Databases Reference
In-Depth Information
80
Threshold
of audibi
l
ity
60
Audible
region
40
20
Inaudible
region
0
20
50
100
200
500
1000
2000
5000
10,000 20,000
Frequency (Hz)
F I GU R E 17 . 1
A typical plot of the audibility threshold.
Original
threshold
of audibility
80
Audible
region
Raised
threshold
of audibility
60
40
20
Inaudible
region
0
20
50
100
200
500
1000
2000
5000
10,000 20,000
Frequency (Hz)
F I GU R E 17 . 2
Change in the audibility threshold.
higher frequencies the bandwidth can be as large as 4kHz. This increase of the threshold
has major implications for compression. Consider the situation in Figure
17.2
. Here a tone
at 1kHz has raised the threshold of audibility so that the adjacent tone above it in frequency
is no longer audible. At the same time, while the tone at 500Hz is audible, because of the
increase in the threshold the tone can be quantized more crudely. This is because increase of
the threshold will allow us to introduce more quantization noise at that frequency. The degree
to which the threshold is increased depends on a variety of factors, including whether the signal
is sinusoidal or atonal.
17.2.2 Temporal Masking
Along with spectral masking, the psychoacoustic coders also make use of the phenomenon of
temporal masking. The temporal masking effect is the masking that occurs when a sound raises
