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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of digital data communication,
and more particularly to wireless data communication to a plurality of
subscribers or users in an interactive manner.
2. Prior Art
Various types of communication systems which include, at least in part,
wireless data communication capabilities for communication to a large
plurality of remote locations are known in the prior art. By way of
example, in U.S. Pat. No. 4,117,405, a narrow band radio communication
system is disclosed. In accordance with that disclosure, a narrow band
system for communication between a transmitter device and a receiver
device is provided by locking both devices to a carrier signal from a
radio broadcast station and synthesizing a precise local frequency at
leach device as a pre-set multiple of the frequency of the carrier. Such a
system is usable in multi-station alarm and status communication including
a multiplicity of independent radio alarm transmitters whose various
carrier frequencies are different from, but phase locked to a local radio
broadcast station, and a central alarm receiving station which employs a
corresponding multiplicity of synchronized detectors. The detectors each
have associated with them a synthesized local oscillator source which is
also phase locked to the same radio broadcast station used by the alarm
transmitters. Both the alarm transmitters and the central receiver are
thereby accurately referenced to a readily available local frequency
source, the broadcast station. As a consequence, very narrow band radio
circuits may be employed and the receiver and transmitter band may be
accurately matched to the information content of the alarm and status
signal source to achieve high signal to noise ratio reliable
transmissions. In U.S. Pat. No. 4,208,630, a narrow band paging or control
radio system is disclosed which employs a similar synchronizing mechanism
using a broadcast station for reference, though with the central
transmitting apparatus transmitting signals to a multiplicity of remotely
located receivers, which receivers may be used for paging purposes, or for
controlling apparatus connected thereto.
The foregoing communication systems are generally directed toward the
communication of identification codes, control signals, paging signals and
the like of relatively limited information, but rather feature the
capability of selective communication with any of a large number of remote
receivers using the narrow band techniques disclosed therein. In other
instances, it is desired to communicate large amounts of data to remote
receivers so as to provide various types of information services for use
in the home and by businesses alike. Such services may be either
subscriber paid for, such as services which provide access to various
types of data banks, electronic publishing or other information services,
or services which are paid for by advertisers and the like, such as public
transmission of catalogue and other advertising information.
Two types of services of this general type are known as videotex and
teletex. Videotex uses the telephone line to provide two-way interactive
communication so that the data requested by the user at the time is
transmitted over the TV transmitter and/or the telephone line for display
on the user's terminal or TV set. Such a system has the advantage of being
interactive in that only the data desired by the user at the time need be
transmitted, though it has the disadvantage of relatively low data
communication rate capability if only the phone line is used in comparison
to that normally desired, particularly in the forward mode, i.e., when
communicating the requested data to the user. Teletex on the other hand is
a one-way information transmission system using signals superimposed on
television transmissions. The system has the advantage of high
communication rate capabilities, though is highly limited because of the
lack of interaction with the user. Consequently, usually teletex revenue
is obtained mainly from advertisers as is customary in the television
industry, while videotex revenues are expected to be derived primarily
from subscribers. Obviously because of the advantages and disadvantages of
each of these two systems, the nature of the information likely to be
accessible over the two systems may be expected to be substantially
non-overlapping, and accordingly both could be successful in the same
geographic area. The purpose of the present invention however, is to
provide a system having most of the advantages of both teletex and
videotex in a single low cost system.
BRIEF SUMMARY OF THE INVENTION
Two-way data broadcast networks capable of taking advantage of the
potentially very high data communication capabilities of conventional
television broadcast stations and television receivers for the forward
link are disclosed. For the return or reverse communication link, the
plurality of receivers, each within the broadcast distance of both a local
television station and a local AM radio station, each contain a local
oscillator which locks onto the carrier of the AM radio station and
generates therefrom a preassigned broadcast frequency for that receiver to
transmit data to the television station at data rates also synced by the
AM carrier frequency of the AM radio station. This television signal
receiver and return transmitter is called the "subscriber terminal". Each
subscriber terminal may transmit data in the form of information requests
to the television station by transmitting on its preassigned frequency,
modulated with a narrow band modulation in accordance with the information
being transmitted. In this manner, each subscriber terminal may
communicate in the reverse link through its own separate antennae. Each
subscriber device transmits on its own preassigned narrow frequency band,
allowing a very large number of subscriber terminals to simultaneously
communicate Simultaneous reception and detection of a large number of
subscriber terminal transmissions is enabled by the syncing of the
subscriber terminal transmitters and an oscillator at the AM station to
the TV station carrier (or subcarrier) frequency, with separation of the
various receiver signals being accomplished at a central receiver through
the use of a fast fourier transform computer, or by other techniques. On
receipt of a request for data from a particular subscriber terminal,
connection to the requested data source and very rapid transmission (e.g.
