5-4 CODING
90
60
40
20
0
deciBels
.02 .05 .1 .2 .5 1 2 5 10 20
kiloHertz
Masking
Threshold
Threshold
in
q
uiet
Inaudible
Signal
Masker
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.
90
60
40
20
0
deciBels
-60 0 60 100 150 50 100 150 200 t (in ms.)
Post- Maskin
g
Simultaneous-Pre-
0
Masking effects in the time domain. Masking occurs both before and after the masking signal.