December 2009



Audyssey DSX Breaks The Surround Barrier
Adding surround channels where they have the most impact
creates a more enveloping and immersive soundstage.

Matching Human Perception
Realistic reproduction of sound relies on the ability of audio systems to match human hearing performance. The three key requirements are: (1) frequency response, (2) dynamic range and (3) accurate spatial rendering. With 96 kHz sampling rate and 24 bit word length, the requirements for frequency response and dynamic range have already been surpassed by today’s digital audio systems. Research has shown that accurate spatial sound rendering requires more information than what 5.1 surround sound systems provide. Audyssey DSX delivers this additional information from the required directions to overcome the limitations faced by 5.1 surround sound systems.

Limitations Of 5.1 Surround Sound
5.1 surround sound was a significant improvement over 2 channel stereo. Imaging was improved by adding a center to the left and right front channels to anchor the center of the soundstage. Envelopment was improved by adding two surround channels that envelop the listeners with ambient sound.

However, 5.1 surround sound systems are limited by some tradeoffs. The first has to do with rendering the front soundstage. Research into imaging showed that spreading the front channels wider than the standard ±30° results in a greater difference at the two ears and produces more envelopment. In concert hall acoustics this is called Auditory Source Width and, increasing it, is a desirable feature. But increasing the spread beyond ±30° for the front speakers was found to cause many imaging problems. This fundamental tradeoff in 5.1 surround sound systems between better frontal imaging and improved envelopment is a limitation.

The second tradeoff is related to the ITU recommended placement of the two surround channels at ±110°. Surrounds at ±90° were found to produce the most envelopment compared to other angles, while surrounds at ±135° produce better rear phantom imaging capability. Thus ±110° was selected as the best tradeoff between envelopment and imaging. This is another limitation of 5.1 surround systems because one must choose between better rear imaging or better envelopment.

More Channels Are Needed
Research in human hearing shows that we can hear many more directions than what current systems provide. We use the direct sound to localize the direction of sources and the reflected sound to perceive the size of the soundstage.

Experiments have shown that human localization is better in front than to the sides or behind. This means that for front-weighted content such as movies and most music, good engineering dictates that we employ more channels in the front hemisphere than the back. Imaging is also better horizontally than vertically and so good engineering also dictates that channels must first be added in the same plane as our ears before going to higher elevations.

Perception is not the only factor. The physics of room acoustics for music have been well studied, and their correlation with subjective impression increasingly understood over the last 30 years. This research has shown that we have strong built-in preferences for the direction, frequency response, and time of arrival of reflected sound. Additional channels and surround sound processing are needed to properly render these components.

Where Should The New Channels Be Placed?
One key finding from the research is that first side wall reflections play a great role in determining subjective impression. The most important direction of reflected sound was found to be ±60° relative to the front. Audyssey DSX provides a pair of Wide channels (LW and RW) at ±60° with appropriate frequency response and perceptual processing to match these requirements of human hearing. These Wide channels are much more critical in the presentation of a realistic soundstage than the Back Surround channels found in traditional 7.1 systems. Adding surround channels behind the listener has a very small impact compared to the increase in envelopment and soundstage width that the Wide channels provide.

The next most important acoustical and perceptual cues come from reflections above the front stage. Audyssey DSX provides a pair of Height channels (LH and RH) that should be ideally positioned at a 45° elevation angle.

Improved Surround Envelopment
In addition to creating new Wide and Height channels, Audyssey DSX also provides Surround Envelopment Processing to enhance the envelopment of the Surround and Back Surround channels (if present). Audyssey DSX Surround Envelopment Processing processes the standard surround signals in the time and frequency domains to improve the perceived sense of envelopment and blending with the other speakers in the surround system.

Audyssey Dynamic Surround Expansion is a scalable system that can adapt to the practical needs of the user. The first level of surround stage enhancement comes from adding the Wide channels. The second level comes from adding the Height channels. If it is practical to have both Wides and Heights then the surround performance will be further enhanced. Audyssey DSX automatically reconfigures its processing to optimize surround rendering over any number of available speakers beyond 5.1.

Audyssey
350 S. Figueroa St., Suite 233
Los Angeles, California 90071

Voice: audyssey.com