5.727272 million BPS as in present day Teletex during the vertical
blanking interval) of the requested data is made to the requesting
subscriber terminal through the television transmitter as in Teletex or
Videotex Various embodiments and variations are disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the radio net system of the present invention.
FIG. 2 is a block diagram of a frequency synthesizing circuit for use with
the present invention.
FIG. 3 is a block diagram illustrating the modification required in a
conventional AM radio station to use with the present invention.
FIG. 4 is a block diagram of one possible form of radio subscriber terminal
and,
FIG. 5 is a block diagram of an alternate form of radio subscriber
terminal.
DETAILED DESCRIPTION OF THE INVENTION
The present invention represents a marriage between commercial broadcast
station resources and private radio to provide a bidirectional and thus
interactive data communication system having a forward data communication
rate of a number of orders of magnitude greater than can be achieved over
a conventional telephone line, and a reverse link data communication rate
which is adequate for most interactive applications, and which may be
increased as may be necessary for other applications. The system is a
relatively low cost system in that it takes advantage in large part of
already installed equipment to minimize installation cost and time. In
accordance with the invention, forward link communication is achieved
utilizing a conventional television broadcast station, either
communicating over the air or by cable, and either utilizing an entire TV
channel or in a shared TV mode, with the return link communication being
achieved using narrow band communication techniques for relatively low
data rate communication through conventional television receiver aerials
to receiver antennas located in the vicinity of the television station
transmitter broadcast antennae. The carrier reference of a local AM radio
station is used as a synchronizing reference for all the data streams and
to allow the very close spacing of each subscriber terminal channel in the
return communication link to allow a very large number of subscriber
terminals to simultaneously communicate in the reverse link.
A basic system in accordance with the present invention may be seen in FIG.
1. The system, referred to by applicant as the "radio net system" or
RADNET, utilizes a conventional television broadcast station 20, a
conventional AM radio broadcast station 22 and a plurality of subscriber
terminals 24 coupled to conventional television antennaes 26, which may be
installed at the subscriber's home, business or the like, or otherwise
available as a portable antennae. A central receiver and data distribution
center 28 provides overall orchestration of the system, in part by
providing traffic synchronization and supervision signals to the AM radio
station 22 for broadcast to the terminals 24 to provide synchronization
and supervision signals thereto. Such signals may include, in addition to
the fundamental frequency reference, time reference signals to identify
time windows within which various functions can or will occur, subscriber
identification codes to momentarily enable reception by one or more
selected subscribers, transmission data content identification to allow
subscriber selected reception of advertiser paid materials, mode control
signals to enable various terminal modes such as immediate display, memory
storage or VCR storage, etc.
For one-way teletex type services, a radio net teletex service center 30 is
used to originate broadcasts comprising advertising, entertainment,
interactive TV, electronic publishing and the like, for reception by the
subscriber. In the case of advertising, or advertising paid entertainment,
all subscribers may be enabled so that a subscriber may selectively
receive whatever advertising or entertainment is desired. In this regard,
it should be noted that while the present invention is primarily intended
for data communication, the inclusion of a stop frame video recorder at a
radio net terminal will allow the transmission and reception of pictures
(analog) also, so that catalogues which include both product pictures and
textural materials regarding product description and prices may be
efficiently transmitted during off peak hours for storage and latter
display by RADNET subscribers. Other teletex type data such as electronic
newspaper and other electronic publishing is made code or address
dependent or otherwise encrypted so that only subscribers for that
particular publication may receive and display the transmission. Here
again, while the present invention is principally intended for data
communication, both text and pictures may be transmitted, the text
preferably, but not necessarily, transmitted in data form.
