These detectors record sounds at their original frequencies, i.e. ultrasonic sounds are not converted to a lower frequency in order to make them audible. They record the full frequency range up to a limit determined by the sampling rate used (maximum recordable frequency is half the recording sampling rate, so if the detector's sampling rate is, for example, 256 kHz, it will record sounds up to 128 kHz, while a sampling rate of 384 kHz will record sounds up to 192 kHz).
These detectors offer the 'best of both worlds' between Time Expansion and Frequency Division detectors. They are able to capture sound in the same high level of detail as Time Expansion, but they record in real time continuous monitoring as with Frequency Division.
The resulting sound files are very large so these detectors tend to use a triggering system so that recordings are made only when sounds are above certain frequency and amplitude thresholds.
These detectors are generally in the higher price range for detectors and are nowadays typically the system of choice for most professionals and researchers, although cheaper models have appeared in recent years enabling them to become increasingly popular with amateur bat enthusiasts.