Editing ATRAC (section)

=== ATRAC3 (LP2 and LP4 Modes) ===
Like ATRAC1 and MP3, ATRAC3 is also a hybrid [[Sub-band coding|subband]]-[[Modified discrete cosine transform|MDCT]] encoder, but with several differences.

In ATRAC3, Three stacked QMF split the signal into 4 parts:
* 0 to 2.75625&nbsp;kHz (DC to ''f''/16)
* 2.75625 to 5.5125&nbsp;kHz (''f''/16 to ''f''/8)
* 5.5125 to 11.025&nbsp;kHz (''f''/8 to ''f''/4)
* 11.025 to 22.05&nbsp;kHz (''f''/4 to ''f''/2)

The four [[Sub-band coding|subbands]] are 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 called [[gain control]] is 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 to [[Quantization (signal processing)#Quantization and data compression|quantization]], 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 hybrid [[Sub-band coding|subband]]-[[Modified discrete cosine transform|MDCT]] codec that is algorithmically very similar to [[MP3]], any advantage is probably exaggerated. Compared to newer formats such as [[Vorbis|Ogg Vorbis]] which use a simple MDCT rather than a hybrid, ATRAC3 must perform an additional computationally expensive inverse-[[Quadrature mirror filter|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&nbsp;kbit/s data rate, the quality of which is advertised to be similar to that of [[MP3]] encoded at a similar bit rate.
However, in an independent [[double-blind]] test (2004/05) without format encoding parameters reference against [[Vorbis|Ogg Vorbis]], [[Advanced Audio Coding|AAC]], and [[LAME]] VBR [[MP3]], ATRAC3 came last.<ref>{{Cite web|url=http://listening-tests.freetzi.com/html/Multiformat_128kbps_public_listening_test_results.htm|title=Results of Multiformat at 128kbit/s Listening Test|website=listening-tests.freetzi.com}}</ref> 

;LP4 Mode

This reduces the data rate to 66&nbsp;kbit/s (half that of LP2), partly by using joint stereo coding and a [[Low-pass filter|lowpass filter]] around 13.5&nbsp;kHz. It allows 324 minutes to be recorded on an 80-minute MiniDisc, with the same padding required as LP2.

;Notes

[[FFmpeg]] has an implementation<ref>{{Cite web|url=http://git.ffmpeg.org/?p=ffmpeg;a=blob;f=libavcodec/atrac3.c;hb=HEAD|archive-url=https://web.archive.org/web/20110726052201/http://git.ffmpeg.org/?p=ffmpeg%3Ba%3Dblob%3Bf%3Dlibavcodec%2Fatrac3.c%3Bhb%3DHEAD|url-status=dead|title=Source code for ATRAC3 decoder|archive-date=26 July 2011}}</ref> of an ATRAC3 decoder, which was converted to [[fixed precision]] and implemented in the [[Rockbox]] series of firmware for [[ARM architecture|ARM]], [[Freescale ColdFire|Coldfire]] and [[MIPS architecture|MIPS]] processors. 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.

The [[PlayStation 3]] video game ''[[Race Driver: Grid]]'' uses 224 simultaneous streams of ATRAC3 compressed audio, with between one and eight channels per stream at sample rates between 24 and 48&nbsp;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-order [[Ambisonic]] mixing.<ref>{{Cite web|url=https://issuu.com/develop/docs/dev86_web|title=Develop - Issue 86 - August 2008|website=Issuu|date=14 August 2008 }}</ref>