Dynamically responsive CATV system with shared fiber optic link

A CATV system uses a single single-mode optical fiber for transmitting a composite optical signal containing television signals in different frequency channels for a plurality of subscriber terminals from a headend terminal to a distribution terminal, and converts the transmitted signals at the distribution terminal into electrical signals for distribution to a plurality of subscriber terminals over a plurality of coaxial cables. The distribution terminal further receives electrical service request signals from each of the plurality of subscriber terminals, and converts each received service request signal into an optical service request signal for transmission to the headend terminal over the same single-mode optical fiber as used from transmitting the composite optical signal. Wavelength-division multiplexer/demultiplexers couple the optical fiber to the headend terminal and the distribution terminal to enable bidirectional transmission. The subscriber terminal is responsive to a control signal transmitted to the subscriber terminal for enabling a receiver therein to receive television signals transmitted to the subscriber terminal over a selected one of said different given frequency channels. The headend terminal includes a digital switch that is operated in coordination with the control signals that are transmitted to the subscriber terminals for enabling reception of only selected frequency channels in the respective subscriber terminals, by frequency positioning the television signal indicated by the service request signal received from a given subscriber terminal within the composite signal for transmission to the given subscriber terminal over the given frequency channel enabled by the control signal that is transmitted to the given subscriber terminal.

BACKGROUND OF THE INVENTION 
The present invention generally pertains to electronic communication 
systems and is particularly directed to the use of fiber optics in cable 
television (CATV) systems. 
In a headend terminal of a typical CATV system, a plurality of television 
signals from separate television signal sources are amplitude modulated 
onto a plurality of carriers and then multiplexed to provide a multiplexed 
electrical signal. The multiplexed electrical signal is then transmitted 
to a distribution terminal (also known as a multi-tap or tap), from which 
it is further transmitted to each of a plurality of subscriber terminals 
over a corresponding plurality of coaxial cables that couple the 
distribution terminal to the plurality of subscriber terminals. Service 
request signals generated in the subscriber terminals typically are 
transmitted back to the headend terminal over telephone lines or through 
CATV distribution plants. 
The use of optical fibers to transmit television signals in CATV systems 
has been suggested because of the superior long-distance transmission 
characteristics of optical fibers in comparison with the long-distance 
transmission characteristics of coaxial cable. Hightower, "Economic FO 
System For New Residential Services", Telephony, Mar. 17, 1986, p. 44, 
describes a CATV system in which the multiplexed electrical signal is 
converted at a central office to a multiplexed optical signal and then 
transmitted over a single-mode optical fiber to a multimode splitter, from 
which the multiplexed optical signal is distributed to a plurality of 
subscriber terminals over a corresponding plurality of multimode optical 
fibers. In the Hightower system, service request signals from the 
plurality of subscriber terminals are provided over separate fiber optic 
links to a second multimode splitter which transmits the service request 
signals over a multimode optical fiber to the central office. A switch 
used for multiplexing the signals at the central office is controlled in 
response to the service request signals. 
The cost of such a CATV system using fiber optic links to each of the 
subscriber terminals is very high in comparison with the cost of a typical 
prior art CATV system, which uses coaxial cable links to each of the 
subscriber terminals. 
SUMMARY OF THE INVENTION 
The present invention provides an economical CATV system that uses a single 
optical fiber for transmitting information signals, such as television 
signals, for a plurality of subscriber terminals from a headend terminal 
to a distribution terminal, and converts the transmitted signals at the 
distribution terminal into electrical signals for distribution to the 
plurality of subscriber terminals over a plurality of electrical signal 
conduits, such as coaxial cables. The CATV system of the present invention 
thereby utilizes the superior long distance transmission characteristics 
of optical fibers for the link between the headend terminal and the 
distribution terminal, and utilizes the lower cost coaxial cable for the 
plurality of links between the distribution terminal and the subscriber 
terminals where the length of such links is such that the difference in 
transmission characteristics between coaxial cable and optical fibers is 
not as significant. 
