Note: Descriptions are shown in the official language in which they were submitted.
PHN 7507
1055597
The invention relates to a cable television system
in which a local program centre is connected via a multiple
coax-tube trunk to a plurality of cable distribution stations
which derive VHF television signals by branching from the
trunk, which signals are divided between a plurality of coax-
tubes of the trunk, each tube carrying a plurality of signals
of different frequencies, in which system a plurality of
subscribers are connected to each cable distribution station
in a star-network configuration, with a coaxial subscriber's
connection for each subscriber between the cable distribution
station and the subscriber, whilst the cable distribution
station contains selection apparatus which can be controlled
from the subscribers' stations for selecting any one of the
television signals supplied by the trunk and transmitting it
to a subscriber's connection.
The conventional cable television systems have a
tree structure in which from a main station first via one or
more main lines but subsequently via an increasingly ramified
branch system the signals are ultimately supplied to the
subscribers. In such distribution systems the number of
television programs which can be distributed is restricted
owing to various circumstances.
When the complete UHF band is used a comparatively
large number of signals can be transmitted, however, through
very limited distances only, because at the higher UHF
frequencies the attenuation in the cables is very great. If
larger distances are to be bridged, many interposed amplifiers
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are required, which renders the entire system expensive.
For this reason large-haul transport always is
effected in the VHF band. Owing to the limited selectivity of
the receivers a channel is frequently left unoccupied between
every two occupied channels, although in the U.S.A. occupation
of all channels is permitted to increase the capacity.
Another method of increasing the channel capacity
in YHF systems is the use of what are generally referred to as
S bands which lie between the standard television bands and are
assigned to other services. This has the disadvantage that
interference of television reception by these services and
conversely occur, unless the entire system is provided with
full high-frequency screening. In addition, the use of the S
bands requires the provision of special converters in the
subscribers' homes, because conventional television receivers
are not designed for reception of these bands.
A selection system as referred to hereinbefore and
described~ for example, in the paper: "Two-way applications
for cable television systems in the seventies", I.E.E.E.
Spectrum, November 1971, pages 53-54, "Discade system", in
principle has an unlimited channel capacity. The star-
configuration of the cable network between the cable
distribution station and the subscribers moreover brings the
use of two-way communication and of communication between any
two subscribers within the range of the technically feasible.
In the aforementioned known selection television
system the cable distribution station has a selector for each
subscriber which connects the subscriber's connection of the
relevant subscriber to one of the tubes (coaxial cables) of
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the multitube trunk. In order to keep the number of tubes in
the trunk within reasonable limits, each coax-tube of the
trunk carries two VHF television signals which both are
transmitted to the subscriber's connection via the selector.
The apparatus provided in the subscriber's home then must have
an additional selection facility (converter including an
oscillator capable of being changed over) to enable a selection
between the two signals.
The technical advantages of the known selection
television system involve, however, high expenses attendant on
its installation. It is an object of the present invention to
provide a system which can be realised at considerably reduced
cost, and for this purpose the cable television system
according to the invention is characterized in that the
selection apparatus is provided with conversion means for
converting at least part of the VHF signals, which are divided
between a plurality of coax-tubes and situated on a plurality
of frequencies within each tube, into converted frequency
division multiplex (FDM) UHF signals, with subsequent splitter
means having an output for each subscriber connected thereto
and with FDM UHF tuners which each are connected to a separate
output of the splitter means, and are able by the relevant
subscriber and the output signal of which after conversion to
a standard television channel (for example VHF channel 2) is
applied to the subscriber's connection of the relevant
subscriber.
In the system according to the invention the
controllable UHF tuner effectively selects both between
television signals which are supplied through different coax-
PHN 7507
1055597
tubes of the trunk (space division multiplex) and between
television signals which are supplied through one coax-tube of
the trunk but at different frequencies (frequency division
multiplex). As a result, the equipment in the subscriber's
home is simplified and moreover the same number of television
programs can be supplied by a trunk comprising a smaller
number of coax-tubes. Furthermore the invention provides a
substantial saving in that in the cable distribution station
channel selectors are used which select a signal from a
plurality of signals at adjacent frequencies (frequency
division multiplex selectors) instead of the selectors which
are used in known selection television systems and which
select a connection from a plurality of connections (space
division multiplex selectors), for the former type of selectors
is cheaply available owing to recent developments in television
receiver technology.
Because in a selection television system of the
type proposed only a comparatively narrow frequency band of
the subscriber's connection is required, the system can simply
be extended by a system on the basis of general distribution.
