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Claims  |
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What is claimed is:
1. Apparatus for integrating disparate audio sources for use in light
aircraft or in other high noise environments comprising unitary driver
means for generating a composite output signal representative of a driver
input signal, audio output means associated with the output of the driver
means for connection of a plurality of remote headsets whereby a
substantially identical composite output signal will be present at all of
the headsets without regard to the source of audio comprising said
composite output signal; audio signal summation means operatively
connected to the driver input having a plurality of audio inputs for
connection of remote microphones, each remote microphone being associated
with and proximate to a headset whereby an occupant of the aircraft may
speak into a microphone and listen to the composite output signal,
including the voice audio of said occupant, through the associated
headset, said audio summation means funtioning to combine the respective
audio inputs to form a single composite signal; each summation means audio
input including an associated audio switch means, audio detector means,
and audio input means for connection of an audio input signal thereto;
each audio switch means connected in series between the associated
summation means audio input and the audio input means, and each audio
switch means having a gate means whereby the switch means may be
selectively controlled to a first closed-circuit condition whereby an
audio input signal at the associated audio input means is substantially
connected to the summation means audio input and a second open-circuit
condition whereby an audio input signal at the associated audio input
means is substantially removed from said audio input; each audio signal
detector means being operatively connected to the gate means of the
associated switch means and to the associated audio input means, each
detector means being unresponsive to audio input signals on other than the
associated audio input means, whereby the detector means controls the
associated switch means into the first closed-circuit condition whenever
an audio input signal of predetermined amplitude is present at the
associated audio input means without regard to the input signal condition
on any other audio input means and into the second open-circuit condition
following a predetermined interval in which an audio input signal at the
associated audio input means has not exceeded the predetermined amplitude
whereby only audio input signals having a predetermined amplitude
contribute to the driver means output signal thereby improving the audio
quality by eliminating noise present on unused audio signal inputs.
2. Apparatus for integrating disparate audio sources, including microphone
signals and other non-microphone input signals, for use in light aircraft
or in other high noise environments comprising unitary driver means for
generating a composite output signal representative of a driver input
signal, audio output means associated with the output of the driver means
for connection of a plurality of remote sound transducer means thereto
whereby a substantially identical composite output signal will be present
at all of the remote sound transducer means without regard to the source
of audio comprising said composite output signal; audio signal summation
means operatively connected to the driver input having microphone input
means and other input means, said audio summation means functioning to
combine respective microphone and other input means to form a single
composite signal; means for interconnecting microphone signals to the
microphone input means in response to predetermined minimum microphone
signal levels, said interconnecting means including switch means and
microphone signal detector means; switchable mute means for
interconnecting said other non-microphone input signals to the other input
means, the mute means operatively connected to the microphone signal
detector means such that the amplitude of the other input signals
interconnected to the other input means is lowered by a predetermined
non-zero fractional degree in response to predetermined minimum microphone
signal levels whereby fractionally muting the other signal enhances
microphone signal intelligibility without appreciably interrupting the
continuity of said other signals.
3. The apparatus for integrating disparate audio sources of claim 2, the
disparate audio sources including radio signals, wherein the audio signal
summation means includes radio signal input means and including radio
signal detector means operatively connected to the other signal mute means
whereby the amplitude of the other signal interconnected to the other
signal input means is lowered by a predetermined non-zero fractional
degree in response to detected radio signals or predetermined minimum
microphone signal levels whereby fractionally muting the other signal
enhances microphone and radio signal intelligibility without appreciably
interrupting the continuity of said other signals.
