(dolby-digital, dolby-ac-3, ac-3) The approved 5.1 channel (surround-sound) audio standard for ATSC digital television, using approximately 13:1 compression Six discrete audio channels are used: Left, Center, Right, Left Rear (or side) Surround, Right Rear (or side) Surround, and a subwoofer (considered the ".1" as it is limited in bandwidth). The bit rate can range from 56 kbps to 640 kbps, typically 64 kbps mono, 192 kbps two-channel, 320 kbps 35mm Cinema 5.1, 384 kbps Laserdisc/DVD 5.1 and ATSC, 448 kbps 5.1. When moving from analog recording to a digital recording medium, the digital audio coding used yields an amount of data often too immense to store or transmit economically, especially when multiple channels are required. As a result, new forms of digital audio coding, often known as "perceptual coding", have been developed to allow the use of lower data rates with a minimum of perceived degradation of sound quality. Dolby's third generation audio coding algorithm (originally called AC-3) is such a coder. This coder has been designed to take maximum advantage of human auditory masking in that it divides the audio spectrum of each channel into narrow frequency bands of different sizes, optimized with respect to the frequency selectivity of human hearing. This makes it possible to sharply filter coding noise so that it is forced to stay very close in frequency to the frequency components of the audio signal being coded. By reducing or eliminating coding noise wherever there are no audio signals to mask it, the sound quality of the original signal can be subjectively preserved. In this key respect, a coding system like Dolby Digital is essentially a form of very selective and powerful noise reduction.