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Патент USA US3022514

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Feb. 20, 1962
v. D. STROUD EI'AL
3,022,504
TWO-WAY RADIO TELEPHONE svs'rm UTILIZING FREQUENCY SUBBANDS
TO PROVIDE TRANSMITTER-RECEIVER ISOLATION
Filed Nov. 18, 1960
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INVENTORS
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l/AROLP 6'.
BY
RANT
3,022,504
United States Patent 0
Patented Feb. 20, 1962
1
2
provide a two-wire to four-wire terminating circuit for
3,022,504
use in a radio telephone system which is at the same time
TWO-WAY RADIO TELEPHONE SYSTEM UTILIZ
lNG FREQUENCY SUBBANDS TO PROVIDE
relatively insensitive to changes in the telephone line char
acteristics, line length and the number of units used on
TRANSMITTER-RECEIVER ISOLATION
Vincent D. Stroud, 3915 Weller Road, Wheaton, Md.,
and Harold S. Grant, 2414 Fairlawn St., Washington,
any particular line.
Other objects and many of the attendant advantages of
this invention will be readily appreciated as the same be
comes better understood by reference to the following de
tailed description when considered in connection with the
D.C.
Filed Nov. 18, 1960, Ser. No. 70,292
5 Claims. (Cl. 343-178)
10 accompanying drawing, the single ?gure of which is a
schematic diagram of a two-station system constructed
The present invention relates generally to communica
tions systems and more particularly to a terminating net
work for providing signal isolation between the transmit- 1
in accordance with a preferred embodiment of the present
invention.
.
Brie?y and in general terms, the above objects of the
ting and receiving components of a radio telephone sys 15 invention are realized, according to the present invention,
tem connected to a standard commercial two-wire tele
by utilizing interleaved comb ?lters to separate the sig
phone line.
nals in the audio portions of the system into two series
The terminating network employed in most radio tele
of mutually exclusive but complementary subbands which
phone communication systems usually includes, a mutual
together ?ll the complete audio spectrum. By means of
inductance bridge or a voice operated gain adjusting de 20 this technique, the voice signals coupled, for example, to
vice (VOGAD). To achieve the required amount of
the radio transmitter from the subscriber’s telephone at
isolation between the receiving radio path and the trans
one station have missing therefrom a particular series of
mitting radio path, it is necessary to maintain a precise
audio subbands, which subbands correspond to those
balance of the mutual inductance bridge. These adjust
which are coupled to the transmitter at the second station
ments, to minimize signal feedover, must be repeated 25 from the subscriber’s telephone located thereat. Thus.
every time a different telephone line is connected to the
in effect, mutually exclusive but complementary audio
terminating network. The VOGAD system attempts to
subbands modulate the transmitters at the two different
solve the problem by the use of voice operated switching
stations. Although the signals supplied to the listener are
relays to prevent feedback. This leaves much to be de
incomplete and have missing therefrom certain frequency
sired; however, since the relays being voice operated must 30 subbands, nevertheless, it has been found that such signals
often make a compromise when one party interrupts the
will convey intelligible speech with little or no change in
other. What is worse, to add to the confusion, the inter
rupting party does not realize that he is not being heard.
the natural sound of the subscriber’s voice as transmitted
over a telephone line. Furthermore, by employing com
In addition, because of the sensitivity limitations imposed
plementary subbands which fall completely within the
by the circuit parameters and speech characteristics, the 35 audio spectrum, the bandwidth of the radio transmission
?rst syllable of each word is frequently unheard or lost.
system is kept within normal limits.
The problem is threefold: (a) Feedback is encountered
Referring now to the drawing, the single ?gure of which
in long distance two-way radio voice circuits when termi
schematically illustrates a two-station radio telephone
nated on a two-wire standard telephone line at each end
communication system utilizing the present invention in
conjunction with a hybrid unit, the subscriber’s telephone
because of inadequate isolation between the receiving
path and the transmitting path. Since it is necessary to
run the radio paths at approximately unity gain, the sys
tem will be on the verge of oscillation if no effective iso
lation between the receiving and transmitting paths is
vset 1 at station A is connected via a two-wire sending
line, represented in simpli?ed form by reference charac
ter 2, to a ?rst bank of parallelly connected audio band
pass ?lters 3. These ?lters are in the input circuit of a
maintained. (b) Insu?icient isolation results in the noise 45 conventional» audio ampli?er 4 which supplies the modu
being enhanced by the cumulative effect of feedback.
