Telos Zephyr Xstream - Manual (Page 128)

USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 116
The block diagram of the AAC encoder is shown below. It is considerably more sophisticated
than the previous MPEG Layer‐2 and Layer‐3 systems, and therefore offers more coding power.
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 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 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 code. 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 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 allocated 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 from discrete L/ R to a matrixed
L+R/ L‐R mode dynamically, depending upon the program material.