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5.1.2 mp3-HD (16-bit lossless)
Currently it is not possible to audition in real time the mp3-HD codec. (The codec library
itself includes writes-to-file that cannot be used in the DSP thread of a plug-in.) If the mp3-
HD codec is selected, and its MON button enabled, the audition path is switched to a
synthesised version of the codec output. This synthesised signal mimics the behaviour of the
mp3-HD codec, in that it will replicate the input but ensure that the signal is only 16 bits
deep, whatever the input word size. The audition path might include dither (see below).
The MONITOR led (see right) indicates INPUT 16, to indicate that the
signal is a modified version of the input that is intended to synthesise the
codec output.
You will notice that the DIFF signal in the FFT display will drop below -96dB. mp3-HD is
lossless to 16 bits, so the DIFF signal has an amplitude smaller than 16 bits down. If the
input signal to the codec is quantised to 16 bits, either by dither and truncation, or because it
is unprocessed data from a 16-bit input file, the DIFF signal will drop below -180dB.
The mp3-HD codec is lossless to 16 bits only. With 16-bit sessions, the files will be truly
lossless, and auditioning the input will accurately replicate the post-decoder audio.
This is not true for a 24-bit session. As the mp3-HD codec is only 16-bit capable, the 24-bit
input can be dithered and truncated internally as part of the encoding process. Thus the mp3-
HD encoded-decoded file will not be bit-identical with either a 24-bit rendered wav file, or an
externally dithered and truncated 16-bit wav file. (Note an mp3-HD file can be decoded to
either 16 or 24-bit audio files.)
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