Gain Sharing Automatic Mixer

Purpose

The Deluxe Gain Sharing Automatic Mixer is a combination of High Pass Filters, Expanders, Band Pass Filters, Gain Sharing devices, and Mixers. The Standard version leaves out the Expanders, and the Economy version leaves out the Expanders and High Pass Filters.

Gain Sharing Automatic Mixers are of particular advantage in teleconferencing systems and sound reinforcement systems for conferences and boardrooms. They work well in Mix-Minus designs.

Gain Sharing Automatic Mixers are designed to smoothly give the available system gain to the microphone(s) in use. Other inactive microphones have their gain reduced at the same time to maintain feedback stability margin. The decisions as to which mics get the gain is based on the energy in the mid band speech frequencies thus greatly reducing the influence of low and high frequency background noise. The Standard and Deluxe versions include high pass filters on each input to reduce the amplification of low frequency noise below the speech frequencies. The Deluxe version also contains Expanders to reduce the influence of room noise on microphone inputs that are not in use.

Part of the reason the Gain Sharing Automatic Mixers work so well is that the mics are never shut off. With no one speaking, all the inputs go to a nominal "Rest State" gain setting. When someone speaks into a microphone, that input goes to full gain, while the other inputs go to a lower gain setting. Thus no input is ever gated off, and the changes in gain are made smoothly. When more than one mic is in use at the same time, the available gain is shared between the microphones in use.

The Gain Sharing Automatic Mixer improves system performance with the following features:

Controls

Output Mix

Normally all these controls are left set at 0.00 dB. Since these controls are after the gain sharing circuitry, they do not affect how much gain a given input gets. They are used in conjunction with the Input Level controls if you wish to weight a particular input so it will obtain a greater amount of the available gain, without making that input louder in the mix. For example, an input could be set 6 dB above the other inputs on the input level controls, but turned down 6 dB on the output mix. Do not try to kill an individual input using these controls, since the input will still be taking part in the gain sharing even though it will not be part of the output mix. Instead use the Input Level or Mute controls.

Input Level

These adjust the relative balance of the incoming audio signals. They are before the gain sharing circuitry and will affect how much gain each input gets.

Input Level Overload indicator

This shows overload of the sum of all inputs that is used to compute the gain sharing. This will only indicate if there are high levels on many inputs at the same time. When lit it does not indicate audible distortion, but only a possible error in the computation of the gain sharing. Turning down the Input Level controls or reducing the levels sent into the automixer will cause it to go out.

Mute

This allows you to mute an individual input.

Channel Gain

This displays the current gain applied to the input channel, and seen at both the direct output and the sum output.

Overload

This shows overload on the individual input or the sum output.

Out

This is the master gain for the sum output. Next to it is the overload indicator for the sum output, and the sum output level meter.

Gain Share Time Constant

This sets the reaction time for the gain sharing. We suggest a starting setting in the 30 to 50 ms range. The best setting will be dependent on many environmental factors, and should be determined in the finished system.

Rest State Attenuation

This sets the attenuation each channel gets when no channel has input audio. If -3 dB was selected, then there would be little advantage of the automatic mixer over a plain mixer. If -24 dB was selected, then background noise would cause a greater modulation of the gain changes. -12 dB is suggested as a starting setting.

Max Depth

This sets the maximum amount of attenuation of a given channel when another channel has the dominant audio, and therefore 0 or close to 0 attenuation. Together with the Rest State Attenuation switches and the Gain Share Time Constant control, these controls determine the reaction of the Gain Share devices to audio. When there is no input signal, all the Gain Sharing devices go to the rest state attenuation. When there is signal on one input, but on no other inputs, then the input with signal goes to zero attenuation and the others go to the max depth attenuation. The rate at which the gains change is determined by the Time Constant.

Ratio and Threshold controls and the Signal indicators

These are part of an expander that is right ahead of the gain share devices. The expander is used to gently reduce the incoming audio level when no one is speaking into a given input. This reduces the amount of gain modulation that occurs due to background noise (see Rest State Attenuation, Gain Share Time Constant, and Max Depth above.)

Threshold

This sets the level below which expansion happens. This should be adjusted so that the threshold is below the softest speech, but above the background noise level. A starting setting of -30 is suggested.

Ratio

This adjusts how much additional attenuation is applied as the input level drops below the threshold level. A low ratio in the range of 1.1 to 1.5 usually works best. Higher settings can produce greater noise reduction, but might also become more audible. A starting setting of 1.3 is suggested.

Signal indicators

These show the input signal level relative to the threshold level, and aid in adjusting the threshold level. When someone is speaking into the input, the "above" indicator should be lit. When no one is speaking into the input, the "below" indicator should be lit.

Invert

Reverses the polarity of the input.