Polk Audio: SDA™ Surround technology
November 17, 2007
SDA™ Surround technology relies on a mixture of acoustical engineering, psychoacoustic principles and a little magic. The original SDA technology, developed in the early 1980′s, was a two-channel technology intended to overcome a fundamental problem of stereo reproduction. SDA™ Surround utilizes the same basic principles but expands the technology to allow for reproduction of surround sound without the need for rear speakers.
Let’s understand the basic SDA Technology, to get a clear understanding of :
What problem does it solve?
The problem targeted by the original SDA technology is a phenomenon known as Interaural Crosstalk (or IAC). Interaural Crosstalk is a fundamental problem not only for stereo reproduction but for any system with more than one speaker. It is because of IAC that we hear the positions of the speakers in a stereo system. It is also Interaural Crosstalk that restricts the sound we hear to the area between the speakers.
Fig. 1. shows the "listening in Live concerts" and Fig. 2. shows the IAC problem:
How it solves?
SDA™ used a system of extra drive units placed at the same distance apart as the spacebetween your ears (also known as the Interaural Distance) (shown in Fig. 3) as the basis for an acoustic system of Interaural Crosstalk Cancellation. The additional “SDA” drivers received an inverted version of the stereo signal from the opposite channel. The geometry and spacing of the drive units insures that this inverted Crosstalk Cancellation Signal arrives at the ear at the same time as the unwanted Interaural Crosstalk and acoustically cancels it. The geometry also insures that proper cancellation will occur regardless of how far apart the two speakers are or how far away the listener sits. Using SDA Technology, the Crosstalk Cancellation Signal was limited to a range of frequencies with longer wavelengths which are psychoacoustically important for image location. Hence it sufficiently provides reasonably effective crosstalk cancellation over a considerable range of seating positions left to right as well as front to back.Fig. 4. shows a "listening environment" with SDA Technology in place.
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