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Automatic Mic Mixers

Microphone mixing circuits are by no means new to the audio industry. The basic resistor-coupled mixer is as old as radio broadcasting itself (the early

Automatic Mic Mixers

May 1, 2003 12:00 PM,
Allan Soifer

Microphone mixing circuits are by no means new to the audio industry. The basic resistor-coupled mixer is as old as radio broadcasting itself (the early 1920s), developed by the need for multiple microphones and other signal sources to be active and level balanced to each other. Without mixers the producer’s command of “music fades up and under” would never have been coined, and the announcer voice-over could not exist. Technical engineering improvements over the decades have brought mic mixers to a high state of the art in low-noise, multichannel, multibus mixing and blending of audio signals.

Back in the ’70s, several manufacturers were developing the next wave of broadcast and pro-audio tools, which became the gated mixer. The idea was simple: eliminate the pumping and breathing effects of overactive compression and provide more control over on-location audio by coupling the gating circuit with a limiter. Holding the audio at the loudest previous instantaneous signal level was an innovation, and it cleaned up the sound of many sports remotes and similar location broadcasts. From there it was a relatively short leap to using the control circuits developed for gating for high-speed switching to raise and lower the attenuation of multiple mics.

The need for many mics to be active at once was first perceived by those A/V contractors who serviced government legislative chambers, city council halls, hotel function rooms, and so on. The common problem with trying to operate and control the levels on even six mics is obvious to anyone who has attempted it — one never has enough hands nor is quite fast enough to really ride herd on levels. If one turns all the mics on, both feedback and excessive background noise can occur. Simply gating mics on and off is not much help, because several mics may gate on simultaneously as participants speak, creating the same problems as having all mics live.

One solution was to allow the loudest sound (the mic with highest nominal level arriving at the input) to gate open first, followed by other mics. However, loud noises would create false triggering. Some form of background level detection was mandated, along with the individual channel gating. A minimum threshold would be set for the gates, and as each additional mic came online, the output signal level would be decreased by a small amount, nominally 3 dB, to prevent feedback. This technique became known in the industry as number of open mics (NOM). Each gate would hold open for a predetermined time, and if no active signal above background level was detected, it would then close.

Industry veteran Dan Dugan is often credited with building the first viable automatic mic mixer in 1971, using the patented Dugan Speech System. The original Model A was licensed to Altec Lansing, but subsequent inferior implementations of his original design by the audio giant caused automatic mic mixers to lose favor until the early ’90s, when Dugan and others began to refine the designs and add useful features. One such refinement allowed each mic to actually stay live but at reduced levels, thus largely eliminating any clipping off of a speaker’s first syllables. More current developments include adaptive thresholds, digitalization of both audio and control functions, and total computer control of mixer functions. A good basic discussion on the technical engineering features of automixers can be found in the January 1998 issue of S&VC or at this link:


Generally, automatic microphone mixers have two simple premises. First, they keep open the mic inputs that are in use and close or attenuate unused inputs. Second, when using two or more microphone inputs simultaneously, automatic mic mixers attenuate the mix of the inputs to keep a stable gain before feedback.

Automatic mic mixers are the ideal answer in many audio installations. Hotel and convention meeting rooms are a prime example — the use of a good automixer can eliminate the need for a live operator if multiple mics are required for a conference. Some providers of simultaneous interpretation services have found that automixing can help when large numbers of delegates or participants are intervening in a discussion (see the sidebar “Using Automixers in Simultaneous Translation Systems”). The resulting confusion can be difficult and frustrating for the technician trying to determine who needs a live mic. Large houses of worship can benefit from being able to run multiple live mics when a typical mix console installation and operator are inconvenient or impractical. For most applications, the main features you should look for are these:

  • ability to discriminate between speech and ambient noise;
  • quick, seamless microphone activation;
  • automatically adaptive threshold;
  • automatic level adjustment based on the number of active microphones;
  • ability to override automatic functions on individual channels as required; and
  • ability to expand with the needs of the facility.


Most product offerings in this category understandably come from the well-known microphone manufacturers — automatic microphone mixers are a natural line extension for these companies. They are available in standalone analog units or totally software-based mixers. For smaller jobs in which a PC is unavailable or unnecessary, a standalone device can make sense. However, as PCs find application in just about every aspect of sound system design, it can be a trivial matter to implement automatic mic mixing as another function of the computer in the installation. There are at least 20 equipment manufacturers that offer automatic microphone mixers, but for now here’s a look at some of the major suppliers and their products.

