Applied Technology: Automixing with Sound Devices 688 Mixer/Recorder

In optimal production environments, electronically mixing microphones is a challenging job. Moreover, as video productions require even more microphones simultaneously, often in unscripted reality programming, the problem of producing a clean mix becomes even tougher.

Unscripted dialog, multiple characters, and wide-and-tight multi-camera coverage often require the use of wireless lavalier mics and multi-track recording. In these situations, even experienced audio pros struggle to achieve desired results. Good, clean, intelligible speech in a mix does not happen by accident. The task of cueing mics in and out can inhibit a location recordist from paying enough attention to the artistic balance.

A choice in mix algorithms

One established tool of the audio trade, the automatic mixer, is now available for film and TV productions.

The Sound Devices 688 12-input, 16-track field mixer/recorder is an extensible mixing and recording system which incorporates both the company’s proprietary automatic mixing technology, MixAssist, and the Dugan Speech System automixing algorithm. Recordists now have a menu-selectable choice in mixing tools.

The Sound Devices 688 mixer/recorder provides dual automixing algorithms; MixAssist and Dugan automixing. Either are available via menu selection.

The Sound Devices 688 mixer/recorder provides dual automixing algorithms; MixAssist and Dugan automixing. Either are available via menu selection.

Under the hood

The first automatic mixers were just glorified audio gates. If the incoming sound was higher than a fixed threshold, the microphone turned on. If the audio dropped below the threshold, the microphone turned off. While useful, it was not a sophisticated solution.

Dan Dugan invented the first real automatic mixer in 1976, for which he received a patent. The core concept of Dugan’s method remains unchanged since first implemented. It is simple to operate as there are no thresholds or other adjustments to set.

Dugan’s system operates on an elegant principle. Each input channel is attenuated by an amount (in dB) equal to the difference (in dB) between that channel’s level and the sum of all channel levels. The gain of all channels is adjusted immediately and continuously based on what each channel is receiving. The key aspect of this mathematical construct is that the total gain through the system always stays at 0dB.

Steve Julstrom invented another well-regarded and established method for automatic mixing while he worked for Shure Brothers. This is the basis of the Sound Devices’ MixAssist algorithm. MixAssist conceptually operates based on a sophisticated gate. The gate turns microphones on or off with very smooth transitions, yet provides finite off-attenuation. Thus, when a microphone is turned off, it is attenuated by a certain amount, typically 15dB, resulting in a more transparent sound.

Each microphone has an automatic threshold, or Noise-adaptive Threshold (NAT), which is continuously calculated. This per-channel threshold has a slow-attack and fast decay, essentially making the NAT a good ‘hole detector’ of the audio. When an incoming microphone’s signal is instantaneously above the threshold, it can be turned on. For each doubling of the number of open microphones, the gain is attenuated by 3dB. Similar to the Dugan concept, the process maintains the total gain through the system at 0dB.

How does it sound?

Automatic mixer systems, such as MixAssist and the Dugan Speech System, attenuate any microphones not actively being used. This reduces the background noise in a mix, making the desired speech cleaner and more intelligible. Both solutions hold system gain between microphones to 0dB, and each offers near-instantaneous (~2mS) turning on of microphones in use.

The results of the Dugan automixing is described by some to sound a bit smoother in operation as all of the microphone gains are continuously and smoothly changing. The mics are neither all the way on or all the way off. Each microphone is continuously mixed in various proportions.

MixAssist offers a lower noise floor, as microphones that are picking up constant noise will be attenuated at all times. MixAssist, due to its Maxbus, will only turn on one microphone even if several mics detect the same source. 

Author: Matt Anderson, President, Sound Devices

Author: Matt Anderson, President, Sound Devices

Both solutions in one package

The Sound Devices 688 field mixer/recorder offers both MixAssist and Dugan automixing in one mixer/recorder and either solution can be used according to the situation. The subtle differences between the two algorithms, combined with any particular environment, may steer a user to select one mix method over another.

Recordists will quickly become familiar with both solutions and be able to determine which automixing tool is best for any situation. The combination of Dugan automixing along with MixAssist gives the professional sound mixer the ultimate choice in automixing tools.

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