285
CHAPTER
16
h[i ] '
sin (2BfC i )
iB
Windowed-Sinc Filters
Windowed-sinc filters are used to separate one band of frequencies from another. They are very
stable, produce few surprises, and can be pushed to incredible performance levels. These
exceptional frequency domain characteristics are obtained at the expense of poor performance in
the time domain, including excessive ripple and overshoot in the step response. When carried out
by standard convolution, windowed-sinc filters are easy to program, but slow to execute. Chapter
18 shows how the FFT can be used to dramatically improve the computational speed of these
filters.
Strategy of the Windowed-Sinc
Figure 16-1 illustrates the idea behind the windowed-sinc filter. In (a), the
frequency response of the ideal low-pass filter is shown. All frequencies below
the cutoff frequency, , are passed with unity amplitude, while all higher fC
frequencies are blocked. The passband is perfectly flat, the attenuation in the
stopband is infinite, and the transition between the two is infinitesimally small.
Taking the Inverse Fourier Transform of this ideal frequency response produces
the ideal filter kernel (impulse response) shown in (b). As previously discussed
(see Chapter 11, Eq. 11-4), this curve is of the general form: , called sin(x)/x
the sinc function, given by:
Convolving an input signal with this filter kernel provides a perfect low-pass
filter. The problem is, the sinc function continues to both negative and positive
infinity without dropping to zero amplitude. While this infinite length is not
a problem for mathematics, it is a show stopper for computers.
The Scientist and Engineer's Guide to Digital Signal Processing 286
w[i ] ' 0.54 & 0.46 cos(2Bi /M )
EQUATION 16-1
The Hamming window. These
windows run from to M, i ' 0
for a total of points. M % 1
w[i ] ' 0.42 & 0.5 cos(2Bi /M ) % 0.08 cos(4Bi /M ) EQUATION 16-2
The Blackman window.
FIGURE 16-1 (facing page)
Derivation of the windowed-sinc filter kernel. The frequency response of the ideal low-pass filter is shown
in (a), with the corresponding filter kernel in (b), a sinc function. Since the sinc is infinitely long, it must be
truncated to be used in a computer, as shown in (c). However, this truncation results in undesirable changes
in the frequency response, (d). The solution is to multiply the truncated-sinc with a smooth window, (e),
resulting in the windowed-sinc filter kernel, (f). The frequency response of the windowed-sinc, (g), is smooth
and well behaved. These figures are not to scale.
To get around this problem, we will make two modifications to the sinc
function in (b), resulting in the waveform shown in (c). First, it is truncated
to points, symmetrically chosen around the main lobe, where M is an M%1
even number. All samples outside these points are set to zero, or simply M%1
ignored. Second, the entire sequence is shifted to the right so that it runs from
0 to M. This allows the filter kernel to be represented using only positive
indexes. While many programming languages allow negative indexes, they are
a nuisance to use. The sole effect of this shift in the filter kernel is to M/2
shift the output signal by the same amount.
Since the modified filter kernel is only an approximation to the ideal filter
kernel, it will not have an ideal frequency response. To find the frequency
response that is obtained, the Fourier transform can be taken of the signal in
(c), resulting in the curve in (d). It's a mess! There is excessive ripple in the
passband and poor attenuation in the stopband (recall the Gibbs effect
discussed in Chapter 11). These problems result from the abrupt discontinuity
at the ends of the truncated sinc function. Increasing the length of the filter
kernel does not reduce these problems; the discontinuity is significant no matter
how long M is made.
Fortunately, there is a simple method of improving this situation. Figure (e)
shows a smoothly tapered curve called a Blackman window. Multiplying the
truncated-sinc, (c), by the Blackman window, (e), results in the windowedsinc
filter kernel shown in (f). The idea is to reduce the abruptness of the
truncated ends and thereby improve the frequency response. Figure (g) shows
this improvement. The passband is now flat, and the stopband attenuation is
so good it cannot be seen in this graph.
