Custom filter bank plot
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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 example:

14;18;A
18;22.4;B
22.4;28
...................

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 window

This would be an example of FLB file with some settings described above:

#X_LABEL=Bands
#Y_LABEL=dBFS
#FULL_SCALE=32768
14;18;A
18;22.4;B
22.4;28;C
...................

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).

graphic
 
 
Custom filter bank plot