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AMM – Automatic Microphone Mixer Concepts

Automatic Microphone Mixers (AMM) are used in teleconferencing to turn on microphones that are currently in use and turn off microphones that are not in use. If four microphones and a standard mixer are used in a teleconference environment with all four microphones turned on, the amount of background noise being transmitted to the far end would be far too loud. In addition, all four microphones would be picking up the talker at different volumes and with different arrival times due to distance from the talker to the microphones, so the talker’s voice would be heavily comb filtered and difficult to understand.

There are two main types of Automatic Microphone Mixers: Gain Sharing and Gated. Each type of AMM performs differently and has different strengths. A Gain Sharing AMM compares each channel’s current signal level to the average signal level of all channels. A Gated AMM compares each channel’s current signal level to a threshold for that channel.

The Gain of each channel in a Gain Sharing AMM varies continuously depending on the current signal level for that channel and all other channels. In a Gated AMM, the amount of gain applied to each channel is based almost solely on whether that channel’s audio signal level is above the current threshold.

A Gain Sharing AMM never completely turns off any channel and varies the level of each channel depending on that channel’s current signal level, as well as the signal level of other channels. A Gated AMM turns off any unused channels. Some Gated AMMs can set a ducking depth which will keep unused channels turned on at a lower level, but this is a static, preset level that does not vary based on the activity of other channels.

In the simplified illustrations below, the chart on the left shows the audio input signal level for four microphone inputs, plus the average signal level of all four inputs. These are the values that would be shown on the audio meters for those channels. The center and right charts show the applied gain from each type of Automatic Microphone Mixer. This is equivalent to the fader position on a traditional mixer. They represent the gain setting of each channel, not the level of the audio signal for each channel.

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.

AMM concepts 1

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, but 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.

AMM concepts 2

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 and 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.

AMM concepts 3

If four channels are active at one time, each would be reduced by 6 dB. With two talkers using the system at the same time, if both are speaking at the same level, Gain Sharing and Gated AMMs provide very similar results.

AMM concepts 4

Two Talkers, Different Levels

When two talkers speaking at different levels use the system simultaneously, the differences between Gain Sharing and Gating AMMs can become more noticeable. With a Gated AMM, each channel is ON or OFF depending on that channel’s current signal level compared to a threshold. The threshold may be a manually set target, or the threshold may be automatically set by the AMM based on the average signal level of all channels. If a second talker begins speaking at a level that is below the threshold for the Gated AMM, the second talker’s channel will remain in the OFF position, either muted or greatly reduced in volume. When the second talker’s signal level increases to a point above the threshold, that channel will become “ON” and the Gated AMM will apply gain to both channels in the same manner as two talkers speaking at the same level. A Gain Sharing AMM handles this scenario differently because a Gain Sharing AMM always applies gain to each channel based on that channel’s current audio level compared to the average audio level. As soon as a talker begins using a second channel of the Gain Sharing AMM, even if that talker is much quieter than the primary talker, the Gain Sharing AMM will begin increasing the amount of gain allocated to the second talker’s channel.

AMM concepts 5

It is important to note that neither of these methods can be considered the “right” way to handle this situation. If the two talkers are engaged in an interactive discussion, or if the second talker is adding a point of clarification, the response of the Gain Sharing AMM will seem more natural. If the second talker is someone having a side conversation, the response of the Gated AMM will be more appropriate.
In addition to these differences in response, there is a large difference in the degree to which each type of AMM can be adjusted. Due to the nature of the Gain Sharing algorithm, a Gain Sharing AMM will have relatively few user adjustable controls. A Gated AMM will often provide controls that allow detailed adjustment of every parameter of each channel’s gate. In a standard room, a Gain Sharing AMM may provide a more natural response with little or no need for configuration. In a more challenging room, a Gated AMM’s ability to precisely tweak each channel’s setting may provide better response. It is impossible to say that the Gain Sharing AMM or Gated AMM is “better”, and each type of AMM will be needed for different installations.