For two-way videotex type services, a return link communication system is
required. Communication in this direction may generally be quite slow, as
bulk data is not normally desired to be transmitted, but rather only
specific data requests, short responses, etc., are required. By way of
specific example, the request for access to specific data within a
specific data bank may only require the transmission of a very brief
digital message. For instance, a request for stock market information and
a specific stock quote would only require the identification of the stock
market data bank, the specific stock information requested, and the
specific quote requested, all of which requested information may be
menu-driven at the radio net terminals 24, with the actual data bank
identification, etc., being transmitted in abbreviated or coded form. For
this purpose an unused television channel is preferably broken up into a
large number of very narrow frequency subchannels so that simultaneous
transmission from a plurality of radio subscriber terminals 24 back to the
TV station 20 may occur without interference between channels. By way of
example, if a number of reverse link subchannels are created, each 200
hertz wide, then theoretically 30,000 such channels may be established in
the 6 megahertz bandwidth of an unused television channel. Communication
from subscribers to the central receiver antenna 29 near the TV station
antennae is possible in part due to reciprocity, that is, if the TV
station broadcast antennae can provide a signal for good reception by the
TV antennae of an individual subscriber, then the individual subscriber
can transmit (providing the transmission power density is sufficient) to
an antenna in the vicinity of the TV station antennae. With respect to the
question of power, a signal on the order of a few watts may readily be
radiated by a subscriber TV antennae which signal, on a power per cycle of
bandwidth, is on the same order as or higher than the signal typically
broadcast by TV stations. Thus, assuming very high selectivity in the
reception and demodulation of the return link signal, a high quality
reverse communication capability may be provided. It should be noted
further that TV station broadcast antennaes are normally positioned on
high terrain to provide maximum line-of-sight communication with receiver
antennaes, which in turn are purposely positioned and oriented for best
reception. Such a configuration automatically maximizes the capability of
communication in the return link, and for that matter minimizes the
likelihood of interference with neighboring TV stations, as a subscribers
TV antennae normally will be relatively low in comparison to the
associated TV station antennae and thus poorly oriented and positioned for
transmitting but very local TV stations, all of which will be sufficiently
separated in frequency to not interfere with each other.
The system of the present invention may also be advantageously used for the
promotion and sale of software. In particular, a subscriber terminal may
be used as a computer terminal, with the user receiving software
promotional material, and being able to "use" the software on a
demonstration basis by the users entry of his own data and receiving the
processed and formatted data on an interactive basis without actually
having the software in the terminal. Upon his decision to buy the
software, he may transmit his order and credit card or credit information
over his preassigned return link channel, after which the entire software
package would be encrypted by a password known only to the user or his
terminal, and transmitted to the user over the forward link.
If the password is, at least in part, part of the software purchase order
transmitted over the return link, piracy of the software from the forward
link transmission by others, while perhaps possible, would be most
difficult and impractical because of the special equipment required to
listen to any return link transmission and the uncertainty in the time and
channel of any such transmission. Obviously software or other items may be
sold and delivered this way, or delivered through the mail or by other
delivery services. Even if the delivery of the software were by mail, the
ability to "demonstrate" the software in the customer's home, on his own
equipment and at his leisure is a tremendous advance in marketing
technique.
A method of generating the frequency and time references for each band for
the reverse communication link, both at the RADNET terminals 24 of the
various subscribers and at the central receiver 29 near the TV station is
illustrated in FIG. 2. In accordance with this figure, an antennae 32
which may be a simple ferrite antennae or a TV antennae, provides a signal
to a narrowly tuned carrier detector circuit 34 locked to the carrier of a
local AM radio station 22 (FIG. 1) to provide an output equal to the
carrier frequency f.sub.c in response thereto. For illustrative purposes,
assume for the moment that the AM radio station has a carrier frequency of
640 Khz, and accordingly the frequency f.sub.c will be the carrier
frequency of 640 Khz with the side bands removed therefrom. The carrier
frequency is then preferably passed through a limiter amplifier 36,
essentially amplifying and clipping the carrier frequency to provide a
square wave output to the phase comparator 38 to emphasize the position of
the zero crossings and de-emphasize the amplitude the output of the
carrier detector 34. The output of the phase comparator 38 is passed
through a low pass filter 40 and applied to a voltage controlled
oscillator 42. The output of the voltage controlled oscillator is divided
by a factor M by the divide circuit 44, and applied to another phase
comparator 46, the output of which is filtered by low pass filter 48 and
used to control the voltage controlled oscillator 50, the output of which
provides the second input to the phase comparator 38. Thus, when the
circuit of FIG. 2 is locked onto the AM broadcast station carrier, the
output of voltage controlled oscillator 50 will be equal to the carrier
frequency. The output of the voltage controlled oscillator 50 is divided
by a factor N by the divide circuit 52 to provide the second input to the
phase comparator 46. For the specific example being described, if N is
3,200, then for the 640 Khz carrier, the input to the phase comparator 46
from the divide circuit 52 will be 200 hertz. Obviously the other input to
the phase comparator 46 from the divide by M circuit 44 will also be 200
hertz, or stated otherwise, the output of VCO 42 will be exactly 200 M
times the carrier frequency f.sub.c of the AM radio station. Obviously
incremental steps in the value of M cause incremental steps in the output
frequency f.sub.c of 200 hertz.
