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.

Other MiniDisc manufacturers such as Sharp and Panasonic also implemented their own versions of the ATRAC codec. The hybrid lossless compression scheme was added to the ATRAC family in 2006.

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.

Bitrate quality compared to other formats

Sony's official claim is that ATRAC3plus at 64 kbit/s rate provides a quality comparable to MP3 at 128 kbit/s, which would place this codec in the same league as HE-AAC, mp3PRO and Windows Media Audio (with similar claims from Microsoft).

Performance

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. This is in contrast to other codecs developed on computers with no regard for the constraints of portable hardware.

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

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.

Two stacked quadrature mirror filters split the signal into 3 parts:

• 0 to 5.5125 kHz
• 5.5125 to 11.025 kHz
• 11.025 to 22.05 kHz

Full stereo (i.e., independent channel) encoding with a data rate is 292 kbit/s.

Quality is generally transparent for many people (meaning that it is not possible to tell an ATRAC encoding from the source) Fact: date=February 2007. This is most possible when using the latest algorithm, Type-S, or Type-R (Type-S only improves LP modes). Like most other audio compression codecs, some signals will "trip" the codec and cause artifacts, though these are not usually severe enough to be obvious.