an external mic is the guaranteed way to get cleaner audio, but it also offers some help to those suffering with AGC. Microphones with tight pickup patterns, such as a shotgun mic, help to minimize background noise. The result is clean, clear audio from the subject that keeps the AGC focused on your project and not distracted by the surroundings. Of course, that won't fix everything, but it will help.

Cloak and Dagger

The next step has to do with the speaking technique of your talent. Professional on-screen talent will speak clearly and consistently, making your recording and editing task easier. However, most of the people we shoot aren't pros, and we'll have to work around their lack of technique. AGC tends to react quickly to short, loud sounds such as coughing, laughing or the random throat-clear. When it hears these types of sounds, it says "whoa, that's loud" and promptly turns the volume down. If the talent immediately picks up from the last line, the recorded volume will be much lower for several seconds, and then it will gradually increase.

The opposite can also occur where there is no sound or a very low audio level. Imagine shooting a meeting in a boardroom, and the speaker pauses to collect his thought. There's no sound being generated during that pause, right? Wrong! There's the hum of the air-conditioner and perhaps a projector for the slideshow presentation he's using. The AGC frantically searches for those low level hums, raises the volume to get them, and then the next time your speaker opens his mouth, your camera is going to freak out trying to bring the audio down to a proper level.

This is where you put on your producer hat. If you're shooting with a script, simply go back a few lines before the noise level changed, and do another take. But if you're shooting an off-the-cuff interview, allow the talent to complete the thought, and then politely ask for a repeat of the section. Not only will you get a cleaner take, you might get something a little more focused. It's a tricky balancing act, and monitoring with headphones will help you get the most consistent audio.

Another trick is to follow the style of TV news reporters. When they prepare to read their scripts, they will recite a mantra right before recording each audio track, usually the common countdown: three... two... one... then using the same time-frame pause as they used for the countdown, they begin to read their lines. They do this for editing purposes, so their editor can quickly find the beginning of the track by the well-known skreet these words create in fast-forward or reverse speed, but it works to fool AGC, too. If that feels odd and unnatural for the speaker, some reporters will speak a short phrase right before each track such as "listen to this". (Just be sure you cut out that first line on the track. We know of one editor who didn't know of this speaking tactic, and thought the reporter just wanted to use that phrase before each track for emphasis!)

Audio Forensics

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AGC complicates the editing process. Your recorded audio may be loud and noisy for one scene and squashed in another. As you're editing, the task is to even out the variances to create a consistent performance. Your first reaction may be to normalize all the clips, but that's not always helpful. In fact, normalizing may exaggerate the volume differences. While it's more time-consuming, the manual approach is usually a better solution. With your edits on the timeline, listen through the project and make notes on which clips sound out of place. Most NLEs allow you to adjust the overall volume of individual clips, and that's a good place to start. But once you've balanced the speaking volume, you will likely notice a jump in background noise between adjacent clips. To minimize - or at least smooth - these abrupt changes, add some volume handles to the clips to gently fade up or down. This is especially helpful at the edit points. You can enhance the effect by crossfading audio clips. Try overlapping the end of one clip with the beginning of the next one. The fades ease out of the first clip and into the second. It takes some tweaking, but this technique can salvage some of the damage caused by AGC.

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You may also find that clips change volume from beginning to end. By placing keyframes around the volume changes, you can minimize the effect and smooth the performance volume. Play through the troublesome clip and note where the audio changes volume. Placing keyframes at those points, you can gently raise or lower the audio volume for the necessary time, then ease back to the original level. It's tweaky and time-consuming, but these techniques can salvage the audio from most AGC recordings.





Stick It to the Man

Camcorder manufacturers don't seem to fully realize how we push the envelope with their creations. But, with a little planning, care and edit time, we can create professional productions with devices that were never meant for much more than vacation video. Ideally, all camcorders would have manual audio controls, but, until that day comes, we can work around the AGC circuit as needed to get the job done. AGC is an evil conspiracy! Videographers unite!

Contributing Editor Hal Robertson is a digital media producer and technology consultant.




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