ATRAC (Adaptive TRansform Acoustic Coding) is an audio compression algorithm used to store information on Minidiscs and other Sony_branded audio players. First developed by Sony in 1991; the higher compression flavors of ATRAC3 and ATRAC3plus followed in 2000 and 2003, respectively. It uses quadrature mirror filters and modified discrete cosine transform to represent encoded audio.

On 23 September 2004, Sony has announced that its music hardware will support the MP3 format along with ATRAC in the future. This may lead to ATRAC format becoming obsolete.

ATRAC SP

Two stacked QMF split the signal into 3 parts:

• 0 to 5.5125 kHz
• 5.5125to 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). This is most possible when using the latest algorithm, Type_S, or Type_R (Type_S only improves LP modes). Some signals will "trip" the codec and cause artifacts, though these are not usually severe enough to be blatantly obvious.

High_frequency lowpass depends on the complexity of the material; some encodings have content clear up to 22.05 kHz.

ATRAC SP Mono

Identical to ATRAC SP, but with only one channel, doubling recording time.

ATRAC3 LP2 Mode

This uses a 132 kbps data rate. The quality is similar to that of 128 kbps MP3, although it came last in a double blind test (http://www.rjamorim.com/test) against Ogg Vorbis, AAC, and LAME VBR MP3.

Three stacked QMF split the signal into 4 parts:

• 0 to 2.75625 kHz
• 2.75625 to 5.5125 kHz
• 5.5125 to 11.025 kHz
• 11.025 to 22.05 kHz

It uses a high_frequency lowpass filter around 17.5 kHz, and like SP, uses full stereo encoding.

Allows for 161:58 min of audio on a standard 80 minute MD blank. This only doubles recording time, despite requiring less than half as much space. 14 kbps of data is added to pad the frames so that they play as silence in older, SP_mode only decks or players.

ATRAC3 LP4 Mode

This reduces the data rate to 66 kbps (half that of LP2), partly by using joint stereo coding and a lowpass filter around 13.5 kHz.. It allows 324 minutes to be recorded on an 80 minute MiniDisc, with the same padding required as LP2.

It has reasonable quality, similar to MP3 at 80 to 96 kbit/s or analog cassette tape.

ATRAC3plus

This codec is used in Hi_MD players (e.g., "Hi_LP and Hi_SP"), memory_stick players, and ATRAC CD players. It is thought to be a hybrid subband/MDCT codec, though not much information has been released. It uses a relatively huge transform window of 4096 samples, 4x bigger than that of ATRAC3. The signal is split into 16 sub_bands before MDCT and bit allocation.

Data rates are 48 kbps, 64 kbps, and 256 kbps.

Minidiscs recorded in this format are not compatible with older players.

The audio quality at 48 or 64 kbit/s is not well regarded in comparison to Ogg Vorbis or HE_AAC

Tests conducted by Sony found ATRAC3plus at 64 kbps to be equal in subjective sound quality to MP3 at 128 kbps. However, the testing may have been biased in the choice of test material.

Contrary to what Sony states, ATRAC files can be played back on IBM compatible PC using the RealAudio software or the Liquid Audio plug_in module for Winamp.

see also: Lossy data compression, MP3