Automatic microphone mixers (AMMs) can be used in multi-microphone phone calls or local conferences, and they can automatically analyze, assign, and control microphone gain and channel switching.
These mixers are useful for simplifying system operation, reducing comb filtering, reducing the system's noise floor, and preventing acoustic feedback.
Relevant Product(s)
- ControlSpace Designer
Table of Contents
3. How each type functions in practice
There are two types of AMM modules built into Bose digital audio processor products: gain sharing and gated.
Each type of AMM has different performances and different advantages.
EX Series: Both types have direct outputs for each channel, and both support up to 32 inputs. EX supports AMM Link for expanding the channel count.
ESP Series: Only the gain sharing type has direct outputs for each channel. The gain sharing type supports a maximum of 32 inputs, and the gated type supports a maximum of 8 inputs. AMM Link is not supported.
1. Gain Sharing AMM
Gain Sharing AMMs, while maintaining the same overall system gain, will open all channels and compare the current input level of each channel to the average level; then it will distribute the gain proportionally to each channel.
The channels with lower input level will receive lower gain, and the channels with higher input level will receive higher gain.
2. Gated AMM
A Gated AMM requires setting the threshold level for each channel. The threshold level can be set manually or can be automatically calculated by the AMM. Each channel is turned on or off depending on whether the input level of that channel is above the threshold level.
Channels above the threshold level will be turned on, releasing the level. Channels below the threshold level will be turned off, limiting the level.
3. How each type functions in practice
Take the example of four microphones.
No Talkers
When there are no talkers, a basic Gated AMM will simply turn off all channels, while a basic Gain Sharing AMM will allocate an equal amount of gain to every channel. This can result in complete silence from the Gated AMM and a large amount of background noise from the Gain Sharing AMM.
Both of these byproducts are a problem, and each type of AMM has a method to solve these problems. Most Gated AMMs include a control for Ducking Depth so that unused channels are turned down by a fixed value rather than being turned off completely. Most Gain Sharing AMMs include an “OFF Attenuation” value which allows the AMM to reduce the gain of unused channels by a selected value. With these two functions, a Gated AMM and a Gain Sharing AMM will perform almost exactly the same when there are no talkers.
One Talker
When there is a single talker using the system, a Gated AMM will open that microphone channel and a Gain Sharing AMM will increase the gain of that channel.
For the Gated AMM, the level of all other channels will remain unchanged. Unused channels in the Gated AMM will be off or at their preset ducking depth. With a single talker, a Gain Sharing AMM will increase the gain applied to that microphone channel, and it will also reduce the gain applied to all other channels. A Gain Sharing AMM is always working to apply the same total system gain, so increasing the talker’s gain means unused channels need to be reduced.
Two Talkers
When two talkers use the system at the same time, and both are speaking at the same level, the Gain Sharing AMM will provide gain to both channels, and any unused channels will be further reduced. The gain applied to the two active channels will be 3 dB lower than the gain applied to a single channel because the Gain Sharing AMM is working to maintain the same total system gain, regardless of the number of channels in use.
A Gated AMM achieves a nearly identical result, but in a different manner. Most Gated AMMs include NOM Attenuation. NOM stands for Number Of Mics. NOM Attenuation means that the gain applied to each channel is reduced by 3 dB for every doubling of the number of active channels. If two channels are simultaneously active, each is reduced by 3dB.
Four Talkers
A Gain Sharing AMM will provide the same gain for all four channels, but because it is working to maintain the same overall system gain, all four channels will be 6 dB lower than the gain of a single speaker's channel.
The Gated AMM has four channels whose level value is higher than the threshold level, so the Gated AMM will release four channels. Each channel is reduced by 6 dB from NOM Attenuation.
Two Talkers, Different Levels
The difference between a Gain Sharing AMM and a Gated AMM becomes more pronounced when people speak with different input levels.
A Gain Sharing AMM will distribute gain proportionally to each channel based on its current level compared to the average level.
Therefore, in the example images below where both Mic2 and Mic3 have active speakers, Mic3 will get the largest gain followed by Mic2.
A Gated AMM has a threshold level for each channel, and if Mic2's input level value falls below that channel's threshold level, Mic2 will remain in the OFF position, either muted or greatly reduced in volume.
When the input level of Mic2 is above the threshold level of this channel, Mic2 will be turned on and has the same gain as Mic3.