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.

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 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.

• 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

• 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 => inverse QMF)