Assume for the moment that channel 18 was to be used for data communication
in accordance with the present invention, and that only the 4 megahertz
band width of the picture upper side band is to be used. The frequencies
f.sub.c would range from 495.25 megahertz to 499.25 megahertz. Similarly
for the return link, the values of M associated with individual
transmitters at subscriber radio net terminals and associated with the
corresponding channel to be demodulated after reception at the TV station
would range from 4,952,500 to 4,992,500, thereby effectively defining
20,000 channels over the 4 megahertz bandwidth. Obviously the 200 hertz
width of each channel could be made larger or smaller depending upon the
desired communication rate, though it is believed desirable to select a
relatively low rate usuable for general purposes, and to assign multiple
channels to a given terminal for those applications where the
communication rate for a single channel for that terminal is not adequate.
By way of example, assignment of 200 Hertz subchannels to a given
subscriber terminal would allow communication of 75 ASCII characters per
second together with a parity bit for each character, and leave a 50 Hertz
guard band.
The advantage of using a circuit such as that of FIG. 2 for establishing
the assigned channel frequency or frequencies at each terminal, and for
establishing the channel frequency of each subchannel being received at
the central receiver 28, is that both locations are generating the same
frequency using the same reference, so that a slight variation of the
carrier frequency of the AM radio station over a period of time will be
exactly tracked at both locations. In that regard, by requirement AM
carrier signals stay within a very few hertz of the preassigned carrier
frequency so that the frequency wander will be highly limited in any
event. Further, this also means that the circuit of FIG. 2 must only
capture and lock onto the AM radio broadcast carrier frequency within a
very limited band so that the low pass filters 40 and 48 may have a
significant time constant, which in turn will provide a "coasting" effect
in the event of momentary dropouts of the received AM carrier signal due
to interference, short term signal reflections, etc. This of course has
advantages in any situation, though may be particularly advantageous in
the case of mobile terminals such as the mobile terminal 54 illustrated in
FIG. 1, which could be used by way of example by police to gain immediate
access to Department of Motor Vehicle data banks, etc.
Now referring to FIG. 3, a typical AM radio broadcast station 22 (FIG. 1)
used with the present invention may be seen. As with all AM radio
broadcast stations, a master oscillator 56, an amplitude modulator 58 and
power amplifier 60 driving the antennae may be found. In addition however,
between the master oscillator 56 and the amplitude modulator 58 is a phase
modulator 62 which provides small angle phase modulation of the carrier in
accordance with the traffic sync and supervision signals, typically
digital signals applied thereto, from the radio net central receiver and
data distribution office 28 (FIG. 1). The modification which must be made
to a standard AM broadcast station in order for the station to function
with the present invention is relatively minor, as the phase modulator 62
operates at master oscillator signal levels rather than high power levels
and may readily be inserted between the master oscillator and the
amplitude modulator without any other modification to the broadcast
station. The modulation of the carrier will still keep the carrier
frequency within FCC requirements if the carrier is modulated at a
subaudible rate with a phase shift of plus or minus 15 degrees. By making
the phase shift plus 15 degrees followed by a shift of minus 15 degrees
for the transmission of a mark or one, and no phase shift for the
transmission of a space or zero, the average carrier frequency is not
disturbed by the modulation and the side bands created by the carrier are
sufficiently narrow to stay within legally prescribed limits of the
nominal carrier frequency, and are undetectable by way of conventional AM
mono or stereo radio receivers. Further, the positive phase shift followed
by a negative phase shift (or vice versa if preferred) occurs sufficiently
rapidly to be essentially transparent to the frequency synthesizing
circuit of FIG. 2 operating from the received (and thus the phase
modulated) AM carrier. In that regard, while the data transmission over a
conventional TV channel of 6 megahertz bandwidth may be on the order of
one megabit per second or higher, a plus or minus 15 degree inaccuracy in
following the AM radio station carrier for a 640 Khz radio station would
represent less than a 30 degree bit sampling error at a one megabit per
second bit rate. Such an error is clearly tolerable in accurately sampling
a bit stream for detecting the bits therein, and accordingly the circuit
of FIG. 2 may readily be used to synchronize the bit stream at the
transmission and reception ends of the forward communication link so that
longer data streams can be transmitted between start and stop bits than
would be possible if the transmitter and receivers operated on separate
clocks, thereby grossly reducing the synchronizing overhead required and
increasing the efficiency of the data transmission.
