Spectrogram settings

Aim

To optimize the appearance of the spectrograms.

How to access these settings

nIn the Call Detection screen or in the Call Labeling screen, under View, click Settings next to Spectrogram.

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nRight-click anywhere on the spectrogram.

SFT length

UltraVox XT uses Short-time Fourier Transform (SFT) to create a spectrogram. In this process, digital data is broken in sliding frames containing a number of samples of the signal. Select here the frame length (minimum = 64, maximum = 2048).

A higher value of SFT length increases results in a higher frequency resolution and a less coarse image in the spectrogram; however, it reduces the time resolution, that is, the ability to discriminate between two events at two time points). That is because at a fixed sampling frequency, there is a trade-off between frequency resolution and time resolution (for details, see Frequency resolution and time resolution).

nThe higher SFT length, the faster the processing time.

nLower SFT length is good for wide-band spectrograms, while higher SFT length is good for narrow-band spectrograms.

nChoose a high SFT length when you are interested in an accurate measurement of the dominant frequency of vocalizations. Choose a low SFT length when you are more interested in the timing of vocalizations.

nChoose SFT length in such a way that waveform frequency resolution is smaller than the minimum spacing between the frequencies of interest.

Overlap

The Overlap setting determines which percentage of the frame of samples is re-used for the next frame.

Select the value you require (0 - 95%; default 50%). High Overlap percentages (90% and higher) result in the highest time resolution (that is, the ability to discriminate events at two different time points).

To speed up call detection, select a lower value. However, if you use two call definitions A and B that differ by a small range of frequencies, two calls of different type may be detected as both A or both B.

See also

nOverlap

Amplitude scale

See Amplitude scale

Frequency scale

See Frequency scale

Notes

nThe Zero pad option has been removed in UltraVox XT 4. No zero padding is applied to the analysis frame, meaning that the length of the spectrum is equal to SFT length divided by 2. That is the same as Zero pad = 1 in UltraVox XT 3.

nNote that zero padding does not affect the waveform frequency resolution, which is the minimum spacing between two frequencies that can be distinguished. The waveform frequency resolution is always equal to R= 1/T, where T is the length of the original (unpadded) signal (see Frequency resolution and time resolution).