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VP-8IP / Sep 2013
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P R O C E S S M O D E S
Because the behavior of perceptual codecs have certain predictable similarities
when operated at various bitrates (as far as how they analyze the incoming audio
for opportunities to discard information within it) it becomes a bit easier to design
processing algorithms that can make the best use of the codec’s ability to mask the
material that it must remove.
This last point is important: if a codec is continuously tasked with coding increasingly
difficult-to-code material its decision-making process will begin to permit it to discard
more and more audible parts of the incoming program material. The task of a codec
pre-processor then is to purposely condition the incoming program in a manner that
makes the codec’s job easier, even if it means making subtle changes to certain aspects
of that program material before it ever gets to the codec.
This task of codec preprocessing is arguably best accomplished by utilizing special
audio processing techniques that purposely alter the program material prior to the codec
in a manner that either removes subtle audio information that the codec does not need
to waste its precious few bits coding, or enhances it to ensure that the codec assigns
enough bits to obtain good quality. Because there is a fixed bit pool in most codecs,
removing unnecessary information prior to the codec leaves “more bits” available to
code the most audible portions of the program, which increases the perceived quality
for the listener.
Audio Processor Tuning for CODECS
We strongly caution the user to never use the sound at the output of the
VP‑8
iP
to
determine how to tune it for best behavior with a codec or make judgments about the
sound quality. This is because the work that the
VP‑8
iP
must do to “fool” the codec
into producing a better overall audio quality may cause it to sound odd or unnatural
with program material. You should always listen at the codec output in order to make
judgments about the sound quality!
CODEC Overview
Though there are many CODECS in use throughout the world in myriad specialized
applications, the most popular CODECS that a typical
VP‑8
iP
user will currently
encounter in the field are:
• AAC (Advanced Audio Coding) has replaced MP3 in many applications due to
its higher efficiency and higher audio quality at equivalent bitrates.
• High Efficiency AAC (HE-AAC) is a lossy data compression scheme and an
extension of the Low Complexity AAC (AAC LC) codec that was originally
optimized for low-bitrate applications such as streaming audio.
• HE-AAC version 1 (HE-AAC v1) uses Spectral Band Replication (SBR) to
enhance compression efficiency in the frequency domain.
• HE-AAC version 2 (HE-AAC v2) couples Spectral Band Replication with
Parametric Stereo (PS) to further enhance the compression efficiency of stereo
signals.
• Microsoft’s WMP Professional codec is quite similar in behavior to HE-AAC.
The same holds true in general terms for a very general comparison between the
MPEG-4 / AAC-LD codec. Note also that the HE-AAC codec is also sometimes
referred to as aacPlus and AAC+.
