Note: Descriptions are shown in the official language in which they were submitted.
i6
BACKGROUND OF THE IN~ENTION
eld of the Invention
This in~ention relates to a conference bridge.
(ii~ Pxior Art
.
Difficulties arise in communications networkst such
as telephone networks, designed to permit multiple
interconnections between a number of csmmunications
devices, such as where a number of telephones are to be
interconnected or the purpose of permitting users to
conduct a conference during which each speaker may speak
10 and be heard as the occasion requires. In principle,
without consideration of practical transmission
performance limitations, if the telephones are analogue
devices with ~wo wire links therebetween, it is possible
to interconnect a number of telephones for conference
15 type calls provided the interconnection routes between
the telephones are also two wire links. Where, howeverr
four wire links are employed, such as where the signals
are digitally encoded at some point in the
communica~ions circuit, an equivalent technique of
20 direct multiple interconnection is not possible and
arrangements must be made for switching the incoming and
outgoing signals from each telephone in such a fashion
26~3
as to direct the signals between telephones in a way
which will permit intelligibl.e communication. To
separate incoming and outgoing signals from each two
wire telephone link, a hybrid is used to convert two
wire to four wire links. It is known to digitise the
outgoing signals from each of the telephones and, hy
using a suitable switch, to block outgoing calls from
all but one selected one of the telephones from reaching
the remainder whilst directi.ng the outgoing signal from
the selected telephone to the remainder. Various means
have been proposed to control such a switch. United
States patents 3,958,084 and 3,947,639 describe an
arrangement in which the determination of which
telephone is to be selected for the time being as
providing the outcoming signal to the remaining
telephones is made in accordance with a comparison of
the instantaneous magnitude of the signal from the last
selected telephone with the magnitude of signals from
the remaining telephones and operating to replace the
prevailing selection by another in the event that
certain conditions are fulfilled. More particularly,
the conditions are if the instantaneous magnitude of a
non-selected signal is greater than the corresponding
instantaneous magnitude of the signal from the pre-
vailing selection, and if that greater instantaneous
magnitude is also greater than a predetermined threshold
value. In the arrangement of these patents, an
arrangement is provided for varying the threshold under
certain conditions. More particularly, the thresho]d is
determined as representing peak amplitudes of the signal
from the selected telephone but, manipulation is made to
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decrea~e the thxeshold stepwise in the event that fresh
samples of the selected signal fall below the preceding
threshold. Thus, in the event that the selected signal
outp~t should fail to reach the threshold level for a
succession of sampling intervals, the threshold will
eventually reach a zero state unless supplanted by a
threshold derived from a supplanting signal. This has
the effect that it is relatively more difficult for a
selected signal to be supplanted where other of the
10 signals are of larger magnitude but are possible echoes
in their local hybrids, whilst still permitting a
relatively free transition to another selected signal in
the case where the earlier selected signal has, say,
fallen to zero for an acceptable interval.
A difficulty arises, however, in implementing
devices as described above, called conference bridges,
in that the signal levels from each of the telephones
may not reach the bridge with equal signal levels for
equal sound input to the respective telephones. This
20 phenomena arises, for example, where there are
differences in the links interconnecting the telephones
with the bridge such as due to different line lengths or
line characteristics. In this case, it will be seen that
unless adjustments are made to the signal levels,
25 improper operation may occur since a signal from a
particular telephone which exhibits, naturally, a
greater signal level than those from other telephones
would tend to tak precedence even where this was not
desired.
An object of ~he present invention is to at least
in part alleviate the above mentioned difficulty.
~L232~;63
BRIEF DESCRIPTION OF TEIE INVENTION
.