The CATV system of the present invention includes a headend terminal and a 
plurality of distribution terminals, with each distribution terminal being 
coupled to the headend terminal by a separate single optical fiber for 
transmitting the composite optical signal from the converting means of the 
headend terminal to each distribution terminal. The headend terminal 
includes means for providing a plurality of information signals for a 
plurality of subscriber terminals; switch means responsive to subscriber 
requests for dynamically selecting the plurality of information signals to 
provide a composite electrical signal; and means for converting the 
composite electrical signal into a composite optical signal for 
transmission. Each distribution terminal includes means for receiving said 
transmitted composite optical signal and for converting it into electrical 
signals for distribution to the different subscriber terminals. 
Accordingly, preexisting CATV systems that use coaxial cable throughout 
their distribution links may be readily retrofitted with the system of the 
present invention without having to replace the coaxial cable links from 
the distribution terminal to the subscriber terminals. 
In another aspect, the present invention provides a distribution terminal 
for a CATV system in which a composite optical signal containing 
information signals for a plurality of subscribers is transmitted 
optically from a headend terminal to a plurality of distribution 
terminals, and then distributed from the distribution terminals to 
different subscriber terminals. The distribution terminal of the present 
invention includes means for receiving said transmitted composite optical 
signal; and means for converting the received composite optical signal 
into electrical signals for distribution to the different subscriber 
terminals. 
In still another aspect, the present invention provides a subscriber 
terminal for a CATV system in which a composite signal containing 
information signals for a plurality of subscribers is transmitted to 
different subscriber terminals in response to service-request signals 
received from the subscriber terminals. The subscriber terminal of the 
present invention includes means for receiving information signals 
transmitted to the subscriber terminal over a plurality of different given 
frequency channels; and means responsive to a control signal transmitted 
to the subscriber terminal for enabling the receiving means to receive 
information signals transmitted to the subscriber terminal over a selected 
one of said different given frequency channels. 
In a further aspect, the present invention provides a CATV system in which 
information signals for a plurality of subscribers are transmitted from a 
headend terminal to different subscriber terminals. The system includes a 
headend terminal including means for providing a plurality of information 
signals for a plurality of subscriber terminals; switch means for 
selecting and processing the plurality of information signals to provide a 
composite optical signal for transmission; and means for controlling said 
switch means in response to optical service-request signals received from 
the subscriber terminals; and a single-mode optical fiber connected to the 
headend terminal by a wavelength-division multiplexing/demultiplexing 
means for both transmitting the composite optical signal from the 
converting means of the headend terminal, and transmitting the optical 
service-request signals to the headend terminal, wherein the 
wavelength-division multiplexing/demultiplexing means is adapted for 
propagating short-wavelength light over the optical fiber in one direction 
and for simultaneously receiving long-wavelength light propagated over the 
optical fiber in the opposite direction. 
In a still further aspect, the present invention provides a bidirectional 
optical transmission system for distributing high bandwidth video signals, 
comprising a conventional single-mode optical fiber with a cut-off 
wavelength greater than approximately 1200 nm, first wavelength-division 
multiplexing/demultiplexing means connected to one end of the single-mode 
optical fiber and adapted for propagating long-wavelength light over the 
optical fiber in a first direction and for simultaneously receiving 
short-wavelength light propagated over the optical fiber in the opposite 
direction; and second wavelength-division multiplexing/demultiplexing 
means connected to the other end of the single-mode optical fiber and 
adapted for propagating short-wavelength light over the optical fiber in 
the opposite direction and for simultaneously receiving long-wavelength 
light propagated over the optical fiber in said first direction. 
Additional features of the present invention are described in relation to 
the description of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a preferred embodiment of a CATV system according to 
the present invention includes a headend terminal 10, a plurality of 
distribution terminals 12 and a plurality of subscriber terminals 14. 
Each distribution terminal 12 is connected to the headend terminal 10 by a 
single single-mode optical fiber 16. Each distribution terminal 12 is also 
connected to a plurality of subscriber terminals 14 by a corresponding 
plurality of coaxial cables 18. 