Several different windows are available, most of them named after their
original developers in the 1950s. Only two are worth using, the Hamming
window and the Blackman window These are given by:
Figure 16-2a shows the shape of these two windows for (i.e., 51 total M ' 50
points in the curves). Which of these two windows should you use? It's a
trade-off between parameters. As shown in Fig. 16-2b, the Hamming
window has about a 20% faster roll-off than the Blackman. However,
Chapter 16- Windowed-Sinc Filters 287
Time Domain
Frequency
0 0.5
-0.5
0.0
0.5
1.0
1.5
fc
a. Ideal frequency response
Sample number
-50 -25 0 25 50
-0.5
0.0
0.5
1.0
1.5
b. Ideal filter kernel
Frequency
0 0.5
-0.5
0.0
0.5
1.0
1.5
fc
d. Truncated-sinc frequency response
Sample number
0 1
-0.5
0.0
0.5
1.0
1.5
M
abrupt end
c. Truncated-sinc filter kernel
Sample number
0 1
-0.5
0.0
0.5
1.0
1.5
M
e. Blackman or Hamming window
Frequency Domain
Frequency
0 0.5
-0.5
0.0
0.5
1.0
1.5
g. Windowed-sinc frequency response
fc
Sample number
0 1
-0.5
0.0
0.5
1.0
1.5
M
f. Windowed-sinc filter kernel
FIGURE 16-1
Amplitude
Amplitude Amplitude
Amplitude Amplitude
Amplitude
Amplitude
The Scientist and Engineer's Guide to Digital Signal Processing 288
Sample number
0 10 20 30 40 50
0.0
0.5
1.0
1.5
a. Blackman and Hamming window
Blackman
Hamming
Frequency
0 0.1 0.2 0.3 0.4 0.5
0.0
0.5
1.0
1.5
b. Frequency response
Hamming
Blackman
Frequency
0 0.1 0.2 0.3 0.4 0.5
-120
-100
-80
-60
-40
-20
0
20
40
c. Frequency response (dB)
Blackman
Hamming
FIGURE 16-2
Characteristics of the Blackman and Hamming
windows. The shapes of these two windows are
shown in (a), and given by Eqs. 16-1 and 16-2. As
shown in (b), the Hamming window results in about
20% faster roll-off than the Blackman window.
However, the Blackman window has better stopband
attenuation (Blackman: 0.02%, Hamming:
0.2%), and a lower passband ripple (Blackman:
0.02% Hamming: 0.2%).
Amplitude (dB) Amplitude
Amplitude
(c) shows that the Blackman has a better stopband attenuation. To be exact,
the stopband attenuation for the Blackman is -74dB (-0.02%), while the
Hamming is only -53dB (-0.2%). Although it cannot be seen in these graphs,
the Blackman has a passband ripple of only about 0.02%, while the Hamming
is typically 0.2%. In general, the Blackman should be your first choice; a
slow roll-off is easier to handle than poor stopband attenuation.
There are other windows you might hear about, although they fall short of the
Blackman and Hamming. The Bartlett window is a triangle, using straight
lines for the taper. The Hanning window, also called the raised cosine
window, is given by: . These two windows have w[i] ' 0.5& 0.5cos(2Bi /M)
about the same roll-off speed as the Hamming, but worse stopband attenuation
(Bartlett: -25dB or 5.6%, Hanning -44dB or 0.63%). You might also hear of
a rectangular window. This is the same as no window, just a truncation of
the tails (such as in Fig. 16-1c). While the roll-off is -2.5 times faster than the
Blackman, the stopband attenuation is only -21dB (8.9%).