In accordance with one aspect of the invention
there is provided a diyital conference bridge whereby
outp~k signals from a plurality of com~unications
davices can be controlled whereby the output signal from
a selected one of the devices is directed to the
remainder, the signal for the time being so selected
being determined by selection means incorporated into
said bridge and receiving time spaced digitised sa~ples
at least representing said signals, characterised in
that said selection means comprises first generating
means for generating the products xn. gn, xi. gi where
Xn and xi are instantaneous magnitudes of the samples
of all of the signals, and of the selected signal
respec~ively and gn and gi are predetermined gain
factors for all of the signals and fox the selected
signal respectively; first store means for storing a
threshold value "T"; first compaxator means for
comparing the proaucts XD . gn with the product xi.gi;
switch means for routing the selected signal to each
non-selected communications device; and control means
responsive to the conditions xn.gn > xi.gi and xn.gn>
T.gn to control said switch means to supplant a
selection of an existing said signal with a newly
selected signal in accordance with that one of the
previously non-selected signals which meets said
conditions~
In this arrangement, the selection means may
further include updating means for updating said first
store means, including a second comparator means for
comparing a presently stored threshold value T in said
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threshold register with products xi.gi and means for
supplanting the existing threshold value by a new value
equal to xi gi in the event that a product xi.gi should
exceed the existing threshold value, said updating means
further including decrementing means coupled to said
comparator for decrementing said threshold value T by a
predetermined amoun~ in the event that the threshold
value T should exceed one or a plurality of successive~
products xi.gi.
1o BRIEF DESCRIPTION OF THE SEVERAL VIh~S OF THE
ACCOMPANYING DRAWINGS
The invention is further described by way of
example only with reference to the accompanying drawings
in which:
Figure 1 is a diagram showing the interconnection
of telephones in a conferencing network, through
encoding devices to buffers;
Figure 2 is a diagram of part of a conference
bridge constructed in accordance with the invention; and
Figure 3 is a circuit diagram of a further part of
a conference bridge constructed in accordance with the
inventionO
DETAILED DESCRIPTION:
-
Figure 1 shows a telephone hook-up arrangement
intended to be used in conjunction with a conference
bridge constructed in accordance with the invention.
In Figure 1, two telephones designated #1 and #n as
shown, being the first and nth respectively of a series
of such telephones, the intervening members of the
series of telephones being omitted for clarity. Each
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telephone #l through #n is connected to a respective
hybrid 20. Each hybrid 20 has an input-output port
coupled by a two wire link 22 to a respective telephone,
an outgoing port coupled t~ an outgoing line 24 for
directing outgoing signal on line 22 from the hybrid and
an incoming port connected to an incoming signal line 26
for receiving incoming signal and directing it from the
hybrid to the line 22. Lines 24~ 26 associated with a
hybrid 20 are connected to a respective digital
encoder/decoder device 28. Devices 28 operate at a high
sampling rate such as 8 Khz. The devices 28 are in turn
connected to respective PCM highways 30, 32, Highway 30
in use carries thereon pulse-code modulated
time-division multiplexed signals from the
encoders/decoder devices 28 and connects to a serial to
parallel buffer 34 via a gain control device 37. Highway
32 in use receives time division multiplexed pulse-code
modulated signals from a parallel to serial buffer 36
for direction to the encoder/coder device 23. As in
20 accordance with conventional practice in pulse-code
modulated time-division multi-plexed communications
arrangements, the coder/decoder device 28 operate to
generate digitised signals from the respective
telephones 18 and hybrids 20 and to appropriately
~5 position them in successive time windows forming~ for
each set of windows corresponding to a set of digitised
signals a respective time frame, for data transmission
on the highway 30 and likewise to receive and decode the
respective correspondingly positioned signals on highway
30 32,to direct signals to appropriate ones of the hybrids
20 and telephones 13.
-
;3
The gain control device 37 is operative to multiply
digitised samples signal Xn, corresponding to
respective ones of the telephones 18 by predetermined
gain factors gn the gain factor being capable of being
different for each signal xn. The device may comprise a
digital multiplier suitably programmed and capable of
being programmed in a predetermined fashion such that,
for constant sound levels at each telephone 18 as
pxovided by a speaker thereat, products xn.gn will be
the same for signals derived from each of the telephones
18. There will of course be a set of products xn.gn for
each set of digitised sample signals from the telephones
18.
The buffers 34, 36 and the device 37 form part of
conference bridge, which bridge includes further
components as shown in Figures 2 and 3.
Figure 2 shows a register 40 for storing a product
Xi gi this corresponding to a predetermined one of the
set of products xn.gn. Register 40 is updated each time
20 a new set of signals representing products xn.gn is
received, the signal "i~ selected to have the product
xn.gn corresponding thereto s~orea being determined in
accordance with the particular telephone 18 which is for
the time being selected as one from which output signal
is to be directed to all of the remaining telephones.
