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Patent No. 6587729 Apparatus for audibly communicating speech using the radio frequency hearing affect (O'Loughlin, et al., Jul 1, 2003)
Abstract
A modulation process with a fully suppressed carrier and input preprocessor filtering to produce an encoded output; for amplitude modulation (AM) and audio speech preprocessor filtering, intelligible subjective sound is produced when the encoded signal is demodulated using the RF Hearing Effect. Suitable forms of carrier suppressed modulation include single sideband (SSB) and carrier suppressed amplitude modulation (CSAM), with both sidebands present.
Notes:
Government Interests
The invention described herein may be manufactured and used by or for the Government
for governmental purposes without the payment of any royalty thereon.
Parent Case Text
This application is a division of U.S. patent application Ser. No. 08/766,687
filed on Dec. 13, 1996, now U.S. Pat. No. 6,470,214, and claims the benefit
of the foregoing filing date.
BACKGROUND OF THE INVENTION
This invention relates to the modulating of signals on carriers, which are transmitted
and the signals intelligibly recovered, and more particularly, to the modulation
of speech on a carrier and the intelligible recover of the speech by means of
the Radio Frequency Hearing Effect.
The Radio Frequency ("RF") Hearing Effect was first noticed during World War
II as a subjective "click" produced by a pulsed radar signal when the transmitted
power is above a "threshold" level. Below the threshold level, the click cannot
be heard.
The discovery of the Radio Frequency Hearing Effect suggested that a pulsed
RF carrier could be encoded with an amplitude modulated ("AM") envelope. In
one approach to pulsed carrier modulation, it was assumed that the "click" of
the pulsed carrier was similar to a data sample and could be used to synthesize
both simple and complex tones such as speech. Although pulsed carrier modulation
can induce a subjective sensation for simple tones, it severely distorts the
complex waveforms of speech, as has been confirmed experimentally.
The presence of this kind of distortion has prevented the click process for
the encoding of intelligible speech. An example is provided by AM sampled data
modulation
Upon demodulation the perceived speech signal has some of the envelope characteristics
of an audio signal. Consequently a message can be recognized as speech when
a listener is pre-advised that speech has been sent. However, if the listener
does not know the content of the message, the audio signal is unintelligible.
The attempt to use the click process to encode speech has been based on the
assumption that if simple tones can be encoded, speech can be encoded as well,
but this is not so. A simple tone can contain several distortions and still
be perceived as a tone whereas the same degree of distortion applied to speech
renders it unintelligible.
SUMMARY OF THE INVENTION
In accomplishing the foregoing and related object the invention uses a modulation
process with a fully suppressed carrier and pre-processor filtering of the input
to produce an encoded output. Where amplitude modulation (AM) is employed and
the pre-processor filtering is of audio speech input, intelligible subjective
sound is produced when the encoded signal is demodulated by means of the RF
Hearing Effect. Suitable forms of carrier suppressed modulation include single
sideband (SSB) and carrier suppressed amplitude modulation (CSAM), with both
sidebands present.
The invention further provides for analysis of the RE hearing phenomena based
on an RF to acoustic transducer model. Analysis of the model suggests a new
modulation process which permits the RF Hearing Effect to be used following
the transmission of encoded speech.
In accordance with one aspect of the invention the preprocessing of an input
speech signal takes place with a filter that de-emphasizes the high frequency
content of the input speech signal. The de-emphasis can provide a signal reduction
of about 40 dB (decibels) per decade. Further processing of the speech signal
then takes place by adding a bias level and taking a root of the predistorted
waveform. The resultant signal is used to modulated an RF carrier in the AM
fully suppressed carrier mode, with single or double sidebands.
The modulated RF signal is demodulated by an RF to acoustic demodulator that
produces an intelligible acoustic replication of the original input speech.
The RF Hearing Effect is explained and analyzed as a thermal to acoustic demodulating
process. Energy absorption in a medium, such as the head, causes mechanical
expansion and contraction, and thus an acoustic signal.
When the expansion and contraction take place in the head of an animal, the
acoustic signal is passed by conduction to the inner ear where it is further
processed as if it were an acoustic signal from the outer ear.
The RF to Acoustic Demodulator thus has characteristics which permit the conversion
of the RF energy input to an acoustic output.
Accordingly, it is an object of the invention to provide a novel technique for
the intelligible encoding of signals. A related object is to provide for the
intelligible encoding of speech.
Another object of the invention is to make use of the Radio Frequency ("RF")
Hearing Effect in the intelligible demodulation of encoded signals, including
speech.
Still another object of the invention is to suitably encode a pulsed RF carrier
with an amplitude modulated ("AM") envelope such that the modulation will be
intelligibly demodulated by means of the RF Hearing Effect. A related object
is to permit a message to be identified and understood as speech when a listener
does not know beforehand that the message is speech.