The headend terminal 10 dynamically selects television signals from a 
number of different sources, such as locally broadcast television programs 
20, television programs broadcast via a communication satellite 22, and 
prerecorded programs from a video-tape library 24, for the plurality of 
subscriber terminals 14 in response to subscriber requests and provides on 
each optical fiber 16 a composite optical signal containing the television 
signals selected for transmission to the subscriber terminals 14 connected 
to the distribution terminal 12 to which the respective optical fiber 16 
is connected. Each composite signal contains a predetermined number of 
frequency channels occupying different portions of a given transmission 
band. 
Each distribution terminal 12 converts the composite optical signal 
received from a single optical fiber 16 into composite electrical signals 
for distribution over the coaxial cables 18 to each of the different 
subscriber terminals 14. Each composite electrical signal provided on a 
coaxial cable 18 from a given distribution terminal 12 contains the same 
predetermined number of channels as the composite optical signal received 
over the single optical fiber 16 connected to the given distribution 
terminal 12. 
Each subscriber terminal 14 includes a receiver for receiving the 
television signals included in the composite electrical signal transmitted 
from the connected distribution terminal 12 over the connecting coaxial 
cable 18; and is responsive to a control signal transmitted to the 
subscriber terminal 14 within such composite signal for enabling the 
receiver of the subscriber terminal 14 to receive a television signal 
transmitted to the subscriber terminal 14 from the distribution terminal 
12 over a selected one of the different given frequency channels of the 
composite signal. 
Service-request signals are generated in the subscriber terminals 14 and 
transmitted electrically over the coaxial cables 18 to the respective 
distribution terminals 12 to which the subscriber terminals 14 are 
connected. Each distribution terminal 12 converts the electrical 
service-request signals received over the coaxial cables 18 from the 
subscriber terminals 14 into optical service-request signals for 
transmission to the headend terminal 10 over the single optical fiber 16. 
The headend terminal 10 converts the optically transmitted service-request 
signals received over the optical fibers 16 from the respective 
distribution terminals 12 and converts them into electrical 
service-request signals, which are processed by the headend terminal 10 to 
dynamically control the selection of the television signals that are 
provided on the different frequency channels included in the composite 
signals transmitted to the different distribution terminals 12 over the 
different optical fibers 16. 
The headend terminal 10 operates in coordination with the control signals 
that are transmitted to the subscriber terminals 14 for enabling reception 
of only selected frequency channels in the respective subscriber terminals 
14, by frequency positioning the television signal indicated by the 
service request signal received from a given subscriber terminal 14 within 
the composite signal for transmission to the given subscriber terminal 14 
over the given frequency channel enabled by the control signal that is 
transmitted to the given subscriber terminal 14. 
Referring to FIG. 2, the headend terminal includes a plurality of 
source-signal conditioning units 26 a plurality of transmission units 28, 
and a control computer 30. The source-signal conditioning units 26 are 
respectively connected to different sources of television signals 32, such 
as programs derived from local broadcasts, satellite transmissions, and 
video tapes of prerecorded programs. The number of source-signal 
conditioning units 26 is limited only by the number of different sources 
that are accessed to provide different television signals for transmission 
to the subscriber terminals 14. The number of transmission units 28 
corresponds to the number of distribution terminals 12. 
Each source-signal conditioning unit 26 includes a control data inserter 
34, an FM modulator 36 and a hard limiter 38. 
The control data inserter 34 inserts control data into the television 
signal provided from a given source 32. The control data inserter 34 
inserts such control data into a blanking interval of the television 
signal, such as the horizontal blanking interval, as described in U.S. 
Pat. No. 4,613,901 to Klein S. Gilhousen, Charles F. Newby, Jr. and Karl 
E. Moerder. The control data includes the aforementioned control signal 
that enables a receiver of a given subscriber terminal 14 to receive a 
television signal occupying a selected predetermined frequency channel. 