Designing the Filter
To design a windowed-sinc, two parameters must be selected: the cutoff
frequency, , and the length of the filter kernel, M. The cutoff frequency fC
Chapter 16- Windowed-Sinc Filters 289
Frequency
0 0.1 0.2 0.3 0.4 0.5
0.0
0.5
1.0
1.5
fC=0.05 fC=0.25
fC=0.45
b. Roll-off vs. cutoff frequency
FIGURE 16-3
Filter length vs. roll-off of the windowed-sinc filter. As shown in (a), for M = 20, 40, and 200, the transition
bandwidths are BW = 0.2, 0.1, and 0.02 of the sampling rate, respectively. As shown in (b), the shape of the
frequency response does not change with different cutoff frequencies. In (b), M = 60.
Frequency
0 0.1 0.2 0.3 0.4 0.5
0.0
0.5
1.0
1.5
M=40 M=200
M=20
a. Roll-off vs. kernel length
Amplitude
Amplitude
M . 4
BW
EQUATION 16-3
Filter length vs. roll-off. The length of the
filter kernel, M, determines the transition
bandwidth of the filter, BW. This is only an
approximation since roll-off depends on the
particular window being used.
is expressed as a fraction of the sampling rate, and therefore must be between
0 and 0.5. The value for M sets the roll-off according to the approximation:
where BW is the width of the transition band, measured from where the curve
just barely leaves one, to where it almost reaches zero (say, 99% to 1% of the
curve). The transition bandwidth is also expressed as a fraction of the
sampling frequency, and must between 0 and 0.5. Figure 16-3a shows an
example of how this approximation is used. The three curves shown are
generated from filter kernels with: . From Eq. 16-3, the M ' 20, 40, and 200
transition bandwidths are: , respectively. Figure (b) BW ' 0.2, 0.1, and 0.02
shows that the shape of the frequency response does not depend on the cutoff
frequency selected.
Since the time required for a convolution is proportional to the length of the
signals, Eq. 16-3 expresses a trade-off between computation time (depends on
the value of M) and filter sharpness (the value of BW). For instance, the 20%
slower roll-off of the Blackman window (as compared with the Hamming) can
be compensated for by using a filter kernel 20% longer. In other words, it
could be said that the Blackman window is 20% slower to execute that an
equivalent roll-off Hamming window. This is important because the execution
speed of windowed-sinc filters is already terribly slow.
As also shown in Fig. 16-3b, the cutoff frequency of the windowed-sinc filter
is measured at the one-half amplitude point. Why use 0.5 instead of the
The Scientist and Engineer's Guide to Digital Signal Processing 290
h[i ] ' K
sin (2BfC (i&M/2))
i&M/2
0.42 & 0.5cos
2Bi
M
% 0.08cos
4Bi
M
EQUATION 16-4
The windowed-sinc filter kernel. The cutoff frequency, , is expressed as a fC
fraction of the sampling rate, a value between 0 and 0.5. The length of the filter
kernel is determined by M, which must be an even integer. The sample number
i, is an integer that runs from 0 to M, resulting in total points in the filter M%1
kernel. The constant, K, is chosen to provide unity gain at zero frequency. To
avoid a divide-by-zero error, for , use . i ' M/2 h[i ] ' 2BfC K
standard 0.707 (-3dB) used in analog electronics and other digital filters? This
is because the windowed-sinc's frequency response is symmetrical between the
passband and the stopband. For instance, the Hamming window results in a
passband ripple of 0.2%, and an identical stopband attenuation (i.e., ripple in
the stopband) of 0.2%. Other filters do not show this symmetry, and therefore
have no advantage in using the one-half amplitude point to mark the cutoff
frequency. As shown later in this chapter, this symmetry makes the windowedsinc
ideal for spectral inversion.
