Patent No. 5539705 Ultrasonic speech translator and communications system
Patent No. 5539705
Ultrasonic speech translator and communications system (Akerman, et al., Jul 23, 1996)
Abstract
A wireless communication system undetectable by radio frequency methods for converting audio signals, including human voice, to electronic signals in the ultrasonic frequency range, transmitting the ultrasonic signal by way of acoustical pressure waves across a carrier medium, including gases, liquids, or solids, and reconverting the ultrasonic acoustical pressure waves back to the original audio signal. The ultrasonic speech translator and communication system (20) includes an ultrasonic transmitting device (100) and an ultrasonic receiving device (200). The ultrasonic transmitting device (100) accepts as input (115) an audio signal such as human voice input from a microphone (114) or tape deck. The ultrasonic transmitting device (100) frequency modulates an ultrasonic carrier signal with the audio signal producing a frequency modulated ultrasonic carrier signal, which is transmitted via acoustical pressure waves across a carrier medium such as gases, liquids or solids. The ultrasonic receiving device (200) converts the frequency modulated ultrasonic acoustical pressure waves to a frequency modulated electronic signal, demodulates the audio signal from the ultrasonic carrier signal, and conditions the demodulated audio signal to reproduce the original audio signal at its output (250).
Notes:
BACKGROUND
OF THE INVENTION
The present invention relates generally to the art of wireless communication
and, more particularly, to a system which utilizes ultrasonic acoustical pressure
waves to transmit and receive audio signals across a medium such as gas, liquid,
or solid material. The invention further relates to the art of modulation of
audio signals to the ultrasonic frequency range, and to the art of demodulation
of audio signals from frequency modulated ultrasonic carrier signals. The invention
further relates to the art of inaudible communication, whereby the information
contained in the signals is secure and undetectable by radio frequency monitoring.
Radio frequency waves , or electromagnetic radiation in the frequency range
of approximately 10 kilohertz to 100 gigahertz, has been utilized for wireless
communication systems by civilian and military personnel for decades. Numerous
applications of radio frequency communication methods include, to name a few,
radio broadcasting, air traffic control, and cellular telecommunications. Radio
frequency communication is limited, for practical purposes, to operation within
mediums such as air and space. Furthermore, radio frequency methods are inappropriate
in some circumstances where communication is required, such as within blasting
zones where explosives may be susceptible to unplanned detonation due to radio
interference. In addition, radio frequency methods are limited in their ability
to provide a secure system to ensure confidentiality of information, which is
required by many applications for communication.
Sound waves, or acoustical pressure waves, have likewise been successfully employed
as a method of wireless ultrasonic communication across various mediums. Ultrasonic
communication is most often utilized in underwater applications because the
physical properties of solids and liquids tend to allow waves traveling via
molecular vibrations to cover relatively long distances, on the order of the
kilometer range. It has been similarly employed for communication over structural
matter such as beams or pipes. Ultrasonic communication has generally not been
utilized in air for long range communication because radio frequency methods
are particularly suitable in air for long range communication, offering suitable
and efficient means for most applications.
Some applications, however, require security and inaudibility by radio detectors.
Examples of these applications include undercover operations where it is necessary
not only that the communication be uninterpretable, but also that the communication
be undetectable so as not to alert the presence of such communication. Other
applications requiring inaudibility include situations where radio frequency
methods are inappropriate, such as, for example, in a blasting zone where the
presence of radio frequency waves could unexpectedly set off a detonator or
in a factory with sensitive electronics or other components sensitive to electromagnetic
radiations.
In applications requiring confidentiality and a high degree of security, numerous
schemes have been employed to minimize detection and eavesdropping. These schemes
often include scrambling a signal prior to broadcasting and then unscrambling
the signal after reception, as well as continual switching from frequency to
frequency. The main problem with these existing techniques is that the simple
detection of any radio frequency transmission whatsoever, even if the transmitted
signals are not decoded or interpreted, indicates the presence of existing communication.
Thus, such schemes may not provide sufficient security in operations requiring
complete inaudibility. It is desirable, therefore, to provide a communication
system which is inaudible by radio frequency detectors.
