The zoom box, situated at the bottom of the Spectral view dialog, shows the distance (expressed in milliseconds) between the centers of consecutive frames. By means of decreasing this value (set by default to 32 msec) you can zoom in on a selected part of a time-frequency plot, i.e. analyze a shorter fragment of the signal. Increasing the time gap (up to approximately one second) will result in widening the spectral panorama - at the expense of time resolution.
When the distance between successive frames is larger than the frame size some parts of the signal are not sampled. To guarantee a 100% coverage of the inspected fragment (which is important, for example, when trying to spot a disturbance), the zoom coefficient must be kept smaller than the length of the analysis frame (set independently for the FFT and AR spectral estimators) divided by the sampling frequency (expressed in kHz). Whenever the above condition is fulfilled the red full coverage indicator placed in the Zoom group is lit.
NOTICE
The spectral panorama shown after zooming in or out is always centered on the active time frame.