Consider a typical conference room setup: there may be a number of microphones distributed around the tabletop or hanging from the ceiling. With multiple people talking, the conference can lose focus if the room’s microphones are not managed effectively. Auto mixers are designed to take the audio present at each microphone and ensure that a person at the opposite end of the room – or at the opposite end of a teleconference call – receives the appropriate mix of voices to understand the discussion and have a clear conversation.

Auto mixers are typically used in this scenario, as they deliver a constant output gain to both near and far ends with no user intervention. In other words, as more people speak, the overall level in the room (or far end) does not increase. This avoids feedback that may result if the microphones are merely summed. There are two primary options: gating auto mixers (as referenced in our previous post), and gain sharing auto mixers. They both do great things for us, but in different ways.

Rather than turning microphones on or off (gating), a gain sharing auto mixer functions by leaving all microphones on, sharing a fixed amount of gain (100%) between all channels. Active channels receive more gain, proportionally, than inactive channels, meaning that a microphone with a person actively speaking into it will receive priority. As the active microphones receive more gain, the others receive less to keep the mixer output to 100% overall.

While gating auto mixers typically have controls for the maximum number of open microphones and whether to leave the last gated mic on (an option I like for ‘comfort noise’ to the far end), gain sharing auto mixers typically have no controls. There’s always a little comfort noise to the far end of the conference, and technically all mics can be given gain at any time. That leads us to one of the drawbacks: since gain is shared between all mics, as the number of microphones increases, less preferential gain can be given to any one microphone. This is especially true if speech bleeds over to adjacent microphones. To counteract this, Biamp’s gain sharing algorithm has a configurable ‘mic isolation factor’ that can allow the mixer to focus gain on the mic with the best speech signal.

One of the advantages of gain sharing over gating auto mixers is simply that the mics are not gating on and off. With gating auto mixers, an adaptive gating threshold is computed from the noise and common signals presented to the inputs. Marginal speech levels or high noise levels in the room can make gating of a mic difficult. This often results in first syllables being lost in the reinforcement (or far end), so your “unimportant” issue could suddenly become “important” to the far end. While the “un” might be lower in level through a gain sharing auto mixer (as the mic is granted more gain), it will still be there.

In a room with good acoustics and proper gain structure, both gating and gain sharing auto mixers can perform quite well. In difficult situations, the gating auto mixer allows for the most control over circumstances. However, I tend to favor gain sharing mixers in rooms with good acoustic properties.

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