Thus, for example, if telephone ~3 is ~o be so selected
n i~ will have the value 3.
The value of n i~ for the preceding set of products
xn.gn i~ stored in a register 60. ~hen the new set of
30 products xn.gn is received, it is the product xi.gi from
that set which is stored in register 40/ where the value
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of "i" is that prevailing from the selection made from
the preceding set of products.
The now stored value xi.gi in register 40 is
applied to one input of a comparator 42 the other input
of which comparator receives the signals representing
products xn.gn. Comparator 42 is operable to compare,
for each set of products xn.g~, the now skored product
xi.gi in store 40 with the current set of products
xn.gn. The comparator 42 has its output connected to an
AND gate 44.
Figure 2 also shows a threshold register 48 having
a stored threshold value "T" therein. Register 48 is
connected to a gain adjusting device 50,~ such as a gain
look-up table) and the output therefrom is connected to
one input of a second comparator 52. The other output
of comparator 52 receives the products xn.gn.
Comparator 52 operates to compare the product T.gn and
xn.gn wh~re T is the stored value in the register 48.
Output ~rom comparator 52 is coupled to AND gate 44.
In the event that output from comparator 42 should
be indicative of the condition that a product xn.gn is
greater than a product xi.gi stored in register 40 the
output of the comparator 42 will be enabled. Similarly,
the outpu~ of comparator 52 will be enabled in the event
that the product xn.gn is greater than the product T~gi
derived from the threshold register and gain look-up
table 50.
In the event ~hat output of both of the comparators
42 and 52 should be enabled, the ou~put fxom gate 44 is
enabled~ Gate 44 has its output oonnected to a register
60 for storing the value of "i", the identification of
the particular telephone for the time being to be the
telephone from which signal i5 to be directed to the
remaining telephones. Enabling of the output of gate 44
causes updating of the regis~er 60 to record the
identity of "in derived from the corresponding set of
products xn.gn Register 60 is able to identify the
selected telephone on the basis that the enabling of the
output of gate 44 will occur at a time period during the
relevant PCM fra~e indicative of a particular telephone.
Register 40 is coupled to register 60 so that register
40 can be conditioned for each time frame to store the
selected product xi.gi as before described. The whole
of the described updating function in resp~ct of
register 60 may be effected under control of
microprocessor 65, which microprocessor may also be
employed for outputting of the products xn.gn to the
circui, of Figuxe 2.
The actual switching of signal as between
tel~phones 18, once "i" has been identified, is effected
2~ by a microprocessor 65 which is coupled to the register
60 and which i~ also coupled to the buffers 34, 36 which
act as controllable switches for directing the signal
from the ~elephone identified in accordanoe with the
last selection of "i" to the remaining telephone. More
par~icularly, the microprocessor 65 is coupled to a bus
80 to which the buff~rs 34, 36 are likewise connected,
for the transfer of signals between buffers under
control of the microprocessor.
In Figure 3, which shows a further part of the
bridge, the means for updating the register 48 is shown.
Here, the register 48 is shown connected to a tristate
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78 which can be enabled to permit the value T stored in
register 48 to be outputted as required to a data bus
80, which bus 80 communicates inter alia with device 50.
The register 40 is also shown, together with a
comparator 66 which receives the output fxom the
register 48 and from the register 40. Comparator 66
operates during each PCM time frame to compare the
threshold value T stored in register 48 with the last
stored product xi. gi in register 400 In the event
that the stored threshold value T in register 48 ls
equal to or less than the product xi.gi one output 66a
of the comparator 66 is enabled. This output is
connected to a tristate 70 and is arranged such that the
tristate 70 is enabled in this condition. The tristate
is coupled to the register 40 so that when enable~ it
provides the product xi.gi ~from the register 40~ on an
output line 70a therefrom to data bus 800 Bus 80
provides a connection to the microprocessor 65 which
updates register 48 by substituting the so outputted
product xi.gi for the previous threshold value T.
