Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Thc present inven-ti~n relates to wired television
~roadc~lsting systems~ and in particular to wired television
broadcasting systems in which any one of a plurality of sub-
scribers may selectively connect his television receiver to any
one of a plurality of separate conductive patl~s defining sel~arate
television programme channels.
Wired broadcasting systems are known in wllich programmes
are made available at a central exchange to which a plurality of
subscribers are connected by respective signal conductors.
Each subscriber is provided with means for controlling a selector
s~ecific thereto located at the exchange for selecting a desired
progran~e for application to his signal conductors. Such a system
is disclosed in our British Patent Specification No. 1,272,594.
- Systems are also known in which programme signals are made
available on separate conductive paths extending to each sub-
scribers premises, each subscriber's television receivex being
provided with a selector for connec-ting the receiver to a desired
one of the conductive paths.
In systems such as those descri~ed above, cross-view
coupling can occur between separate channels. The effects of
cross-view coupling are however generally mitigated by random
fre~uellcy and phase relationships which occur in actual in-
stallations. For example in a practical system of the type des-
cribed in Specification 1,272~594 two groups of carriers are
derived from respective high stability crystal oscillators which
are operated in precision offse-t. Each oscillator feeds a "tree"
of hybrid transformers to pxovide one carrier wave ou-tlet in
respect of each programme wllich is to be made available. The
carrier waves from these outlets are applied to modulators or to
automatic frequency control inputs of special off-air receivers.
If no attention is paid to the polarity of the windings and
wirin~-u~ of the various hybrids and other radio fre~uency
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transformers wlli.cl~ are provided, and if no particular relationship
is o~served bet~een the programme channels and tlle two carrier
wave oscillators, tne random frequency and phase relationsllips
which resul~ reduce the cross~view coupling between separate
cha~ els. This is not ent::irely satisfactory however as the
cross-view protec-tion is not as good as it could ~e nor wholly
predictable.
It is an ob]ect of the present invention, to improve
cross-view protection in wired television broadcasting systems.
According to the present invention, -t1~ere is provided
a multiple channel wired -television .,roadcasting syste~ for making
a pluralit~ o programr,~e signals with a preselected line fre-
quency ciharacteristic available on separate conductive paths to
each of a plurality of progran~e selectors controllable ~y res-
pective subscribers to the system, means arranging the conductive
paths defining channels in two groups of channels, means providing
a first one of said two groups of channels a first operating
carrier frequency and the second of said two groups o~ channels
a second carrier frequency oEfset by an odd integral multiple j~
of haif line frequency to tne first, means establislling said
carriers of eacih said group of channels in quadrature or in
phase opposition to each other, and means allocating the cilannels
to respective conductive paths arranged in pairs of adjacent
conductive paths such tnat one conductive patil of each pair
operates at said first-carrier frequency and t'ne otller con~uctive
path operates at said second carrier frequencr.
It will be appreciated tilat the required phase difference
of an integral multiple of 90 between the pairs in a group
precludes there being more than four pairs in any one group, tile
four pairs l~aving relative phases of 0, 90, 180 and 270.
An embodiment of the present invention wi11 now be
described, by way of example, with reference to the accompanying
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~q~g~3~
drawings, in which;
Fig. 1 illustrates the arranc3ement of one group of
eight channels in a tllirty six channel s~stem;
Fig. 2 i.s a circuit diagram of one of two 180 phase
shifters shown in Fig. l; and
Fig. 3 is a circuit diagram of one of four 90~ phase
shifters shown in Fig. 1.
In the illustrated system thirty six television programme
signals are made availa~le at an exchange and applied in -tlle
exchange to a plurality of thirty six terminal selector switches.
Fi~. 1 illustrates the exchange circuitry and switches which handle
the first eight of these programme signals, the remaining pro-
gramMe signals ~eing handled in a like manner. Each switch is
connected by an individual conductive path 6 to a respective
subscriber 7 to the system 8, and each subscriber is provided
with means 9 for controlling his selector switch. One way in
whicll the necessary subscriber/switch connections and controls
can be achieved is described in the afore-mentioned British
Patent Specification No. 1,272,594.
Referring to Fig. 1 two high stability crystal oscillators
11 and 12 operate in precision offset, the nominal frequency of
~ach oscillator being for example 8.9 MHz and the off set being
one half line frequency. The oscillators 11, 12 feed respective
"trees" of phase shifters. Each "tree" comprises three phase
shifters which together handle Eour programme signal channels.
The first of the two "trees" shown comprises a 180 phase
shifter 13 and two 90 phase sllifters 14, 15 and the second
comprises a 180 phase sllifter 16 and two 90 phase shif-ters
1;', 18.
