Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to wired broadcasting
systems.
I According to the present invention, there is provided
a wired broadcasting system comprising a wideband coaxial cable
for carrying multi-channel programme signals from at least one
programme source to a plurality of exchang,ers to each of which
plurality of subscriber units for receiving a particular fre-
quency band are connected, the said at least one programme
source being constructed to supply the signals to the cable in
different frequency channels, and each exchange comprising for
each subscriber a switchable frequency translator for selectively
translating any one of the said frequency bands to a predeter-
mined other frequency band receivable by the subscriber unit,
switching means controllable from the respective subscriber unit `~
to selectively switch the frequency translator, and an output
circuit constructed to supply the said predetermined other
frequency signal to a feeder cable to which the subscriber is
connected, a selector unit at each subscriber unit for generating
control tones, wherein said switching means located at the
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exchange is responsive to said tones, a tone generator unit `
constructed to provide at least one master pilot tone to the
said wide band coaxial cable, and a tone extractor unit at each
exchange constructed to extract the said at least one master -
pilot tone from the said wideband coaxial cable for application
to each subscriber unit connected to the exchange, the selector
unit comprising tone generators responsive to the said at least
master pilot tone.
Each of the subscribers may be connected to -the res-
pective exchange by a separate cable in which case the said
other frequency may be exactly the same for each subscriber, or
may be held in some other precise relationship to obtain optimum
crossview protection on multi-pair twisted cables. This is a
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particularly advantageous arrangement for a system where HF :~
amplitude modulated
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. signals are sent .~rom exchange.s~ Alternclti.vely,
tlle su~)scribers may be corulected to their respec-tive
exchaage by a coml~on cable~ in tlhich case -the said
other :t`requency is diffe.rent .~or each subscr.ibera
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q`he switching means located at the exchange rna~
be controlled by tones from a selector unit at the sub-
scriber unit. rr`he selector unit ma~ comprise harmon.ic
a~d divider generators locl~ed to a.master pilot tone or
tones provided to the subscriber from the excha~geO
If it is desired to prevent a subscriber from
selecting a particular programme or programmes9 means
may be provided to prevent the.or one or more o~ the
maæter pilot tones reaching the su.bsc.riber, thereby
disabling one or more oX -the harmonic and divi.dex
generators and mal~ing it impossible for the svbscriber
to provide the necessary selection tone or tones to
seleot the programme or programmes.
Embodiments of the present in~ention will now
be described~ by way of example, with reference to ~he
accompanying drawings; in wllich:
~ ig~l schematically illustrates an embodiment
of the invention in which a plurali.ty of subscribers
are li~ed to an exchange by separate respective cables;
and
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2 schematically illustxates a further
embodiment of the inY~ntion ~~ ich a plurality of
~u.~sc.ribers are l mked to an exch~e by a common
cable O . '~
Re~'erring to ~ig. 1, a programme car.rier signal
generator 1 which comi)rises master fxequency gene.rators
and ~0~O modulators is provlded with sound and vision
pro~ramme sig~als on inputs 2, ~'he signal generator 1
transmits programme sig~als on low deviat-ion frecluenc~
modulatecL carriers stacked one upon the other in l.crequency
to a wide band coaxial cable 3 which is in t~rn connected,
to a plurality of exchanges 4 only one of which is shown~
~ow deviation F~Mo carriers are used as this reduced the
'problem of intermodulation distor-tion in frequency -trans-
lators and permits adjacent chan~el selection after -t.rans--
lation to a lower frequency band. ~lternatively l~.ere
the problem of intermocluJ.ation distortion may be reduced
by controlled frecluerLcy banc~ fi].terin.g the programme
carrier signal generator may include stal~dard V~ and
~HF modulators. ~he cable ~ may have a bandwidth of
for example 600 MEz. A tone generator unit 5 controlled
b~ a~ i~put 6 is located between the signal generator 1
and the cable 3 and arranged to provide to the cab].e 3
one or more mast;er pilot tones.,
Each exchan~e 4 is co~nected to a plurali-t~ of
~ubscriber units 7 (only one of which is shown) b~
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respective ones o.~` a plllrality o~ two or four w:i.re
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~Qwist' c~les~ such as are described in oux Paterlt
Spec:i.~ical;ioll Nou 1,272,59/l. Each su~scribe~ cable
8 is co~.mected to a respective o~e of a seri.es of
baluns li~ed to a respec-tive one of a series of
fretluency trc-~slators 10 arr~nged to translate the
carrier frequencies to a fre~uency acceptable -~o
.the subscri.bers unit. A tone extractor and control
unit 11 i.s arranged to extract -the master pi.lot tone
or tones provided by the unit 5 and apply them to the
baluns 9.
.
