SUMMARY

Produces a graphic display of a filter's digital vs. analog characteristics for: amplitude, phase, and impulse response curves, and the group delay.

SYNTAX

FILTERDESIGN [PRINT [pname] ] [FILE [prefix] ][filteroptions] [delta]

where filteroptions are the same as those used in the various filter commands in SAC, including the filter type. delta is the sampling interval of the data

Note Order of options is important. If the PRINT option is used, it must be the first option. If the FILE option is used, it must precede the filter options.

INPUT

PRINT {pname}:
Prints the resulting plot to the printer named in pname, or to the default printer if pname is not used.

Note this must be the first option given on the command line. (This makes use of the SGF capability.)

FILE {prefix}:
Writes three SAC files to disk. Theses files contain the digital responses determined in the FILTERDESIGN:

[prefix].spec:
is of type IAMPH, and contains both the amplitude and phase information from the FILTERDESIGN.

[prefix].gd:
is of type ITIME, and contains the group delay information from the FILTERDESIGN.

Note that in spite of the fact that the file is of type ITIME, group delay is a function of frequency. It is incumbent upon the user to remember that even though the plots will have seconds for units, the actual units are hertz.

[prefix].imp:
is of type ITIME, and contains the impulse response.

In each of these SAC files, the user header fields are set as follows:

user0:
pass code

1: low pass

2: high pass

3: band pass

4: band reject

user1:
type code

1: Butterworth

2: Bessel

3: C1

4: C2

user2:
number of poles

user3:
number of passes

user4:
tranbw

user5:
attenuation

user6:
delta

user7:
first corner

user8:
second corner if present, or -12345 if not

kuser0:
pass (lowpass, highpass, bandpass, or bandrej)

kuser1:
type (Butter, Bessel, C1, or C2 )

PRINT {pname}:	Prints the resulting plot to the printer named in pname, or to the default printer if pname is not used. Note this must be the first option given on the command line. (This makes use of the SGF capability.)
FILE {prefix}:	Writes three SAC files to disk. Theses files contain the digital responses determined in the FILTERDESIGN:
[prefix].spec:	is of type IAMPH, and contains both the amplitude and phase information from the FILTERDESIGN.
[prefix].gd:	is of type ITIME, and contains the group delay information from the FILTERDESIGN. Note that in spite of the fact that the file is of type ITIME, group delay is a function of frequency. It is incumbent upon the user to remember that even though the plots will have seconds for units, the actual units are hertz.
[prefix].imp:	is of type ITIME, and contains the impulse response.

DEFAULT VALUES

Only the delta parameter has a default (0.025 seconds). Options for filter type and related parameters must be supplied.

DESCRIPTION

The FILTERDESIGN command is implemented through XAPiir, a basic recursive digital filtering package (see REFERENCES). XAPiir implements the standard recursive digital filter design through bilinear transformation of prototype analog filters. These prototype filters, specified in terms of poles and zeros, are then transformed to highpass, bandpass and band reject filters using analog spectral transformations. FILTERDESIGN displays digital filter responses as solid lines and analog responses as dashed lines. On color monitors, digital curves are blue while analog curves are amber.

EXAMPLES

The following example shows how the FILTERDESIGN command is used to produce the digital and analog response curves for a highpass, 2 Hz., six pole, two pass filter on data with a sampling rate of .025 seconds.:
SAC> fd hp c 2 n 6 p 2 delta .025

SEE COMMANDS

HIGHPASS, LOWPASS, BANDPASS, BANDREJECT UCRL-ID-106005. XAPiir: A Recursive Digital Filtering Package. David Harris. September 21, 1990 In Xwindows, a linestyle problem may cause both analog and digital traces to plot as solid lines.

LATEST REVISION

July 22, 1991 (Version 0.58)