Taking the outputs of the oscillators 11 and 12 to be
at pllase 0, the phase sh1fter 13 provides a 0 output 19 to
phase shifter 14 and a 180 output 20 to phase shifter 15.
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Ti~e ~hase sllif~er 16 provic,es a o output 21 to p]lase sllifter 17
and a 180 ou~ut 22 to p~lase shifter 18. The phase shifter 14,
15, 17 and 18 eac~l provide outputs thepilases of which are res~
pectively +45 and -45 relative to their inputs. T]IUS phase
shifters 14 and 17 ~rovide -4~ out~uts 23 and +4~ outputs 24, and
phase shifters 15 and 18 provide 13~ outputs 25 and 225 outputs
26.
The outputs 23, 24, 25 and 26 are applied to a ~eries
of eight modulators 27 each of which receives a respec~ive pro-
gralllme signal 28 at video. The modula-tors 27 supply carrier
siynals modulated with the respective programme signals 28 to the
bus~ars of a switching network 29 that are dedicated to channels
1 to 8 as shown. The switching network 29 comprises switches
adapted to connect a respective su~scribers outlet to.ani~ one of
the bus~ars. Suitable switching systems are well known, and will
therefore not be described in detail herein~
The carriers of the system shown in Fig. 1 have relative
~hase differences of 0, 90, 180 and 270 and frequencies which
are relatively either high or low in accordance with the following
sec~uence:
CHANNEL RELATIVE PHASE RELATIVE
FREQUENCY
1 0 High
2 0 Lo~
3 go High
4 go Low
180 High ~
6 180 Low
7 270 High
8 270 Low
~here more than eight channels are provided, the above
sequence repeats itself every eight channels, viz:
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C~A~N~:LRELATIVE l~-'llASE :REL,ATIVE
FRT:~QVENCY
9 0 Higll
0 Low
11 90 High
12 90 Lo~
alld so on.
Tlle elgllt channels 1 to 8 form two groups a firs-t one of
whic~l (cllannels 1,3,5 and 7) operates at a ]ligh relative frequency
10 clnd the second o~ wllich (challnels 2, 4, 6 and 8) operates at a low
relative frectuellcy. The'carrier~ of eacll group are placed in
cluadrature or in p`lase opposition to eacll other. The channels
are allotted to respective conductive paths arranged in pairs
(1 and 2, 3 and 4, S and 6, 7 and 8) of adjacent col~ductive
paths. As a result one conduc-tive path of each pair operates at
the high relative fre~uency, the other at the low relative fre-
quency .
In systems of the type to whicll-the present invention
is applica~le, a major component of po-tential cross-view coupling
2Q results from direct switch cross-view to the individual subscribers
out~ut line. To il]ustrate the enhanced cross-view protection
<~forded by the present invention, this component is analysed
~low.
In a thirty six channel system, thirty five channels
interfere at the switch. Of these (from an extrapolation of the
carrier allocation table given above) eigilteen channels are in
half line frequency offset to the wanted. The eighteen offset -.
channels are placed in ~uadrature or in phase opposition to each
other. The resulting multichannel residual carrier is Eor most
of the time cancelled by the opposition of the two sets oE residual
carriers.
The remaining seventeen channel5 appearing at the sw,itch
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are s~ncllronized to the wanted channel. Of the~e four will be
substantially in ~he sarne phase and four in antipllase to the wanted.
The resulting multichannel residual carrier is for most of the
time cancelled by the opposi-tion of the two sets of residual
carrlers .
The remaining nine channels will be substantially in
quadrature with the wanted channel i.e. the 0 or 180~ control of
pllasing ensures that the resulting residual carrier is sub-
stantially cancelled and the sideband signals at the output o~
the swi~ch are in ~uadrature. The television receiver does not
however respond readily to quadrature phased sideband signal
com~onents and therefore provides added protection.
Referring now to Fig. 2, the phase shifter 13 of Fig. 1
is illustrated as comprising a core 30 supporting an input winding
31 and an output winding 32. The input winding 31 is connected
to the respective oscillator 11 (Fig. 1) and the output winding
32 has a grounded centre tap 33 so as to provide outputs 19 and
20 respec~ivel~ at 0 and 180 relative to ~he output of -the
oscillator 11.
Referring now to Fig. 3, the phase shifter 14 is
illustrated as comprising an inductance 34 in parallel with a
capacitor 35 providing outputs 23 and 24. The reactance of the
inductance 34 is the same as that of the capacitor 35, for example
75 ohms at 8.9 MHz.
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