lation signal for radio frequency transmitter 5 having a
This, in effect, increases the radio path noise which results
radiating antenna 6.
in a very poor signal-to-noise ratio. (0) The nominal
Telephone set 1 is also connected via a two-wire receiv
level established by one standard prior art system‘ for a
ing line, represented in simpli?ed form by reference
voice transmission at the receive side of a two-wire tele 60 character 7, to the output of an audio ampli?er 8 which
phone line is —25 db. The high noise level brought
has in its input circuit a second bank of parallelly con
about by the lack of isolation mentioned above frequent
nected audio band-pass ?lters 9. These ?lters are sup
ly exceeds the nominal level present in this system. Fur
plied with signal energy from the audio output of radio
ther, modulating the radio transmitter with this low level
frequency receiver 10 associated with detecting antenna 11.
voice signal (-25 db) results in an effective decrease of
Filters F1, F3, F5, F7 and F, of ?lter bank 3 in the
the signal-to-noise ratio at the distant receiver station.
transmitting portion of the system are tuned to different
It is accordingly a primary object of the present inven
frequencies within the audio band. As an example, these,
tion to provide a two-wire to four-wire terminating net
frequencies could be spaced one from the other by twice
work for use in a radio telephone communication system.
the frequency spread of the band-pass ?lters. Thus, if all
Another object of the present invention is to provide a 60 the above ?lters have a band-pass width of 200 cycles,
two-wire to four-wire radio telephone terminating unit
?lter F1 may be tuned to 200 cycles, F3, to 600 cycles,
which utilizes interleaved comb ?lters to divide the audio
band between the transmitting and receiving circuits. Each
link has a comb of (n) ?lters for its use spaced across
F5, to 1,000 cycles, F7, to ‘1,400 cycles and F9, to 1,800
cycles. Likewise, ?lters F2,’ F4, F6, F8 and F10 of ?lter
the audio band. The second set of (n) ?lters is conq 65 bank 9, associated with the receiving part of the system,
are tuned to different frequencies within the audio band,
structed to ?t between the pass-bands of the ?rst set.
with the separation between adjacent frequencies again
A still further object of the present invention is to pro
vide a radio telephone terminating network which, by it
self or with complementary apparatus, improves the sig
nal path isolation between the receiving and transmittin 70
components of the system.
>
A yet still further object of the present invention is to
being twice the band-pass spread of these ?lters. How
ever, these last-mentioned frequencies are midway be
tween those to which the individual ?lters in band 3 are ,
tuned. Thus, for example, if the receiving ?lters each
have a band-pass width of 200 cycles, F2 may be tuned
8,022,604
4
to 400 cycles, F4, to 800 cycles, F6, to 1600 cycles and
ever, in order to provide a greater degree of signal iso
F10, to 2000 cycles.
lation in the system, a hybrid transformer can be inserted
at the junction formed by the above two lines and the
'
It would be pointed out at this time that the particular
number of ?lters employed in each bank and the band
width and center frequency of each ?lter is purely a matter
of design. Two speci?c requirements should be con
common subscriber line.
It would also be pointed out that in the modi?cation
above described audio ampli?ers were inserted in the
transmitting lines before each of the transmitters 5 and
sidered in the above design. One, the pass-bandyof all
21. These ampli?ers are present to permit the operator
the ?lters stay within the limits of the audio spectrum
to adjust the line level to that required by the transmitter.
which may be considered as extending from 200 to 3,000
cycles. Two, in order to preserve the naturalness of the 10 Furthermore, in order to prevent complete dropout of the
groups of frequencies between the ?lters in either the
voice transmission, it has been determined by experiments
transmitting or receiving circuit, a frequency admittance
that the band-pass of the individual ?lters should not be
level ampli?er may be connected in parallel with each
equal and the center frequencies not equally spaced.
