Unity gain is a term used when establishing the balance between pieces of audio equipment. The idea is that input should equal output, level-wise. Audio that goes into a device at one level and comes out of that device at the same level is said to be at unity gain. The devices can be as simple as a guitar amplifier and a microphone or as complex as a chain of interacting equipment.
Explanation of Gain
When we talk about "gain," we're usually talking about one device's ability to take the lower level of one signal and bring it to a higher voltage level.
A great example of this is a microphone preamp. The preamp applies gain to amplify the signal coming from the microphone.
Explanation of Unity Gain
In the concept of unity gain, the input and the output between two devices are at the same level. That's to say, when a microphone is outputting a gain of factor 1, which is equivalent to 0 decibels, the mixer also outputs at 0 dB. The input and output are at the same voltage and impedance. Unity gain is established by calibrating two pieces of equipment to interact at the same level.
Frequently, the best way to set unity gain is to adjust a microphone or line signal gain to 0 dB, measured both at the preamp and output stage, and then match that 0 dB level simultaneously on the input of the second piece of equipment—whether an amplifier, recording software or mixer.
The more gear that is involved, the longer the process of reaching unity gain on all equipment.
It isn't unusual for traveling sound professionals to carry equipment specifically designed for bringing all devices in a studio into unity gain.
So, what do you do with equipment that is made specifically to boost gain, such as amps? You still use them as intended, but first calibrate for unity gain to achieve the best possible signal you can and then boost it to your liking.
Benefits of Achieving Unity Gain
Unity gain is useful for several reasons:
It gives a significantly cleaner and undistorted signal. If each piece of gear in an audio studio adds even a tiny bit of distortion, the accumulated effects degrade the signal.
Microphones sound much fuller and have much better dynamic range.
It is easier to apply outboard effects, as most effects units are set to accept a unity signal.
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