True Stereo

Most electronic musicians would probably apply the word exclusively to 2-channel audio systems. So you might be surprised to learn that stereo has nothing

Most electronic musicians would probably apply the word “stereo” exclusively to 2-channel audio systems. So you might be surprised to learn that stereo has nothing to do with the number two per se; it is derived from “stereos,” the Greek word for “solid.” The word stereo was applied to 2-channel sound systems when they first became available because those systems rendered a solid sonic image. With 2 channels, the placement of individual audio elements (instruments, voices, and so on) is much more specific than monaural systems. Similarly, 5.1-channel surround-sound systems (with one low-frequency subwoofer and five full-range speakers placed in an array around the listener) are far more “stereo” than 2-channel systems.

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FIG. 1: Each IOSONO panel has 8 tweeters, 8 midrange drivers, and 16 power amps. Designed primarily for commercial cinema and other public-venue applications, IOSONO (pronounced EE-oh-so-no) places literally hundreds of individually addressable speaker drivers around a room, forming a continuous line that completely encircles the audience. The drivers include 1-inch dome tweeters and 5.5-inch midrange cones mounted in rectangular panels. Each panel holds 8 pairs of drivers and 16 amplifiers to power them (see Fig. 1). Several powered subwoofers are also placed around the room to handle the low frequencies.

The IOSONO system utilizes Wave Field Synthesis (WFS) technology. WFS is based on the work of 17th-century Dutch mathematician and astronomer Christiaan Huygens, who proposed that the wave field of a single source can be accurately emulated by many closely spaced sources located on the perimeter of the original wave field. Thus, any virtual sound source can be realistically reproduced by an array of actual sound sources, such as the drivers in the IOSONO panels.

Sounds reproduced by IOSONO can come from any playback system, including a DAW. Each input is encoded with localization coordinates for that source and other metadata. Information about the physical configuration of the speaker array and the room in which it is installed is also encoded.

WFS algorithms render signals for the speaker array in real time to simulate the sound of the original sources at the designated coordinates. That involves more than simple amplitude panning; complex changes in a signal's phase, delay, and spectrum are also important components of the process. The software lets a mixing engineer control the position and movement of each sound source graphically on the screen. Using multiple computers, the current system has enough horsepower to render as many as 32 separate sound sources in this manner, and the signals are transmitted from the computers via MADI (Multichannel Audio Digital Interface), which is converted to ADAT Optical format in each panel.

I recently attended a demonstration of the IOSONO system at Todd-AO in Los Angeles. That system had 38 panels (304 2-way speakers) and 8 subwoofers. The panels were fed by 8 rendering PCs, each equipped with a Pentium 4/2.8 GHz CPU and 1 GB of RAM. The demo material included specially produced clips that had many sound sources moving around and a single-source “whispering ghost” that attendees could move around the room with an electronic pen and tablet.

The effect was quite startling and much more natural sounding than even 5.1 surround-sound systems, in which the sweet spot is often relatively small. In this demo, the sweet spot essentially encompassed the entire room, and the sonic image remained completely stable no matter how I turned my head. Even the apparent distance from each source was clearly evident. It was one of the best holographic sound demos I've ever heard, expanding the sound field well beyond the physical boundaries of the room — the true epitome of stereo audio reproduction.