Section 7 AUDIO CODING PRINCIPLES
107
Most important for audio coding are the effects in the frequency domain. Research into
perception has revealed that a tone or narrow- band noise at a certain frequency
inhibits the audibility of other signals that fall below a threshold curve centered on a
masking signal.
The figure below shows two threshold of audibility curves. The lower one is the typical
frequency sensitivity of the human ear when presented with a single swept tone. When
a single, constant tone is added, the threshold of audibility changes, as shown in the
upper curve. The ear’s sensitivity to signals near the constant tone is greatly reduced.
Tones that were previously audible become “masked” in the presence of “masking
tones,” in this case, the one at 300 Hz.
All signals below the upper “threshold of audibility” curve, or Masking Threshold are
not audible, so we can drop them out or quantize them crudely with the least number of
bits. Any noise which results from crude quantization will not be audible if it occurs
below the threshold of masking. The masking depends upon the frequency, the level,
and the spectral distribution of both the masker and the masked sounds.
80
60
40
20
0
dB
.02
.05
.1
.2
.5
1
2
5
10
20
kHz
Threshold
in quiet
Inaudible
Signal
Masker
Masking
Threshold
Masked Sound
Masking effects in the frequency domain. A masking signal
inhibits audibility of signals adjacent in frequency and
below the threshold.
To benefit from the masking effects, perceptual coders use a filterbank to divide the
input audio into multiple bands for analysis and processing. The maximum masked
noise level is calculated depending upon the spectral content, and the available bits are
allocated so as to keep the quantization noise below the masking threshold at every
point in the spectrum.
While coding efficiency increases with more bands and better frequency resolution, the
time domain resolution decreases simultaneously owing to an inevitable side- effect of
the band filtering process: higher frequency resolution requires a longer time window –
which limits the time resolution. Happily, masking works also in the time domain. A
short time before and a longer time after a tone is switched on and off, other signals
below a threshold amplitude level are not noticeable. Filterbanks with higher frequency
resolution naturally exploit the ear’s time- masking properties.
Summary of Contents for Zephyr
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Page 39: ...39 SECTION 3 ZEPHYR AT A GLANCE ...
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Page 53: ...Section 4 INSTALLATION BASIC OP 53 SECTION 4 INSTALLATION BASIC OPERATION ...
Page 84: ...Section 4 INSTALLATION BASIC OPERATION 84 ...
Page 85: ...Section 5 ISDN 85 SECTION 5 ISDN ...
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Page 119: ...Section 8 DETAILED MENU REFERENCE 119 SECTION 8 DETAILED MENU REFERENCE ...
Page 157: ...Section 9 REMOTE CONTROL 157 SECTION 9 REMOTE CONTROL ...
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Page 177: ...Section 10 ADVANCED PROBLEM SOLVING 177 SECTION 10 ADVANCED PROBLEM SOLVING ...
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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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Page 229: ...Section 15 APPENDICES 229 SECTION 15 APPENDICES ...