After and M have been selected, the filter kernel is calculated from the fC
relation:
Don't be intimidated by this equation! Based on the previous discussion, you
should be able to identify three components: the sinc function, the M/2 shift,
and the Blackman window. For the filter to have unity gain at DC, the constant
K must be chosen such that the sum of all the samples is equal to one. In
practice, ignore K during the calculation of the filter kernel, and then normalize
all of the samples as needed. The program listed in Table 16-1 shows how this
is done. Also notice how the calculation is handled at the center of the sinc,
, which involves a division by zero. i ' M/2
This equation may be long, but it is easy to use; simply type it into your
computer program and forget it. Let the computer handle the calculations. If
you find yourself trying to evaluate this equation by hand, you are doing
something very very wrong.
Let's be specific about where the filter kernel described by Eq. 16-4 is located
in your computer array. As an example, M will be chosen to be 100.
Remember, M must be an even number. The first point in the filter kernel is
in array location 0, while the last point is in array location 100. This means
that the entire signal is 101 points long. The center of symmetry is at point 50,
i.e., . The 50 points to the left of point 50 are symmetrical with the 50 M/2
points to the right. Point 0 is the same value as point 100, and point 49 is the
same as point 51. If you must have a specific number of samples in the filter
kernel, such as to use the FFT, simply add zeros to one end or the other. For
example, with , you could make samples 101 through 127 equal to M ' 100
zero, resulting in a filter kernel 128 points long.
Chapter 16- Windowed-Sinc Filters 291
Filter kernel
Sample number
0 100 200 300 400 500
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
b. fC = 0.015
M = 500
Sample number
0 100 200 300 400 500
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
a. fC = 0.015
M = 500
Sample number
0 100 200 300 400 500
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
d. fC = 0.04
M = 500
Sample number
0 100 200 300 400 500
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
c. fC = 0.04
M = 500
Sample number
-175 0 175
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
150
e. fC = 0.04
M = 150
Step response
Sample number
0 100 200 300 400 500
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
f. fC = 0.04
M = 150
FIGURE 16-4
Example filter kernels and the corresponding step responses. The frequency of the sinusoidal oscillation is
approximately equal to the cutoff frequency, , while M determines the kernel length. fC
Amplitude
Amplitude
Amplitude Amplitude
Amplitude Amplitude
Figure 16-4 shows examples of windowed-sinc filter kernels, and their
corresponding step responses. The samples at the beginning and end of
the filter kernels are so small that they can't even be seen in the graphs.
Don't make the mistake of thinking they are unimportant! These samples may
be small in value; however, they collectively have a large effect on the
The Scientist and Engineer's Guide to Digital Signal Processing 292
FIGURE 16-5
Example of windowed-sinc filters. The alpha and beta rhythms in an EEG are separated by low-pass and highpass
filters with . The program to implement the low-pass filter is shown in Table 16-1. The program M ' 100
for the high-pass filter is identical, except for a spectral inversion of the low-pass filter kernel.
Frequency
0 0.1 0.2 0.3 0.4 0.5
0.0
0.5
1.0
1.5
a. Low-pass filter
Frequency
0 0.1 0.2 0.3 0.4 0.5
0.0
0.5
1.0
1.5
b. High-pass filter
Amplitude
Amplitude
Alpha wave
Beta wave
Alpha wave
Beta wave
performance of the filter. This is also why floating point representation is
typically used to implement windowed-sinc filters. Integers usually don't have
enough dynamic range to capture the large variation of values contained in the
filter kernel. How does the windowed-sinc filter perform in the time domain?
Terrible! The step response has overshoot and ringing; this is not a filter for
signals with information encoded in the time domain.
Examples of Windowed-Sinc Filters
An electroencephalogram, or EEG, is a measurement of the electrical
activity of the brain. It can be detected as millivolt level signals appearing
on electrodes attached to the surface of the head. Each nerve cell in the
brain generates small electrical pulses. The EEG is the combined result of
an enormous number of these electrical pulses being generated in a
(hopefully) coordinated manner. Although the relationship between thought
and this electrical coordination is very poorly understood, different
frequencies in the EEG can be identified with specific mental states. If you
close your eyes and relax, the predominant EEG pattern will be a slow
oscillation between about 7 and 12 hertz. This waveform is called the
alpha rhythm, and is associated with contentment and a decreased level of
attention. Opening your eyes and looking around causes the EEG to change
to the beta rhythm, occurring between about 17 and 20 hertz. Other
frequencies and waveforms are seen in children, different depths of sleep,
and various brain disorders such as epilepsy.