This control signal is provided to the control data inserter 34 by the 
control computer 30, which also keeps track of subscriber data. The 
control data may further include such data as is necessary for 
descrambling the television signal if such signal is transmitted in a 
scrambled format. The control data inserter 34 may further convert the 
audio portion of the provided television signal into digital audio signals 
and insert the digital audio signals into the horizontal blanking 
intervals together with the control data, as also described in U.S. Pat. 
No. 4,613,901. 
The FM modulator 36 modulates the television signal at 60 MHz after the 
control data has been inserted; and the modulated television signal is 
hard limited by the hard limiter 38 for provision to a transmission unit 
28. 
Each transmission unit 28 includes an N.times.M digital switch 40, M 
upconverters 42, a signal combining unit 44, a laser diode driver 46, a 
laser diode 48, a wavelength-division multiplexer/demultiplexer (WDM) 50, 
a PIN-FET receiver 52, a data detector 54, a channel selection controller 
56 and a FSK modulator 58. 
M is the predetermined number of different frequency channels that are 
provided in the transmitted frequency band; and N is at least the number 
of different source-signal conditioning units 26. The source-signal 
conditioning units 26 are connected in the parallel to the digital 
switches 40 in the different transmission units 28. The digital switch 40 
provides television signals at M different outputs to the M different 
upconverters 42. Each upconverter 42 modulates the signal that it receives 
from the digital switch 40 to a different frequency channel. The 
interconnection of the different upconverters 42 to the respective 
source-signal conditioning units 26 is controlled by the channel selection 
controller 56. A given source-signal conditioning unit 26 is 
interconnected by the digital switch 40 to more than one upconverter 42 
when more than one subscriber terminal 14 is to receive the television 
signal provided by the given source-signal conditioning unit 26, or when 
more than one receiver at a given subscriber terminal 14 is to receive the 
television signal provided by the given source-signal conditioning unit 
26. 
The signal combining unit 44 combines the television signals on the 
different frequency channels provided by the upconverters 42 to provide a 
composite electrical signal on line 60 containing the upconverted 
television signals on the M different frequency channels. 
The laser diode driver 46 and the laser diode 48 convert the composite 
electrical signal on line 60 to a composite optical signal 62. 
The wavelength-division multiplexer/demultiplexer (WDM) 50 couples a 
conventional single-mode optical fiber 16 to both the laser diode 48 and 
the PIN-FET receiver 52 for enabling both the transmission of the 
composite optical signal 62 from the laser diode 48 over the conventional 
single-mode optical fiber 16 to the optical receiver 72 (FIG. 3) at the 
distribution terminal 12 (FIG. 1), and the receipt by the PIN-FET receiver 
52 of optical service-request signals 64, or other low bandwidth signals 
(e.g., 1.5 Mbit/sec T1 telephone transmission), transmitted over the 
optical fiber 16 to the headend terminal 10. 
The WDM 50 provides for simultaneous propagation of long-wavelength light 
into conventional single-mode optical fiber 16 and short-wavelength light 
received from optical fiber 16 through known wavelength-division 
multiplexing/demultiplexing techniques. One form of WDM comprises a pair 
of optical fibers which are held together, heated, stretched and fused to 
create opposite facing beam broadening tapers separated by an elongated 
coupling region. An example of a stretched and fused WDM designed for use 
in a system with purely single-mode transmission at 1320 and 1550 nm is 
given in U.S. Pat. No. 4,834,481 to Lawson et al. Preferably, the WDM is 
formed in a multiple index fiber-in-tube structure of the sort described 
in U.S. patent application Ser. No. 223,423, filed Jul. 25, 1988, which is 
copending with the present application. The manufacturing process for 
these fiber-in-tube WDMs does not require etching of the fiber. With this 
design, the fiber-in-tube WDMs exhibited isolation of 10-15 dB for 1300 nm 
and greater isolation at 850 nm. 