Each subscriber unit Gomprises a receiver 12
adapte~ to receive the ~ or ~ signals and a programme
.selector 13 which may compri.~e a bank of push buttons 14,
actuation of which provides one or more selection ton.es
to the e~change along the respective cclble 80 The
.æelection tones or Xrequencies are generated by hc~moni.c
and divlde~ generators locked to the master pilot to~.e
or tones se~t from the baluns 9 in the exchan~e l~ along
either of the palrs of wires in the cable ~.
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When a subscriber wishes to receive a particular
progra~e, he p.resses an appropriate push-button generating
the required tone which identifies -the frequency channel
in the cc~ble 3 on ~lhich tha-t; programme is provided. ~he
frequellcy tra~slator and ballul syste~ of the subscriber
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~ is arranged so -tha-t~ on receipt of ar~ one tone or
combi~a-tion o~ -to~es, the f.requency of the signaJ.
assoc,iated with that -tone or tones i5 transla-ted
from the ba~d occupied on the coaxial cable 3 to
the low frec,~uency band reqvired by the subscribers
u~it 70 '
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Where only one master tone is pro~ided o~ the
li~ 3 b~ the unit 59 the sub~Griber can be denied all
the progra~mes on the line by preventing t'he ma~ter to~e
being transmitted to him from the exchange via -tone
~xtractor arld control u~it 11, the absence of the master
tone clisabling the selec,tor 130 Where more than one
maæter tolle is provided, particular programmes or groups
of programmes can be denied to ~ subscriber whils-t
other programmes or groups of programmes are supplied,
by preventi~g the tra~smissi.on of selected ones of the
' 'master tones from the exchange to..the subscriber~ ~his
æelective denial facility may be usef~ æystems where
for e~ample payment ~or some but not all of the pxogrammes ~:'
is requ.ired, or where access to particular programmes has :-
~to be li~ited ~or security reasons.
:,
, Where the to~es are associated with programmes ~or
whish additio~al pa~ment is required th`e corresponding tone
generator ma~ be inhibited until the subscriber takes some
action to inclicate that he has paid or is pxepared to
accept payment for receiving the programmeO Alter~atively
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se:l.ectio~ of a "pay" progrratmne may autornati.cc311y i.ndicat;e
that pa~mer.~t ~ accep-ted~ In one application all tone
extrac-tor a~1d control ~lits 11 may be v~lder the control
of a cen-tral pay ~V com~uter a~d the co-mpute.r may
generate si~n.als such as digital information over ths
k 3 or a secondary connectionO
~ eferring now to Fig. 2, an alternative arrange~
m~nt i.s shown in which the i.ndividual cables 8 o~ ig. 1
are replaced by a wide band coa~ial ca~l.e 15 commo~ tv
all of the subscribers co~n.ected -to a particular e}~change
4. Corresponding co~ponents of` the ~ig. 1 a~d l~ig~
embodiments are given corresponding reference ~umexals~
~ ach subscriber is li~ked by a subsidi.ary coc~xial
cab:Le lG to the main cable 15, and is allocated a particular
frequency band i.n the cables 15 and 16~ Subscribers near
to the e~ch~ngc are allocated high frequency bauds and
subscribers remote from the excha~ge are allocated low
~reque~cy bands to ob-tai.n a maximum reach for excha~ges
at a given sending level~
The excha~ge 4 is similar to that described ~ith
re~erence to ~igo 1 but i.ncludes in respect of each sub~-
scxiber a characteristic fxeque~cy geuera-tor to provide
sel~cted programme signals on an ~o~l- carrier~ VEF or
UEF sta~dard ~ravefor~ns could alternatively be provided~
~he subscxiber unit is also similar to that described
~r.ith refere~ce to ~igD 1~ bu-t additio~all~lcludes
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r.ecluency ~ranslator for transla-ting the characteristic
carr:ier -~o a frequency accepta~le to th~ subscril:~ers
rece:Lver 12~ and &. c}laracteristic selectioll tone ~eneratcr
1~, the e~change being arranged on receipt of the
characteristlc selection tone -to alloca-te the frequency
of a selectecl programme signal to a band dedicated to
the particular subscriber. ~he local oscill~.~tors of
the subscribers frequency txv~lslator may be controlled
by pilot slgnals sent from the exchange to increase
-the security of the system. ~lternativel~ or in
addition this translator cc~ be located outside the
subscribers premises~ for example on the pole route
-outside the premises, or a-t the end of the road in
- which the premises are located~
Alter~a:tive arr~n~ements to those illust.rated
may of course be provided. ~or example~ provision ~y
be made for the ~ariable tuning o~ exchanKe filters
upon receipt of single or combination tone lrequencies
from the subscriber selector u~it.
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