4. Apparatus for integrating disparate audio sources for use in light
aircraft or in other high noise environments comprising unitary driver
means with a driver input for generating a composite output signal
representative of a driver input signal, audio output means associated
with the output of the driver means for connection of a plurality of
remote sound transducer means thereto whereby a substantially identical
composite output signal will be present at all of the remote sound
transducer means without regard to the source of audio comprising said
composite output signal; audio signal summation means operatively
connected to the driver input having microphone input means and radio
input means, said audio summation means functioning to combine outputs
from the microphone input and radio input means to form a single composite
signal; means for interconnecting microphone signals to the microphone
input means in response to predetermined minimum microphone signal levels,
said interconnecting means including microphone switch means and
microphone signal detector means; radio signal detector means; microphone
mute means for lowering the microphone input signal by a predetermined
first non-zero fractional degree; radio mute means for lowering a radio
input signal at the radio input means by a predetermined second non-zero
fractional degree; means for manually selecting priority, said priority
selecting means having at least respective microphone and radio priority
positions, said priority selecting means coupled to the microphone and
radio signal detector means and to the microphone and radio mute means
whereby at least one of said microphone or radio signals may be selected
and operatively connected to the radio or microphone mute means,
respectively, thereby lowering the non-selected input signal by a
predetermined fractional degree in response to a detected selected signal
whereby fractionally muting the non-selected signal enhances the selected
signal intelligibility without interrupting the continuity of the
non-selected signal.
5. The apparatus for integrating disparate audio sources of claim 4
including other signal summation input means; other signal mute means
operatively coupled to the microphone and radio signal detector means for
lowering the other input signal by a predetermined third non-zero
fractional degree in response to detected radio or microphone signals
whereby fractionally muting the other signal enhances microphone and radio
signals intelligibility without appreciably interrupting the continuity of
said other signals.
6. The apparatus for integrating disparate audio sources of claim 4 wherein
the priority selecting means defines a first radio priority position
wherein the radio signal detector means is operatively coupled to the
microphone mute means and a second microphone priority position wherein
the microphone signal detector means is operatively coupled to the radio
mute means.
7. The apparatus for integrating disparate audio sources of claim 6 wherein
the priority selecting means defines a third non-priority position wherein
the microphone and radio signal detector means are not coupled to the
microphone and radio mute means.
8. Aircraft audio intercom apparatus including unitary amplifier means
having output means for connecting a plurality of remote headsets whereby
the same amplifier output signal is available to each headset; a plurality
of input means for connecting microphone audio to each such means, said
microphone audio defining a first signal level generally representative of
ambient cabin aircraft noise and a second signal level generally
representative of said ambient cabin aircraft nosie and voice audio of an
aircraft occupant using said microphone; microphone switch means
operatively coupling selected microphone input means to the amplifier
means; microphone actuator means operatively connected to the switch means
and to the input means, said actuator means including means for separately
detecting an audio input signal at each of the microphone input means
whereby the actuator and switch means automatically couple each input
means having said second signal level audio to the amplifier means without
regard to audio input signal levels on the remaining audio input means.
9. The aircraft intercom apparatus of claim 8 wherein said switch means is
comprised of a plurality of electrically operated switches, one switch
interposed between each of said plural audio input means and the amplifier
means and wherein said means for separately detecting an audio input
signal at each microphone input means includes an audio detector coupled
to each of said plural audio input means and to the respective audio input
switch.
10. The aircraft audio intercom apparatus of claim 9 including light
indicator means operatively coupled to each audio detector thereby
providing independent visual indications of voice audio on each microphone
input means.
11. Aircraft audio intercom apparatus including unitary amplifier means,
means for connecting the output of the amplifier means to a plurality of
remote headsets whereby the same amplifier output signal is available to
each headset; a plurality of microphone input means; a plurality of
microphone switch means, each of the switch means operatively coupling a
respective one of the microphone input means to the amplifier means; a
plurality of audio detector means, each of the detector means having a
detector input operatively connected to one of the audio input means and a
detector output representative of the presence of an audio signal on the
corresponding input means, the detector output operatively connected to
the respective audio switch means associated with said one audio input
means whereby only microphone input means having audio signals thereon are
coupled to the amplifier means; aircraft radio audio input means
operatively coupled to the amplifier means; entertainment input means
operatively coupled to the amplifier means; means for partially muting a
signal from the entertainment input means; aircraft audio detector means
having an input operatively connected to the aircraft audio input means
and a detector output representative of the presence of an audio signal on
the aircraft audio input means; means operatively interconnecting the
microphone detector means and the aircraft audio detector means with the
entertainment mute means whereby the level of a signal at the amplifier
means from the entertainment means may be partially muted in response to
detected microphone input means signal or aircraft radio input means
signals.
