Surround Sound Formats

Dolby Pro Logic
Dolby Pro Logic, like Dolby Stereo in the movie theater, converts two tracks of sound into four channels. In the days of Laserdisc and Stereo VHS it literally was the only thing going. While dated and overshadowed by newer technology, Dolby Pro Logic remains vital: It is used whenever the medium is limited to two channel and may even be used when technology permits more, in order to save space or preserve the artistry of an original audio work.

Dolby Digital

Dolby Digital (also known as the AC-3 Codec) was first made available to consumers on special Laserdiscs. It came into the mainstream when it was chosen as the multi-channel codec for the DVD Video format. It is an extremely flexible codec which can carry from 1 to 6 completely separate and independent channels of digital sound. The full six-channel configuration consists of the front left, center, front right, rear left and right, and the LFE channel (or Low Frequency Effects channel). The LFE channel has a restricted frequency range, while all the other channels are full range (8 Hz - 20 kHz), so the system is often called "5.1".

While two channel CD Audio has about a 1.5 mbps bitstream, Dolby Digital uses as little as 384 kbps in total for all six channels. Obviously, this requires considerable compression of the audio data (about 10:1). Dolby utilizes "Perceptual Encoding" for this process, putting only those sounds that the brain would pay attention to in the sound tracks. This necessitates very sophisticated algorithms (a series of mathematical formulas) to determine what to include and what to eliminate. In practice, the sound quality is spectacular. While initially a very expensive system for consumers, every surround sound decoder on the market now has Dolby Digital as part of the feature set.

DTS

Digital Theater Systems, or DTS, is another 5.1 digital surround sound format, first developed for movie theaters and then later appearing in homes on Laserdiscs. It is now technically an optional format for DVD. The system utilizes a completely different lossy compression codec from AC-3, and only manages a 5:1 compression. The term "Lossy" means that some of the data are lost during compression, and cannot be restored when the bitstream is decoded. This is contrasted to the term "Lossless", which means that the compression codec (the combination of algorithms used to compress and uncompress the digital data) does not throw away any data, and the original data are restored when they are decoded.

There are some who feel that the DTS codec results in better sound because it uses less compression, but because the two codecs are different, the bitrates cannot be compared at face value (i.e., it could simply be that DTS is less efficient than AC-3 as opposed to actually sounding better). We at Secrets have never been able to definitively show that DTS sounds any better than Dolby Digital. Whereas DTS used to come at a premium price, it is now found on virtually every surround sound decoder on the market.