This fledgling video producer bought the best equipment his meager budget would allow, and, fueled by his creative drive, flew headlong into his first production shoot. Back home in his edit suite/living room, the images he captured were stunning – but what was that awful whirring in the audio? He tried everything, but alas, the noise would not go away. He would have to either re-shoot the segment or live with the poor audio – neither of which would make his client happy. What to do? Our young friend had fallen prey to the evils of AGC, also known as automatic gain control.
Actually, this story is played out all over the world as more and more people discover desktop video production. Most beginners and hobbyists can’t afford a $5,000-plus professional video camera with all the bells and whistles, so they buy something that better suits their budget, usually a Mini DV or Digital8 consumer-grade camcorder. These cams have excellent images and digital video effects. They represent a complete production studio in the palm of your hand except for the audio features. Specifically, the inclusion of AGC instead of an adjustable audio input.
How Does it Work?
The automatic gain control circuit in your camcorder "listens" to the incoming audio and keeps the loud sounds from getting too loud and the soft sounds from getting too soft. It’s a great idea on paper but, in application, the results are often unpredictable, even under the best of circumstances.
When the AGC hears a loud sound, it turns the audio input down to keep from overloading the recorded signal. Of course, a simple electronic circuit can’t make a distinction between a voice that you want to record and the loud truck driving by that you don’t want to record. It only measures the loudest sound and adjusts to compensate for it.
In contrast, AGC treats soft sounds in the opposite manner, turning up the input until it hears "enough" signal. This can work to your advantage in a quiet environment, but those locations are very difficult to find. It seems everywhere we go, there are noises in the background: air-conditioning systems, traffic rumble, even crickets, which all seem loud when assaulted by an AGC.
How Do I Make it Stop?
You don’t. More accurately, unless you spent more than $2,500 for your camcorder, you can’t. Those who can afford a Canon XL1S, Sony VX-2000 or other high-end camcorder, have access to manual volume control.
Since AGC isn’t going away anytime soon, here are several techniques to help you minimize the effects of AGC and produce more professional-sounding video.
Minimize Background Noise
The simplest way to lessen the effects of AGC on your audio is to minimize the background noise at your shooting location. Remember, the AGC is always listening to your audio input. If it doesn’t hear enough sound, it will try to automatically turn up the volume. Soft sounds that seem completely harmless could ruin your audio if the AGC turns them up too loud. Listen closely through your headphones before the shoot and try to identify potential sources of noise. This could be a simple matter of turning off the air conditioning system during an interview. You should also turn down (or off) any background music or paging systems if possible. Shooting after regular business hours may help too. Just make sure you know the schedule of the cleaning crew, unless you want to add vacuum cleaners to the list of sounds in your video. Under more drastic circumstances, you may have to relocate or postpone the entire shoot to eliminate these effects.
In my hometown, there used to be a hotel with a large, working water wheel in the lobby. It was an imposing sight and made for some wonderful visuals. Unfortunately, the local news crews liked to interview visiting dignitaries with this water wheel as a backdrop. The resulting audio was always a disaster. The point here is simple; don’t sabotage your next video production by shooting in an environment that you know will cause audio problems, especially with AGC.
Use an External Microphone
The typical built-in camcorder microphone can be a dreadful way to capture audio in the field. Most are physically attached to the unit (if not inside) and consistently pick up all the noises within your cam — tape drive, zoom and focus motors, etc. Worse yet, since it is attached to the camera, this microphone can never get any closer to your subject than your camera is. The ratio of the desired audio to the noise of its background decreases as this distance increases. This is an AGC nightmare waiting to happen.
Take control of the situation by using an external microphone. Most video cameras have small jacks on the side to plug in another mike. Pick one you like and use it. AGC is still an issue, but now you have the microphone directly in front of (or clipped on) the subject of your video. This helps to minimize background noise, giving the AGC circuit less "bad" sound to turn up. An added benefit is that you will get much cleaner sound from your subject.
Don’t forget that microphones have different pickup patterns. Omnidirectional mikes pick up sound equally in all directions and tend to pick up more background noise. Unidirectional or cardioid mikes pick up sound primarily from a single direction and can help minimize AGC problems with background noise. Just make sure the microphone points at the talent and not up in the sky or toward the street (See the December column entitled The Sweet Spot for more on mike techniques.)
External mixers/volume controls can limit how loud is "loud" and how soft is "soft." By minimizing the difference in these variables with a mixer before the camera/AGC, the circuit will work less to "fix" your sound.
Work With the Talent
Even if you follow all these tips, you can still encounter audio problems caused by the AGC. You’ll uncover these problems when editing segments of video from the same person. Take One may be strong and clear, but you have to edit it against some weaker material from Take Three.
The best way to eliminate this problem (and, as a rule, all production problems) is to take care during the shoot. Before each take, have your performers count down from three just before they deliver their lines. This will get the AGC working in roughly the same range for each take, making a more consistent volume on the tape. If shooting an interview, have the talent start each thought with some throwaway phrase or the sentence just before the thought you want to capture. With some practice, both methods can be very effective.
Alternatively, you can adjust the volume of each clip in the computer. Most editing programs allow adjustments during the clip too, if necessary. With some careful listening, you can get most clips "in the pocket." Adobe Premiere has an audio feature called "rubber bands" that let you draw large or small volume changes within each clip. Using this tool gives you microscopic control over each word if necessary. You can also insert an audio effect called a Noise Gate into your audio path. This tool automatically mutes all audio below a threshold that you set, eliminating any background noise that the AGC may have amplified.
One of the most irritating things about AGC is hearing the circuit work – constantly adjusting the volume up and down. You can also hide some of the more inconsistent moments by masking them with a nice layer of background music. By giving our ears something else to listen to, they don’t focus on the audible and sometimes bothersome effects of the automatic gain control.