ATRAC

ATRAC (Adaptive TRansform Acoustic Coding) is a proprietary audio compression algorithm used to store information on Minidiscs and other Sony-branded audio players. It was 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 (QMF) and modified discrete cosine transform to represent encoded audio.

Choice of a bitrate with which to record

The latest version, ATRAC3plus, gives FM quality at 64 Kbit/s, and CD quality at 256 Kbit/s. Surprisingly, ATRAC3plus does not provide a recording rate at around 128 Kbit/s. This is provided, instead, by the previous generation of ATRAC3 at 132 Kbit/s.

This leaves the user in something of a quandry as to which bitrate to use for near CD quality - the 64 Kbit/s rate does not provide the quality of a mp3 at 128 Kbit/s, the 256 Kbit/s bit rate uses a lot of space, and the 132 Kbit/s rate uses the previous, and less efficient, version of ATRAC. Unfortunately -- and this is a serious problem for Sony -- it means that ATRAC does not use less storage space than mp3s when one wants to record at near CD quality.

ATRAC1

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.

ATRAC1 can also be used in mono (one channel) mode, doubling recording time.

ATRAC3 LP2 Mode

This uses a 132 kbit/s data rate. The quality is commonly said to be similar to that of MP3 encoded at 128 kbit/s, but is actually worse; it came last in a double blind test [1] against Ogg Vorbis, Advanced Audio Coding (AAC), and LAME VBR MP3.

Three stacked QMF split the signal into 4 parts:

0 to 2.75625 kHz (DC to f/16)
2.75625 to 5.5125 kHz (f/16 to f/8)
5.5125 to 11.025 kHz (f/8 to f/4)
11.025 to 22.05 kHz (f/4 to f/2)

ATRAC3 LP4 Mode

This reduces the data rate to 66 kbit/s (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, comparable to cassette tape.

ATRAC3plus

This codec is used in HiMD players (e.g., "Hi-LP and Hi-SP"), Memory Stick players, VAIO Pocket, PSP console 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 large transform window of 4096 samples, four times bigger than that of ATRAC3. The signal is split into 16 sub-bands before MDCT and bit allocation.

The available data encoding rates are 48 kbit/s, 64 kbit/s, and 256 kbit/s. In the recently released Sonic Stage version 3.2 and 3.3 some more bitrates have been introduced, the available bitrates are: 48, 64, 96, 128, 160, 192, 256, 320 and 352 kbit/s.

Minidiscs recorded in this format are incompatible with older players.

Tests conducted in-house by Sony found ATRAC3plus at 64 kbit/s to be equal in subjective sound quality to MP3 at 128 kbit/s. However, this testing may have been biased in the choice of test material. Double-blind tests have generally found other versions of ATRAC to be inferior to most modern codecs (MP3, AAC, Vorbis), raising even more doubt about the validity of Sony's ATRAC3plus claims.

To convert audio to ATRAC3plus you need Sony's SonicStage software, which you get with Sony digital audio players.

Contrary to what Sony states, unprotected ATRAC files can be played back on IBM compatible PC using the RealAudio software or the Liquid Audio plugin module for Winamp. The problem: nearly all ATRAC files contain DRM, with the exception of those encoded with a very old version of a leaked ATRAC3 ACM codec.

External links

InfoAnarchy Atrac3 page - A somewhat more technical and less theoretical page on ATRAC3. Includes links to audio output conversion software.