Connected Region Search
Whistle and Moan Detector
Connected Region Search
Once the threshold has been applied to the spectrogram data in the noise removal stage, the Whistle and Moan detector searches for connected regions in the spectrogram matrix.
Min Frequency and Max Frequency
Usually, you will want the Whistle and Moan detector to search the whole range of available frequencies, from 0 Hz to half the sample rate. However, these parameters give you the opportunity to limit the connected region search to a specific frequency band. This can be particularly useful if your data are contaminated by a lot of noise at a particular frequency.
Connection Type
When searching for connected regions, two main connection types are available.
Connect 4 tries to connect each pixel to other pixels directly above, below, to the left or to the right (i.e. 4 possible connections).
Connect 8 tries to connect to the sides and on the diagonals (i.e. 8 possible connections).
Clearly connect 8 will detect more whistles and can also detect whistles sweeping more rapidly, however connect 8 may also be prone to a higher rate of false alarm if it connects things which shouldn’t be connected.
Minimum length and Minimum total size
There will always be a small number of random spectrogram pixels which are above threshold but are not part of a whistle or tone. Generally (hopefully !) these will few in number and the chances of them connecting to other random pixels to make a large connected region will be small.
All connected regions which are shorter than the minimum length, or have fewer pixels than the minimum total size will be discarded.