Signal Processing Toolbox |
Convert zero-pole-gain filter parameters to second-order sections form
Syntax
[sos,g] =
zp2sos(z,p,k)
[sos,g] = zp2sos(z,p,k,'
order
')
[sos,g] = zp2sos(z,p,k,'
order
','
scale
')
sos = zp2sos(...)
Description
zp2sos
converts a discrete-time zero-pole-gain representation of a given digital filter to an equivalent second-order section representation.
Use
to obtain a matrix [sos,g] =
zp2sos(z,p,k)
sos
in second-order section form with gain g
equivalent to the discrete-time zero-pole-gain filter represented by input arguments z
, p
, and k
. Vectors z
and p
contain the zeros and poles of the filter's transfer function H(z), not necessarily in any particular order.
where n and m are the lengths of z
and p
, respectively, and k
is a scalar gain. The zeros and poles must be real or complex conjugate pairs. sos
is an L-by-6 matrix
whose rows contain the numerator and denominator coefficients bik and aik of the second-order sections of H(z).
The number L of rows of the matrix sos
is the closest integer greater than or equal to the maximum of n/2 and m/2.
specifies the order of the rows in [sos,g] = zp2sos(z,p,k,'
order
')
sos
, where '
order
'
is
'down'
, to order the sections so the first row of sos
contains the poles closest to the unit circle
'up'
, to order the sections so the first row of sos
contains the poles farthest from the unit circle (default)
specifies the desired scaling of the gain and the numerator coefficients of all second-order sections, where [sos,g] = zp2sos(z,p,k,'
order
','
scale
')
'
scale
'
is
'none'
, to apply no scaling (default)
'inf'
, to apply infinity-norm scaling
'two'
, to apply 2-norm scaling
Using infinity-norm scaling in conjunction with up
-ordering minimizes the probability of overflow in the realization. Using 2-norm scaling in conjunction with down
-ordering minimizes the peak round-off noise.
sos
embeds the overall system gain, = zp2sos(...)
g
, in the first section, H1(z), so that
Note Embedding the gain in the first section when scaling a direct-form II structure is not recommended and may result in erratic scaling. To avoid embedding the gain, use ss2sos with two outputs. |
Examples
Find a second-order section form of a Butterworth lowpass filter:
Algorithm
zp2sos
uses a four-step algorithm to determine the second-order section representation for an input zero-pole-gain system:
cplxpair
function.
zp2sos
groups real poles into sections with the real poles closest to them in absolute value. The same rule holds for real zeros.
zp2sos
normally orders the sections with poles closest to the unit circle last in the cascade. You can tell zp2sos
to order the sections in the reverse order by specifying the down
flag.
zp2sos
scales the sections by the norm specified in the '
scale
'
argument. For arbitrary H(), the scaling is defined by
See Also
cplxpair
, filternorm
, sos2zp
, ss2sos
, tf2sos
, zp2ss
, zp2tf
References
[1] Jackson, L.B., Digital Filters and Signal Processing, 3rd ed., Kluwer Academic Publishers, Boston, 1996, Chapter 11.
[2] Mitra, S.K., Digital Signal Processing: A Computer-Based Approach, McGraw-Hill, New York, 1998, Chapter 9.
[3] Vaidyanathan, P.P., "Robust Digital Filter Structures," Handbook for Digital Signal Processing, S.K. Mitra and J.F. Kaiser, ed., John Wiley & Sons, New York, 1993, Chapter 7.
zerophase | zp2ss |
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