provide the gain stability and low reverberation of a single microphone even when multiple microphones are being utilized. As input signals are processed and mixed, DTS® (Dynamic Threshold Sensing), combined with IRP’s new Level-Matic II® Automatic Gain System on the Main output of the mixer, provide consistent, predictable, performance.
Set-up adjustments for the automatic operation are simplified by an LED level indicator which illuminates when an input channel sensitivity control is optimally calibrated. Last Mic Hold circuitry keeps the most recently activated microphone in the "on" state, providing continuity of room ambience in the sound system and preventing microphone shutoff during pauses. Each balanced microphone input is independently convertible for line level signals and includes switchable phantom power; switched input padding; a defeatable 100Hz High-Pass filter and 3-band TEQ® equalization.

The individual screwdriver adjustable 3-band TEQ® equalizer controls on each mic/line input allow compensation for various frequency response characteristics from a mixture of different microphone types. All automatically mixed input channels route through a Level-Matic II® Automatic Gain Control. This exclusive two-stage circuit compensates for level changes between talkers and variations in talker-to-microphone distances automatically and unobtrusively with no "breathing" or "hunting" artifacts.

OPERATING PRINCIPLES
The Voice-Matic® Microphone Mixer is unique in operating principle. It uses Dynamic Threshold Sensing (DTS) to differentiate between active and inactive microphones. Dynamic Threshold Sensing utilizes a combination of the amplitude and history of the signal to determine channel access. DTS generates a threshold reference which decays over an 80dB range from a high level to a low level in a 10ms or less interval. All microphone inputs are referenced to the threshold simultaneously. The first channel whose instantaneous amplitude equals or exceeds the sweeping threshold is given temporary ON status for 200ms. Simultaneously, the threshold is reset high for 7ms and another sweep is initiated. Since any active input will equal or exceed the decaying threshold earlier in ist sweep, the average sweep time is only 3 to 4ms and the frequency of the intervals will be increased.

On a repeated sweep the same or a different channel may have the highest instantaneous amplitude and receive temporary ON status. In this way, the active microphone channels are detected and their ON status is renewed. Several microphones can have simultaneous ON status and be effectively updated without dropouts. Rather than turning an inactive microphone OFF it is attenuated by an internally pre-set 30dB. The microphone is rapidly turned ON when it receives a signal, preventing upcutting of leading syllables. A continuous count of the number of ON microphones is used to adjust the master amplifier gain according to the rule of reducing gain by 3dB for each doubling in the number of simultaneously active microphones.

The Level-Matic II ® AGC incorporated in the VM-4083 mixer performs an important function typically performed by a manual operator in a sound system. The Level-Matic II ® AGC circuitry automatically adjusts the master output gain to maintain a uniform output signal level for input level variations of as much as 10dB. The Level-Matic II ® gain control is based on the loudness versus frequency and loudness versus time response of the human ear. A loud talker causes the master gain to decrease. If the talker stops talking the gain holds as established by the talkers average speaking level. If a quiet talker then begins to use the system, the gain increases to a new value as established by his average speaking level. Two control voltages are created in the circuitry, one circuit establishes a semi-stationary control voltage and another circuit creates a voltage which follows the signal peaks. At any instant, an attenuation circuit is governed by one, but not both, of these control voltages. Gain corrections are made at a constant slew rate to minimize gain "hunting".

The three-band equalizer incorporated on each of the Mic/Line inputs on the VM-4083 is designed to compensate for an undesired frequency response in a source. The equalizer utilizes transversal technology (TEQ ® ) to produce three adjustable minimum phase response curves focused on the speech spectrum of the typical human voice. The transversal equalizer utilizes an analog all-pass phase shift network and the application of Fourier Transform theory to synthesize a filter by its time response. This creates an equalizer with a continuous smooth response curve without band-edge ripple, excellent transient response and minimum coloration of the original signal.