In this case the cable television system according to the
invention is characterized in that part of the VHF television
signals and/or VHF frequency-modulation radio signals either
directly or after conversion, for example to the UHF band, are
applied to all the subscribers' connections.
In a selection television system each selector in
the cable distribution station must be remotely controlled by
the relevant subscriber. According to a further aspect of the
invention the UHF tuners in the cable distribution stations are
PHN 7507
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not directly controlled by the subscribers but indirectly by
a computer in the local program centre via a first commun-
ication channel, using a known interrogating system by means
of which the local program centre collects the selection in-
formation from the subscribers via a second communication
channel. This permits further simplification of the equipment
to be installed at the subscribers' ends.
The invention will now be described, by way of
example, with reference to the accompanying diagrammatic
drawings, in which
Fig. 1 shows schematically a local program centre
and the cabling configuration which may be used in a cable
television system according to the invention,
Fig. 2 shows schematically a cable distribution
station and a subscriber's station which may be used in a
television distribution system according to the invention,
and
Fig. 3 shows the frequency positions of the sig-
nals transmitted through the subscriber's connection.
Referring now to Fig. 1, a local program centre
1, which includes at least one program source, for example a
first program source 16 for standard television signals, a
second program source 17 for local television signals, a
third program source 18 for selection television signals and
a fourth program source 19 for still videotheque pictures,
transmits these television signals via a multiple coax-tube
trunk 2 to a plurality of cable distribution stations 3. The
trunks in actual fact form a tree network, the same informat-
ion being supplied to all the cable distribution stations.
Signal transmission via the trunk is effected in
PHN 7507
1.055597
the VHF bands and may, for example, take place at standard
frequencies, one channel being left unoccupied between each
two occupied channels. In this case, at a maximum 6 television
signals can be transmitted in each coax-tube and the overall
capacity of a trunk comprising, say, 4 coax-tubes is 24
television signals. If the capacity is to be increased without
using more coax-tubes in the trunk, there is no serious ob-
jection to the use of non-standard television frequencies
~for example in the S-band of 118 to 174 MHz situated below
the band III). Transmission via the trunk is restricted to
the VHF frequencies and any further extension of the capacity
of the trunk is to be found in the use of more coax-tubes in
the trunk in order to maintain attenuation in the trunk at a
sufficiently low value so that in general the trunk amplifiers
required in the trunk can be incorporated in the cable dis-
tribution stations and hence only in exceptional cases where
two cable distribution stations are a large distance apart
additional amplifiers between these stations are required.
To each cable distribution station a number of
(for example about 300) subscribers are connected, each sub-
scriber being connected to a cable distribution station via
a separate coaxial subscriber's connection 5 (star config-
uration).
In the cable distribution station of Fig. 2 each
coax-tube Pl, P2, P3, P4 of the incoming trunk 2 is connected
to the input of a VHF amplifier Vl, V2, V3, V4 respectively,
The outputs of the amplifiers feed coax-tubes Ql, Q2, Q3, Q4,
respectively of the outgoing trunk which supplies further cable
distribution stations. Consequently the amplifiers Vl to V4
act as trunk amplifiers.
PHN 7507
~055597
A small part of the output signal power of the
trunk amplifier Vl is supplied to the inputs of VHF/UHF chan-
nel converters Cl. Each of these converters converts one of
the VHF channels supplied via the coaxial pipe Pl to a given
UHF channel in a manner such, however, that the resulting UHF
channels are arranged in regular sequence, every two occupied
channels being separated by an unoccupied intermediate chan-
nel. Similarly a small part of the output signal power of the
trunk amplifier V2 is supplied to the inputs of further VHF/UHV
channel converters Cl' which convert the VHF signals from the
coax pipe P2 to the UHF band. The output signals of the con-
verters Cl and Cl' are amplified by UHF channel amplifiers Ul
and Ul' respectively, brought to substantially equal levels
and then combined and applied to the input of a splitter net-
work Sl. If, for example, six television signals are supplied
in VHF channels 2, 4, 5, 7, 9 and 11 through the coaxial pipe
Pl and if another six television signals in the same VHF chan-
nels are supplied through the coaxial pipe P2, these 12 tele-
vision signals can all appear at the input of the splitter Sl
side by side in the UHF band, for example in odd-numbered UHF
channels 21, 23, ..., 43.
The incoming VHF signals which are divided between
a plurality of coax-tubes (space division multiplex) and also
between a plurality of frequencies (frequency division multi-
plex) thus all are converted to frequency division multiplex
in the UHF band.
