Science Fair Project Encyclopedia
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.
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 similar to that of 128 kbit/s MP3, although it came last in a double blind test against Ogg Vorbis, 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, similar to MP3 at 80 to 96 kbit/s or analog cassette tape.
This codec is used in HiMD players (e.g., "Hi-LP and Hi-SP"), MemoryStick players, VAIO Pocket, 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 kbit/s, 64 kbit/s, and 256 kbit/s.
Minidiscs recorded in this format are not compatible with older players.
Tests conducted 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.
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.
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