ATRAC3: Difference between revisions

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(Added introduction.)
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  7  352 Kbps (352800) 1024 bytes Normal stereo 1024 per channel
  7  352 Kbps (352800) 1024 bytes Normal stereo 1024 per channel


= Specification =
== ATRAC3 encoding algorithm ==
 
* split the input signal into four bands using Quadrature mirror filter (QMF)
* gain control analyze and obtain gain control data
* convert all four bands into freq domain using Modified Cosine Transform (MDCT or MLT)
* find tonal components
* quantization
* encode the bit stream
 
== ATRAC3 decoding algorithm ==
 
* parse the bitstream and extract the following sound data:
** gain control data, tonal components and quantized spectral coefficients
* inverse quantization of the tonal components and spectral coefficients
* merge tonal components and other spectral coefficients together
* reconstruct the timedomain signal using IMDCT
* gain compensation
* apply the QMF synthesis filter to reconstruct the sound
 
= Decoding Specification =





Revision as of 07:44, 2 December 2006

Found in some old RealMedia files. Suspected to be similar to, or the same as, Sony ATRAC.

Introduction

ATRAC (Adaptive TRansform Acoustic Coding) is the collective name for audio compression technologies developed by Sony Corporation. This codec family includes today the following codecs: ATRAC, ATRAC3, ATRAC3plus and ATRAC Advanced lossless. You can read about it here: http://www.sony.net/Products/ATRAC3/overview/index.html#family

This ATRAC codec was introduced in 1992 with the MiniDisc. There is a good description of this algorithm you can get here: http://www.minidisc.org/aes_atrac.html This codec is used in the MiniDisc portable players by many companies.

ATRAC3 is the next generation of the ATRAC codec. There are two major implementations of it for the PC: RealAudio8 atrc codec and the Sony ATRAC3 codec for Audio Compression Manager (ACM) operates under Windows.

The ATRAC3 codec supports many constant bit rates ("flavours"). The following table shows the bit rate, the size of a frame and the coding mode for each flavour respectively:

Nr Bitrate		Frame size (stereo)	Coding mode	Samples per frame
-- -----------------	-------------------	-------------	-----------------
0  66 Kbps (66150)	192 bytes		Joint stereo	1024 per channel
1  94 Kpbs (93713)	272 bytes		Joint stereo	1024 per channel
2  105 Kbps (104738)	304 bytes		Normal stereo	1024 per channel
3  132 Kpbs (132300)	384 bytes		Normal stereo	1024 per channel
4  146 Kbps (146081)	424 bytes		Normal stereo	1024 per channel
5  176 Kbps (176400)	512 bytes		Normal stereo	1024 per channel
6  264 Kbps (264600)	768 bytes		Normal stereo	1024 per channel
7  352 Kbps (352800)	1024 bytes		Normal stereo	1024 per channel

ATRAC3 encoding algorithm

  • split the input signal into four bands using Quadrature mirror filter (QMF)
  • gain control analyze and obtain gain control data
  • convert all four bands into freq domain using Modified Cosine Transform (MDCT or MLT)
  • find tonal components
  • quantization
  • encode the bit stream

ATRAC3 decoding algorithm

  • parse the bitstream and extract the following sound data:
    • gain control data, tonal components and quantized spectral coefficients
  • inverse quantization of the tonal components and spectral coefficients
  • merge tonal components and other spectral coefficients together
  • reconstruct the timedomain signal using IMDCT
  • gain compensation
  • apply the QMF synthesis filter to reconstruct the sound

Decoding Specification

Extra data format

In rm files the extra data is as follows (big endian order):

INT32	id, always == 4
INT16	samples per frame, always 1024 * 2
INT16	delay, not used but always 0x88E
INT16	stereo coding mode, 2 - normal stereo, 0x12 - joint stereo

It is always 10 bytes of data.

Transforms

QMF

MLT

The transform is the same as the MPEG AAC SSR profile with the exception of a special window.

