Triana Help Index,
Frequency-Domain
Filtering in Triana
Time-Domain Filtering in Triana
Index
Triana
Units for Time-Domain Filtering
There are a number of units in Triana that
perform filtering operations in the time domain. In this help file we list
the units and then suggest ways to use them to solve important filtering
problems.
The time-domain filtering units are in the SignalProc/Filtering toolbox.
They are:
-
FIR_LowPass -- A low-pass filter that smooths
the result with a windowing function.
-
FIR_HighPass -- A high-pass filter that
smooths the result with a windowing function.
-
FIR_BandPass -- A band-pass filter that
smooths the result with a windowing function.
-
FIR_Filter -- A general FIR filtering unit
that will apply any filter whose coefficients are supplied by the user.
-
IIR_Filter -- A general IIR filtering unit
that will apply any filter whose coefficients are supplied by the user.
The Filtering toolbox also contains group units that are useful for inspecting
filters and deciding which ones to use. These are LookFIR_Band, LookFIR_Low,
LookFIR_Hig, SpectFIR_Band, SpectFIR_Low, and SpectFIR_High.
In addition there are units that are useful for display: Grapher,
MultiGraph,
ImageMapper,
and ImageView.
For an introduction to frequency-domain filtering in Triana, see the help
file Frequency-Domain
Filtering in Triana.
Using Time-Domain Filtering Tools
Most of the tools offer the user options in their parameter windows. Naturally,
filtering units must be told the frequencies they are to select. In addition,
three choices appear in many of the units: "Filter Order...",
"Window",
"...filter
continued across successive input sets". They are explained here.
-
"Filter Order ..." The user gives here the
number of points used in convolving the filter with the data in the time
domain. The more points, the sharper will be the filter edges, but the
longer the computation will take.
-
"Window" Sharp edges in the Fourier transform
version of the filter produce oscillations in the time domain after filtering.
If these are unwanted, then the edges can be smoothed by applying a window
function to the time-domain representation of the filter before it is convolved
with the data. Window functions are smooth functions that have a value
close to 1 over most of their domain, falling symmetrically to very small
values at either end. Triana units use the
window functions provided by the unit WindowFnc.
The default window is "Rectangle", which does no smoothing at all.
-
"Check here if you want filter continued across
successive input sets" Select this checkbox to ensure that the input
data stream is treated in a continuous manner even if it is split between
successive input data sets. The previous data set is remembered in order
to ensure that filtering on the first elements of the current data set
is done properly.
Group Units
The group units provide means of inspecting the filters. The groups called
"Look..." plot a graph of the output of a filter on a unit impulse: this
is the time-domain impulse response of the filter, and it shows the oscillations
or lack of them created by the filter. The groups called "Spect..." plot
a graph of the power spectrum of the output of the "Look..." units, which
is then the frequency-domain impulse response function. This shows how
much spectral leakage there is out of the pass-band. Graphs are displayed
with logarithmic vertical axes.
To use the group units, drag them onto the working area of Triana and
double-click them to open a separate window showing the individual units,
or right-click the group unit and choose "Ungroup" to see all the individual
units. Then double-click the filtering unit to get the unit's parameter
window, which will allow you to choose the window function that you want
to use. Then run the group in Single Step mode.