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For each quantization unit with non-zero quantizer, the quantized spectral data is stored using one out of many many many prefix code trees, see Main Trees List. There is no data stored if the quantizer is zero. The format of the spectral data is also described in the main tree list page.

Dithering is only enabled if some quantization units except the lowest two contain data. In that case, for all dither groups needed to cover the range up to the highest quantization unit with non-zero quantizer, a dither level value (4 bits) is stored in a pure binary encoding. A higher dither level means a lower dither amplitude, with a level of 15 meaning no dither at all.