ATRAC

ATRAC

Filename extension	.aa3,.aea,.at3,.at9,.atp,.hma OpenMGcontainer .oma,.omg
Internet media type	audio/ATRAC3, audio/ATRAC-ADVANCED-LOSSLESS, audio/ATRAC-X
Developed by	Sony Corporation
Type of format	Audio
Open format?	No
Free format?	No

Adaptive Transform Acoustic Coding(ATRAC) is a family of proprietaryaudio compression algorithmsdeveloped bySony.MiniDiscwas the first commercial product to incorporate ATRAC, in 1992. ATRAC allowed a relatively small disc like MiniDisc to have the same running time asCDwhile storing audio information with minimal perceptible loss in quality. Improvements to the codec in the form of ATRAC3, ATRAC3plus, and ATRAC Advanced Lossless followed in 1999, 2002, and 2006 respectively.

Files in ATRAC3 format originally had the.aa3extension; however, in most cases, the files would be stored in anOpenMGAudio container using the extension.oma.Previously, files that were encrypted with OpenMG had the.omgextension, which was replaced by.omastarting inSonicStagev2.1. Encryption is no longer compulsory as of v3.2.

Other MiniDisc manufacturers such asSharpandPanasonicalso implemented their own versions of the ATRAC codec.

History

ATRAC was developed for Sony'sMiniDiscformat. ATRAC was updated with version 2, then version 3, version 4, version 4.5, and Type R and Type S.

The first major update was ATRAC3 (not to be confused with version 3 of original ATRAC) in 1999.ATRAC3 was used on MiniDisc as well as theNetwork Walkmanand Vaio Music Clip. ATRAC3plus launched in 2003 forHi-MD,but was also compatible with somePlayStation,VAIOand Xplod devices.

On 31 March 2008 Sony all but dropped the ATRAC-related codecs in the United States and Europe, and in theirSonicStagepoweredConnect Music Store(Sony's equivalent ofiTunesandiTunes Music Store). This was partly due to low adoption of the format, with a source claiming that 90% of European Walkman users did not use ATRAC. Walkman digital players outside Japan no longer worked with ATRAC after September 2007.

Until 1 October 2012, ATRAC was the only codec available to download music frommorauntil they transitioned to aDRMfree model and began offeringFLACfiles the next year.

ATRAC9 was designed for PlayStation audio and debuted with the PlayStation Vita.

Bitrate quality

ATRAC's 292 kbit/s bitrateused on the original MiniDiscs was designed to be near to CD audio quality. Years later ATRAC was improved over earlier versions at similar bitrates. For comparison, CDs are encoded at 1411.2 kbit/s, and lossless encoders can encode most CDs below 1000 kbit/s, with further bitrate reduction for easier-to-encode content such as voice.

Performance

ATRAC algorithms were developed in close cooperation withLSIintegrated circuit development engineers within Sony in order to deliver a product that could encode at high speeds and with minimal power consumption.This contrasts with other codecs developed on computers without regard for the constraints of portable hardware. This is reflected in the design of the ATRAC codecs, which emphasize processing smaller groups of samples at a time to save memory at the cost of compression efficiency and additional multiplies. These trade-offs are logical forDSPsystems, where memory was often at a premium compared to multiplier performance.

SonyWalkmansoffer better battery life when playing ATRAC files than when playingMP3files. However, as Sony only pushed ATRAC compatibility inSony EricssonWalkman series phones in the Japanese market, it is not supported inGSM/UMTSmarket phones. Sony'sXplodseries of car audio CD players supportATRAC CDs.Minidiscs with ATRAC format songs have, in the past, been supported on Eclipse brand car stereos.

Formats

ATRAC (1) (versions 1.0–4.5, Type R/S)

ATRAC1 was first used in Sony's own theater formatSDDSsystem in the 1990s, and in this context is a direct competitor toDolby Digital (AC3)andDTS.SDDS uses ATRAC1 with 8 channel encoding, and with a total encoding rate over all the channels of 1168 kbit/s.

Two stackedquadrature mirror filterssplit thesignalinto 3 parts:

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

Fullstereo(i.e., independent channel) encoding with a data rate of 292 kbit/s.

High-frequencylowpass depends on the complexity of the material; some encodings have content clear up to 22.05 kHz.

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

FFmpeghas an implementationof an ATRAC1 decoder.

ATRAC3 (LP2 and LP4 Modes)

Like ATRAC1 and MP3, ATRAC3 is also a hybridsubband-MDCTencoder, but with several differences.

In ATRAC3, Three stacked QMF split the signal into 4 parts:

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

The foursubbandsare then MDCT encoded using a fixed-length transform. Unlike nearly all modern formats, the transform length cannot be varied to optimize coding transients. Instead, a simpler transient encoding technique calledgain controlis used, in which the gain of different subbands is varied during a transient prior to MDCT and then restored during decoding after the inverse MDCT to try to smooth over transients. Additionally, prior toquantization,tonal components are subtracted from the signal and independently quantized. During decoding, they are separately reconstructed and added back to reform the original MDCT coefficients.

