Sonic Assault (audio device)

The audio device nicknamed in the Hollywood sound community as the Sonic Assault was designed and fabricated by the late Joseph A. Brennan and Gary L.G. Simpson in the early 1990s. At the time, both Joe and Gary worked for Skywalker Sound South.

Original Concept

The idea was to build a device with greater sonic fidelity and control than Dolby Laboratories' Container. For those unfamiliar, the Container was a custom audio device, built by Dolby personnel for use exclusively on Dolby sanctioned LtRt Print-master sessions, mainly its use was reserved for loud action shows. Dolby would bring the Container for the period of the session. Like the Container, the device was to control dynamic range and maximum level, each channel would be split in to five discrete audio bands spanning the audio spectrum. Unlike the Container, each frequency band would have its own adjustable dynamics processor, instead of a limiter. After processing, the bands would be recombined to a full spectrum channel.

Original Purpose

In order to understand the purpose we must first understand the problem. The 35 mm magnetic sound tracks that were being produced, were an intermediate step in the making of a stereo analog optical sound track for a release print. When optical sound was shot, prior to the invention of the laser camera, white light was shown through a light valve. A typical light valve is a very delicate mechanical device which utilizes electrically charged metal ribbons and magnetism to modulate light, exposing high contrast black and white film. If signal level exceeded 100% modulation the ribbons could collide and cause a harsh form of distortion called in the industry "clash". This was to be avoided if possible, as exposed film could not be erased or reused and would cost both time and money.

The Challenge

Many big action filmmakers demanded loud sound tracks yet optical sound had dynamic range limitations. One technique clever sound editors and mixers used was to "fill the track" filling the audio spectrum 20 – 20 kHz with sound energy. On the gear side, a multi-band limiter was another trick. This would ensure no audio band would exceed 100% optical level and cause clash. Since all bands were independent of one another, this in effect was a dynamic equalizer also serving to fill or pack the track. So for the ultimate in loudness, in the days of optical, full audio spectrum at full level with precise level control was the challenge.

Evolution

The first few prototypes' design was based around fairly simple textbook filter concepts. The gain limiting circuits, incorporated diodes in the feedback path of garden variety op amps. Needless to say, they didn't improve the standard set by the Container. It wasn't until about the fifth prototype, when finally things started to come together for the team. Instead of filters based around basic NE5532's, Burr-Brown UAF-42 laser trimmed filter ICs were used. A filter design program also created by Burr-Brown called FilterPro became absolutely instrumental in designing filters sections and determining correct component values. High end FET based analog switches were used to route audio in around filter sections . Balanced transformerless I/O and metering circuitry was designed and implemented. Instead of a simple limiter circuit design, a Precision Monolithics SSM-2120 analog dynamics processor was used. The 2120 was the real magic in the design, it provided for incredibly flexible gain, limiting and AGC style control. It allowed for softer, far less audible, yet very precise gain control.

With the prototype finally a success, Joe spent the next two months laying out a printed circuit board (PCB) using borrowed CAD software. The circuit design barely fit on a one square foot multilayer printed circuit board. Fabrication of a run of boards this size and with this level of complexity would be expensive. This fact proved to be yet another hurdle as the company was resistant to the expenditure and sited that the business of the company was to mix sound and not to make custom audio gear. Thankfully, this heated situation was defused and the project funded when a senior staff mixer felt that the device with a few frequency center adjustments could be used as a dialogue mixing tool.

This variation became the device known as the Sonic Assault. The changes requested and implemented were nine bands instead of the original five. The frequency centers of the bands were reengineered to be more closely spaced, as the new goal was to control sibilance as well as level for dialogue tracks. To achieve this without a complete redesign of the PCB, a decision was made to lash two channels of one board together to achieve the nine filter sections, one left unused. Two boards would be needed for a two channel device so a new larger enclosure was designed and built.

For greater than fifteen years the Sonic Assault was used behind the scenes on many top tier films because of its ability to control level and smooth dialogue.

A few years ago the Sonic Assault's analog behavior was modeled digitally by McDSP and the SA-2 plugin was created.