The cut-off wavelength of a characterized optical fiber is defined as the 
wavelength at which transmission over such a fiber results in propagation 
of more than one mode, i.e., multimode transmission. Transmission or two 
of more modes is considered to be multimode transmission as the term is 
used in this application. Long-wavelength light, as used herein, means 
light with a wavelength greater than the cut-off wavelength of the 
conventional single-mode fiber used in the system, Short-wavelength light, 
as used herein, means light with a wavelength less than said cut-off 
wavelength. Typically, using conventional single-mode fiber with a cut-off 
wavelength around 1240 nm and above, and using widely available laser 
sources, the range of long-wavelength single-mode propagation over the 
fiber is approximately 1240 nm to 1550 nm. At short wavelengths below 
approximately 1240 nm the propagation over such fiber is multimode. 
"Conventional" single-mode fiber as used herein means a matched clad or 
depressed clad fiber without a special refractive index profile designed 
to increase the bandwidth of multimode transmission. 
In the present system, two preferred short wavelengths are 750 nm and 850 
nm; and a preferred long wavelength is 1300 nm, as economical laser 
sources are available at these wavelengths. The part numbers for the 
short-wavelength lasers: are Mitsubishi FU-04LD-N for the 850 nm laser, 
and FU-01SLD-N(4406) for the the 750 nm laser, as referenced in the 
Mitsubishi 1988 Optoelectronic Components Catalog. For most systems, using 
such short wavelengths would be a fatal drawback, as the available 
bandwidth of the multimode transmission at 850 nm (approximately 200-400 
MHz km) is two orders of magnitude less than the available bandwidth of 
the single-mode transmission over the single-mode fiber at 1300 nm 
(greater than 20 GigaHz km). Special fibers have been used to provide 
greater bandwidth multimode transmission. An example of such a special 
fiber is disclosed in U.S. patent application Ser. No. 259,723, filed Oct. 
19, 1988, which is copending with the present application. (See also, 
Turesdale and Nolan, E.C.O.C. 1986 Technical Digest, Vol. 1, p. 181 
(1986), Barcelona, Sp. 
In the present system, the bandwidth disparity is matched with the system 
requirements and the preferred opto-electronic components to provide an 
economical bi-directional system using conventional single-mode optical 
fiber with maximum transmission distance and minimum cost. The high 
bandwidth single-mode transmission link is suitable for transmission of 
the television signals over a moderate distance (e.g., 12 km), and the 
lower bandwidth multimode transmission link is sufficient for transmission 
of the service-request signals and other low bandwidth signals over the 
same distance. Preferably, the system design may be selected so that a 
substantial fraction of the available bandwidth in each direction may be 
utilized without wasting bandwidth in one direction because of the 
bandwidth/distance limitations of the transmission link in the other 
direction. 
Moreover, the present system enables the use of low-cost conventional 
single-mode optical fiber, and low-cost compact disk (CD) lasers, while 
enabling 3dB demultiplexing so that no light is wasted simply due to 
system design. Outside the CATV field, some bi-directional transmission 
systems have been suggested for use with a single fiber. However these 
systems have substantial drawbacks when contrasted with the present 
invention. For example, such systems transmitting at 1300 nm in both 
directions typically require the waste of half of the signal in each 
direction. As another example, such WDM-based systems transmitting solely 
single-mode light at 1300 nm and 1550 nm require expensive 1550 nm 
transmitter/receiver technology. 
The particular fiber-in-tube design of WDM described herein operates a very 
effective mode filter, in part because of the increased taper angle that 
this design provides. This makes possible the filtering of the multimode 
750 nm (or 850 nm) signal into a single-mode signal with reduced power for 
reception by the PIN-FET receiver 52. This is an optional feature; it may 
be used to increase the signal to noise (S/N) ratio of the lower bandwidth 
signal over the multimode transmission link by eliminating the mode 
partition noise. When used in the present invention, the WDM 70 converts 
the optical service-request signals generated at 750 nm (or 850 nm) into a 
single-mode signal that is launched into the optical fiber 16. Because the 
750 nm (or 850 nm) signal is below the cut-off wavelength of the 
conventional single-mode optical fiber 16, the 750 nm (or 850 nm) 
single-mode signal is regenerated into multimode as it is transmitted over 
the length of the optical fiber 16. However, the WDM 50 reconverts the 750 
nm (or 850 nm) signal into single-mode for processing by the PIN-FET 
receiver 52, thereby increasing the S/N ratio. 