Sony claims the major advantage of ATRAC3 is its coding efficiency, which was tuned for portable DSP which provides less computing power and battery life. However, as ATRAC is a hybridsubband-MDCTcodec that is algorithmically very similar toMP3,any advantage is probably exaggerated. Compared to newer formats such asOgg Vorbiswhich use a simple MDCT rather than a hybrid, ATRAC3 must perform an additional computationally expensive inverse-QMF,although the hybrid system significantly reduces memory usage, which was likely a factor given the limited memory available when ATRAC was first developed.

LP2 Mode

This uses a 132 kbit/s data rate, the quality of which is advertised to be similar to that ofMP3encoded at a similar bit rate. However, in an independentdouble-blindtest (2004/05) without format encoding parameters reference againstOgg Vorbis,AAC,andLAMEVBRMP3,ATRAC3 came last.

LP4 Mode

This reduces the data rate to 66 kbit/s (half that of LP2), partly by using joint stereo coding and alowpass filteraround 13.5 kHz. It allows 324 minutes to be recorded on an 80-minute MiniDisc, with the same padding required as LP2.

Notes

FFmpeghas an implementationof an ATRAC3 decoder, which was converted tofixed precisionand implemented in theRockboxseries of firmware forARM,ColdfireandMIPSprocessors. RealAudio8 is a high-bitrate implementation of ATRAC3 (up to 352.8kbit/s). Atracdenc is an open source implementation of ATRAC3 compatible encoder which also can use RealAudio container.

ThePlayStation 3video gameRace Driver: Griduses 224 simultaneous streams of ATRAC3 compressed audio, with between one and eight channels per stream at sample rates between 24 and 48 kHz, each filtered using 512 frequency bands of adaptive equalisation, routed via six reverb units running on the same SPU co-processor (one of eight on the PS3's Cell chip), alongside 7.1 channel hybrid third-orderAmbisonicmi xing.

ATRAC3plus

Thiscodecis used in SonyHi-MDWalkmandevices (e.g., "Hi-LP and Hi-SP" ), NetworkWalkmanplayers,Memory Stickplayers,VAIOPocket,PS3andPSPconsole, andATRAC CDplayers. It is a hybrid subband/MDCTcodec based on a 16 channelQMFfollowed by a 128-pointMDCT.Prior to MDCT coding, Generalized Harmonic Analysis (GHA) is used to extract tonal components, an improved version of the process used in ATRAC3. As in previous ATRAC versions, gain control is used to control preecho rather than variable sized transforms, although different MDCT windows are apparently possible.

AnATRAC3plus CDplaying on a Sony car stereo.

SonicStageversion 3.4, released in February 2006,introduced ripping CDs in bitrates 320 and 352.The available bitrates are: 48, 64, 96, 128, 160, 192, 256, 320 and 352 kbit/s. The newer bitrates are not always compatible with all older hardware decoders, however, some of the older hardware has been found to be compatible with certain newer ATRAC3plus bitrates.

MiniDiscs recorded in this format are incompatible with older players.

In a test conducted by an independent firm, but financed by Sony, it was concluded that ATRAC3plus at 64 kbit/s is equal in subjective sound quality to an obsolete MP3 encoder at 128 kbit/s.Performance against modern high quality MP3 encoders was not evaluated.

ATRAC Advanced Lossless

ATRAC Advanced Losslessis a "scalable" lossless audio codec that records a lossy ATRAC3 or ATRAC3plus stream, and supplements it with a stream of correction information stored within the file itself that allows the original signal to be reproduced, if desired. A player/decoder can extract and use just the ATRAC3 or ATRAC3plus data, or it can combine that with the correction stream to perfectly reproduce the original audio information. This allows the file to be decoded as either lossless or lossy. It is implemented in such a way that allows the file size to be smaller than uncompressed or compressed versions of the same file. Compression is approximately 30–80% of the original file. Benefits of scalable compression include providing backward compatibility, such that older devices that are not AAL-aware can still have the ATRAC3 stream available for playback without understanding the AAL format, and faster transfer speed between portable audio devices and PC.

ATRAC Advanced Lossless is widely supported in olderWalkmanplayers and SonicStage version 4 or later. SonicStage 4 allows download of ATRAC Advanced Lossless to MiniDisc Players,PlayStation Portable,andPlayStation 3.Recent Walkman players do not support ATRAC Advanced Lossless/ATRAC.

AAL's use of a "core" (lossy) and "residual" (correction) stream is similar to the idea behindOpus,MPEG-4 SLS,DTS-HD Master Audio,Dolby TrueHDand Ogg Vorbisbitrate peeling.In fact, AAL was the first to be released in the commercial market with this scheme for backward compatibility.

WavPackhybrid mode andOptimFROGDualStream are in the same category, but store the correction stream in a separate file.

ATRAC9

According to Sony ATRAC9 is a high-compression audio codec optimized for games, offering low delay (granularity) and low CPU and memory usage. It is used in thePS5,PS4andPS Vitaconsoles. Audio middleware such asFMODandAudiokinetic Wwisesupports it.

FFmpeghas an implementationof an ATRAC9 decoder.

See also

• Lossy compression
• OpenMG
• SonicStage
• Walkman

External links

• Sony.net,ATRAC technology page.