Spectrograms

A spectrogram is processed as a matrix of real values but visualized as a grey-scale image. Each row of pixels is a frequency bin and each column of pixels is a time-frame. The value in each spectrogram/matrix cell (represented visually by one image pixel) is the acoustic intensity in decibels with respect to the background noise baseline. Note that the decibel values in a noise-reduced spectrogram are always positive.

Throughout AP you'll see references to spectrogram parameters, such as in the parameters for generic recognizer algorithms. They are FrameSize, FrameStep, WindowFunction and BgNoiseThreshold.

FrameSize

Sets the size of the FFT window used to make the spectrogram. A good default value for detecting typical animal calls is 512. If your target syllable is of long duration with little temporal variation (e.g. a one-second long bird whistle) then FrameSize can be increased to 1024 or even 2048.

Note

The value of FrameSize must be a power of 2.

FrameSize determines the time and frequency resolutions along the x-axis and y-axis (respectively) of the spectrogram. There is a trade-off between these; that is, increasing the resolution of one will decrease the resolution of the other. Finding the best compromise is really a matter of trial and error.

FrameStep

Sets the number of samples between the start of one frame and the next. Therefore it controls frame overlap. FrameStep must be less than FrameSize but need not be a power of 2. By default FrameStep equals FrameSize but it is frequently set to half the frame size.

Note

To capture more temporal variation in your target syllables, decrease FrameSize and/or decrease FrameStep.

WindowFunction

Sets the FFT window function. It can be one of the values from WindowFunctions. Hanning is the default because we find it the most versatile. There should never be a need to change this but you might like to try a HAMMING` window if you are not satisfied with the appearance of your spectrograms.

BgNoiseThreshold

Sets the degree of severity of noise removal from the spectrogram. The "Bg" in BgNoiseThreshold means background. The units are decibels. Zero sets the least severe noise removal. This is the safest default value and probably does not need to be changed. Increasing the value to say 3-4 decibels increases the likelihood that you will lose some important components of your target calls. For more on the noise removal algorithm used by AP see Towsey, Michael W. (2013) Noise removal from wave-forms and spectrograms derived from natural recordings of the environment..