Now referring to FIG. 4, a block diagram of one possible configuration for
the radio subscriber terminals 24 (FIG. 1) of the present invention may be
seen. In FIG. 4, the television antennae 26 is coupled to conventional RF
and video circuits comprising a high frequency (VHF or UHF) tuner 64 and
intermediate frequency amplifier 66 driving a video detector 68 and video
amplifier 70, and also driving a sync detector circuit 72. These circuits
of course provide conventional video and video sync signals which may be
used to drive a conventional black and white or color video display 75,
these circuits and display essentially comprising a conventional
television receiver. For data reception in the forward link, the output of
the video amplifier 70 will provide base band data information to a data
decoder 74 which is synced in part by the sync detector circuit 72. The
bit stream, of course, corresponding to any horizontal sweep line could be
transmitted in the "vertical blanking interval" as in Teletex, and could
be synced by conventional techniques using start and stop bits and a
locally generated block signal. However, as previously described, it is
preferable to connect the AM tuner 76 either to the television antennae 26
or alternatively to a ferrite antennae and to provide a relatively narrow
band filter 78 on the carrier frequency of a predetermined AM station to
provide the AM radio station carrier on line 80, as phase modulated by the
traffic synchronization and supervision signals. (The AM carrier frequency
itself could be locked to the TV chroma subcarrier, for reasons which will
be subsequently described. ) These signals in turn are decoded by a phase
demodulator 82 and used by the data decoder 74 as control signals thereto.
The carrier frequency on line 80 is also passed through a frequency
synthesizer 84, generally constructed in accordance with the circuit of
FIG. 2, to provide a timing reference frequency for the data bit stream
transmitted through the television channel using the same form of
reference. In that regard, the frequency f.sub.c provided by the circuit
of 84 may be set equal to the data rate or at some multiple of the data
rate and then counted down in the data decoder 74 starting at the
mid-point of a start bit to provide near mid-bit sampling of the following
data stream. The serial bit stream is assembled by the data decoder 74
into eight bit bytes and passed through a data and timing I/O interface to
a microprocessor based system bus 88, as controlled by a microprocessor
90. The microprocessor 90 will receive data in byte form from the data and
timing I/O circuit 86 and store that data in data storage unit 92, which
normally will include random access memory sufficient to store a number of
pages of display information and which may include tape or disk storage or
other bulk data storage of relatively large quantities of information.
Typically, for display purposes, a video generator and control circuit 94
is used to generate the required video sync signals and provide timing and
alphanumeric character generation to appropriately convert digital data
presented thereto to appropriate alphanumeric display information for the
various sweep positions of the raster of the display 75. As is done in
some microprocessor based systems such as in Apple II computers, counter
circuitry in the video generator circuits operating off of the same clock
reference as the microprocessor may provide the addressing to random
access memory and to the alphanumeric character generator as well as the
video sync signals required for operation of the display system utilizing
the processor on a 50 percent duty cycle for display refresh purposes, so
that the other 50 percent of the time the processor is available for any
other tasks assigned thereto such as data input, output, etc. Of course,
the overall system is controllable through a conventional keyboard 96 to
carry out the various functions of the system.
