|
Long range telescopic video imagery of distant terrestrial scenes, aircraft, rockets and other aerospace vehicles can
be a powerful observational tool. But what about the associated acoustic activity? A new technology, Remote
Acoustic Sensing (RAS), may provide a method to remotely listen to the acoustic activity near these distant objects.
Local acoustic activity sometimes weakly modulates the ambient illumination in a way that can be remotely sensed.
RAS is a new type of microphone that separates an acoustic transducer into two spatially separated components: 1) a
naturally formed in situ acousto-optic modulator (AOM) located within the distant scene and 2) a remote sensing
readout device that recovers the distant audio. These two elements are passively coupled over long distances at the
speed of light by naturally occurring ambient light energy or other electromagnetic fields. Stereophonic, multichannel
and acoustic beam forming are all possible using RAS techniques and when combined with high-definition
video imagery it can help to provide a more cinema like immersive viewing experience.
A practical implementation of a remote acousto-optic readout device can be a challenging engineering problem. The
acoustic influence on the optical signal is generally weak and often with a strong bias term. The optical signal is
further degraded by atmospheric seeing turbulence. In this paper, we consider two fundamentally different optical
readout approaches: 1) a low pixel count photodiode based RAS photoreceiver and 2) audio extraction directly from
a video stream. Most of our RAS experiments to date have used the first method for reasons of performance and
simplicity. But there are potential advantages to extracting audio directly from a video stream. These advantages
include the straight forward ability to work with multiple AOMs (useful for acoustic beam forming), simpler optical
configurations, and a potential ability to use certain preexisting video recordings. However, doing so requires
overcoming significant limitations typically including much lower sample rates, reduced sensitivity and dynamic
range, more expensive video hardware, and the need for sophisticated video processing. The ATCOM real time
image processing software environment provides many of the needed capabilities for researching video-acoustic
signal extraction. ATCOM currently is a powerful tool for the visual enhancement of atmospheric turbulence
distorted telescopic views. In order to explore the potential of acoustic signal recovery from video imagery we
modified ATCOM to extract audio waveforms from the same telescopic video sources. In this paper, we
demonstrate and compare both readout techniques for several aerospace test scenarios to better show where each has
advantages.
|
