Manual

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4.2 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 the 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.
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 provides the 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.
4.3 Overloading Pre Codec (Codec Headroom)
It is not desirable, but it is possible, that the plug-in will be fed by an overloading input (this
is separate to the possibility that a lossy codec could overload from a non-overloading input).
The input meters will show clipping.
If an overloading input is presented to the 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 the decoder. The Bitstream Level max dB will accurately indicate the
maximum level of the signal, and the monitor output will not clip in the digital domain.
This is not true with the lossless codecs, which do not have internal headroom. This applies
to both the mp3-HD and the HD-AAC codecs.
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