Dolby Signal Processing

Dolby Stereo is the name given to the four-channel surround sound developed by Dolby Laboratories and introduced into movie theaters in the 70's. It employed a matrix encoding scheme called Dolby Surround which recorded four channels of information on two channels. The two channels are decoded into L, R, Center and Surround upon playback. The center channel is recorded identically on the left and right channels.

Riggs, Michael, Digital Surround Comes Home, Stereo Review, May 1995 p 62

Ranada, David, "Inside Dolby Digital", Stereo Review 61, Oct 96 p81-84.

Digital Surround Sound

Digital surround refers to surround sound systems which employ discrete digital recordings of five channels of sound information. Digital surround sound has been introduced into movie theaters in a form called Dolby Stereo Digital. At the heart of Dolby Stereo Digital is and encoding scheme called AC-3. The AC-3 based systems are now often referred to as just "Dolby digital" in the consumer market. It's hard to tell which is changing faster: the technology or the terminology.

Dolby Stereo Digital uses a digital data stream running at 320 kilobits per second. The HDTV and laserdisc version of Dolby Surround AC-3 Digital runs at 384 kilobits per second and dynamically allocates the bits to the channel with the most demanding signal. Use is made of perceptual encoding to decide which parts of the audio signal would not be heard and therefore can be eliminated. The system provides a slight delay in the center channel sound to achieve a more realistic experience of the sounds arriving at the listeners location from the other speakers.

Riggs, Michael, Digital Surround Comes Home, Stereo Review, May 1995 p 62

Ranada, David, "Inside Dolby Digital", Stereo Review 61, Oct 96 p81-84.

Perceptual Encoding for Digital Sound

Perceptual encoding refers to systems which dynamically determine the number of bits of data given to a given channel of audio information based on judgements of its importance to the sound perceived by the listener. Sounds which are below the audibility threshold for the human ear should not waste bits which could be devoted to a higher fidelity reproduction of an important sound. Also, certain sounds are masked by others, and if it is judged that a certain sound would be masked anyway, why not give those bits to another sound which would be heard?

Perceptual encoders divide the sound into frequency bands and determine which bands contain essential audio information, based on rules for audibility and masking. It can also deal with the signal just before and just after a given point in the recording since the temporal environment also affects human perception of sound.

Perceptual encoding is used in the AC-3 system of Digital Surround sound.

Riggs, Michael, Digital Surround Comes Home, Stereo Review, May 1995 p 62

Ranada, David, "Inside Dolby Digital", Stereo Review 61, Oct 96 p81-84.