Dither and truncation, 3 dither and truncation – Sonnox Fraunhofer Pro-Codec User Manual
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5.9 HD-AAC
5 THE PRO-CODEC PLUG-IN
5.9.3 Dither and Truncation
When encoding online, or preparing a session for render through the DAW, it is essential
to provide the input to the HD-AAC codec with a correctly dithered and truncated audio
data stream. As stated above, the plug-in should be the last element of the processing
chain, and a dither/truncation plug-in should immediately precede it. This is particularly
important if HD-AAC is to be used.
When HD-AAC is selected for audition, the FFT display area changes slightly to include an
input bit-meter for the left-hand scale. This bit-meter is included specifically to help select
the correct dither/truncation settings for the signal into the codec.
There is one segment per bit for bits 1 to 24 (see right). One large segment at the bottom
indicates any activity below the 24th bit.
With a 16 bit input you would
expect activity in bits 1-16, and with a 24 bit input you would
expect activity in bits 1-24. As noted in the introduction
to this section, if there is processing before the plug-in,
you would expect ‘inters’ and activity on all bits, including
bits higher than 24 (even if you start with a 16 bit file).
The following
images show the plug-in FFT displaying the
INPUT and DIFF
signals with varying degrees of dither and truncation.