Shure is one of the largest manufacturers and vendors of automatic mic mixing devices. The company has two categories of automatic mic mixers: the Automatic Microphone System, which includes specially matched microphones, and the SCM410, SCM810, and FP410 standalone automatic mixers. All three standalone models use a patented circuit design called IntelliMix, which combines three key features: noise-adaptive threshold activates a microphone for speech but not for constant room noise, Maxbus limits the number of activated microphones to one per talker, and last microphone lock-on keeps the most recently activated microphone open until a newly activated microphone takes its place.

The FP410 is a 4-input mixer ideal for portable use with both battery and AC operation. For large permanently installed systems, Shure offers the SCM810. This 8-channel mixer features adjustable EQ per channel, 48V phantom power, active balanced mic- or line-level inputs, line-level outputs, highly RF-resistant chassis and circuitry, complete logic control of microphone activation, and linking capacity for as many as 400 microphones. The logic control feature is interesting, because it can be coupled to many video switching decks (with suitable interfacing) to provide automatic switching of multiple cameras to match the active mics. The logic circuitry can also be used to fire LEDs for channel/mic indication of use. Smaller installations can benefit from the same features minus the logic control with the 4-input SCM410.

Audio-Technica is another microphone manufacturer that offers auto mic mixers. From its line of audio devices comes the model AT-MX341a SmartMixer. This automixer offers its own proprietary digital algorithms in the mixing, adaptive threshold, and logic control functions. A number of different setups can be achieved, with different mixes of priority override. Phantom power is 12V, and multiple units can be easily linked. The packaging is nice and light — half-rack size — but is available only in AC powering through an outboard plug-in transformer. Audio-Technica’s model AT-MX351 goes up to full-rack size, internalizes the power supply, and offers an auxiliary input, variable level monitoring, and switchable level indicator functions. One feature that is especially noteworthy of this and most of Audio-Technica products is the instruction brochure. Even seasoned pros will appreciate the installation tips and suggestions offered for using the mixer.

Peavey Electronics’ Architectural Acoustics brand has two automixers, the 8-channel Automix 2 and a smaller 4-input unit, the Automix 4. The Automix 2 automatic mixer has eight transformer balanced mic/line inputs. Each channel provides 48V phantom power (mic inputs), a low-cut filter, activity/clipping LED, an aux send control, and a choice between manual and automatic operation. Each channel also provides a defeatable insert point and a 5V TTL status output and can be muted individually. Multiple channels can be muted simultaneously through an assignable mute bus. In addition, channels 1 and 2 provide an adjustable priority control. The master section provides a gain trim control, three 1/9-octave sweepable notch filters, a downward expander, transformer balanced outputs (main and aux), and remote volume connections.

The Automix 2 has been designed to easily link multiple units together to form a single mixer with many more inputs (16, 24, 32, and so on). The Automix 2 is supplied with a see-through Plexiglas security panel to prevent changes to the installer’s settings. Gain-sharing automatic mic mixing is also a feature on Peavey MediaMatrix new version 3.4.4 MWare.

Lectrosonics is well known to broadcasters and video field producers as a manufacturer of top-quality wireless mic systems. It also makes a wide variety of audio-processing equipment and offers some distinct circuitry in automatic mic mixing. Under the trade name of Lectro, the company provides the AM8, a standard automixer with eight inputs and one output bus. For more complex audio needs, Lectro has mixers that offer as many as 12 separate output buses. Boasting quite a few specialized features, the model 16/12 offers a fully programmable 16-by-12 matrix mixer, the ideal tool for complex multimic and multiroom installations. Additionally, this model has built-in four-room combining capability, expandable mixer linking for more channels, proprietary software for Windows operating systems, and AMX and Crestron compatibility. The output matrix is ideal for teleconferencing applications when coupled with a digital phone hybrid.

Rane’s analog automixer offering is a bit different, in that the mixer is not standalone and must be installed with a base unit. The ECM 82e 8-channel conference system and ECB 62e base unit constitute the system. As many as six mixers are supported by the base unit, which stores the mixing and control intelligence. The programmed operating parameters are uploaded to the mixers when the system is powered up. Computer control is available but only under Windows 95 or 98SE. In the digital domain, DragNet, Rane’s Windows-based software application, directly controls the RPM series of DSP audio microprocessors (the RPM 22, 44, and 88). Offering two, four, and eight inputs (and as many as eight outputs on the RPM 88) and infinite daisy-chain ability, these units include a variety of automixer functions.

