Note: Descriptions are shown in the official language in which they were submitted.
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TECHNIC~L FIELD OF THE INVENTION
This inven-tion relates to a c~mmunications system
and more particularly to a system Eor voice transmission.
BACKGROUND ART
Voice transmitting systems employ a microphone,
that is to say a -transducer whereby sound waves are
caused to generate or modulate an electrical signal.
The microphone is connected to the input side of a
circuit which transmits the electrical signal by wire or
wireless means to a receiver. A telephone is an example
of a voice transmitter in which the voice o-f a person
speaking is detected by a microphone, the microphone
signal is amplified and then delivered to a receiver
comprising the earphone of a listener. Commonly used
radio transceivers provi~e a voice transmitting syst~m
on which the voice of a person speaking is detected by a
microphone adapted to modulate the frequency or
amplitude of a radio signal which may be detected by a
receiver.
There are various circumstances in which t'ne use o~
a microphone in existant voice transmitting systems is
disadvantageous. In a noisy environment the electrical
signal is modulated by environmental noise as well as by
the voice leading to poor communication between a person
speaXing and the receiver~
A person wearing a respirator or other mask is
unable to communicate by microphone unless the
microphone is incorporated within the mask adding
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substantially to the co~t of the mask and often making
the mask bulkier than need be since -the microphone must
be spaced from the mouth.
In many circumstanc-es a person wishes to be mobile,
to have bo-th hanas free, and to use a radio transceiver
without the need to holdl a microphone before his mouth.
Sometimes, as during surveillence, it is desira~le that
voice communication be p~ossible without the presence of
a microphone being readily apparent.
Although emphasis has been placed on voice
transmission, it is clearly desirable that the person
speaking also be able to receive and hear voice
communications transmittea to him.
When a person speaks sound is conveyed via his
oto-laryngeal system to his ear. It has been proposed
to employ an earphone for the dual purpose of producing
sound waves in the ear of a wearer and as a micro~hone
to detect sounds conveyed via the oto~laryngeal system
of the wearer. The earphone transducers hitherto
employed have a low impedance typically of 400-600 ohms
at lkHz. Such earphone transducers, while having the
advantage of being able to be worn beneath a mask and of
permitting hands free operation have not proved
satisfactory, especially when used in noisy environments.
DISCLOSURE OF THE INVENTIO~
According to one aspect the present invention
consists in a voice transmitting system comprising:
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a microphone having a high impedance as ~erein
deEined,
earplug means a~apted to ~old the micro~hone in or
adjacent an outer ear of a person 50 as substantially to
enclose air between the microphone and the tympanic
mPmbrane of the ear, ancl
a circuit for transmitting a signai producea or
modulated by the microphone in response to sound waves,
if any, which are transmitted via the pexsons
oto-laryngeal system to the tympanic membrane and
propogated in the ~nclosed ai r .
By "high impedance" is meant that the microphone
has an impedance equivalent to at least 1000 ohms, and
preferably greater than 2000 ohms at lkFIz.
For preference the microphone is worn by means of
an earplu~ individually cast for the ear of the wearer
in the manner of a hearing aid earplug and is a
transducer which is also operable as an earphone.
It was previously ~elieved that increasing t~e
sensitivity of a microphone plugged to the oto-laryngeal
system would merely aggrevate pxoblems of extraneous
noise.
The present invention is based on the surprising
discovery that when air is enclosed between the tympanic
membrane and a sufficiently high impedance microphone,
the effect of external noise and vibration is greatly
reduced, and in preferred embodiments, is halved.
Apparently when a low impedance (400-800 ohms)
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transducer is employed as a ~icrophone a high proportion
o~ the detected audio energy is transmitted via bone
structure and is thus subjected to noise vibrations from
the general environ~en-t. It has been found that when a
high impedance microphone is employed in accordance with
the present invention, a high proportion oE the detected
audio energy is substantially transmitted from the
tympanic membrane by means of the air column in -the
auditory canal and is much less influenced by vibrations
in the bone which originate externally.
DESCRIPTION OF PREFERRED EMBODIMENT
By way of example only the invention will now be
described with reference to -the accompanying circuit
schematic diagram (FIG. 1).
The circuit is an interface circuit for use with a
two way radio transceiver 5no-t shown) having connec-tors
Eor external speaker and microphone.
The interface circuit of Fig. 1 comprises a high
impedance transducer T, a two stage amplifier and an
electronic switch and is powered by a 1.5 ~ battery.
In use terminal Al is connected to the audio output
of the radio transceiver.
~ Yhen yanged switch SlB is in the "NC" position as
shown in Fig. 1 an audio signal from the transceiver
connected at the "audio-in" terminal Al is routed to
transduc~r T via capacitor C8 and resistor Rll.
Capacitor C8 provides DC blocking and resistor R7
provides overload protection Eor transducer T which is a
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type AP2000 earpiece supplied by Crystal Aid Pty. ~td.,
~ustralia, having an impedance of around 2000 ohms at
lkHz and developed for use as a hearing aid.
When ganged switch SWlA/SWlB is in the "NO"
position, that is to say in the "transmit" mode~ power
is applied to electronic switch Ql and the two ampli~ier
stages Q2 and Q3 via swiltch SlA. Transistor Ql is
turned on causing the PTT ("PUSH TO TALK") line
connected via terminal A3 to the associated transceiver
to go low. With switch SlB in the "N0" position the
audio signal Erom transclucer T is applied at the base of
the first amplifier staqe Q2. Transistor Q3 provides
further amplification and impedance matching.
The audio signal i5 then attenuated to the required
leve] by variable resistance RVl and fed to the
associated transceiver via terminal A2 "audio out"~
~ s mentioned above "Earpiece" T comprises a high
impedance coil ~aving a DC resistance of about 2000 ohms
and preferably higher and has a magnet moveable relative
to the coil by a diaphram affixed either -to the magnet
or the coil. T~e "earpiece" is thus able to function
either as a microphone to modulate a signal in response
to movement of the diaphram or as an earphone when fed
with a modulated signal.
For preference the "earpiece" is oE a size similar
to a hearing aid earpiece and is held in the ear o-f a
wearer by a custom made hollow casting such as is often
used to seal a hearing aid earphone to an ear. However
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the earpiece may be otherwi~e adapted '~y shape ko fit in
or adjacent the ear so as to enclose air between the
transducer and tympanic membrane.
Switch ~Wl serves to disconnect the audio reception
line from the transceiver to the transducer when the
transducer is used Eor voice transmission. Because the
apparatus operates from a l.5 battery it is non-sparking
and useable in hazardous environments~
Fl is a ferrite bead providing shielding. Ql, Q2
and Q3 are each type BC 549 transistors. The circuit
parameters shown in Fig. l and indeed the circuit itself
may be varied in ways which will be apparent to those
skilled in the art from the teaching hereof.
For preference the interface circuit is adapted to
be worn on the belt.
As will be apparent to those skilled in the art,
ear protectors if desired, may be worn over the earpiece
to further reduce extraneous noise. In other
embodiments the microphone is built into ear protectors.