?lter bank. These ampli?ers 25, 26 of station A and 27,
The ?lters at the low frequency end of the audio spectrum
should be designed with narrow band-pass and center fre 15 28 of station B permit the isolation ?gure of the ?lter
system to be controlled.
quencies closely spaced. As the frequency of the ?lters
It would also be mentioned that instead of using a single
increases, the band-pass of the ?lters can be wider and
telephone set at each station a multiplicity of such sets
the center frequencies can be spaced farther apart. It
may be connected to the common subscriber line without
would also be mentioned that the various ?lters should be
designed to provide from 50 db to 60 db attenuation at 20 unbalancing or otherwise disturbing the operation of the
system.
the ?lter characteristic crossover point between adjacent
Obviously many modi?cations and variations of the
?lters, such as F3 in bank 3 and F4 in bank 9.
present invention are possible in the light of the above
It will be seen from an examination of the construction
teachings. ‘It is therefore to be understood that within the
of station A that, when a subscriber speaks into telephone
set 1, his voice signals will pass to the ?lter bank 3, and 25 scope of the appended claims the invention may be prac
ticed otherwise than as speci?cally described.
these ?lters will effectively sample the signal at a number
What is claimed is:
of discrete frequency bands corresponding to the band
1. A two-station radio telephone communication sys
pass frequencies of the individual ?lters making up this
tem comprising, in combination, means at said ?rst station
array. These frequency components will thus pass with
out any appreciable attenuation to the input of audio 30 for radiating a radio frequency carrier wave which is mod
ulated with a locally generated subscriber’s voice signal
ampli?er 4, modulate radio transmitter 5 and be sent out
from which a ?rst series of audio subbands have been
to remote station B by antenna 6.
suppressed, means at said second station for radiating a
The modulator carrier wave radiated from antenna 6
radio frequency carrier wave which is modulated with a
when station A is sending is detected by antenna 13 at
locally generated subscriber’s voice signal from which a
station B, and these RF signals are supplied to a conven
second series of spaced audio subbands have been sup
tional receiver 14 whose audio output feeds a bank of
pressed, the subbands of said ?rst and second series oc
parallelly connected audio band-pass ?lters 15. These
cupying mutually exclusive but complementary portions
?lters, it will be appreciated, are counterparts of those of
of the complete audio frequency spectrum, and means at
bank 3 located in the transmitting line of station A, being
each station for detecting and converting the modulated
tuned to the same band-pass characteristics. Consequent
ly, these ?lters present little impedance to the incoming
radio frequency carrier wave radiated by the other station
to an audio signal.
audio signals, and they pass via audio ampli?er 16 to the
subscriber’s telephone set 17.
2. A two-station radio communication system compris
Station B also includes a transmitting bank of ?lters
ing, in combination, means at said ?rst station for radi
ating a radio frequency carrier wave which is modulated
18 which are counterparts of those located in the receiv
ing line of station A, being tuned to the same frequencies
with a locally generated subscriber’s voice signal from
and having the same band-pass characteristics. This bank
which a ?rst series of audio subbands have been sup
of ?lters is energized with audio signals from telephone
pressed, means at said second station for radiating a radio
set 17 via two-wire sending line 19 and feeds audio am
frequency carrier wave which is modulated with a locally
pli?er 20 which provides the modulating signal for radio
generated subscriber’s voice signal from which a second
frequency transmitter 21 coupled to radiating antenna
series of spaced audio subbands have-been suppressed, the
22. From a comparison of station A and station B, it
audio subbands of said second series occurring at those
will be seen that the operating equipment located at each‘
portions of the audio spectrum which are not occupied
station is essentially the same, the only difference being in
by said ?rst subbands, said ?rst and second series of sub
the location of the similar banks of ?lters in the trans
bands substantially occupying the complete audio spec
mitting and receiving lines.
trum, and means at each station for detecting the modu
The procedure, whereby a subscriber speaking into
lated radio frequency carrier wave radiated by the other
telephone set 17 at station B communicates with a party
station and for converting it to an audio signal.
attending telephone set 1 at station A, is similar to that
3. In a two-station radio telephone communication sys
previously described. However, the voice signals now 60 tem, the combination of an “A” set of parallelly connected
modulating transmitter 21 contain a different set of audio
audio band-pass ?lters at each station, the band-passes of
subbands, which subbands correspond to those deleted
the various ?lters of each “A” set extending from F1 to
vfrom the voice signal modulating transmitter 5 when
F2 cycles, F3 to F4 cycles, F5 to F6 cycles, etc., a “B” set
station A is broadcasting to station B. Consequently,
of parallelly connected audio pass-band ?lters at each sta
when the modulated carrier wave detected by antenna 11
tion, the band-passes of the various ?lters of each “B” set
is converted by radio receiver 10 to the audio portion of
extending from F2 to F3 cycles, F4 to F5 cycles, F6 to F?
