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.TH HF 1 "Ultrasonics Laboratory"
.SH NAME
hf - Homorphic Filter Waveforms
.SH SYNOPSIS
hf [options...]
.SH DESCRIPTION
This program uses a homorphic filter to remove
multipath artifacts from ultrasonic waveforms.
The program reads in each received waveforms,
applies a homorphic filter and then
estimates the attenuation using either
the frequency shift method or
the spectral division method.
Normally the input data consists of
128 element data representing the magnitude
of the Fourier Transform of each received waveform.
.PP
After homorphic filtering
two types of
post processing are done.
The resulting
attenuation estimates are first processed
to remove any slant
in the projection data.
Because the transmitter and receiver do
not travel along parallel paths there
will be a linear slant to the scanner data.
This slant is removed after making the
attenuation estimate.
In addition,
adjancent scans are done in opposite directions.
Normally both of these artifacts are removed
before the attenuation projection data
is written out to a file for
filtering and back projection.
See 
.B scan(1)
for more details.
.PP
There are many variables that can be set from
the command line to control the filtering.
The variables and their default values 
are:
.TP 13
n=100
the number of rays per projection.
.TP
k=1
the number of projections.
.TP
gamma=1
is the power that frequency is raised to in the
attenuation model.
.TP
sigma=3
is the variance of the Gaussian waveform model.
.TP
gammap=1
is gamma prime.
.TP
dt=.05
is the sampling interval (in micro-seconds) of the input data.
.TP
b1=10
is the lower limit of the band for spectral
division in FFT sample units.
.TP
b2=50
is the upper limit of the band for spectral
division in FFT sample units.
.TP
band=20
is the number of samples in the cepstrum that are kept in the
homorphic filter.
This corresponds to a low pass filtering of the waveform.
.TP
if=udata
is the input file with each of the received waveforms
If the spectrum of each waveform is used as input the data
should be floating point numbers.
Otherwise the input should be integers (as output by scan)
representing time samples of the waveform.
.TP
of=for003.dat
is the file where the
resulting attenuation data is stored
by projection.
.TP
sf=spectrum
is the file where the
spectrum of each received waveform
is stored.
Setting this variable also sets the
.B -s
flag.
.TP
cf=cepstrum
is the file where the
cepstrum of each of the received waveforms
is stored.
Setting this variable also set
the 
.B -c
flag (see below).
.PP
The flags that can be set from the command line
are shown below.
The default value of each flag is shown
in parenthesis.
.TP 13
-p(off)
do not print the program parameters.
.TP
-d(off)
use spectral division to estimate attenuation.
If this flag is off then the frequency shift method
is used to make the estimation.
.TP
-n(off)
do not apply a homorphic filter to the signal.
.TP
-r(off)
do not post process each projection.
No calculation is made of the slant
and all data is assumed to be in the
proper order.
.TP
-w(off)
do not swap
alternating projections.
The slant in each projection is removed unless the
.B -r
flag is also set.
.TP
-t(off)
input data is in the time domain.
Otherwise the input data is assumed
to be the magnitude of the Fourier
Transform.
.TP
-s(off)
save the magnitude of the spectrum of each
received waveform.  
The spectrums are stored in the
file indicated by the variable 
.B sf.
.TP
-c(off)
save the cepstrum of each received waveform.
The cepstrums are stored in
the file indicated by the variable
.B cf.
.TP
-S(off)
save the magnitude of the spectrum after
filtering in the file 
.B fspec
.SH FILES
.TP 13
udata
is the default input file.  
The data is the magnitude of the
Fourier Transform unless the 
.B -t
flag is set.
.TP
for003.dat
is the default output file.
The resulting output data is single
precision binary numbers that
estimate the
attenuation for each ray in the projection.
.TP
spectrum
is the file where the spectrum of each received waveform is
stored.
This file can be changed with the
variable
.B sf.
.TP
fspec
is the file where the spectrum after filtering is
stored.
See the 
.B -S 
flag above.
.SH SEE ALSO
Crawford, Carl, 
"Multipath Artifacts in Ultrasonic Transmission Tomography,"
TR-EE 81-43,
December 1981.
.SH DIAGNOSTICS
All IO errors are indicated.