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The bytes of the ATRAC3+ frame make up a bitstream. The most significant bit of the input bytes is consumed first. In multi-bit-fields, also the first of the bits that make up the field is the most significant one.

The substream data first contains the number of quantization units that have to be processed. For some reason, the total number of quantization units should not be 29, 30 or 31. The maximum number of quantization units, 32, is fine, though. For the quantization units, the quantizers are encoded. The quantizers may be 0, meaning that no data for that quantization unit is present.

• 5 bits: Count of quantization units in this block-1 (should not be 28, 29 or 30).
• 1 bit: Mute if set (but frequency-space decoding is still done)
• For each channel: serialized quantization unit quantizers
• If there are quantization units with non-zero quantizer
  • For each channel: serialized quantization unit levels
  • 1 bit: Enable full spectral data tree access (if clear, only most common trees are accessible)
  • For each channel: serialized quantization unit tree choice info
  • For each channel: serialized quantization unit spectral data and dither noise levels
• For each channel: serialized band joint stereo info
• For each channel: serialized window shape info
• For each channel: serialized band envelopes
• 1 bit: Enable tone data. If that bit is set:
  • serialized tone data
• 1 bit: enable time domain dither
  • if enabled: 4 bits noise level
  • if enabled: 4 bits noise phase