Sound Advice: AGC Coping Skills
Psychologists are fond of reminding us that humans have built-in mechanisms to help them cope with various situations. Some of these responses are automatic, while others are learned. Shock, denial and blame are all different ways of coping with the difficulties of life. One of the most troublesome areas of the videomaker’s life is a little circuit called the Automatic Gain Control or AGC. This ubiquitous annoyance attempts to be helpful, but usually causes problems with audio volume and consistency throughout your production. Let’s look at why it’s there in the first place, how it works and how to work around its inherent limitations.

Conspiracy!
All right, it may not be a conspiracy, but virtually every consumer camcorder on the market today contains an AGC circuit. It’s there for one simple reason: cost. Cameras get smaller every year and the shrinking real estate on the case limits the number of knobs, buttons and displays it can hold. Manual audio control is easy to target for removal since it requires one or two knobs and some kind of display for viewing audio levels. How much does this cost the manufacturer? It could be $1 or even less, but with budget Mini DV camcorders starting at around $300, every penny counts.

It is also argued that consumers don’t understand all of the features already peppering the surface of their camcorders; manual audio settings are just one more thing to get wrong. This reflects poorly on the manufacturer if everyone records bad sound, so they install an AGC circuit and call it good. The simple fact is, unless you have a camera that cost somewhere upwards of $2,000, all your audio levels are probably set by the AGC. Notable exceptions include the Canon Xi, the Sony TRV950 and the Panasonic DV953.

Automatic gain control is one of those things that sounds like a great idea at first, but it doesn’t work so well in the real world. As audio comes into your camera through the microphone, the automatic gain circuit analyzes the audio volume and decides whether to turn the gain (volume) up or down to get the strongest possible signal. What’s wrong with that? Well, it’s great for the screaming kids at your three-year-old’s birthday party, any time the audio level remains fairly strong or even when the audio is constantly very quiet. It can cause serious problems while shooting an interview, however, and that’s exactly when you need the best sound. When your interview subject pauses, as people frequently do when they talk, the AGC decides there isn’t enough audio and cranks up the gain. When they start speaking again, it’s too loud at first and then the AGC kicks in again to correct the level. The burst of sound can last for just a fraction of a second, but you can usually hear it. The reverse problem crops up during loud bursts of sound like laughter or coughing. The AGC hears a loud sound and quickly turns the gain down. Unfortunately, the normal volume of the speaking voice is much lower and now you can barely hear it until the AGC attempts to fix the discrepancy. The ramping back up of the gain can be a longer process. A third problem occurs when you can hear some background noise, either the camera itself or maybe an air conditioner. When the interviewee pauses, the gain ramps up and the background noise can become very apparent. The noise then disappears again once the subject speaks. This creates a kind of breathing background to the interview.

Muddling Through
The reality is that you often have no choice but to use a camera with AGC. Here’s a typical scenario. You arrive at the shoot to discover your location is just outside a mechanical room. Pumps kick on and off every few minutes and the roar from the air handler is deafening. You lobby for another location, but there are none available, so you decide to make the best of it. Next, you meet your interview subject who, as Jerry Seinfeld would say, is a low talker and you can barely make out what she says in person, let alone on the recording. By now, you’ve resigned yourself to the fact that the AGC will be wide open all morning and your recording will preserve, in pristine digital quality, every noise you never wanted to hear.

So now, you’re depressed and wondering if you can ever record decent audio again, right? It’s not all bad. There are situations when AGC can be quite helpful. Legal video and board meetings often use one or two boundary microphones on a conference table. Unlike a lapel mike, these are located several feet from the various speakers and must pick up every word. AGC can often compensate for variations in speaking voices to rescue a potentially poor recording. Shooting a play or musical is also a challenge from an audio standpoint and AGC can help even out the recording volume, allowing you to concentrate on your camera technique. In fact, if you really want to leverage the AGC, run your microphones through a mixer first. The additional boost will push almost every sound into the limiting range of the AGC and you’ll get a very loud recording. Yes, there will be background noise, but the dialog will be intact and, if there’s music, it won’t overwhelm the voices.

Five Ways To Deal
Since you’re likely stuck with AGC during your shoot, having a few tricks up your sleeve would be helpful. First, experiment with the AGC circuit in your camera before a critical shoot. Hook up the same microphone you’ll use and simulate the shoot conditions as best you can. Record some test tape and listen to the result. Listen with headphones and make a mental note of how microphone placement and speaking volume interact with the AGC. Second, get a handle on the background noise at your shoot location. Minimizing background noise such as traffic sounds, air conditioning or computer fans will give the AGC less to turn up when it senses low volume. This quiets some of the telltale signs of AGC (as the breathing discussed earlier) and will help when you’re editing. Third, have your talent start their re-takes with a sentence or two before the part you really need to record. This gets the AGC working in a normal range and keeps your audio at a level similar to previous takes. Fourth, monitor the whole shoot with headphones. You will hear the AGC working as you shoot and you may be able to make changes to minimize its effects or reshoot poor takes. Finally, when all else fails, you can repair some AGC problems in post-production. With careful normalizing of takes and simple adjustment of incoming and outgoing audio levels, you may be able to mask many of the standard problems.

Is this really all that important? Can’t we just record with AGC and forget about it? In many video forms, AGC is not an issue. In dialog-based projects that include interviews, AGC often rears it’s ugly head. For reference, check out the making-of documentary on a popular DVD such as the Lord of the Rings series. These documentaries are cut together from many interviews and subjects, yet the audio is consistent throughout. This did not happen by accident and shows what a little knowledge and planning can achieve.

Author Hal Robertson has been producing stereo soundtracks for over 24 years. He owns a consulting company that specializes in media production.

[Sidebar: The Manual Method]
If you’re one of the few with a manual audio adjustment on your camcorder, let me ask you one question: have you ever used it? You gain more creative control by using manual white balance and aperture settings for the video and using the manual audio level controls will enhance your sound. After you’ve hooked up your microphone, flip the switch (or make the menu selection) that disengages the AGC. Set the microphone in the position you plan to use for the shoot and adjust the level controls until the loudest possible sound will barely push the meters to the top. This setting will ensure consistent levels and nothing will overload or distort the audio recording. While it would be best if you could wear headphones and ride the gain throughout the shoot, but, reasonably, you can probably get by with a thorough set up, a good mike check and an occasional glance at the meters.

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