In this example, we will assume that the EEG signal has been amplified by
analog electronics, and then digitized at a sampling rate of 100 samples per
second. Acquiring data for 50 seconds produces a signal of 5,000 points. Our
goal is to separate the alpha from the beta rhythms. To do this, we will design
a digital low-pass filter with a cutoff frequency of 14 hertz, or 0.14
Chapter 16- Windowed-Sinc Filters 293
Frequency (discrete)
0.15 0.2 0.25
0.00
0.50
1.00
1.50
a. Frequency response
Frequency (hertz)
1500 2000 2500
-120
-100
-80
-60
-40
-20
0
20
40
b. Frequency response (dB)
FIGURE 16-6
Example of a windowed-sinc band-pass filter. This filter was designed for a sampling rate of 10 kHz. When
referenced to the analog signal, the center frequency of the passband is at 2 kHz, the passband is 80 hertz, and the
transition bands are 50 hertz. The windowed-sinc uses 801 points in the filter kernel to achieve this roll-off, and a
Blackman window for good stopband attenuation. Figure (a) shows the resulting frequency response on a linear
scale, while (b) shows it in decibels. The frequency axis in (a) is expressed as a fraction of the sampling frequency,
while (b) is expressed in terms of the analog signal before digitization.
Amplitude (dB)
Amplitude
of the sampling rate. The transition bandwidth will be set at 4 hertz, or 0.04 of
the sampling rate. From Eq. 16-3, the filter kernel needs to be about 101 points
long, and we will arbitrarily choose to use a Hamming window. The program in
Table 16-1 shows how the filter is carried out. The frequency response of the
filter, obtained by taking the Fourier Transform of the filter kernel, is shown in
Fig. 16-5.
In a second example, we will design a band-pass filter to isolate a signaling
tone in an audio signal, such as when a button on a telephone is pressed. We
will assume that the signal has been digitized at 10 kHz, and the goal is to
isolate an 80 hertz band of frequencies centered on 2 kHz. In terms of the
sampling rate, we want to block all frequencies below 0.196 and above 0.204
(corresponding to 1960 hertz and 2040 hertz, respectively). To achieve a
transition bandwidth of 50 hertz (0.005 of the sampling rate), we will make the
filter kernel 801 points long, and use a Blackman window. Table 16-2 contains
a program for calculating the filter kernel, while Fig. 16-6 shows the frequency
response. The design involves several steps. First, two low-pass filters are
designed, one with a cutoff at 0.196, and the other with a cutoff at 0.204. This
second filter is then spectrally inverted, making it a high-pass filter (see
Chapter 14, Fig. 14-6). Next, the two filter kernels are added, resulting in a
band-reject filter (see Fig. 14-8). Finally, another spectral inversion makes
this into the desired band-pass filter.
Pushing it to the Limit
The windowed-sinc filter can be pushed to incredible performance levels
without nasty surprises. For instance, suppose you need to isolate a 1 millivolt
signal riding on a 120 volt power line. The low-pass filter will need
The Scientist and Engineer's Guide to Digital Signal Processing 294
100 'LOW-PASS WINDOWED-SINC FILTER
110 'This program filters 5000 samples with a 101 point windowed-sinc filter,
120 'resulting in 4900 samples of filtered data.
