Adaptive Transform Acoustic Coding

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Adaptive Transform Acoustic Coding (ATRAC) is a family of proprietary audio compression algorithms developed by Sony. MiniDisc was the first commercial product to incorporate ATRAC in 1992. ATRAC allowed a relatively small disc like MiniDisc to have the same running time as CD while storing audio information with minimal loss in perceptible quality. Today ATRAC is used in many Sony-branded audio players. Improvements to the codec in the form of ATRAC3, ATRAC3plus and ATRAC Advanced Lossless followed in 1999, 2002 and 2006 respectively.[1]

Other MiniDisc manufacturers such as Sharp and Panasonic also implemented their own versions of the ATRAC codec.


General bitrate quality

ATRAC's original 292 kbit/s bitrate was designed to be close to CD quality acoustically. This is the bitrate used on original MiniDiscs. Years later ATRAC was improved and is generally considered better than earlier versions at similar bitrates. For purposes of comparison, CDs are encoded at 1411.2 kbit/s, and lossless encoders can encode most CDs below 1000 kbit/s, with significant bitrate reduction for easier-to-encode content such as voice.


According to ATRAC engineers, ATRAC algorithms were developed in close cooperation with LSI development engineers within Sony in order to deliver on a tangible product that could encode at high speeds and with minimal power consumption.[2] This is in contrast to other codecs developed on computers with no regard for the constraints of portable hardware. This is reflected in the design of the ATRAC codecs, which tend to emphasize processing smaller numbers of samples at a time to save memory at the cost compression efficiency and additional multiplies. These trade-offs are entirely logical on DSP systems, where memory is often at a premium compared to multiplier performance.

Sony Walkmans offer better battery life when playing ATRAC files as compared to MP3 files. However, Sony only pushed ATRAC compatibility in Sony Ericsson Walkman series phones in the Japanese market, it is not supported in GSM/UMTS market phones. Sony's Xplod series of car audio CD players support ATRAC CDs. Minidiscs with ATRAC format songs have, in the past, been supported on Eclipse brand car stereos.


ATRAC1 was first used in Sony's own theater format SDDS system in the 1990s, and in this context is a direct competitor to Dolby Digital (AC3) and DTS. SDDS uses ATRAC1 with 8 channel encoding, and with a total encoding rate over all the channels of 1168 kbit/s.

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