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Description  |
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FIELD OF THE INVENTION
My present invention relates to a videotelephone system comprising a number
of stations within a limited area, such as outlying stations associated
with a private telephone exchange.
BACKGROUND OF THE INVENTION
It is known, in such a system, to equip each outlying station with a video
transmitter and a video receiver in addition to the usual telephone
apparatus, these instruments being linked by audio and video lines with
the common central office. Each station also includes a source of
synchronizing pulses which controls the sweep circuits of its own video
transmitter as well as those of a remote receiver temporarily connected
thereto by way of the central office. The synchronizing pulses are
generated by individual crystal-stabilized oscillators operating
independently of one another; such oscillators are relatively expensive
and their duplication at each of the several stations weighs heavily in
the overall cost of the system.
OBJECTS OF THE INVENTION
The general object of my present invention, therefore, is to provide a
simplified videotelephone system in which this cost factor is
substantially reduced.
A related object is to provide means in such a system for improving the
operation thereof with suppression of crosstalk.
SUMMARY OF THE INVENTION
I realize these objects, in conformity with my present invention, by the
provision of a single source of synchronizing pulses at the central office
of such a videotelephone system, each of the associated outlying stations
including a pulse extractor for separating the sync pulses from the
accompanying video signals. The common pulse source is connected at the
central office to a set of first links in parallel, namely the incoming
transmission lines originating at the video transmitters of the outlying
stations; a second set of links, i.e., the corresponding outgoing lines
terminating at the associated video receivers, are connected at the
several stations to the respective pulse extractors thereof. In this way,
the sync pulses are transmitted through the switching equipment of the
central office to the pulse extractor of any station included in an
established videotelephone connection; the pulse extractor, in turn, feeds
both the video transmitter and the video receiver of that station.
Where transit time is a factor, i.e., where the outlying stations are
separated by substantial distances from the central office as may be the
case in a network serving a large industrial complex, the sweep circuits
of the video transmitter and the video receiver of a station cannot be
operated in step with one another since the video signals sent over the
intervening line links to the remote station will then be lagging behind
the sync pulses arriving at that remote station directly from the central
office. In accordance with an important feature of my invention,
therefore, each station includes delay means for relatively staggering the
operation of the transmitting and receiving sweep circuits to compensate
for this time lag. Such compensation, in the case of a relatively compact
communication network as here considered, will generally be required only
for the line scan or horizontal sweep but not for the frame scan or
vertical sweep.
BRIEF DESCRIPTION OF THE DRAWING
The above and other features of my invention will now be described in
detail with reference to the accompanying drawing in which:
FIG. 1 is a schematic view of a conventional videotelephone system of the
general type here considered;
FIG. 2 is a block diagram of video equipment provided at a typical station
of the conventional system of FIG. 1;
FIG. 3 is a view similar to FIG. 1, illustrating my present improvement;
FIG. 4 is a view similar to FIG. 2, showing the video equipment at a
typical station of the system of FIG. 3;
FIG. 5 is a block diagram of an amplification and equalization unit
included in the system of FIG. 1 or FIG. 3; and
FIG. 6 is a circuit diagram of an amplification and equalization unit
included in the system of FIG. 3.
SPECIFIC DESCRIPTION
In FIG. 1 I have shown a prior-art videotelephone system comprising a
central office or exchange CO and a multiplicity of outlying stations
partly illustrated at U.sub.1, U.sub.2, . . . U.sub.i. The central office
CO includes the usual terminal equipment CT for establishing communication
channels among the associated stations as well as between the latter and
subscribers outside the system. Each station includes a telephone set
T.sub.1, T.sub.2, . . . T.sub.i linked with the terminal equipment CT via
a respective audio line L.sub.1, L.sub.2, . . . L.sub.i ; it further
includes a video unit V.sub.1, V.sub.2, . . . V.sub.i (generally
designated V in FIG. 2) divided into a transmitting section VT and a
receiving section VR. The transmitting sections are connected to the
central office CO by way of respective first links L'.sub.1, L'.sub.2, . .
. L'.sub.i representing incoming video lines as seen from the central
office; receiving sections are served by second links L".sub.1, L".sub.2,
. . . L".sub.i constituting outgoing video lines. Each video line is
provided at its far end with an amplification and equalization unit
E'.sub.1, E'.sub.2, . . . E'.sub.i at the central office in the case of
the first links and E".sub.1, E".sub.2, . . . E".sub.i at the
corresponding station in the case of the second links.