12. The aircraft intercom apparatus of claim 11 including microphone audio
partial mute means and aircraft radio audio partial mute means; priority
means selectively interconnecting the microphone and aircraft detector
means outputs with the microphone and aircraft partial mute means whereby
a signal on one of said microphone and aircraft input means partially
mutes a signal on the other of said input means.
13. The aircraft audio intercom apparatus of claim 11 wherein the amplifier
means includes stereo left channel amplifier means and right channel
amplifier means and wherein the entertainment input means includes stereo
left channel input means operatively connected to the left channel
amplifier means and right channel input means operatively connected to the
right channel amplifier means and including microphone signal splitter
means operatively associated with each microphone input means for
connecting microphone audio signals to both of said left and right channel
amplifier means; aircraft radio signal splitter means operatively coupling
the aircraft radio input means to both of said left and right channel
amplifier means whereby the monophonic voice microphone audio signals and
monophonic aircraft radio audio signals will be present at the output
means of both of said amplifier means.
14. The aircraft audio intercom apparatus of claim 11 including aircraft
radio audio switch means having a push-to-talk input whereby signals on
the aircraft audio input are gated off in response to a push-to-transmit
signal on said push-to-transmit input.
15. Aircraft audio intercom apparatus including unitary amplifier means
having output means for connecting a plurality of remote headsets whereby
the same amplifier output signal is available to each headset; a plurality
of microphone input means; microphone switch means operatively coupling
selected microphone input means to the amplifier means; microphone
actuator means operatively connected to the switch means and to the input
means, said actuator means including means for separately detecting an
audio input signal at each of the microphone input means whereby the
actuator and switch means automatically couple each input means having an
audio signal of predetermined amplitude thereon to the amplifier means
without regard to audio input signal levels on the remaining audio input
means; wherein the amplifier means includes stereo left channel amplifier
means and right channel amplifier means and including microphone signal
splitter means operatively associated with each microphone input means for
connecting microphone audio signals to both of said left and right channel
amplifier means whereby the microphone voice audio signal will be present
at the output means of both of said amplifier means.
16. Apparatus for integrating disparate audio sources for use in light
aircraft or in other high noise environments comprising unitary driver
means for generating a composite output signal representative of a driver
input signal, audio output means associated with the output of the driver
means for connection of a plurality of remote sound transducer means
thereto whereby a substantially identical composite output signal will be
present at all of the remote sound transducer means without regard to the
source of audio comprising said composite output signal; audio signal
summation means operatively connected to the driver input having at least
two audio inputs, said audio summation means functioning to combine the
respective audio inputs to form a single composite signal; each summation
means audio input including an associated audio switch means, audio
detector means, and audio input means for connection of an audio input
signal thereto; each audio switch means connected in series between the
associated summation means audio input and the audio input means, and each
audio switch means having a gate means whereby the switch means may be
selectively controlled to a first closed-circuit condition whereby an
audio input signal at the associated audio input means is substantially
connected to the summation means audio input and a second open-circuit
condition whereby an audio input signal at the associated audio input
means is substantially removed from said audio input; each audio signal
detector means being operatively connected to the gate means of the
associated switch means and to the associated audio input means, each
detector means being unresponsive to audio input signals on other than the
associated audio input means, whereby the detector means controls the
associated switch means into the first closed-circuit condition whenever
an audio input signal of predetermined amplitude is present at the
associated audio input means without regard to the input signal condition
on any other audio input means and into the second open-circuit condition
following a predetermined interval in which an audio input signal at the
associated audio input means has not exceeded the predetermined amplitude
whereby only audio input signals having a predetermined amplitude
contribute to the driver means output signal thereby improving the audio
quality by eliminating noise present on unused audio signal inputs;
including a single dc level potentiometer operatively interconnected to
each of the microphone audio detector means whereby the predetermined
detection amplitude of all of the detectors may be simultaneously
adjusted. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to intercom apparatus particularly adapted
for use in light aircraft. More specifically, the present invention
relates to a voice activated intercom enabling hands-free intra-cabin
communications between pilot, co-pilot, and passengers including means for
interfacing existing aircraft communications and navigation equipment
(avionics) and optional stereophonic entertainment sources such an as FM
tuner or cassette recorder or the like. In particular, the present
intercom incorporates multi-VOX (voice or radio enabled audio) for
maximizing voice intelligibility and for activating the partial-mute audio
prioritizing circuitry of the present invention. In addition, the
partial-mute of the present invention reduces the stereo source volume
upon VOX actuation enhancing voice intelligibility without distracting
interruption of the stereo audio source.