The invention is disclosed herein in the context of utilizing ultrasonic waves
for relatively long range, secure, wireless communication through air. However,
by way of example, and not limitation, the disclosed invention is useful in
a variety of applications including undercover operations, industrial applications,
and many commercial uses in various media.
Prior art ultrasonic communication systems involving the conversion of audio
signals to ultrasonic acoustical pressure waves encompass a variety of methods
and applications. In the context of the present invention, it may be noted that
there are no known prior art communication systems which employ ultrasonic acoustical
pressure waves for signal transmission through air for relatively long distances.
Prior art ultrasonic communication systems employ a means of carrying a modulated
ultrasonic frequency signal from a transmitter to a receiver. One approach has
been disclosed for use in electrical power networks, whereby a two-tone control
signal frequency modulates an ultrasonic subcarrier which is then used to frequency
modulate the broadcast of a local FM station. The frequency modulated ultrasonic
signal is demodulated from the FM broadcast program on the receiving end by
receiver circuitry. In this particular approach, however, communication is entirely
through radio frequency waves and telephone lines, whereby although a signal
is used to modulate an ultrasonic subcarrier, the modulated ultrasonic subcarrier
is never transformed from radio frequency signals to acoustical pressure waves.
The communication thus remains detectable by radio frequency detectors. It is
desirable to employ an alternate communication carrier other than radio frequency
waves such that the system is not limited to the use of radio facilities or
wire lines.
Another prior art approach for transmitting a modulated ultrasonic frequency
signal across a medium is through the conversion of the electronic audio signals
to acoustical pressure waves. This technique is employed in many communication
systems where radio waves cannot travel useful distances due to the attenuation
caused by the properties of the carrier medium, as in underwater communication.
Prior art ultrasonic communication systems employ a means of modulating an ultrasonic
frequency signal with an audio frequency signal. Methods utilized have included
both amplitude modulation and angle modulation, which encompasses both frequency
and phase modulation.
The amplitude modulation techniques used in prior art have encountered the inherent
limitation that medium disturbance, e.g. air or water currents, causes additional
amplitude modulation of a carrier signal. Thus, unwanted signals from medium
disturbance become superimposed on the amplitude modulated carrier, which often
results in difficulty recovering a clean original audio signal. Furthermore,
amplitude modulation, even when superimposed on a carrier of ultrasonic frequency,
may still be audible.
Another prior art approach for modulating an ultrasonic frequency signal with
an audio signal is through frequency modulation. One prior art technique feeds
an audio signal through a modulator to produce a frequency modulated (FM) radio
frequency signal at a predetermined intermediate center frequency. The FM radio
frequency signal is then fed into one input of a balanced modulator having a
second input of fixed frequency from a local oscillator. The balanced modulator
produces two outputs including the sum and the difference of the two input signals,
whereby proper selection of the fixed intermediate frequency for the first input
and the fixed frequency for the second input produces at the difference output
the frequency modulated signal in the ultrasonic range. It would be desirable
to eliminate the additional intermediate carrier frequency step.
Prior art techniques for demodulating audio signals from frequency modulated
ultrasonic carrier signals in ultrasonic communication systems have utilized
digital integrated circuit techniques.
The present invention reveals a technique for inaudible, long range communication
through air, as well as other media such as water or solid pipes and beams.
Prior art techniques have involved limitation to liquid or solid media or to
very short ranges in air.
The present invention also reveals a technique for simple, direct modulation
of audio signals onto ultrasonic frequency carrier signals for use in ultrasonic
communication systems. Prior art techniques have always involved indirect techniques
resulting from the high cost of quality components required to build systems
with high noise immunity operating at high frequency with a wide bandwidth.
Similarly, the present invention reveals a technique for simple, direct demodulation
of audio signals from ultrasonic frequency carrier signals for use in ultrasonic
communication systems. Again, prior art techniques have involved more complicated,
indirect techniques for demodulation.
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From
the foregoing description of the invention, it will be appreciated that the
ultrasonic speech translator and communication system encompasses a wide range
of desirable and useful applications. While specific embodiments of the invention
have been illustrated and described herein, it is realized that numerous modifications
and changes will occur to those skilled in the art. It is therefore to be understood
that the appended claims are intended to cover all such modifications and changes
as fall within the true spirit and scope of the invention.
Comments