Databases Reference
In-Depth Information
16
14
12
10
8
Amplitude
6
4
2
0
0
20
40
60
Sample number
80
100
120
140
F I GU R E 17 . 7
Source output sequence.
10
5
Transform
coefficient
0
−
5
−
10
0
20
40
60
Coefficient number
80
100
120
140
F I GU R E 17 . 8
Transformed sequence.
the stationary periods (that is a block size of 18 plus 50% overlap from neighboring blocks).
This window is called the
long window
. If a sharp attack is detected, the algorithm shifts to
a sequence of three
short windows
of length 12 after a transition window of length 30. This
initial transition window is called the
start
window. If the input returns to a more stationary
mode, the short windows are followed by another transition window called the
stop
window
of length 30 and then the standard sequence of long windows. The process of transitioning
between windows is shown in Figure
17.10
. A possible set of window transitions is shown
in Figure
17.11
. For the long windows we end up with 18 frequencies per subband, resulting
in a total of 576 frequencies. For the short windows we get 6 coefficients per subband for a
total of 192 frequencies. The standard allows for a mixed block mode in which the two lowest
subbands use long windows while the remaining subbands use short windows. Notice that
while the number of frequencies may change depending on whether we are using long or short