It is well known that the light aircraft cabin environment is characterized
by high engine and slip-stream noise levels which, in turn, contribute to
pilot (and passenger) fatigue and, more significantly, severely limit
normal intra-cabin conversation, particularly between front and rear seat
passengers. In addition, these high noise levels compromise the
intelligibility of aircraft radio communications and so detract from the
quality of stereo music to render listening unpleasant and impractical.
The use of noise cancelling headsets to attenuate these inherent noises
greatly reduces pilot fatigue and, where connected to conventional
aircraft avionics equipment, improves the radio communications
intelligibility. Such headsets are well known to the art. Where multiple
users must be accommodated, however, such headsets are commonly interfaced
with an intercom system which functions, in part, to electronically
interconnect the various headsets and aircraft avionics. The use of
aircraft intercoms is also well known to the art. The present invention,
therefore, pertains to an improved aircraft intercom system adapted to
maximize the users voice intelligibility and to provide an optimum
integration between the multiple intercom audio sources, taking into
consideration the changing communications requirements occasioned by the
ever differing flight conditions.
A principal objective and advantage of any aircraft intercom system is the
reduction in aircraft noise and the corresponding improvement in voice or
radio intelligibility afforded by the noise cancelling headphones
generally used in conjunction with the intercom system. As previously
discussed, modern aircraft headsets block substantially all aircraft noise
allowing the user to easily understand voice conversation and to
appreciate locally generated music, where provided.
Unfortunately, the microphones associates with the various passengers'
headsets represent significant potential sources of noise since they are
of necessity positioned within the noisey aircraft cabin. Several known
aircraft intercom systems have addressed this problem by incorporating
voice actuated circuitry (VOX) to simultaneously enable the several
headset microphones, but only when voice audio is detected from any one of
the microphones. This prior art arrangement has proved somewhat
satisfactory at least to the extent that noise entering these microphones
is not amplified nor transferred to the occupants' headsets until one of
the users speaks. In this manner the fatigue level is substantially
reduced as virtually no noise is present in the headsets during periods of
non-conversation. However, when any one of the occupants speaks,
substantial noise is present due to the summation of aircraft noise
contributed by the plurality of simultaneously enabled headsets.
Therefore, this solution, which reduces fatigue caused by the constant
drone of the aircraft, does little to enhance the voice intelligibility of
any given cabin occupant. The noise, switched `off` during lulls in
conversation, is again present in the headsets whenever an occupant
speaks. More specifically, conventional aircraft intercoms interconnect
all passenger (or crew) microphones to a common VOX arrangement which
functions to enable all such microphones upon the detection of legitimate
voice audio from any one microphone. Due to the nature of these prior art
VOX circuits, `information` identifying which passenger, at any given
instant, is speaking is lost and, therefore, the single output from these
prior art VOX circuits must necessarily and simultaneously enable all
microphones. It will be appreciated that enabling all aircraft
microphones, where only one is in use, causes additional noise from the
`inactive` microphones to be needlessly added to the voice signal thereby
reducing the signal-to-noise ratio and the corresponding intelligibility
of the desired voice signal. For example, in a system used by four
occupants, noise from all four microphones is added to the voice audio
signal from the one active microphone. One voice signal--four noise
`signals`.