The UHF signal energy is divided by the splitter
Sl between a plurality of outputs, amplified in wide-band UHF
amplifiers U2, divided again by splitters S2 and then supplied
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PHN 7507
1055597
to UHF television tuners T. As a result, all the UHF signals
produced in the converters Cl and Cl' are available at the
input of each tuner T. For each subscriber equipped with means
for receiving selection television signals at least one tuner
T is provided which at the command of the relevant subscriber
is tuned by a control unit CU. The tuners T may be conventional
UHF tuners which are generally used in television receivers
and are inexpensive in manufacture. When using a varactor tuner
the control unit CU supplies the desired tuning voltage for
the varactors of each tuner. The tuner T selects from all the
UHF signals applied via the splitter S2 that UHF signal which
is set by the control unit CU and is desired by the relevant
subscriber and converts it to a standard television channel,
for example the VHF channel 2 (47 to 53 MHz). This signal is
applied to the coaxial subscriber's connection of the relevant
subscriber via a first low-pass filter LPl and a second low-
pass filter LP2. Thus the selection television channel sup-
plied via the coaxial pipes Pl and P2 are always converted to
the channel 2. This channel is particularly suited for further
transport of the selected television signal to the subscriber,
because it is substantially not used by broadcast television
transmitters. Owing to the imperfections in radiation-tight-
ness of the cable 5 and of the apparatus in the subscriber's
station, transport through another channel may suffer from in-
terference by signals which are on the air in the same channel.
If a subscriber wants to be able to receive two
selection television signals simultaneously, for example for
two receivers, this wish can simply be fulfilled by assigning
two tuners to him, adjusting the output frequency of one tuner
PHN 7507
1055597
to channel 2 and that of the other tuner to, for example,
channel 4, and connecting the outputs of both tuners to the
same low-pass filter LPl.
In addition to the aforedescribed selection tele-
vision system using signals supplied via Pl and P2, the de-
vice of Fig. 2 has facilities for supplying television signals
to all subscribers in accordance with conventional television
distribution systems. For this purpose VHF signals supplied
via the coaxial pipe P3 and amplified in the amplifier V3,
which signals all lie in band III (174 to 230 MHz), are ap-
plied via a further VHF amplifier V5 to a splitter S3 having
one output for each subscriber. The output signal of this
splitter is also applied via a high-pass filter HPl, which
passes the VHF band III, to the input of the low-pass filter
LP2 which passes the YHF bands I and III. Thus the television
signals from the coaxial pipe P3 are continuously, and without
conversion, supplied to all subscribers. Through the pipe P3
the frequency-modulation radio signals of the band II also can
be supplied to all subscribers.
The VHF television signals from the coaxial pipe
P4 are amplified in the YHF amplifier V4 and then supplied to
six VHF-UHF converters C2 by which each of the VHF channels of
the pipe P4 is converted to a UHF channel, for example to one
of the low UHF channels 21, 23, 25, 27, 29, 31. After amp-
lification in UHF channel amplifiers U3 these signals are
added together. The resulting UHF television band is divided
in a UHF splitter S4, amplified in UHF-band amplifiers U4,
divided again in second UHF splitters S5, again amplified in
UHF-band amplifiers U5 for part of the subscribers, and then
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PHN 7507
1055597
via a high-pass filter HP2 which passes the UHF band supplied
to the subscriber's connection 5.
Fig. 3 is an illustrative diagram showing the fre-
quency positions of the signals in the subscriber's connection
5. The total frequency band consists of a distribution band
above 40 MHz comprising signals transmitted to the subscriber
and of a return band comprising signals from the subscriber.
The UHF band IV (channels 21 to 31) accommodates
6 television signals supplied via P4 to all subscribers.
The VHF band III (channels 5 to 11) accommodates
4 television signals which are supplied through P3 and are
directly, without conversion, transmitted to all subscribers.
The frequency-modulation radio band II supplied through P3 is
also directly transmitted to all subscribers.
The VHF band I (channels 2 and 4) accommodates one
or two television signals which a subscriber has selected from
the plurality of television signals which are supplied via
the coaxial pipes Pl and P2.
It should be mentioned that the cable television
system described makes it possible that part of the subscribers
can receive only normal television distribution of the sig-
nals supplied via P3 and P4 if they do not want the selection
television facility with regard to the signals from the coaxial
pipes Pl and P2.
