Sonnox Fraunhofer Pro-Codec - Manual (Page 22)

5.3 Online
Auditioning
and
Configuring
Codecs 5 THE PRO-CODEC PLUG-IN
5.3.3 Overloading
Pre-Codec
(Codec
Headroom)
It is possible, but not desirable, that the plug-in will be fed by an overloading input
(sample values greater than 0 dBFS). In this case, the input meters will show clipping.
This is separate to the possibility that a lossy encoder given a non-overloading input
could produce a bitstream that overloads a decoder output.
If an overloading input is presented to lossy codecs such as AAC-LC, the codec output
will endeavour to follow the input as accurately as possible, using internal headroom in
both the encoder and decoder. Bitstream Level Max
dB will accurately indicate the
maximum level of the signal, and the signal will not clip in the plug-in.
Note:
This is not true for the HD-AAC codec (lossless), which does not have internal
headroom for the lossless layer.
5.3.4 Overloading
Post-Codec
(Audition
Level
Matching)
As discussed above, any form of filtering (including encoding and decoding) can increase
the peak level of a signal. Some codecs such as HE-AAC can add several dBs, hence the
provision of Bitstream Level
Trim to avoid post-decoder clipping in the audition chain. Up
to five codecs can be selected for audition, and each could require a different trim setting.
To keep the audition process palatable, the trim is reversed on each channel before the
audition point, so that switching between codecs does not introduce audition level
changes.
Note:
However, this means that you are not auditioning the true level at the input of the
encoder. If there are significant trim values involved, it is possible that the encoded
files will demonstrate differing average levels when played back, and this may be
undesirable.
The Pro-Codec plug-in provides the ability to observe and monitor bitstream level
behaviour, and tools for the user to mitigate this behaviour. It is convenient to monitor
many codecs simultaneously for comparison, but it might be best practice to treat each
codec individually when producing a final master. In this way any level change can be
kept in the master mix, rather than in the plug-in.
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