Abstract

Many animals and systems radiate ultrasound that contains valuable information, from bats to high-voltage power lines. We set out to develop a real-time bandwidth compressor that can convey the prominent features of ultrasound in the human hearing band of 50 Hz to 16 kHz that requires no assumption of where in the ultrasonic frequency band or when in the time domain the information is encoded.

A primary application is in dolphin communication, which is believed to be both sophisticated and ultrasonic. Real-time studies of their acoustic communication patterns together with their behavior with the added capability to be able to react and respond in a timely manner to interact with them could rapidly generate important findings, greatly improving the efficiency of dolphin-human interactions. This is not possible without a real-time interface between ultrasound and human hearing.

As is well known, there is no pictorial representation that readily conveys the richness of a sound. We are therefore driven to find an efficient acoustic interface, translating ultrasound into audible sounds. This is easy to do in post-processing (simply play back at reduced speed), but continuous streaming real-time processing presents a challenge. The total information-carrying capacity of a signal can be represented by the time-bandwidth product. If the time is constrained to be the same and the bandwidth must be reduced, some information must be discarded. Choosing how and where to do this is the key to a successful algorithm.

In this paper we present an algorithm that compresses ultrasound signals into the audio band of human hearing while maintaining the overall signatures and structures of the signal, regardless of the signal type. This algorithm can be demonstrated to be optimal under the applied constraints. This is followed by the design of a prototype system that provides real-time bandwidth compression and a preliminary test result of the system capability.

The algorithm has a time-domain implementation that makes it possible to downshift signals sampled at up to 1MSa/s to audio range using a DSP. The system is autonomous and compact so that it can be carried by operators, including divers, allowing them to swim among dolphins while listening to their communications. The system is demonstrated using high frequency acoustic signals from a bottlenose dolphin.