130 '
140 DIM X[4999] 'X[ ] holds the input signal
150 DIM Y[4999] 'Y[ ] holds the output signal
160 DIM H[100] 'H[ ] holds the filter kernel
170 '
180 PI = 3.14159265
190 FC = .14 'Set the cutoff frequency (between 0 and 0.5)
200 M% = 100 'Set filter length (101 points)
210 '
220 GOSUB XXXX 'Mythical subroutine to load X[ ]
230 '
240 ' 'Calculate the low-pass filter kernel via Eq. 16-4
250 FOR I% = 0 TO 100
260 IF (I%-M%/2) = 0 THEN H[I%] = 2*PI*FC
270 IF (I%-M%/2) <> 0 THEN H[I%] = SIN(2*PI*FC * (I%-M%/2)) / (I%-M%/2)
280 H[I%] = H[I%] * (0.54 - 0.46*COS(2*PI*I%/M%) )
290 NEXT I%
300 '
310 SUM = 0 'Normalize the low-pass filter kernel for
320 FOR I% = 0 TO 100 'unity gain at DC
330 SUM = SUM + H[I%]
340 NEXT I%
350 '
360 FOR I% = 0 TO 100
370 H[I%] = H[I%] / SUM
380 NEXT I%
390 '
400 FOR J% = 100 TO 4999 'Convolve the input signal & filter kernel
410 Y[J%] = 0
420 FOR I% = 0 TO 100
430 Y[J%] = Y[J%] + X[J%-I%] * H[I%]
440 NEXT I%
450 NEXT J%
460 '
470 END
TABLE 16-1
a stopband attenuation of at least -120dB (one part in one-million for those
that refuse to learn decibels). As previously shown, the Blackman window
only provides -74dB (one part in five-thousand). Fortunately, greater
stopband attenuation is easy to obtain. The input signal can be filtered
using a conventional windowed-sinc filter kernel, providing an intermediate
signal. The intermediate signal can then be passed through the filter a
second time, further increasing the stopband attenuation to -148dB (1 part
in 30 million, wow!). It is also possible to combine the two stages into a
single filter. The kernel of the combined filter is equal to the convolution of
the filter kernels of the two stages. This also means that convolving any
filter kernel with itself results in a filter kernel with a much improved
stopband attenuation. The price you pay is a longer filter kernel and a
slower roll-off. Figure 16-7a shows the frequency response of a 201 point lowpass
filter, formed by convolving a 101 point Blackman windowed-sinc with
itself. Amazing performance! (If you really need more than -100dB of
stopband attenuation, you should use double precision. Single precision
Chapter 16- Windowed-Sinc Filters 295
100 'BAND-PASS WINDOWED-SINC FILTER
110 'This program calculates an 801 point band-pass filter kernel
120 '
130 DIM A[800] 'A[ ] workspace for the lower cutoff
140 DIM B[800] 'B[ ] workspace for the upper cutoff
150 DIM H[800] 'H[ ] holds the final filter kernel
160 '
170 PI = 3.1415926
180 M% = 800 'Set filter kernel length (801 points)
190 '
200 ' 'Calculate the first low-pass filter kernel via Eq. 16-4,
210 FC = 0.196 'with a cutoff frequency of 0.196, store in A[ ]
220 FOR I% = 0 TO 800
230 IF (I%-M%/2) = 0 THEN A[I%] = 2*PI*FC
240 IF (I%-M%/2) <> 0 THEN A[I%] = SIN(2*PI*FC * (I%-M%/2)) / (I%-M%/2)
250 A[I%] = A[I%] * (0.42 - 0.5*COS(2*PI*I%/M%) + 0.08*COS(4*PI*I%/M%))
260 NEXT I%
270 '
280 SUM = 0 'Normalize the first low-pass filter kernel for
290 FOR I% = 0 TO 800 'unity gain at DC
300 SUM = SUM + A[I%]
310 NEXT I%
320 '
330 FOR I% = 0 TO 800
340 A[I%] = A[I%] / SUM
350 NEXT I%
360 ' 'Calculate the second low-pass filter kernel via Eq. 16-4,
370 FC = 0.204 'with a cutoff frequency of 0.204, store in B[ ]
380 FOR I% = 0 TO 800
390 IF (I%-M%/2) = 0 THEN B[I%] = 2*PI*FC
400 IF (I%-M%/2) <> 0 THEN B[I%] = SIN(2*PI*FC * (I%-M%/2)) / (I%-M%/2)
