SV SLA-PA201, Руководство пользователя

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where
N
is buffer size. There are all kinds of window function, but generally hanning, flat-top and
rectangular(no window) window is used.
[Fig. V-2-5] Various Windowing Functions
However windowing causes the leakage of time signal’s energy as [Fig. V-2-6].
Energy Leakages
[Fig. V-2-6] Windowing causes the leakage of energy of time signal
2. 2 Overlap
Overlapping is done to protect energy leakage. If you hope to understand the overlapping, you have
to know no overlapped time signal. For example, let’s suppose that the first time data
1
x
has 0 ~ N-1,
the second time data
2
x
has N ~ 2N-1 and so on.
1
FFT
is the FFT spectrum of
1
x
and
2
FFT
is
the FFT spectrum of
2
x
. See [Fig. V-2-7]. The left figure is no overlap that means the time signal
buffer for FFT is not repeated. 50% Overlapping uses the second half data of
1
x
and the first half
data of
2
x
together.
Over 66% overlapping doesn’t usually have the loss of energy in the time signal.
No Overlap
50% Overlap
1
FFT
2
FFT
3
FFT
1
FFT
3
FFT
2
FFT
[Fig. V-2-7] Overlapping
10 20 30 40 50 60
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Samples
A
m
pl
it
ud
e
Time domain
0 0.2 0.4 0.6 0.8
-140
-120
-100
-80
-60
-40
-20
0
20
40
Normalized Frequency (
×π
rad/sample)
M
a
g
n
it
ude (
d
B
)
Frequency domain