Telos Z/IP ONE User Manual page 78

Because AAC was built on a similar structure to Layer 3, it therefore retains some of its powerful features:
Redundancy Reduction – a Huffman encoding process causes values that appear more frequently to be
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coded with shorter words, while values that appear only rarely are coded with longer words. This results
in an overall increase in coding efficiency with no degradation, since it is a completely lossless process.
Bit Reservoir Buffering – often, there are some critical parts in a piece of music that cannot be encoded
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at a given data rate without audible noise. These sequences require a higher data rate to avoid artifacts.
On the other hand, some signals are easy to encode. If a frame is easy, then the unused bits are put into
a reservoir buffer. When a frame comes along that needs more than the average amount of bits, the
reservoir is tapped for extra capacity.
Ancillary Data – the bit reservoir buffer offers an effective solution for the inclusion of such ancillary data
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as text or control signaling. The data is held in a separate buffer and gated onto the output bit stream using
some of the bits allocate for the reservoir buffer, when they are not required for audio.
The Joint Stereo mode takes advantage of the redundancy in stereo program material. The encoder switches
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from discrete L/R to a matrixed L+R/L-R mode dynamically, depending on the program material
The result of all this is that the researchers succeeded: AAC provides performance superior to any known codec at
bitrates greater than 64kbps, and excellent performance relative to the alternatives at bitrates reaching as low as 16kbps.
AUDIO CODING REFERENCE |
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