times of interest more quickly when you get the data back. The downside of course is that you do
not have the full wideband recording on the SoundTrap. This is mitigated in SoundTrap’s detector by
recording a short snippet of wideband sound around each detection. A few 100's of µs is often all
that is needed to classify odontocete clicks into general classes. The detector can be configured to
also make a continuous recording at lower bandwidth that allows you to quantify the general
ambient noise conditions, an important factor affecting detector performance, as well as the
presence of low frequency whistles that can help with species identification.
4.1.1
Click detector
The click detector comprises an approximate pre-whitening filter followed by a power-in-band
Constant False Alarm Rate (CFAR) detector. The whitening filter (a 35kHz high-pass filter) corrects
the typical low-frequency emphasis of underwater ambient noise creating a more spectrally-
balanced signal for the detector. The power-in-band detector consists of a highpass Finite Impulse
Response (FIR) filter (approximate 3 dB bandwidth 115 - 160 kHz) followed by a moving average
power estimator. The duration of the moving average window is a user parameter (see below). In
parallel, the power of the full bandwidth whitened signal is measured using a much longer
exponential window to give an estimate of the ambient noise power. This is used to adjust the
absolute detection threshold: a detection will be made if the power-in-band is greater than the
ambient power by more than the Relative Detection Threshold (RDT) which is a user parameter.
When a detection is made, the time of the detection and the ambient noise power at the time are
passed to the snippet extractor. A user-selectable blanking time must then elapse before another
detection can be made. This blanking-time is useful to avoid detecting echoes from a click or to
prevent detecting a large number of clicks if an animal is buzzing near the recorder.
4.1.2
Snippet extractor
For each detection, an entry is added to the detection log and a snippet of wideband sound
surrounding the detection time is saved. The snippet starts and ends a user-selectable time before
and after the detection (user parameters:
pre-trigger
and
post-trigger
). Evidently, saving longer
snippets per detection means that the memory is used up more rapidly so this feature allows the
user to trade-off the amount of context that is saved for each detection against the total recording
time. It is important to note that the snippet is taken from the whitened sound and so is high-pass
filtered.
Содержание SoundTrap ST400
Страница 1: ...SoundTrap ST400 User Guide September 2021...
Страница 9: ...Figure 1 The Device List...