The present invention, by contrast, utilizes a multi-VOX arrangement
wherein `information` identifying which microphone(s) is active
selectively enables only that microphone(s). Thus, in the above example,
only one `noise` signal, rather than four, is added to any given voice
signal. In this manner the voice signal-to-noise ratio and intelligibility
are significantly improved over conventional intercom systems.
It has also been found that music is quite pleasurable in light aircraft
where, as described above, proper noise cancelling headsets are employed.
Music is conventionally accommodated by interfacing a stereo FM tuner or
cassette tape player with an aircraft intercom. In addition, it is
generally desirable to interface the aircraft avionics equipment into the
intercom to permit the pilot to communicate with, or monitor, the various
aviation frequencies as required for the safe conduct of the flight. Thus,
as many as three or more disparate audio sources may be operatively
connected through the intercom at any given time. It will, therefore, be
appreciated that a proper `mix` or integration between these various
intercom inputs is required in order that each of these audio input
sources may be utilized to the maximum potential. As will be described in
more detail below, the present invention provides a truly unique and
functional interplay between the intracabin intercom, air-to-ground
communications, and music sources wherein each attains a high degree of
utility approaching that which would obtain were the given source the only
source.
Intercoms known to the art have never achieved a very satisfactory or high
level of input source integration. Severe source integration limitations,
characteristic of prior art intercoms, often restrict the use of these
intercoms to one `function` at a time or, alternatively, segregate the
various aircraft occupants into groups, each group utilizing the intercom
system only in its partial capacity. For example, it has long been
recognized that music can interfere with important air-to-ground
communications and, consequently, one known prior art approach restricts
music listening to the passengers and, similarly, the radio communications
to the pilots. This, of course, is highly unsatisfactory since many
passengers enjoy listening and following the progress of the flight by
monitoring the air-to-ground communications and feel more a part of the
flight when privy to such communications. At least as important, this
solution is entirely unsatisfactory since it precludes the pilot from the
highly enjoyable music feature of the intercom system.
Recognizing that segregation of aircraft occupants is highly
unsatisfactory, other known systems have adopted a music muting
arrangement whereby audio from either the communications radio or a
passenger's microphone totally `mute` the music. Although this arrangement
assures maximum voice intelligibility, the repeated and nearly constant
interruption or `punctuation` of the music occasioned by routine radio
communications, particuarly when monitoring a busy ATC frequency or unicom
channel, or by `idle` cabin chatter has been found to be unduly
distracting and annoying to the extent that passengers refuse to talk
while the music is playing. In addition, the annoyance of repeated
interruptions during an IFR flight usually requires that the music simply
be turned-off.
The present invention, by contrast, incorporates a partial mute arrangement
whereby the level or volume of the music is significantly, but not
totally, reduced whenever an intercom microphone or communications radio
audio signal is present. It has been found that reducing the level of
music affords a high degree of voice audio intelligibility without the
obnoxious interruptions common to prior art total mute systems. In fact,
experiments demonstrate that a relatively large reduction in volume can be
implemented with only a barely perceptible loss of music listening
pleasure. Therefore, it will be appreciated that the partial mute of the
present invention permits all occupants of the aircraft, including the
pilot, to enjoy the full benefit of airborne music even when monitoring
crowded ATC frequencies or engaging in protracted intra-cabin
conversations.