Terminal equipment CT includes selector switches for interconnecting the
audio lines L.sub.1, L.sub.2, etc., of calling and called stations and for
controlling, through an interface unit I, a cross-bar switch UA for
simultaneously linking up the corresponding incoming and outgoing video
lines L'.sub.1, L'.sub.2, etc., and L".sub.1, L".sub.2 etc.
In FIG. 2, where a generic video line L' originates at the transmitting
section VT whereas a generic video line L" terminates at the receiving
section VR, the transmitting section is shown to comprise a camera tube TP
with associated horizontal-deflection and vertical-deflection coils HD'
and VD' whereas the receiving section VR includes a picture tube RC with
horizontal-deflection and vertical-deflection coils HD" and VD". Also
included in section VT is a sync-pulse generator GS individual to this
station, generator GS controlling the energization of coils HD' and VD'
through respective line-scan and frame-scan generators DO' and DV'. The
output of sync-pulse generator GS is further delivered to a mixer MS also
receiving video signals from a screen s of tube TP, this mixer working
through a line amplifier AL into transmission line L'.
Section VR has a sync-pulse extractor SS energized from line L" through a
video amplifier FV which may be part of the associated unit E" (of FIG.
5), the video signals on that line going to the intensity-control grid g
of picture tube RC. Extractor SS controls the energization of coil VD"
through a frame-scan generator DV", and that of coil HD" through a
line-scan generator DO" by way of a crystal-stabilized automatic
sweep-frequency-control circuit CAF.
In the operation of the conventional system of FIGS. 1 and 2, a temporary
connection between two stations such as U.sub.1 and U.sub.2 is established
by the equipment CT for voice signals and by the cross-bar switch UA for
video signals, the latter passing from line L'.sub.1 through unit E'.sub.1
to line L".sub.2 as well as from line L'.sub.2 through unit E'.sub.2 to
line L".sub.1. The video signals on lines L'.sub.1 and L".sub.2 are
accompanied by sync pulses generated at station U.sub.1 whereas the video
signals on lines L'.sub.2 and L".sub.1 are accompanied by sync pulses
generated at station U.sub.2. Owing to the mutual proximity of these lines
and the absence of synchronization between the two sync-pulse generators,
a certain amount of interference between the transmitted and received
signals is practically unavoidable.
For a description of my improved system, reference will now be made to
FIGS. 3 and 4 in which elements corresponding to those of FIGS. 1 and 2
have been identically designated and need not be redescribed.
A shown in FIG. 3, central office CO includes a common sync-pulse generator
GS.sub.c working in parallel into the amplification and equalization units
E'.sub.1, E'.sub.2, . . . E'.sub.i of lines L'.sub.1, L'.sub.2 , . . .
L'.sub.i. If the terminal equipment CT of the central office is of the
time-division-multiplex (TDM) type, generator GS.sub.c may also deliver
sampling pulses on a lead Q to that equipment at a frequency of 8 kHz
which corresponds to the international line frequency for videotelephone
systems. The sync pulses for the video equipment, delivered to units
E'.sub.1, etc., via a lead P.sub.1, thus include line-synchronization
pulses with a cadence of 8 kHz (corresponding to a scanning cycle of 125
.mu.s) and frame-synchronization pulses with a cadence of 50 Hz
(corresponding to a scanning cycle of 20 ms).
As will be seen from FIG. 4, the receiving station VR of my improved system
is practically identical with that of the conventional system (FIG. 2) but
the associated transmitting section VT lacks the sync-pulse generator GS,
the frame-scan generator DV' and the mixer MS of FIG. 2, the video signals
from screen s being delivered directly to line amplifier AL.
Vertical-deflection coil VD' of section VT is connected in parallel with
coil VD" of section VR to the frame-scan generator DV of the latter
section; on the other hand, line-scan generator DO' of section VT is
connected to the output of automatic sweep-control circuit CAF in section
VR, in parallel with line-scan generator DO", through a delay circuit CR.
The purpose of this delay circuit is to compensate for the transit time
which the video signals from screen s undergo in traveling to the remote
station over the interconnected video links, i.e., over the line L'
emanating at the station here considered and the line L" originating at
the remote station communicating therewith.