MLT Window table

atrac_window[256] = {
   9.41253620112548e-06,8.47234623506665e-05,0.000235401996178553,0.000461561692645773,
   0.000763372576329857,0.00114106200635433,0.00159491400700063,0.00212526903487742,
   0.00273252697661519,0.00341714196838439,0.00417962903156877,0.00502056023105979,
   0.00594056397676468,0.00694032711908221,0.00802059844136238,0.00918218307197094,
   0.0104259504005313,0.0117528103291988,0.013163760304451,0.0146598499268293,
   0.0162421800196171,0.017911909148097,0.019670270383358,0.0215185508131981,
   0.023458119481802,0.0254903696477413,0.0276167895644903,0.0298389103263617,
   0.0321583412587643,0.0345767587423325,0.0370958782732487,0.0397174991667271,
   0.0424434691667557,0.04527572914958,0.048216238617897,0.0512670800089836,
   0.0544303394854069,0.0577081888914108,0.0611028708517551,0.0646167024970055,
   0.0682519972324371,0.0720112174749374,0.0758968368172646,0.0799113884568214,
   0.084057442843914,0.0883376598358154,0.0927547365427017,0.0973114371299744,
   0.102010503411293,0.106854900717735,0.111847400665283,0.116990998387337,
   0.122288599610329,0.127743199467659,0.133357793092728,0.139135494828224,
   0.145079404115677,0.151192396879196,0.157477602362633,0.163938298821449,
   0.170577302575111,0.177397698163986,0.184402495622635,0.191594794392586,
   0.198977395892143,0.20655320584774,0.214325100183487,0.222295597195625,
   0.230467602610588,0.238843500614166,0.247425600886345,0.256216287612915,
   0.265217810869217,0.274431705474854,0.283860206604004,0.293504595756531,
   0.303366303443909,0.313446491956711,0.323745906352997,0.334265112876892,
   0.345004290342331,0.355963587760925,0.367142498493195,0.378540098667145,
   0.390155404806137,0.401986807584763,0.414032310247421,0.426289200782776,
   0.438754886388779,0.451425611972809,0.464297711849213,0.477366387844086,
   0.490626811981201,0.504073321819305,0.517699420452118,0.531498670578003,
   0.545463383197784,0.559585571289062,0.573856592178345,0.588267087936401,
   0.602807104587555,0.617465972900391,0.632232785224915,0.647095382213593,
   0.662041485309601,0.677057981491089,0.692131519317627,0.707247793674469,
   0.722392320632935,0.737549722194672,0.752704918384552,0.767841517925262,
   0.782943487167358,0.797994315624237,0.812976777553558,0.827874422073364,
   0.842669785022736,0.857345581054688,0.871884822845459,0.886269986629486,
   0.900484383106232,0.91451108455658,0.928333282470703,0.941935122013092,
   0.955300509929657,0.968414008617401,0.981260776519775,0.993826687335968,
   1.00609803199768,1.01806199550629,1.02970600128174,1.04101896286011,
   1.05199003219604,1.06261003017426,1.07287001609802,1.08276200294495,
   1.09228003025055,1.10141599178314,1.11016702651978,1.118528008461,
   1.12649703025818,1.13407099246979,1.14124798774719,1.14803004264832,
   1.15441703796387,1.16040897369385,1.16601002216339,1.17122399806976,
   1.17605304718018,1.18050396442413,1.18457996845245,1.18828999996185,
   1.19163799285889,1.19463300704956,1.19728195667267,1.1995929479599,
   1.20157599449158,1.20323896408081,1.20459198951721,1.2056440114975,
   1.20640504360199,1.20688498020172,1.20709502696991,1.2070449590683,
   1.20674395561218,1.20620501041412,1.20543599128723,1.20444905757904,
   1.20325398445129,1.20186197757721,1.20028102397919,1.19852304458618,
   1.19659698009491,1.19451296329498,1.19228100776672,1.18991005420685,
   1.18740999698639,1.18478798866272,1.18205404281616,1.17921698093414,
   1.17628395557404,1.17326295375824,1.17016303539276,1.1669909954071,
   1.16375398635864,1.16045904159546,1.15711295604706,1.15372204780579,
   1.15029203891754,1.14683103561401,1.14334201812744,1.13983201980591,
   1.13630700111389,1.13276994228363,1.1292279958725,1.12568402290344,
   1.12214398384094,1.11861002445221,1.11508798599243,1.11158096790314,
   1.10809302330017,1.10462701320648,1.1011860370636,1.09777402877808,
   1.09439396858215,1.09104800224304,1.08773899078369,1.08446896076202,
   1.08124196529388,1.07805800437927,1.07492101192474,1.07183194160461,
   1.06879305839539,1.06580603122711,1.062873005867,1.05999398231506,
   1.05717301368713,1.05440902709961,1.05170500278473,1.04906094074249,
   1.04647898674011,1.0439590215683,1.04150295257568,1.03911197185516,
   1.03678703308105,1.03452801704407,1.03233599662781,1.03021097183228,
   1.028156042099,1.02616894245148,1.0242520570755,1.02240598201752,
   1.02063000202179,1.01892495155334,1.01729202270508,1.01573097705841,
   1.01424205303192,1.01282596588135,1.01148295402527,1.01021301746368,
   1.00901699066162,1.00789403915405,1.00684499740601,1.00587105751038,
   1.00497102737427,1.00414502620697,1.0033940076828,1.00271797180176,
   1.00211596488953,1.00159001350403,1.00113797187805,1.00076198577881,
   1.00046098232269,1.0002349615097,1.00008499622345,1.00000894069672,
};

