Over the years, many a newcomer to the industry has asked: Why do I need a mixer to do audio? Well, the answer is quite simple: For the same reason that we need to use lights for image.
They appear to understand until I tell them that lighting is all about creating shadows and contrast, not light.
Next I explain that manipulation of these two elements, combined with a few others, helps to create the illusion of 3-D space in the two-dimensional world of the mono-optical camera lens. Without shadows and contrast the image would appear flat and separation of the subject from the background is almost impossible.
OK, I think I understand now, but what does that have to do with sound?
Well, in order for our audience to hear what is important, we need to suppress background ambiance. But much like the depth perception problem of the single lens, a microphone cannot distinguish desired sound from ambient noise.
Use of a mixer in tandem with the microphone allows us to maintain a high level of desired signal while minimizing the noise floor, which is commonly referred to as a good signal-to-noise ratio. This allows the listener to separate the dialog (or other focal audio content) from the background.
So the bottom line with both tools is that they give you control over the environment.
Now that we have established the need to have control, let’s see how a mixer can help us.
First and foremost, even with the most complete set of onboard audio features imagineable, the controls on a camera can't be adjusted when the camera is in use. These controls are inaccessible while the camera is in motion, and when static, any attempt to adjust levels might bump the camera giving the appearance of an earthquake on the screen.
Use of automatic gain control on the camera may not work well enought either because these devices cause dramatic fluctuations in the signal and with it the noise floor. When you get into the editing room, the audio edit may not work because the noise level in one shot does not match the noise level in the cut. Use of a mixer allows us to gradually and smoothly manipulate the levels in relation to the signal; this minimizes pumping of that noise floor. But this is just part of the picture.
Mixers are devices dedicated to sound and as such manufacturers put their money into the electronics that will guarantee quality audio: the preamps on mixers are typically of much greater quality that those on a camera. The preamps take the signal from microphone-level to line-level.
Once this is done, we would prefer to stay at line level; again because the signal-to-noise ratio of line-level is substantially better than mic-level. So, we run at line-level output from the mixer to line-level input at the camera.
Mystere ENG audio mixer
Mixers also have additional features such as high-pass filters and limiters. The high-pass filter can cut much of the low frequency noise present in all environments. Reduced noise enhances clarity of the signal. Limiters allow us to inhibit over-modulation of the signal that might clip the signal or introduce analog distortion.
Now, let’s add to this the bonus of confidence monitoring which allows us to audition either the mixer or the camera so we can isolate any problems in the signal by more easily determining the source.
Oh, and did I mention that we could have more than two inputs? Yes, even ENG mixers these days provide as many as six inputs. All these features combined make mixers a powerful tool for ensuring usable audio.
And, it only takes one time in the editing room trying to salvage bad audio to realize that the cost of a mixer is nominal in relation to the ongoing expense of audio post due to poor production sound.