With all of the advantages that the HDSLR revolution has given video producers and filmmakers, there are a number of well-known drawbacks to be aware of: moiré, aliasing, lack of headphone monitoring and sensors that quickly overheat. One of the lesser-known drawbacks is an audio feature called AGC. The initials stand for Automatic Gain Control, which means that the camera will continuously adjust the audio levels so that loud sounds won’t overload and distort, and soft sounds won’t go unheard. In some situations AGC is a really nice feature to have. However, there are other shooting scenarios where AGC can cause major problems and to make matters worse, most cameras don’t give you the option to turn it off.
One common AGC problem occurs when you’re shooting video with dialog in a quiet location. When the onscreen subject speaks, the camera hears a relatively strong signal, and the AGC quickly adjusts the audio level to properly record their voice. The problem arises in the silence between their words. The very moment the onscreen subject stops speaking, the AGC quickly turns up the audio levels in the camera. This creates an uneven ambiance that the viewer can hear. Between every pause in the dialog you can hear the camera quickly turning up the audio level, which sounds like little pumps of noise between the words. It’s very unnatural sounding and totally distracting. When you’re doing your best to create a high-quality production, AGC can present a very big problem.
If you have a camera that doesn’t allow you to turn off the AGC (such as the Canon 7D), there are ways to defeat it. One option is to purchase a somewhat expensive dedicated external device that has built-in AGC defeat functionality, like the juicedLink DT454 and Beachtek DXA-SLR camcorder XLR adapters. The nice thing about these products is that they allow you to use multiple professional XLR microphones with cameras that only feature a single mini-plug input (learn all about Camcorder XLR Adapters in this B&H Buyer’s Guide). However, if you don’t need this capability and you just want to defeat the AGC in your camera, there’s a less expensive way to do it.
Before we get started with step-by-step instructions, it’s important that you understand how a typical camera records audio. Cameras like the Canon 7D have a mini-plug stereo microphone input. “Stereo” means that the camera records two separate tracks of audio. This can be confusing because some people associate the word “stereo” with good, and “mono” with bad. This is not the case. You can make excellent-sounding recordings in mono. Stereo simply means two and mono means one. When you plug a stereo microphone (such as the Audio Technica AT2022) into a camera, two separate tracks of audio are recorded (one for the left stereo channel, another for the right).
When you defeat AGC with an MP3 player or another external device, you will only be able to record audio on one of the two tracks in the camera. You can’t make stereo recordings using this method. Before you write this off as a deal breaker, keep in mind that most shotgun microphones (like the Rode VideoMic Pro and the Sennheiser MKE 400) are mono microphones. In many cases, the advantages of defeating the AGC will outweigh the disadvantages.
The reason you can’t make stereo recordings when you’re defeating AGC with an external device is that a tone will be recorded onto one of the two tracks in the camera. In this tutorial, you are going to plug your MP3 player into the camera with a special Y-cable and then play a sound file of a constant tone. The special Y-cable lets you attach your MP3 player and an external microphone to the camera. The constant tone tricks the AGC function in the camera, keeping it from raising or lowering the audio levels. This way the microphone level will not fluctuate and the audio will sound more natural.
The special Y-cable that you need is the Sescom DSLR-AGCY. It’s specifically designed for this purpose. You need to download the MP3 file with the AGC defeat tone from Sescom’s website. The tone is an hour-and-a-half long, so once you press play on the MP3 player, you don’t have to think about it again for a comfortable amount of time. Here’s the list of gear that’s used in this tutorial:
the MP3 Player of your choice
a rubber band from a head of asparagus (not sold at B&H)
This is how you set it up:
1) Assemble the various components as in the animation above. The Rycote Hot Shoe Extension Bar goes on first, followed by the other equipment. Use the broccoli rubber band to fasten your MP3 player to the Sennheiser CA2 Shoemount Adapter.
2) The end of the DSLR-AGCY cable that’s labeled “GEN,” which is short for generator, plugs into your MP3 player’s headphone output. The MP3 player is referred to as a generator because it’s supplying the tone which defeats the AGC. Be sure to have downloaded the MP3 sound file from Sescom’s website so you can play it on your device. Some smart phones (like the Apple iPhone) allow you to stream the MP3 file directly from Sescom’s website.
3) Plug the cable on the Sennheiser MKE 400 Microphone into the female end of the DSLR-AGCY cable (which is labeled “MIC”).
4) The other end of the DSLR-AGCY cable (which is labeled “CAM”) plugs into the mic input on your camera.
5) Press PLAY on your MP3 player and turn on the power on the MKE 400 microphone. With everything plugged in, your camera will be receiving the tone from the MP3 player on one track, and the signal from the mic on the other.
When you shoot video, one channel will be recording the AGC defeat tone from the MP3 player, the other will be recording the microphone signal. One drawback is that when you playback footage, the built-in speaker on your camera will blend the recorded AGC defeat signal with the audio from the microphone. This makes it difficult to monitor the audio of the footage that you playback on your camera.
You won’t hear the benefit of what this AGC defeat method provides until you load your footage into your NLE software. Once your footage is transferred into your editing software, you’re going to need to either delete or mute the track with the recorded AGC defeat tone. In Final Cut Pro, I usually mute the audio track with the AGC defeat tone, and pan the track with the microphone audio to the center.
It’s strongly recommended that you do a few test runs of this workflow before you do any serious shoots. This way you can get used to using all of this equipment, and familiarize yourself with handling the audio tracks in post production. When you’re conducting your tests, I suggest recording a take with all of the AGC defeat equipment running, and recording another take with the microphone plugged directly into the camera. This way you can hear how much the AGC defeat method is improving your sound.