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F-POD vs. other loggers

How do F-PODs and conventional recorders compare?

Conventional sound recorders provide useful data for sound level studies and capture dolphin whistles and baleen whale calls. F-PODs do none of those and the data does not allow the Fourier Transform to be used to determine the frequency spectrum of complex sounds.

To collect the same cetacean click data as an F-POD, a conventional recorder must sample at approx. 500kHz, and use 16bit or larger values for each sample, so they require 1TB of memory every 10 days or less, which is 2,000 times as much memory as an F-POD requires. These huge data volumes are reduced by data compression but still force the use of intermittent logging by WAVE recorders, and require archiving that is not carbon neutral.

F-POD cetacean click data does not suffer clipping when the sound is very loud – this is handled in real-time processing.

F-PODs run for over 4 months continuously (or 8 months on lithium batteries) while conventional recorders sampling at 500kHz require more servicing visits due to battery or memory constraints. An F-POD can log alternate days for more than a year.

The F-POD data is optimised for the process of cetacean click train detection that is carried out during post-processing on a PC by the KERNO-F classifier. This is fast, consistent and free. Methods used on conventional data can be very slow and subjective .

The KERNO-F classifier also pulls out the click rates in fast click bursts that are used as social communication in many toothed cetaceans. This is proving to be of real biological significance.

The F-POD records the temperature and its angle to vertical every minute.

The F-POD housing is very robust and is externally simple so that bio-fouling can be removed easily.

F-PODs are buoyant and marked with a web URL so many have been recovered having been lost and subsequently washed up on a distant shore.

Data streaming and an integrated acoustic release are under development.

Conventional WAVE file recorders need to operate at 400kHz or above to record porpoises and they then typically record 1TB of data every 10days.

Such WAVE file data allows the use of the Fourier Transform (as FFT) to give frequency spectra. However very short sounds, like cetacean clicks, are not well suited to the FFT which also ‘loses’ half the information it receives. That data is not really lost, but is in a set of phase values that is of such limited practical use that it is rarely even mentioned. But it contains the information on when things happened in the click, i.e. the time domain.

The F-POD data is not suitable for FFT analysis, except in the case of the full waveform capture data.

Data from WAVE file instruments logging above 330kHz can be converted to ‘virtual F-POD files’ to get the advantages of data processing in the F-POD software. The classifier performance is then a long way below the normal performance on real F-POD data but can be useful.

F-POD features

F-PODs key features include: automation, long operation times, robustness, low false positive rates, high sensitivity and precise calibration. An F-POD:

stores very high resolution (250ns) time-domain data on each click to enable powerful train detection and species classification, so the need for visual editing of data is generally removed, although some visual inspection of each data file is mandatory.

detects and characterises short dolphin clicks more efficiently.

has automated adaptation to noise so that it does not often max out, even in severe conditions.

writes normal files to any micro SD card, up to 256 GB. 32GB is more than enough for a year of data.

for sites with many sonars it can run two real-time sonar detectors that can filter out boat sonars, and record their detections.

has a simple on-board train detection that selects some clicks from trains so that some representative full waveforms can be saved.

has an extremely robust, sensitive, hydrophone.

has a real-time clock which can be set by users, e.g. to local time rather than UTC.

the standard F-POD takes primary lithium batteries without any modification.

the rechargeable lithium version can run continuously for 8months +

has a deep-sleep mode which enables the POD to run for years, sampling every nth minute.

can be set to start at a later date to enable ‘daisy-chaining’ of instruments.

can be set to switch on and off at different angles to the vertical.

Comparing PODs to alternative instruments

Comparisons between different instruments are difficult because sensitivity can always be increased by accepting a greater rate or risk of false positives.

This outlines some of the issues and illustrates some of the errors made!

Updated: 2024