===== ATRAC3+ format documentation =====

This is not the only ATRAC3+ documentation that floats around in the internet. A mostly independently derived specification can be found at [[http://wiki.multimedia.cx/index.php?title=ATRAC3plus]].

==== intro =====
ATRAC3+ is a hybrid subband codec like MP3. The signal is split into 16 bands, each band is then divided into two parts: A superposition of sinusodial tones and the spectral residue that is encoded in the frequency domain. For the frequency-domain encoding, some of the low frequency bands are split themself into smaller parts, called //[[ATRAC spectrum anatomy|quantization units]]//, to a total of 32 quantization units. The purpose of the quantization unit is that each unit has its own quantization parameters.

ATRAC3+ can have more than 2 channels - in that case more than one substream is present. This case is not described here.

==== Parsing ====
  * First bit must be zero
  * Array of blocks follows, each block:
     * two bits type followed by data
        * type 0 = single channel [[substream data]]
        * type 1 = dual channel (stereo) [[substream data]]
        * type 2 = non-audio data (ignored)
        * type 3 = end of data marker
  
==== Synthesis ====
  * Spectral residual synthesis
    * [[quantization unit joint stereo coefficient cloning]]
    * [[quantization unit dequantization]]
    * [[band frequency domain dithering]]
    * [[band joint stereo post processing]]
    * [[muting]]
    * [[band IMDCT]]
    * [[band envelope processing]]
  * Sinusodial tone synthesis
    * [[band tone adding]]
  * Noise synthesis
    * [[time domain dithering]]
  * [[final band combination (IMDCT + FIR filtering)|final band combination (IMDCT + FIR filtering => inverse QMF)]]