Telos Zephyr Xstream, User Manual

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USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 121
standardized in ISO/MPEG IS ‐ 11172. Prior to the availability of AAC, Layer ‐ 3 was specifically
recommended for 56 and 64 kbps channel.
Layer-3 Features
Psychoacoustic Masking
The audio in Layer ‐ 3 is divided into 576 frequency bands. First, a polyphase filter bank performs
a division into the 32 “main” bands which correspond in frequency to those used by the less
complex Layer ‐ 2. Filters are then used to further subdivide each of the main bands into 18
more. At a 32 kHz sampling rate, the resulting bandwidth is 27.78 Hz – allowing very accurate
calculation of the masking threshold values. Sufficient frequency resolution is available to
exceed the width of the ear’s critical bands (100 Hz below 500 Hz; 20% of the center frequency
at higher frequencies) across the audible spectrum, resulting in better hiding of noise than
would otherwise be possible.
Redundancy Reduction
Redundancy reduction is accomplished by a Huffman coding process to take advantage of the
statistical properties of the simplified signal output from the psychoacoustic stage. Values that
appear more frequently are coded with shorter words, whereas values that appear only rarely
are coded with longer words. This results in an overall decrease in the data rate, with no
degradation, since it is a lossless reduction scheme.
Notice that this redundancy reduction process is the ideal supplement to psychoacoustic
masking. In general, maskers with high tonality have more redundancy but allow less masking,
while noise ‐ like signals have low redundancy and high masking effect.
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. Layer ‐
3 uses a short time “bit reservoir” buffer to address that need. Similar to a savings account, this
buffer is filled in “easy times” with data bits that are not required for the actual frame. If a
critical part occurs, the encoder can use the saved bits to code this part with a higher data rate.
Ancillary Data
The bit reservoir buffer offers an interesting capability: 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 bitstream using the bits allocated for the reservoir buffer when they are not
required for audio.
Layer ‐ 3 J ‐ Stereo
A joint stereo mode permits advantage to be taken from 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 content. The matrixed mode of operation takes of advantage of
the usual redundancy of the “center” channel information and therefore significantly improves
overall fidelity.