Section 7 AUDIO CODING PRINCIPLES
109
Layer III Features
Psychoacoustic Masking
The audio in Layer III 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 II. 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 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 III 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.
Joint 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.
Summary of Contents for Zephyr
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Page 53: ...Section 4 INSTALLATION BASIC OP 53 SECTION 4 INSTALLATION BASIC OPERATION ...
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Page 119: ...Section 8 DETAILED MENU REFERENCE 119 SECTION 8 DETAILED MENU REFERENCE ...
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Page 197: ...Section 11 TECHNICAL INFORMATION 197 SECTION 11 DETAILED TECHNICAL INFORMATION ...
Page 219: ...Section 12 SCHEMATICS 219 SECTION 12 SCHEMATICS ...
Page 221: ...Section 13 MANUFACTURER S DATA SHEETS 221 SECTION 13 MANUFACTURER S DATA SHEETS ...
Page 223: ...Section 14 SPECIFICATIONS WARRANTY 223 SECTION 14 SPECIFICATIONS AND WARRANTY ...
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