ATRAC

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ATRAC (Adaptive TRansform Acoustic Coding) is a family of proprietary audio compression algorithms used to store information on MiniDiscs and other Sony-branded audio players. The original ATRAC-1 was first developed by Sony in 1992; the higher compression versions known as ATRAC3 and ATRAC3plus followed in 1999 and 2002, respectively.

Besides Sony, other MiniDisc manufacturers such as Sharp and Panasonic also have their own versions of the ATRAC codec. In 2006, a hybrid lossless compression scheme was added to the ATRAC family.

1 General bitrate quality
2 Bitrate quality compared to other formats
3 Performance
4 ATRAC1
5 ATRAC3 (LP2 and LP4 Modes)
6 ATRAC3plus
7 See also
8 External links

[edit] General bitrate quality

There seems to be a general agreement that ATRAC1 usually gives transparent quality at 292 kbit/s. This is the bitrate used on original minidiscs, and the quality was usually accepted to be quite good. Later versions of ATRAC can usually be expected to be slightly better than earlier ones.

For purposes of comparison, CDs are encoded at around 1400 kbits/s, and lossless encoders can encode most CDs at around 1000 kbit/s with bitrate possibly going down to around 250 kbit/s or even lower for easy content.

There is some disagreement about the ATRAC quality (and its variants) at the lower bitrates. Judgements of sound quality are necessarily subjective. Sony claims that ATRAC3plus gives "near CD quality" at 64 kbit/s. Unfortunately this general assertion does not specify which type of source is being encoded, as a recording of a choral symphony might be more difficult to encode at 64 KBits/s than a recording of someone talking.

[edit] Bitrate quality compared to other formats

Sony's official claim is that ATRAC3plus at 64 kbit/s rate provides a quality comparable to MP3 at 128 kbit/s, placing this codec in the same league as Windows Media Audio (with similar claims from Microsoft), MPEG-4 AAC (the MP3 technical successor).

[edit] Performance

According to ATRAC engineers, ATRAC family is much more friendly regarding decoding chips than other schemes, and so would need far less processor power to decompress a stream [1].

Sony Walkmans offer better battery life when playing ATRAC files as compared to MP3 files. This is how the Sony NW-HDx series (NW-HD1, NW-HD3, NW-HD5) of Walkmen declare a 30-hour battery life. But, even though this is established, it fails to point out why Sony didn't push for ATRAC compatibility in their Sony-Ericsson Walkman series phones (W800i, W700i, W550i, W600i and so on). It would only make sense, given the prospect of longer battery life which is necessary in multimedia phones. Sony's Xplod series of car audio CD players support ATRAC CDs - relevant, but not necessary considering the almost unlimited supply of electrical power that a car can provide.

[edit] ATRAC1

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

Two stacked quadrature mirror filters 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) ^{[citation needed]}. 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.

[edit] ATRAC3 (LP2 and LP4 Modes)

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)

RealAudio8 is an implementation of ATRAC3.

LP2 Mode

This uses a 132 kbit/s data rate, the quality of which is advertised to be similar to that of MP3 encoded at a similar bit rate.

A blind test[2] (2003/02) funded by Sony (with most test tracks provided by Sony) supports the claim that ATRAC3 at 132 kbit/s produces similar quality to MP3 at comparable bitrate.

However, in an independent double-blind test[3] (2004/05) against Ogg Vorbis, AAC, and LAME VBR MP3, ATRAC3 came last.

Common criticisms of similar tests include the supposition that Sony did not implement the top quality ATRAC3 encoder/decoder, the DSP TYPE S, on the PC. However, ATRAC encoders are usually implemented on portables with limited power, probably making heavy-duty compression (and hence high quality compression) difficult.

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 top quality FM radio.

[edit] ATRAC3plus

Atrac3plus CD playing on a Sony car stereo.

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. However the newer bitrates are not compatible with older hardware decoders.

Minidiscs recorded in this format are incompatible with older players.

In a test conducted by an independent firm [4], but financed by Sony, it was concluded that ATRAC3plus at 64 kbit/s is equal in subjective sound quality to MP3 at 128 kbit/s, or AAC at 96 kbit/s.

While ATRAC3plus at 64 kbit/s is quite competitive, such a strong result seems quite surprising, considering that another Sony-funded test concluded that ATRAC3 at 132 kbit/s produces similar quality to MP3 at comparable bitrate (see upper section), and that it is unlikely that ATRAC3plus at 64 kbit/s and ATRAC3 at 132 kbit/s would provide similar quality.

To convert audio to ATRAC3+ you will need Sony's SonicStage software (free download), or a professional suite like Sony's SoundForge Pro 8.0d or Vagas 7.0

[edit] See also

[edit] External links

ATRACLife - News portal, discussion forums and downloads related to ATRAC.
ATRAC technology page - Official site about ATRAC hosted on Sony webpages.
InfoAnarchy Atrac3 page - A somewhat more technical and less theoretical page on ATRAC3. Includes links to audio output conversion software.