DESCRIPTION

The VM-4083 / VM-4043 Voice-Matic automatic mixer includes all the features needed to head the signal processing chain of any automatically mixed sound system.
All electronically balanced microphone inputs are independently convertible for line level signals and include switchable phantom power; switched input padding; defeatable 100 Hz High-Pass filters; 3-band TEQ equalization; Automatic/Standard mode selection; channel-status logic outputs; two single-ended buffered preamplifier outputs - one pre-level control and the other post-gate; plus pre/post gate assignment to an auxiliary output mixing bus. Individual three band input TEQ tone controls allow compensation for various frequency response characteristics from a mixture of microphone types.
Automatically mixed input channels route through an enhanced Level-Matic II ® AGC circuit (with DIP switch bypass). Two line level sources may be mixed to the outputs via an accessory pair of electronically balanced auxiliary inputs. DIP switch programming allows removal of these auxiliary signals from the mix to selected outputs. This feature can be used to make a mix-minus connection of a teleconferencing interface to the mixer; and to use a single tape machine for both recording and playback without the need of a switch. The electronically balanced Main output is a mix of the gated microphone signals with NOM count attenuation and Level-Matic II ® AGC, ungated signals from input channels switched to STANDARD mode, plus the auxiliary input signals assigned to this output. An auxiliary mix bus bypasses both the NOM attenuation and Level-Matic II ® circuitry. Dual electronically balanced Aux outputs are provided for tape recordings, teleconferencing, hearing assistance systems, and other off premises transmissions.
The Auxiliary output signals include pre/post gate selection from all microphone channels, plus the auxiliary inputs assigned to these outputs (each auxiliary input is defeatable from the Aux output bus). Both auxiliary outputs have an independent level control to compensate varying input sensitivity requirements of external devices wired to these outputs. To prevent power surge transients, a supervisory sensing circuit mutes the Main output and Auxiliary output #1 during power interruption to the mixer. All audio inputs and outputs use convenient and reliable, removable screw-clamp terminal blocks for ease of installation. Set-up adjustments for the automatic mode are simplified by a level indication which illuminates when an input channel sensitivity control is optimally positioned.
The mixer may be set to LAST MIC HOLD mode for the continuity of room ambience in the sound system and to other output devices. The LAST MIC HOLD feature keeps the most recently activated input channel held to the ON status until another active input is detected. The VM-4083 / VM-4043 is linkable for the expansion of automatically mixed inputs. MixLink ILS ® combines all the features of the automatic mixers, including Dynamic Threshold Sensing, NOM count attenuation, Level-Matic II ® AGC, LAST MIC HOLD and Automatic/Standard mode selection among all linked chassis. Identical signals are transmitted to the Main Outputs of all linked mixers.
Aux bus signals are similarly combined and transmitted to the linked chassis Auxiliary Outputs. Easy configuration of room combining projects is possible via remote activation of link mode among the VM-4083 / VM-4043 Voice-Matic ® Mixers.
Complete remote control capability is provided for all standard end-user adjustments. Each of the Automatically mixed inputs, Auxiliary mix inputs and the Main output may be connected to independent 10kW linear potentiometers for up to 60dB remote level adjustment range. Remote switching activates 90dB muting of the corresponding inputs and/or Main output. Remote selection of Automatic/Standard mixer operation is also provided.


APPLICATIONS

The Voice-Matic Microphone Mixer is ideal for use in multiple microphone sound systems installed in Board Rooms, Court Rooms, City Council Chambers, Churches, Schools, Conference Rooms, Teleconferencing Systems, Hotels, and Convention Centers. The VM-4083 / VM-4043 automatic mixers are particularly effective in reducing the pick up of background noise that may be amplified by a sound system, or for replacing standard mixers in systems that produce feedback howl without adequately loud sound levels. Rooms which sound reverberant with standard mixers will have much better quality and improved intelligibility with a VM-4083 / VM-4043 Voice-Matic Mixer.


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 / VM-4043 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 / VM-4043 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.