This sort of shift is also used in most bat detectors.

The instrument used initially was a wide band receiver in the 25-80kHz range, adapted from a 'bat detector'.

This makes the call sound different on a bat detector:

Some bat detectors combine two or all three types.

Thus it is less labour intensive than using a manned bat detector in real time.

For readers not familiar with the different types of bat detector, there is further information below and elsewhere.

On a bat detector it sounds like a sharp click.

A whole world of ultrasound opens up when a bat detector is switched on.

Note the reaction of one cricket when approached by a bat detector.

This is heard as a characteristic peeping sound on a bat detector.