Another difficulty with known intercom systems is audio source interference
arising between regular intra-cabin intercom usage and aircraft radio
reception. It is, simply, difficult, if not impossible, to listen to and
comprehend two voices at the same time. In addition to this obvious loss
of intelligibility, the continuing presence of a `second` voice is quite
distracting to the intercom users. Known prior art systems have wholly
failed to address this significant shortcoming. Use of such systems
generally requires that the aircraft radios be turned-down during flight
or, where radio communications is necessary for instrument flight (IFR) or
the like, that normal intra-cabin conversations be discontinued or, at the
least, frequently interrupted by the continuous flow of radio
transmissions. This, of course, discourages the monitoring of ATC, flight
watch, or unicom frequencies during visual (VFR) flight and, importantly,
severly restricts the utility of the intercom for normal intra-cabin
conversation during IFR flight.
The present invention solves this source integration problem by combining
the features of partial muting, discussed above, with pilot selectable
intercom/aircraft radio priority. In arriving at the present arrangement,
it was first recognized that changing flight conditions necessarily alter
the relative importance of the respective audio source inputs. Thus, for
example, on a clear day, the flight might be conducted with only marginal
interest and attention given to the aircraft radio while, on an instrument
flight in bad weather, aircraft radio communication is paramount in
importance. But notwithstanding the relative importances assigned to the
various intercom inputs during a given flight, or flight segment, it
remains highly desirable and necessary to accommodate the relatively
lesser importance inputs. Thus, during a VFR flight where intra-cabin
conversation will likely be given priority over the aircraft
communications radio, monitoring of certain radio channels such as
flight-watch and unicom is highly desirable--so long as such monitoring
can be effectuated without obnoxious interruption to intra-cabin use of
the intercom. During instrument flight conditions, by contrast, the
communications radios may well be given priority but, again, it remains
desirable to accommodate normal intra-cabin intercom usage. The present
invention accomplishes these objectives. First, it permits the pilot to
select the input to be given `higher` priority during any given flight
segment. Second, an audio signal from the non-selected input is assured.
The use of the present partial mute feature, discussed above with respect
to the stereo input source, is essential to the extent that total muting
of the non-selected input does not permit the occupants to `keep-tabs` on
the activities of such non-selected source. In short, the partial mute
feature places the non-selected audio `in the background` whenever the
prioritized input is active. This has been found to facilitate the
monitoring of such audio source as the users can, notwithstanding the
reduction in audio level, still obtain the `gist` of the activity on that
input.
It is therefore an object of the present invention to provide an intercom
adapted for use in light aircraft or other high noise environments. It is
a further object that the intercom shall be capable of efficiently
integrating several audio sources including, for example, user microphone
audio, communications audio, and entertainment program audio. The intercom
shall maximize audio source intelligibility by reducing environmental
noise and by reducing interference between competing intercom input
sources. In furtherance of these objects, each user shall be provided with
a separate intercom input and, preferably, with separate voice activation
circuits whereby only the microphones of those persons actually speaking
at any given instant shall be enabled. Partial muting shall be provided on
the entertainment input source to facilitate reduction of the level
whenever user microphones or other important input sources are active.
Selectable input priority shall be combined with partial muting to enable
the pilot to choose the input to be assigned the highest priority and to
automatically lower the audio signal level of those non-selected inputs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a panel arrangement for the aircraft intercom of the present
invention.
FIG. 2 is a block representation of the present intercom configured for
stereo, four-user operation.
FIG. 3 shows an embodiment of a signal summer.
FIG. 4 shows an embodiment of the partial-mute gates of the invention.
FIG. 5 shows an embodiment of a channel splitter.
FIG. 6 illustrates the independent microphone voice activation (VOX)
circuitry of the invention.
FIG. 7 shows an arrangement of LED's for voice activity indication.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A typical panel arrangement for the aircraft intercom 10 of the present
invention is illustrated in FIG. 1. Front panel headset and microphone
access is provided, generally for two users, at jacks 12 and 14
respectively. To avoid front panel cluttering, connection to the intercom
by other passengers is made through a rear chassis connector (not shown).
