Dolby Digital™

The 5.1-channel digital format created by Dolby Laboratories, first used in 1992 for “Batman Returns.” In current usage, the term applies to both the Dolby 35mm theatrical format, which contains the data printed optically between the sprocket holes, and for video formats, such as DVD, laserdisc, and DTVAC-3, as Dolby Digital was first called, used RF modulation of the digital signal onto one of the analog tracks, making it possible to fit an entire movie, along with the already existing digital tracks, onto a conventional laserdisc; a demodulator was needed to recover the audio back into a digital bitstream. The Dolby Digital format is a surround-soundsplit-bandperceptual coding scheme. AC-3 was designed as a 5.1 multichannel format , using approximately 13:1 lossy compression, and is specified as the matrixing format for DVD and DTV. Also used in HDTV broadcasts, SR-D, and DSD cinema productions. Versatile, in that parameters such as bit-rate and number of channels can be tailored to particular applications, unique in that the data bits are distributed dynamically among the filter bands as needed by the particular frequency spectrum or dynamic nature of the program. Data rates vary from 32kbps for a single mono channel to as high as 640 kbps for 5.1 format. The data rate is 320kbps for film, 384kbps for laserdisc, and 384kbps or 448kbps for DVD, although the maximum throughput for the specification is 640kbps. Dolby’s current decoder can accept incoming data at 32kHz, 44.1kHz, or 48kHz sample rates, with bit depths of 16, 18, or 20 bits. The commercial competitor to the Dolby Digital format is DTS. See metadataaudio coding mode.

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