Decimator

Overview

Decimators can be used to drop the frequency of raw audio data.

For example, you may have a raw audio data stream sampled at 96kHz (you might have sampled at this search for odontocete clicks); you also want to search for baleen whale sounds at around 100Hz. It would be inefficient to run the baleen whale detector on the 96kHz data. However, by using the Decimator to produce a new raw data stream at a few 100Hz and running the baleen whale detector on this, the total amount of processing will be reduced, spectrogram displays will also be more clearer.

A decimator can be inserted anywhere in the PAMGuard data model where there are raw audio data.

Creating a Decimator

From the File>Add modules>Sound Processing menu, or from the pop-up menu on the data model display select “Decimator”. Enter a name for the new decimator module (e.g. “2kHz”, etc) and press Ok.

Configure the Decimator

The Decimator module requires a source of raw data before it can operate. This may come directly from a Sound Acquisition module (e.g. a sound card or a National Instruments board) or from processed data such as the output from a filter, another decimator, etc.

To configure the decimator, go to the Detection>“your decimator name …”> menu

Select the data source and specify the sample rate of the output data.

Filter Settings

For a decimator to work correctly, you must correctly filter the data before it is downsampled.

Press the filter settings dialog and the filter dialog will appear (see the IIRF Filter help). You need to set up the filter so that most of the energy is filtered out below the Nyquist frequency (half the output sample rate). A low pass 4th or 6th order Chebyshev filter with a cut off frequency of 0.8 times the Nyquist frequency should be adequate.