The channel selection controller 56 provides polling signals that are 
modulated by the FSK modulator 58 for transmission to the subscriber 
terminals 14 with the composite signal provided by the signal combining 
unit 44. The polling signals prompt the subscriber terminals 14 to 
transmit stored service-request signals to the headend terminal 10. The 
polling signals may also be transmitted to the subscriber terminals 14 
from the channel selector controller 56 over telephone lines 67 via a 
telephone interface unit 68. 
The PIN-FET receiver 52 converts the received optical service-request 
signals 64 to electrical service-request signals on line 66. The 
service-request signals on line 66 are detected by the data detector 54 
and provided to the channel selection controller 56. 
The channel selection controller 56 responds dynamically to the received 
service request signals by controlling the digital switch 40 and the 
control computer 30 so that the digital switch 40 interconnects the 
source-signal conditioning unit 26 providing the television signal 
indicated by the service request signal received from a given subscriber 
terminal 14 with the upconverter 42 that frequency positions the 
television signal within the composite signal for transmission to the 
given subscriber terminal 14 over the given frequency channel that is 
enabled by the control signal provided by the control computer 30 that is 
transmitted to the given subscriber terminal 14. 
The channel selection controller 56 responds in a like manner to service 
request signals received from subscribers over telephone lines 67 via the 
telephone interface unit 68. 
Referring to FIG. 3, each distribution terminal includes a 
wavelength-division multiplexer/demultiplexer 70, an optical receiver 72, 
a power splitter 74, a FSK demodulator 76, a laser driver 78 and a laser 
80. An ELED driver and an ELED may be included in lieu of the laser driver 
78 and the laser 80, respectively. 
The wavelength-division multiplexer/demultiplexer 70 couples the optical 
fiber 16 to both the optical receiver 72 and the laser 80 for enabling 
both the receipt of the composite optical signal 82 from the single-mode 
optical fiber 16 by the optical receiver 72 and the transmission by the 
laser 80 of optical service-request signals 84 over the single-mode 
optical fiber 16 to the headend terminal 10. 
The connection of the wavelength-division multiplexer/demultiplexer 50 to 
one end of the single-mode optical fiber 16 at the headend terminal 10 and 
the connection of the wavelength-division multiplexer/demultiplexer 70 to 
the opposite end of the single-mode optical fiber 16 at the distribution 
terminal 12 enables bidirectional communication over the single 
single-mode optical fiber 16 between the headend terminal 10 and the 
distribution terminal 12, and thus effects a substantial savings in the 
quantity of optical fiber required for the system. 
The wavelength-division multiplexer/demultiplexer 70 is similar to or 
identical with the WDM 50. The WDM 70 is adapted for propagating 
short-wavelength light over the single-mode optical fiber 16 to thereby 
transmit the optical service-request signal, or other low bandwidth 
signals, from the distribution terminal 12 to the headend terminal 10 and 
for simultaneously receiving long-wavelength light propagated over the 
same optical fiber 16, thereby enabling transmission of the optical 
television signal from the headend terminal 10 to the distribution 
terminal 12. 
The optical receiver 72 converts the composite optical signal received over 
the optical fiber 16 from the headend terminal 10 into a composite 
electrical signal on line 86. 
The power splitter 74 conditions the composite electrical signal on line 86 
for transmission to the plurality of subscriber terminals 14 over the 
corresponding plurality of coaxial cables 18. 
Electrical service-request signals 88 received over the coaxial cables 18 
from the subscriber terminals 14 are demodulated by the FSK demodulator 76 
and converted into optical service-request signals 84 by the laser driver 
78 and the laser 80 for transmission to the headend terminal 10 over the 
optical fiber 16. 
