Abstract: According to the present invention, a system for audibly recognizing a received signal which is aurally unclassifiable because it is too short or too long in its duration and/or its frequency band is too wide or too narrow, such as occurs in sensor systems, for example, infiltration radars, comprises a signal-storage device such as a magnetic tape recorder/reproducer and motor means for driving the tape at one speed during recording or storage of a received signal and at a different speed, for example a lower speed, during reproducing or retrieval of the signal therefrom. The storage device has means for storing the signal therein as received and means for retrieving the signal from the storage device at a time-rate different from the storing rate, for example a slower rate.