In the above-discussed case of a connection between stations U.sub.1 and
U.sub.2, for example, the sync pulses generated at the central office CO
at a time t.sub.o reach the station U.sub.1 with a delay d.sub.1 and the
station U.sub.2 with a delay d.sub.2. They arrive, therefore, at the
transmitter of unit V.sub.1 at a time t.sub.o + d.sub.1 and at the
receiver of unit V.sub.2 at a time t.sub.o + d.sub.2. Furthermore, the
video signals emanating from unit V.sub.1 require a time d.sub.1 + d.sub.2
to travel to unit V.sub.2, the first video signal of a new scanning line
arriving therefore at the latter receiver at a time t.sub.o + 2d.sub.1 +
d.sub.2 which is two transit-time periods d.sub.1 after the arrival of the
corresponding sync pulse (if it is assumed that this pulse coincides with
the commencement of the line sweep). It thus becomes necessary to
compensate for this lag by either delaying the line scan of the receiving
section VR of unit V.sub.2 or advancing the line scan of transmitting
section VT of unit V.sub.1 by the same time interval.
In the arrangement shown in FIG. 4, the latter solution is adopted by
inserting the delay circuit CR between control circuit CAF and line-scan
generator DO', the lag introduced by this delay circuit being equal to the
duration of a line-scanning cycle less the combined transit time (2d.sub.1
in the case of unit V.sub.1) on the associated line links L' and L". If
these line links are metallic circuits having a length of 1 km, for
example, the delay d.sub.1 will be about 5 .mu.s which is negligible
compared with a frame cycle of 20 ms but is rather significant for a line
cycle of 125 .mu.s. Thus, the delay time of circuit CR in that instance
should be 125 .mu.s - 10 .mu.s = 115 .mu.s, involving an entirely
unobjectionable downward shift of the received picture by the width of one
line.
Alternatively, the circuit CR could be inserted between sweep-control
circuit CAF and line-scan generator DO" of the receiving section VR to
introduce a delay of 2d, here assumed to equal 10 .mu.s. For the reasons
stated, it will generally be unnecessary to stagger the frame cycle of the
two sections so that a single frame-scan generator DV can be used for both
sections.
Units E".sub.1, E".sub.2, . . . E".sub.i in FIG. 3 may each consist in
well-known manner of several cascaded amplifier stages A".sub.1, A".sub.2,
A".sub.3 in series with an equalizer network RE", as shown at E" in FIG.
5, the same as the units E".sub.1, etc., and E".sub.1, etc., in the
conventional system of FIG. 1. Equalizer network RE" is shown inserted
between the first two amplifier stages A".sub.1 and A".sub.2, the latter
being separated by a coupling capacitor C" from the third amplifier stage
A".sub.3. Stage A".sub.1 is an amplifier of the balanced-to-unbalanced
type, whereas stage A".sub.3 is a complementary amplifier of the
unbalanced-to-balanced type. Stage A".sub.1 suppresses line noises
cophasally applied to its two inputs. Network RE" provides attenuation and
phase equalization over the transmitted band of video frequencies.
FIG. 6 shows a unit E' representative of units E'.sub.1, etc., in the
system of FIG. 3, this unit comprising amplifier stages A'.sub.1, A'.sub.2
and A'.sub.3 as well as an equalizer network RE' and a capacitor C' having
the same functions as their counterparts in FIG. 5. Output stage A'.sub.3,
however, includes not only an unbalanced-to-balanced amplifier A' but also
an electronic shunt circuit SH connected to the line between an input of
amplifier A' and coupling capacitor C', this circuit SH comprising an
electronic switch in the form of a PNP transistor TR whose emitter and
collector are bridged by a resistor R.sub.1 and whose base is returned to
its emitter through a biasing resistor R.sub.2. Operating voltage is
supplied to the emitter from a negative battery terminal -B through a
resistor R.sub.o, resistors R.sub.o and R.sub.1 forming a voltage divider
which provides a suitable biasing potential for the aforementioned input
of amplifier A'. The other input of that amplifier is connected to output
lead P of pulse generator GS.sub.c to receive synchronizing pulses Si
therefrom. Quenching pulses Sp of line-scanning frequency, which may or
may not coincide with the synchronizing pulses Si, are applied through a
resistor R.sub.3 to the base of transistor TR.
The occurrence of a quenching pulse Sp during the flyback phase of a
line-scanning cycle discharges the capacitor C' and grounds the
corresponding input of amplifier A', thereby introducing a d-c component
into the transmitted video signals to determine the voltage level assigned
to black picture elements. Thus, residual voltages held over from a
preceding scanning cycle are promptly eliminated and cannot accumulate
into objectionable biasing potentials. Amplifier A' superimposes the line-
and frame-synchronizing pulses Si upon the video signals passing through
network RE'.
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