In the subscriber's station the subscriber con-
nection 5 is connected to a conventional television receiver
6 having VHF/UHF tuning capabilities and, via a band-pass
filter BPl for the band II, to a frequency-modulation radio
PHN 7507
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receiver 7. Via band-pass filters BP2 and BP2' and a mod-
ulation-demodulation device 8 the subscriber's connection
5 is also connected to a control device 9.
Control of the entire system preferably is ef-
fected by means of digital techniques, binary interrogating
signals being transmitted from a central control unit 10
(Fig. 1) in the local program centre sequentially for each
subscriber via an interrogating communication channel D'
(Fig. 3) which may lie in an unused frequency band of the
S channels, for example in the band of 112 to 116 MHz. For
this purpose interrogating signals from the cont~ol unit
10 modulate a carrier of 114 MHz in a modulation-demodulation
device 11 and then are applied via a band-pass filter BP3' to
the coax-tube (P3) of the trunk 2 which ensures VHF trans-
mission to the subscriber. Thus, in the cable distribution
station the interrogating communication channel comprises
V3, V5, S3, HPl and LP2. ln the subscriber's station the
interrogating signals pass through the band-pass filter BP2,
then are demodulated in the device 8 and subsequently reach
the control device 9. The interrogating signals from the cen-
tral control unit 10 include an address code which is specific
for each subscriber. When the control unit 9 receives an in-
terrogating signal containing the address code which is specific
to this control unit, it returns a reply code which includes in-
formation inserted by the subscriber into a store of the control
device. This store may be filled with selection television in-
formation, but also with other subscriber replies, such as re-
plies to polls held via television, intruder or fire alarms, etc.
In the device 8 the reply signal modulates a carrier of 30 MHz.
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1055597
This signal (D) which lies in the return band returns to the
central control unit 10 via BP2', a band-pass filter BP5,
a matching network 12, a return band amplifier V3' connected
in shunt across V3, BP3 and the device 11. When using a bit
frequency of 1 Megabit/s, interrogation of a subscriber and
the subsequent reply for the subscriber need take no more than
0.2 ms so that 5,000 subscribers can be interrogated per
second.
The information collected by the central control
unit 10 in this manner is primarily used to set the control
unit CU of the cable distribution station and hence the tun-
ing of the tuners T via a control channel in the.unused fre-
quency-modulation band of the coax-tube Pl via a 100 Mhz
modulator-demodulator 13, a band-pass filter BP6', the coax-
tube Pl of the trunk 2, a band-pass filter BP7' and a modu-
lator-demodulator 14. By means of a channel reserved for this
purpose and situated in the return band (for example at 26
MHz) the resulting setting of the control unit is checked.
The return path then comprises the modulator-demodulator 14,
a band-pass filter BP7, a return amplifier Vl' connected in
parallel across Vl, the coax-tube Pl, a band-pass filter BP6
and the device 13.
The information in the central control unit 10 is
further stored in a central store 15 for fee registration.
Finally the central unit 10 controls one or more
of the program sources of the local program centre in order to
transmit a given television signal at the demand of a sub-
scriber.
It should be mentioned that the capacity of the
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selection television system shown can be expanded by increas-
ing the number of coax-tubes in the trunk. However, the cap-
acity of the system is limited by the number of television
signals which can be applied to the tuner T. When using UHF
tuners capable of handling adjacent signals without mutual
interference the capacity can be expanded to about 48 tele-
vision channels. By using a continuous UHF/VHF tuner as the
tuner T, i.e. a tuner capable of handling also the VHF bands
I and III and the intermediate channels, further capacity
increase is possible. In addition, or instead, the incoming
signals can be divided into two groups and each subscriber
may be provided with two tuners, one for selection from the
second group of signals. Selection between the two groups
then is effected by switching the tuners from the control
unit CU, for example by switching their supply voltages on
and off.
The frequency space from 17 to 22 MHz which still
is free in the return band both in the trunk and in the sub-
scriber's connection can be used in a non-detailed manner to
a limited extent for transmitting a television signal gen-
erated in the subscriber's station, (for example for ad-
vertising purposes) to the cable distribution station and
from there via one of the coax-tubes of the trunk to the
local program centre, by which it can be distributed to
one or more other subscribers.
The frequency space below 17 MHz which still is
free may be used for a videophone connection, for example
the space from 6 to 8 MHz for transmission to the subscriber
and the space from 10 to 12 MHz for transmission from the
subscriber.
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PHN 7507
1055597
In general, the telephone network is preferably
isolated entirely from the television distribution network.
However, during construction, considerable savings are ob-
tainable by localising the cable distribution stations of
both networks at the same location and by using the same
cable ducts for both networks.
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