the spectrum, the resultant subbands match the band
cycles, etc., with the band-passes of the ?lters of both “A”
pass characteristics of ?lters F2, F4, F6, etc. Hence, these
and “B” sets substantially ?lling the audio spectrum, a
signals suffer little attenuation in the receiving line and
telephone, a radio transmitter and a radio receiver at each
pass via audio ampli?er 8 and two-wire line 7 to the sub 70 station, means for connecting the telephone at each station
scriber‘s telephone at this station.
to one side of an “A” set and a “B” set of ?lters, means for
In the above description, the telephone sets at each
connecting the other side of one of the “A” sets of ?lters
station were directly connected by two-wire lines, such as
to a radio transmitter, means for connecting the other side
2 and 7 of station A, to the transmitting ?lter bank 3 and
of the other “A” set of ?lters to a radio receiver, means
the output of the audio ampli?er 8, respectively. How 75 for connecting the other side of one of the “B” sets of ?lters
8,022,504
5
6
of said second group of ?lters, and means for connecting
the output of said second group of ?lters to said telephone
to a radio transmitter, means for connecting the other
side of the other set of “B” ?lters to a radio receiver
whereby one of said stations radiates a carrier wave mod
ulated by a voice signal from which a ?rst series of audio
set.
5. In an arrangement as de?ned in claim 4, a third
group of parallelly connected audio band-pass ?lters
located at said second station, the band-passes of the vari
ates a carrier wave modulated with a voice signal from
ous ?lters of said third group extending from F1 to F3
which a second series of audio subbands have been sup
cycles, F3 to F4 cycles, F5 to F6 cycles, etc., a fourth group
pressed.
of parallelly connected audio-band-pass ?lters located at
4. In a two-station radio telephone communication sys
tem, the combination of a ?rst group of parallelly con 10 said second station, the band-passes of the various ?lters
of said fourth group extending from F2 to F3 cycles, F4 to
nected audio pass-band ?lters located at said ?rst station,
F5 cycles, F6 to F7 cycles, etc., with the band-passes of
the band-passes of the various ?lters of said group ex
both the third and fourth groups of ?lters covering sub
tending from F1 to F, cycles, F3 to F4 cycles, F5 to F6
stantially the complete audio spectrum, a telephone set lo
cycles, etc., a second group of parallelly connected audio.
band-pass ?lters, the band-passes of the various ?lters of 15 cated at said second station, means for connecting the voice
signal generated by said telephone in response to the sub
said second group extending from'Fz to F3 cycles, F4 to F5
scriber’s use thereof to the input of said fourth group of ?l
cycles, F, to F7 cycles, etc., with the band-passes of both
ters, a second radio frequency transmitter, a second radiat
groups substantially ?lling the complete audio spectrum,
subbands have been suppressed and the other station radi
a telephone set located at said ?rst station, means for con
in g antenna, means for connecting the output of said fourth
response to the subscriber’s use thereof to the input of said
?rst group of ?lters, a ?rst radio transmitter, a ?rst radi
ating antenna, means for connecting the output of said
?rst group of ?lters to said ?rst radio frequency trans
mitter, thereby to modulate said ?rst transmitter, means 25
means for connecting the output of said second radio fre
necting the voice signal generated by said telephone in 20 group of ?lters to said second radio frequency transmitter,
for connecting the output of said ?rst transmitter to said
?rst radiating antenna, a ?rst detecting antenna, a ?rst
radio frequency receiver, means for connecting the output
of said ?rst detecting antenna to the input of said ?rst
radio frequency receiver, means for connecting the audio
output of said ?rst radio frequency receiver to the input
quency transmitter to said second radiating antenna, a sec
ond detecting antenna, a second radio frequency receiver,
means for connecting the output of said second detecting
antenna to the input of said second radio ‘frequency re
ceiver, means for coupling the audio output of said second
radio receiver to the input of said third group of ?lters,
and means for connecting the output of said third group
of ?lters to the telephone set located at said second station.
30
No references cited.
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