What is AGC?

We hear and read blogs on the net discussing the problems of AGC noise interfering with the audio on many DSLR cameras.

This problem is well documented – but what exactly is it?

Quite simply, AGC stands for AUTOMATIC GAIN CONTROL. Most cameras with the capacity to shoot moving images (film/video) employ an AGC circuit to control the level of the audio for the picture being recorded.

DSLR cameras were primarily designed as still-shot cameras. It was only the intuitive users of these cameras that discovered the potential for using DSLR cameras as moving picture cameras. Hence the audio facilities of the cameras are quite basic and this is demonstrated in the manner in which the recording level of the audio recorder is set.

An AGC electronic circuit aims to provide a consistent audio level. It does this by monitoring the incoming audio level, and raising or lowering the level of the sound being recorded. If the sound is too loud then the AGC will reduce the input recording level and if the sound is not high enough an amplifier will increase the level.

The image below illustrates how this works. As you can see from the top image (red) the level of the audio is rising and falling. As seen in the lower image (green) the AGC circuit in the camera is ensuring a consistent level is maintained at all times.

agc

Interfering AGC noise is most noticeable when recording dialog, or any low-level sound, particularly in a quiet situation. As the sound level is reduced or the audio stops (i.e. the person stops talking), the camera compensates and raises its input level to find the now non-existent signal. The AGC circuit keeps increasing the input record level as there is no signal, which in turn increases the self noise of the cameras audio amplifier. This leads to the sound of the unwanted ‘hiss’. There is no way to stop this in a camera with AGC and no manual override. In fact, many postproduction edit suites will not edit audio that is presented with this ‘hiss’ as it is not easily removed.

How does MyMyk address this?

Smartlynk is equipped with a clever audio circuit (AGC BLOCKER) activated by a slider switch at the rear of Smartlynk. When engaged, this will trap the left hand channel of the camera’s audio amplifier and deceive the camera into thinking it has a consistent audio input, and therefore it will not engage the AGC circuit. So, independent of any activity on the right hand channel there will be no noise introduced, as the AGC is not engaging.

In play back we will find there is no audible sound recorded to the left channel and all of the desired information has been recorded to the right channel. In the editing program there is a simple process to merge the right channel to center. This defeats the left channel and we’re now left with a very clean audio track with no unwanted ‘hiss’ from the camera’s AGC.



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