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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 mincompression 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 will likely lead to ATRAC format becoming obsolete.
Two stacked QMF split the signal into 3 parts:
Full stereo (i.e., independent channel) encoding.
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.
Double play time, mono (one channel).
Three stacked QMF split the signal into 4 parts:
Full stereo encoding. Allows for 161:58 min of audio on a standard 80 minute MD blank.
Quality similar to that of 128 kbit/s MP3; verified to be somewhat poor in comparison to Ogg Vorbis, AAC, and LAME VBR MP3 @128 kbit/s in double-blind ABX tests (www.rjamorim.com/test).
High-frequency lowpass around 17.5 kHz.
132 kbit/s data rate. (Not simply 292 kbit/s (SP)/2=146 kbit/s; 14 kbit/s is used to pad frame to prevent spurious digital noise from occurring if disc is played back on older, SP-mode only decks or players. Sony designed the older players to ignore non-standard frames, but for some reason, they attempt to play them back anyway. Thus, 14 kbit/s is lost due to a technical oversight).
66 kbit/s data rate (half that of LP2). Allows 324 min to be recorded on an 80 minminute blank.
Joint stereo encoding. Joint stereo (M/S) model poorly implemented compared to that of LAME or other modern codecs.
Low quality, similar to MP3 at 80 to 96 kbit/s or an old FeO2 Type-I cassette tape. High-frequency lowpass around 13.5 kHz.
Thought to be a hybrid subband/MDCT codec (not much info on this.) Uses huge transform window of 4096 samples, 4x bigger than ATRAC3. Signal is split into 16 sub-bands before MDCT and bit allocation.
New codec used in Hi-MD players (e.g., "Hi-LP and Hi-SP"), memory-stick players, and ATRAC CD players
Data rates are 48 kbit/s, 64 kbit/s, and 256 kbit/s.
Not compatible with standard MiniDisc. (Works on newer Hi-MD).
Quality is not all that great at 48 or 64 kbit/s; Ogg Vorbis and HE-AAC are much better at these bitrates.
A biased test by Sony, in which the sample and encoder were provided BY Sony, "found" ATRAC3plus at 64 kbit/s to be equal in subjective sound quality to MP3 at 128 kbit/s. However, this is very unlikely using a good encoder like LAME 3.90.3 or higher, and a hard sample.
see also: Lossy data compression, MP3
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