Polycom, the company famous for voice and videoconferencing systems, offers the all-digital Vortex EF2280. This is an automixer with output matrix, echo, and noise cancellation. The Vortex has a host of new features, including 12 inputs and outputs; acoustic echo and ambient noise cancellation on the mic/line inputs; a 25-by-18 matrix with adjustable cross-point gains; voice-activated “neural network” processing, a design feature that activates only on valid voice signals; and 5-band parametric EQ on each input and output. Twenty-four-volt phantom power is available on the mic inputs. Such a mixing device is intended for specialized custom installs where the contractor is prepared to do a fair amount of system programming.

Ivie, a manufacturer known for audio test and analysis gear, also offers a series of automixers. The 1026 integrated automixer/digital processor combines a 10-input automatic matrix mixer with a full-function, digital signal processor to provide mixing and signal-processing functions in one rackspace package. The 1026 is easily configurable and intuitive to use. Controlled through Ethernet, it comes with free software with a user-friendly graphic user interface. The usual functions are provided, such as configurable automatic level control on each input and multiple signal-processing functions, including delay, crossovers, a full range of parametric functions, matrixing and routing, comp/limiting, and presets. The 880 series features many of the same functions but with fewer inputs.

Biamp offers several analog models as well as fully software-based automixers as part of its Audia systems. The autoOne, with eight mic/line inputs, is affordable, versatile, and easy to use. Features include NOM attenuation, Adaptive Threshold Sensing (ATS), speech-frequency filtering, manual priority override, selectable channel-off attenuation, and selectable last mic hold. Biamp also has VRAM, a 10-input/2-output programmable automatic mixer. It is completely tamperproof and provides no external controls. All mixer parameters are under microprocessor control and are easily programmed through Windows software. A second model, VRAMeq, includes 3-band channel equalization with variable midfrequency.

The Audia system provides a software-based approach that includes combiners. The combiners function has a linking capability, allowing you to combine the outputs of multiple automixers in room-combining applications. The outputs of two or more automixers are routed to the combiner inputs. Within the combiner, these inputs can be put into groups where audio as well as control data (ATS, NOM) is shared between the automixers.

Several other familiar manufacturers also offer auto mic mixers, with different combinations of features sets and control capabilities. In automixing, as in everything else, you must pay for what you get. With their superb speech detection, easy setup, and wide range of selectable features, today’s automatic mixers bring sound quality and system flexibility to new levels in A/V installations.

Allan Soifer lurks and works as a consultant in eastern Ontario, Canada. A 33-year-plus veteran of audio and broadcasting, he can be reached at

Using Automixers in Simultaneous Translation Systems

One of the more interesting uses of automatic mic mixers is found in simultaneous interpretation and conference translation. Typically, digital conference mics are installed for these meetings, the type in which each delegate must press a button on his or her mic to activate it or signal the chairperson of his or her desire to speak. Many brilliant politicians and otherwise competent people are totally dumbfounded when confronted by a little red button that just requires one press to turn on and another to turn off. The operating technician must be constantly vigilant to observe the proceedings in order to perform the mic on-off functions (through his or her control panel) for those who cannot or will not press the button.

Inevitably, glitches will occur, because the tech may not be able to see every delegate; some delegates barely move their lips when speaking, making it even harder to determine who is the talker; and in a heated debate, delegates may rapidly intervene with each other while forgetting to turn on their mics. The spoken words come so rapidly that the interpretation team members cannot hope to follow the action unless they can clearly hear what is said. Interpreters are highly nervous individuals, constantly fretting over the quality of the sound (audio pickup) in a given room, because it can affect their translations. Given that these professionals are often the ones who book and pay for the audio and technical support, it is wise to be in the customer service mode at all times. In short, this is a perfect application for the use of automatic microphone mixers.

I personally had the pleasure of relieving the tensions and hair pulling by the interpreters at a government workshop, which required French interpretation from English with 36 delegates and highly specialized technical subject matter. Rather than the usual conference mic setup, I used good concert-quality mics on low-rise table stands with neoprene padded bottoms for reduction of finger-drumming noises and so on. A series of linked automixers was set up, giving the chairperson’s mic override priority — when the chairperson spoke, all other mics were muted.

The main output was sent directly to the interpretation console, which fed both the interpreters’ headsets and the IR transmission system in the room, supplying signal to each delegate’s interpretation receiver. The suboutput from the mixers fed a room P.A. system, which was well tuned for feedback suppression and not pushed to deliver high gain. The lack of feedback and background noise was immediately noted and appreciated by the interpretation team, as was the clarity of speech of each delegate. The cassette recordings from this meeting told the same story. The proof in the pudding was the repeat bookings from this set of clients, demanding that I use the same equipment because it “sounded so good.”

For More Information

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Crest Audio
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Dan Dugan Sound Design
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Ivie Technologies
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Protech Audio Corp.
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Rane Corp.
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