In the event that the pre~existing threshold T,
exceeds the stored product xi gi, an output 66b of
comparator 66 is enabled. This output is connected to
one input of each of two AND gates 74 and 90. The other
input of gate 74 is connected to the output of a counter
76. The other input of gate 90 is connected via an
inverter 89 to the output of counter 76. The input of
counter 76 is selectively couplable to threshold
register 48 and to register 90 in a manner described
30 later. When a carry over of the contents of the counter
76 occurs the output from the counter is enabled and, on
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simultaneous enabling of output 66b, gate 74 is Qnabled
and gate 90 is disabled. Enabling of gate 74 disenables
tristate 78 and enables a further tristate 64. Tristate
64 then provides a connection from the ou~put of
register 48, through a table look-up device 51 to bus
80. Device 51 is constructed so as to operate on the
digital signal applied thereto, representing the
threshold value T stored in register 48, to decrement
the value repxesented thereby ~such as by digi~al ~ln)
so that a new~ lower, value threshold value T is
generated.. By operation of the microprocessor 65, the
new value is substituted for the previously existing
value T in the register 48.
It is possible however that under the condition of
lS enabling of output 66a of comparator 66, the counter 76
does not overflow~ In this case, gate 74 is disabled
and gate 90 is enabled. Then, tristate 64 is disabled
and tristate 78 is enabled allowing output from register
48 to reach bus 80. Microprocessor 85 in this case
reads the threshold value stored in register 48,
. directly, for transfer to bus 80. Thus, each time a new
frame of PCM signals is processed, the threshold vallle
held in register 48 will either be updated by replacing
the value therein with a new value xi.gi repr~senting a
25 new, higher threshold or if the condition T < xi.g
prevails, the existing threshold will either be
maintained lif counter 76 has not overflowed~ or be
reduced (if counter 76 overflows). The look-up table
device 51 is preferably constructed, however, so that
30 decrementation of the threshold value does not occur
once a zero value is reached.
12 ~3~3
The counter 76 is provided in view of the fact that
the threshold value stored in register 48 will normally
be a 7 bit A-law codad P~M signal, ancl if that were
directly acted on to effect decrementation of the value
S of ~T" in register 48, undesired steps in the
decrementation of the threshold value in the register
would occur. In use counter 76 is preloaded to a
predetermined count less than its full capacity.
Outputs of tristates 81 and 82 are joined on a common
input bus 84 to a look-up table 83. When output 66b is
- enabled (i.e. when T > xigi), tristate 81 is enabled;
when output 66a is enabled (i.e. when T < xigi),
tristate 82 is enabled. The counter 76 is preloaded
with the output of look-up table 83 (which output is in
that case conditioned in a predetermined manner in
accordance with the value "T" stored in register 48)
whenever output 66a is enabled (i.e. when T ~ xigi) or
the counter overflows. The input of tristate 82 is
connected to the register 40 and enabling of tristate 82
is effecti~e to increment the count in counter 76 in a
manner proportional to the value xigi then stored in
register 40. Thus the number of enablings of output 66b
necessary to produce an overflow from the counter 76 is
dependent upon the magnitude of the stored values in
register 48 and in register 40. In this regard, the
look up table 83 may be constructed so as to effect
preloading of counter 76 with a value which is in some
manner inversely proportiona~ to the value of "T"
presented thereto. Then, since the output of the
counter is enabled only at the overflow condition, the
time taken for a decrementation of the stored threshold
i3
13
value to occur is correspondingly proportional to the
magnitude of the stored threshold value. Similarly 7
each incrementation of the counter vi.a tristate 82 from
register 40 may be made by a value in some manner
inversely proportional to the respective xi.gi value
presented to the look up table 83~
The operation of the bridge can be described by the
following alogrithm written in Pascal:-
i := Instant Speaker;
For n :- l to Number of conferees do
If (Xn*Gn > Xi*Gi) AND (Xn*Gn > Threshold*Gn)
~hen i : = n; ~l)
If Xi*Gi~ Threshold Then threshold := ABS
~Xi*Gi);
If Threshold ~ O Then
Threshold : = Threshold - l;
For n := 1 to Number of conferees do
If n <> i Then Yn := Xi*Gi;
Instant speaker ~
The described arrangement has been advanced merely by
way of explanation and many modifications may be made
thereto without departing from ~he spirit and scope of
the invention as defined in the appended claims.
