E N G L I S H
AUTOMIX™
Thank you for purchasing the Automix™! The Architectural Acoustics Automix is a high quality automatic mixer with eight transformer balanced, Mic/Line inputs. Each Mic/Line preamp provides a gain control, phantom power (mic inputs), low cut filter, activity/clipping LED, and a choice between manual or automatic operation. Each channel can be muted externally or multiple channels can be muted externally via an assignable mutebus. In addition, channels one and two provide adjustable priority and a signal processor loop. The master section provides a gain trim control, three sweepable notch filters, a downward expander, a transformer balanced output, and a remote volume port.
The Automix has been designed so that multiple units can easily be linked together to form a single mixer with many more inputs (16, 24, 32...). The Automix is supplied with a plexiglass security panel to prevent changes to the installers settings. The Automix is packaged as a
THEORY OF OPERATION
The Peavey Automix is a classic "automatic mixer" combining several time proven techniques to deliver maximum gain before feedback in an easy to setup and easy to use configuration.
The Automix uses VCAs (voltage controlled amplifiers) and gain computing circuits in each channel to ride the gain downward as more microphones become active. By dropping the gain 3 dB every time the number of active microphones double, the overall system gain remains at unity for ideal gain before feedback performance.
A precision rectifier and logarithmic converter in each channel computes the amplitude of the audio signal present at each microphone in real time. This channel amplitude, in decibels, is then compared to the amplitude of the sum of all the channels, also in decibels. A simple computing circuit calculates the math- ematical difference between these two amplitudes and feeds the product to the VCA as a gain control signal.
To better grasp how this works, let me offer a few examples. If we take the case of one person speaking into a microphone, the amplitude in that channel will be virtually identical to the "sum of all channels" amplitude. The difference between these two equal amplitudes is "0." When 0 dB is fed to the VCA control port, the result is unity gain for that channel. All of the other channels, with no one speaking, will exhibit significantly lower amplitudes. When compared to the "sum," which contains a nominal signal, negative numbers result. These negative numbers, presented to the VCAs of non active channels, command further attenuation of the noise and leakage present at these non active channels.
A second more interesting example is two people speaking simultaneously. To simplify this example, let's assume they're both speaking with the same loudness. As these two sources are incoherent, i.e. not identi- cal, they will sum as the square root of the sum of the squares or 3 dB more than either individually. When each channel's amplitude is compared to this +3 dB sum, it's VCA will be commanded to attenuate 3 dB. As
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