Image Processing Reference
In-Depth Information
installation but not all equipment is compatible. Choose AES audio or balanced audio as
your base format. Whichever you select, it is very likely that you will have quite a lot of
hardware just for converting between them. This is affectionately called audio-visual glue
and accounts for a large proportion of the cost of building a studio. It is very often not fac-
tored into the price of the build and will take you by surprise when you get the bill. Avoid
this problem by selecting your equipment more carefully and making sure that adequate
systems diagrams are drawn up to include a description of the types of signal being wired
in and where the A-V glue must be inserted.
7.7
The New Standards: HE-AAC
The audio standards developed as part of the MPEG-4 standard are becoming popular.
The name for the new kind of audio coding is AAC. The latest variant is called high effi-
ciency HE-AAC but this was previously known as AAC +. It adds an extra layer of coding
called spectral band replication. This synthesizes frequencies that are under the guidance
of the coder. Those frequencies are then recreated and added to the output at the decod-
ing end.
Figure 7-13 shows how the SBR coding takes place prior to the AAC coding. The SBR
tool output is a coded data stream and the remainder is what gets coded with the AAC
tool. At the decoding end of the equation, a decoder that only supports AAC will not see
the SBR data. It will decode the AAC signal and deliver good-quality audio. If an SBR
encoder is available, it will use the extra information to enhance the audio that has been
recreated by the AAC decoder.
The coding pipeline divides the audio range into low and high frequencies. The low
frequency components are coded by the AAC tool, and the high frequencies by the SBR
tool. Adding this SBR tool to an audio coder improves the efficiency by 30% over what was
being achieved with the AAC tool on its own.
AAC decoder
Good audio out
Transmission
system
SBR coding
AAC coding
AAC decoder
SBR decoder
Fantastic audio out
Figure 7-13 AAC and SBR pipeline.
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