To create different equalizer or octave plots for audio analysis, it is often needed to define a bank
of bandpass filters with different, possibly overlapping, frequency ranges. This task can be
performed by using "Custom filter bank" option in SIGVIEW.
Creating filter bank files
For each custom filter bank, a definition file with the extension "FLB" is created and stored in the
"filters" subdirectory of the SIGVIEW application data directory. Two example files will be stored
there during SIGVIEW installation, "octave_filter.flb" and "1_3_octave_filter.flb". These files are
used for Octave plots functions.
Each FLB file contains multiple rows, each representing one bandpass filter in the filter bank. For
First row defines a filter with bandpass from 14-18 Hz. As a label for this band in a SIGVIEW graphics, a "A" will be shown.
Second row defines a filter with bandpass from 18-22.4 Hz. As a label for this band in a SIGVIEW graphics, a "B" will be shown.
Second row defines a filter with bandpass from 22.4-28 Hz. Since a label is not defined for this
filter, a band index, i.e. "3" will be used as label.
Each filter bank has to contain at least 4 filters in order to be used.
Settings in filter bank files
Besides frequency limits for bandpass filters, you can define some additional settings for the
calculation in the FLB file as well. All settings are added as text lines beginning with "#" character.
The following settings are possible:
#X_LABEL defining label for X-axis (for example #X_LABEL=Hz )
#Y_LABEL defining label for Y-axis (for example #Y_LABEL=dB )
#FULL_SCALE defining the value to be considered as a "full scale" value in signal units when
calculating energy of the filtered signal in dB ( for example#FULL_SCALE=32768 )
#LINEAR_VALUES is a flag (without parameters) which switches the calculation of energy in
single bands from dB to linear values. In that case, FULL_SCALE option has no effect. It is also
possible to switch fast between dB/linear values from the context menu of each octave plot
This would be an example of FLB file with some settings described above:
Please note that the settings names are case-sensitive.
Applying filter bank function
To apply one of defined filter bank files to a signal, use "Signal Tools/Custom filter bank..." menu
option and choose the FLB file you would like to apply.
After a specific filter bank file is applied to a signal, the following procedure will performed:
1. The signal will be filtered with each filter from the filter bank
2. The energy of the filtered signal (RMS) in dB (logarithmic scale) will be stored as a next value
in the "Custom filter bank plot".
3. After all values for all filters are calculated, a plot will be shown. By default, these plots are
displayed by using histogram display type (min-reference). As for all other plots, you can change
the display type by using a context menu or toolbar buttons (Display type option).