Huffman coding

VLC coding is used to compress the spectral coefficients.

Huffman tables

huffcode1[9] = {
  0x0,0x4,0x5,0xC,0xD,0x1C,0x1D,0x1E,0x1F,
};

huffbits1[9] = {
  1,3,3,4,4,5,5,5,5,
};

huffcode2[5] = {
  0x0,0x4,0x5,0x6,0x7,
};

huffbits2[5] = {
  1,3,3,3,3,
};

huffcode3[7] = {
  0x0,0x4,0x5,0xC,0xD,0xE,0xF,
};

huffbits3[7] = {
  1,3,3,4,4,4,4,
};

huffcode4[9] = {
  0x0,0x4,0x5,0xC,0xD,0x1C,0x1D,0x1E,0x1F,
};

huffbits4[9] = {
  1,3,3,4,4,5,5,5,5,
};

huffcode5[15] = {
  0x0,0x2,0x3,0x8,0x9,0xA,0xB,0xC,0xD,0x1C,0x1D,0x3C,0x3D,0x3E,0x3F,
};

huffbits5[15] = {
  2,3,3,4,4,4,4,4,4,5,5,6,6,6,6,
};

huffcode6[31] = {
  0x0,0x2,0x3,0x4,0x5,0x6,0x7,0x8,0x9,0x14,0x15,0x16,0x17,0x18,0x19,0x34,0x35,
  0x36,0x37,0x38,0x39,0x3A,0x3B,0x78,0x79,0x7A,0x7B,0x7C,0x7D,0x7E,0x7F,
};

huffbits6[31] = {
  3,4,4,4,4,4,4,4,4,5,5,5,5,5,5,6,6,6,6,6,6,6,6,7,7,7,7,7,7,7,7,
};

huffcode7[63] = {
  0x0,0x2,0x3,0x8,0x9,0xA,0xB,0xC,0xD,0xE,0xF,0x10,0x11,0x24,0x25,0x26,0x27,0x28,
  0x29,0x2A,0x2B,0x2C,0x2D,0x2E,0x2F,0x30,0x31,0x32,0x33,0x68,0x69,0x6A,0x6B,0x6C,
  0x6D,0x6E,0x6F,0x70,0x71,0x72,0x73,0x74,0x75,0xEC,0xED,0xEE,0xEF,0xF0,0xF1,0xF2,
  0xF3,0xF4,0xF5,0xF6,0xF7,0xF8,0xF9,0xFA,0xFB,0xFC,0xFD,0xFE,0xFF,
};

huffbits7[63] = {
  3,4,4,5,5,5,5,5,5,5,5,5,5,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,7,7,7,7,7,
  7,7,7,7,7,7,7,7,7,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
};