Referring to FIG. 4, each subscriber terminal 14 includes one or more sets 
of subscriber equipment 90. A power splitter 92 in each subscriber 
terminal 14 provides the composite signal received over the respective 
coaxial cable 16 to the different subscriber equipment sets 90. Each 
subscriber equipment set 90 includes a directional coupler 94, a 
programmable FM tuner 96, an FM demodulator 98, a signal processor 100, a 
microcomputer 102, an FSK demodulator 104, an FSK modulator 106 and an 
infrared (IR) receiver 108. Given subscriber equipment sets 90 also may 
include a telephone interface circuit 110. 
In each subscriber equipment set 90, the composite television signal 
received from the coaxial cable 18 via the directional coupler 94 is 
passed through the programmable FM tuner 96 and the FM demodulator 98 to 
the signal processor 100. When the composite signal is received the 
programmable FM tuner 96 is tuned to one of the M different frequency 
channels included in the frequency band over which the composite signal is 
transmitted. The signal processor 100 extracts the control data contained 
in the television signal in the frequency channel to which the tuner 96 is 
then tuned and provides such control data to the microcomputer 102. The 
microcomputer 102 ascertain whether the extracted control data contains a 
control signal that enables the subscriber terminal to receive a 
television signal in the frequency channel to which the tuner 96 is then 
tuned. If not the microcomputer 102 programs the programmable FM tuner 96 
for receiving a television signal over a different frequency channel. This 
process is repeated with the tuner 96 being reprogrammed for the M 
different frequency channels in a predetermined sequence until the 
microcomputer 102 ascertains that the extracted control data contains a 
control signal that enables the subscriber terminal to receive a 
television signal in the frequency channel to which the tuner 96 is then 
tuned. The signal processor 100 provides the demodulated television signal 
on line 112, which may be connected to a television set, VCR or other 
television signal output device. 
When the received television signal on the enabled frequency channel is 
scrambled, the microcomputer 102 processes the extracted control data to 
provide control signals that are sent back to the signal processor 100 to 
enable the television signal to be descrambled by the signal processor 
100. 
In order to provide a service-request signal for transmission back to the 
headend terminal 10, a remote control unit (RCU) 114 is operated. The RCU 
114 provides a coded infrared (IR) signal which is detected by the IR 
receiver 108, and stored in the microcomputer 102. When a polling signal 
is received by the microcomputer 102 from either the coaxial cable 18 via 
the directional coupler 94 and the FSK demodulator 104, or telephone lines 
109 via the telephone interface unit 110, the microcomputer 102 provides 
the stored service-request signal onto the coaxial cable 18 via the FSK 
modulator 106 and the directional coupler 94 for transmission to the 
distribution terminal as a electrical service-request signal. 
Cable systems providing CATV services sometimes also provide telephone 
services. The CATV system described herein optionally includes equipment 
for providing television services together with the CATV services. In the 
headend terminal a second telephone interface unit 116 receives and sends 
telephone from and to telephone lines 117. The second telephone interface 
unit 116 conditions received telephone signals for combination by the 
signal combining unit 44 with the television signals that are being 
transmitted. The telephone signals are likewise converted to optical 
signals and transmitted over a single-mode optical fiber 16 to the 
distribution terminal 12, where after reconversion to electrical signals, 
the telephone signals are transferred by the power splitter 74 to a 
telephone system interface unit 118. Telephone signals are communicated 
between the interface unit 116 and telephones 120 in the subscriber 
terminals 14 via twisted pairs 122. Telephone signals received by the 
distribution terminal 12 from the subscriber terminals 14 over the twisted 
pairs 122 are provided by the interface unit 118 to the laser driver 78 
and laser 80, which convert such telephone signals into optical telephone 
signals for transmission over the conventional single-mode optical fiber 
16 to the PIN-FET receiver 52 in the headend terminal 10. In the headend 
terminal 10, the PIN-FET receiver 52 converts the optical telephone 
signals into electrical telephone signals and provides the same to the 
second telephone interface unit 116 which sends such telephone signals 
over the telephone lines 117.