Sonnox Fraunhofer Pro-Codec - Manual (Page 31)

5.9 HD-AAC 5 THE PRO-CODEC PLUG-IN
5.9 HD-AAC (Lossless
Codec)
A lossless codec is one that produces an output data stream that is an exact duplicate of
the input audio data stream. In the simple case of a 24-bit wav file as input, the output of
the HD- AAC lossless codec will be a wav file with identical audio data.
When HD-AAC is used in a DAW via the Pro-Codec Plug-In, the situation is slightly more
complicated because usually the input is not a simple 16 or 24 bit data stream. Most
DAWs use floating point processing, and so do most plug-ins. A floating point datum will
have many more states than an audio integer as taken from a 24 bit WAV file (this is
another way of saying that the floating point format has higher resolution than the
fixed-point wav format). Whenever processing is performed in a DAW or plug-in, the
processing is performed at the higher resolution of the floating point system, and the
resulting audio data cannot be represented perfectly on a 24 bit scale any more. The data
stream will contain floating point values between the quantised values of the 24 bit scale,
and these in-between values are called ‘inters’. This interactivity can be readily seen
using the
DIFF signal, apparent up to -144 dB for HD-AAC. (One of the main reasons the
DIFF signal is included is to aid visibility of this situation.)
HD-AAC will not reproduce these inter values as it only uses a 16 or 24 bit scale, and the
correct terminology is ‘lossless to 24 bits’, or ‘lossless to 16 bits’.
Therefore, if the audio channel includes any processing (even DAW fades or gain
changes), it is important to present correctly truncated data to the input of the lossless
encoder. When an audio data stream is truncated, it must first be dithered at the correct
level. See section
5.9.3 (Dither and Truncation) for details.
5.9.1 Auditioning
HD-AAC
Fraunhofer’s HD-AAC has a very clever feature; the single compressed lossless file
includes a lossy core channel. It therefore acts as a lossless archival format, a lossless
distribution format for the masters, and a final playback format for both lossless and lossy
decoders — and all of this in a single file.
During HD-AAC playback, for example, if the decoder in your player has full HD- AAC
capability, you will be able to listen to perfect replication of the original wav file. However,
the same HD-AAC file will still play through a decoder that doesn’t have HD- AAC
capability, and instead you will be listening to the lossy embedded AAC channel.
It is possible to audition HD-AAC in real time. If the HD-AAC is selected, and its MON
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