A master volume control 16 simultaneously varies the audio output level to
all user headsets. Individual volume controls may preferably be
incorporated on each user's headset to permit the selective adjustment of
volume as desired. A voice activation (VOX) sensitivity control 18
facilitates compensation for changing noise conditions which often occur
in response to aircraft engine power settings or changes in aircraft
speed. An aircraft radio microphone switch 20 permits the selective
interconnection of either the pilot's or co-pilot's microphone to the
aircraft radio for air-to-ground communications. Priority switch 22
selects between three differing priority conditions thereby allowing the
pilot to change the `weighting` assigned to the various intercom inputs in
accordance with changing flight conditions. Finally, indicators 24 provide
a visual indication of mic enabling VOX activity. The functions of these
items is discussed in more detail below.
It will be appreciated that the present panel may be rearranged to include
or delete controls, connectors, switches or indicators without departing
from the scope of the invention.
FIG. 2 is a block representation of the present intercom 10 configured for
stereo, four-user operation. The present invention may equally be utilized
with, or in, a `mono` arrangement, or, with greater or fewer users as will
become apparent from the following descriptions. A pair of integrated
amplifiers 30a,b are provided for left and right channels, respectively.
These amplifiers are necessary to boost the relatively lower voltage
signal levels from the microphones, aircraft radio, and stereo inputs to
the higher voltage and power levels necessary to drive typical aircraft
stereo headsets. The output of amplifiers 30 should preferably be of low
impedance, 1 to 10 ohms or less, in order to assure proper operation when
connected to a plurality of headsets. Satisfactory operation has been
achieved with the National Semiconductor LM-380 integrated amplifier. This
device exhibits a fixed voltage gain of approximately 34 and has
sufficient power output capability to drive any reasonable number of
headsets. The LM-380 is merely representative and other amplifiers,
including discrete transistor arrangements, may be used with similar
results.
Front panel volume controls 16a and 16b vary the level of the summed audio
signals 32a,b connected to the input of amplifiers 30a,b. Controls 16a and
16b may be ganged for simultaneous volume adjustment of both the left and
right channels or they may be independently operable thereby permitted
`balancing` of the respective channel outputs. Signal summers 34a,b
combine the various audio signals into respective left and right summed
signal outputs 32a,b. Signal summers 34 may be of any conventional type
although a discrete common-base transistor current summing arrangement
such as shown in FIG. 3 has proven quite satisfactory. Biasing of summer
transistor 38 is accomplished in the conventional manner using resistors
40, 41, and 42. Capacitor 44 functions as a by-pass for signal energy
thereby electrically grounding the base of transistor 38 for audio
signals.
The principal intercom voice audio input to summers 34a,b is from
partial-mute gates 46a,b shown in more detail in FIG. 4. Each partial-mute
gate includes an electronic switch 48 having a partial-mute resistor 50
bridged from switch input to output. Switch 48 exhibits substantially open
circuit characteristics when the control voltage on gate line 52 is `low`
and a substantially short circuit or low impedance characteristic when the
gate control line is `high`. Switch 48 may be a conventional bi-lateral
integrated circuit analog switch commonly known within the trade as a
"4016" or it may be a junction FET device. Satisfactory results have been
obtained with either approach.
The partial-mute operation of the present invention cannot be achieved
using a standard electronic switch 48 as the high open circuit impedance
of the switch totally mutes the incoming audio signal. Therefore, a
bridging resistor 50 must be provided to establish a predetermined maximum
resistance through partial-mute gate 46. Operation of the partial-mute
gate 46 is as follows. In the `non-mute` mode, switch 48 is electrically
closed thereby effectively shorting resistor 50. In the `partial-mute`
mode, switch 48 is opened thereby connecting bridging resistor 50 in
series with the respective summing resistor 56, FIG. 5. The reduction in
intercom gain is determined by the ratio between resistor 56 and the sum | | |