410 B[I%] = B[I%] * (0.42 - 0.5*COS(2*PI*I%/M%) + 0.08*COS(4*PI*I%/M%))
420 NEXT I%
430 '
440 SUM = 0 'Normalize the second low-pass filter kernel for
450 FOR I% = 0 TO 800 'unity gain at DC
460 SUM = SUM + B[I%]
470 NEXT I%
480 '
490 FOR I% = 0 TO 800
500 B[I%] = B[I%] / SUM
510 NEXT I%
520 '
530 FOR I% = 0 TO 800 'Change the low-pass filter kernel in B[ ] into a high-pass
540 B[I%] = - B[I%] 'filter kernel using spectral inversion (as in Fig. 14-5)
550 NEXT I%
560 B[400] = B[400] + 1
570 '
580 '
590 FOR I% = 0 TO 800 'Add the low-pass filter kernel in A[ ], to the high-pass
600 H[I%] = A[I%] + B[I%] 'filter kernel in B[ ], to form a band-reject filter kernel
610 NEXT I% 'stored in H[ ] (as in Fig. 14-8)
620 '
630 FOR I% = 0 TO 800 'Change the band-reject filter kernel into a band-pass
640 H[I%] = -H[I%] 'filter kernel by using spectral inversion
650 NEXT I%
660 H[400] = H[400] + 1
670 ' 'The band-pass filter kernel now resides in H[ ]
680 END
TABLE 16-2
The Scientist and Engineer's Guide to Digital Signal Processing 296
Frequency
0 0.1 0.2 0.3 0.4 0.5
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
60
a. Incredible stopband attenuation
round-off noise
single precision
!
FIGURE 16-7
The incredible performance of the windowed-sinc filter. Figure (a) shows the frequency response of a
windowed-sinc filter with increased stopband attenuation. This is achieved by convolving a windowed-sinc
filter kernel with itself. Figure (b) shows the very rapid roll-off a 32,001 point windowed-sinc filter.
Frequency
0.1995 0.2 0.2005
0.0
0.5
1.0
1.5
b. Incredible roll-off !
Amplitude (dB)
Amplitude
round-off noise on signals in the passband can erratically appear in the
stopband with amplitudes in the -100dB to -120dB range).
Figure 16-7b shows another example of the windowed-sinc's incredible
performance: a low-pass filter with 32,001 points in the kernel. The frequency
response appears as expected, with a roll-off of 0.000125 of the sampling rate.
How good is this filter? Try building an analog electronic filter that passes
signals from DC to 1000 hertz with less than a 0.02% variation, and blocks all
frequencies above 1001 hertz with less than 0.02% residue. Now that's a
filter! If you really want to be impressed, remember that both the filters in Fig.
16-7 use single precision. Using double precision allows these performance
levels to be extended by a million times.
The strongest limitation of the windowed-sinc filter is the execution time; it can
be unacceptably long if there are many points in the filter kernel and standard
convolution is used. A high-speed algorithm for this filter (FFT convolution)
is presented in Chapter 18. Recursive filters (Chapter 19) also provide good
frequency separation and are a reasonable alternative to the windowed-sinc
filter.
Is the windowed-sinc the optimal filter kernel for separating frequencies? No,
filter kernels resulting from more sophisticated techniques can be better. But
beware! Before you jump into this very mathematical field, you should
consider exactly what you hope to gain. The windowed-sinc will provide any
level of performance that you could possibly need. What the advanced filter
design methods may provide is a slightly shorter filter kernel for a given level
of performance. This, in turn, may mean a slightly faster execution speed. Be
warned that you may get little return for the effort expended.