In a Teletex type mode, requests for information entered through the
keyboard 96 may be temporarily accumulated into partial or complete
requests by the microprocessor in the data storage 92 and then passed by
the microprocessor to the data and timing I/O circuit 86 to be passed
serially in digital form to a phase modulator 98. This serial bit stream
is also preferably synchronized using a reference also accessible at the
television broadcast station so that the bit stream being transmitted by
each terminal would be synchronously detected once that channel has been
demodulated. While such synchronization is not mandatory, it can be
provided by synchronizing the AM carrier to the TV Chroma subcarrier and
is advantageous to increase the efficiency of the return link
communication by cutting down on the required synchronization overhead.
For such synchronizing purposes even the 60 hertz power line could be
used, but preferably a reference frequency derived from the AM radio
station carrier in the aforesaid manner may be used. Thus as shown in FIG.
4, the output of the frequency synthesizer 84 is used not only for forward
link data synchronization but also is counted down by the data and timing
I/O circuit 86 to provide the relatively low frequency reference for the
data stream of the return link. The output of modulator 98 of course is
amplified by the amplifier 100 to provide a broadcast signal to the
antennae 26, preferably on the order of 2 watts. As previously described,
the phase modulation provided by modulator 98 is limited to a small angle
phase modulation, with the data rate itself being limited to approximately
75 bits per second so as to keep the bandwidth of the signal broadcast by
the return link within the preassigned channel limits, centered on the
preassigned frequency of that channel.
While the serial data rate in the forward link may be relatively high (on
the order of a megabit per second or higher) the data once assembled in
byte form may readily be received and processed on-line by conventional
microprocessors such as microprocessor 90 and stored in limited random
access memory in data storage unit 92, which normally will be adequate
when operating in a Videotex mode as normally a subscriber will be asking
for and receiving specific limited information at any one time. However,
in the typical Teletex mode, large amounts of data may be received such as
newspapers, catalogues and the like, which for practical reasons would
exceed that which may be readily stored in a reasonable size random access
memory. Accordingly, for this purpose, a video recorder may be provided
such as video recorder 102 operative under control through the keyboard 96
and microprocessor 90 through the video recorder control circuit 104,
either in an immediate response mode or as controlled by clock 106 for
operation in accordance with a programmable schedule. The video recorder
102 as shown of course is bilaterally operable from the video and video
sync signals of the video amplifier 70 and sync detector 72 so as to be
usable to record incoming signals in bulk and later play back parts of
those signals if desired for processing through the data decoder 74 as if
then being received.
Currently conventional video recorders usually include their own tuner,
etc., so as to connect directly to a TV antennae to provide the capability
of recording one program while the conventional TV receiver is used to
view another program. As such a conventional recorder such as recorder 108
may also be used if desired to record directly off of the antennae signal
and to play back portions of the tape for viewing under manual control of
the user.
It will be appreciated on review of FIG. 4 that substantially all of the
analog video circuitry and display is characteristic of conventional
television receivers and that the digital circuitry is characteristic of
conventional personal computers, using in addition of course some
additional specialized circuits such as the data decoder 74 to
characterize the personal computer for this particular function. The
remaining synchronization and return link communication capabilities are
achieved by specialized, though relatively low cost circuitry so that the
entire function of the radio subscriber terminal may be readily achieved
with a conventional TV set, a personal computer and some additional
communication circuitry, together with a conventional video recorder if
desired.
Now referring to FIG. 5, a block diagram for an alternate radio subscriber
terminal configuration may be seen. This embodiment is the same as that
shown in FIG. 4 with the exception that horizontal and vertical sync
signals for the video signal are not transmitted. Such a configuration has
the advantage of not having to time share the television channel between
data and sync signals and of simplifying the radio subscriber terminal,
though of course has the disadvantage of no longer enabling the use of a
video recorder or the display of anything other than digital data in
alphanumeric character form. Also such a configuration does not have any
special advantage if the system is being used on a time shared basis (in
the nonviewable lines) with conventional TV broadcasts, as horizontal and
vertical sync signals are being broadcast anyway. In that regard one of
the advantages of the present invention is that ability to use the system
with conventional or substantially conventional television receivers which
can be used as such when not being used as a radio subscriber terminal. In
this manner, the incremental cost to provide a radio subscriber terminal
in a home already having a conventional television set, and particularly
having both a television set and a personal computer would be very low in
comparison to competing systems.
The embodiments heretofore described have utilized the carrier of a local
AM radio station for synchronization and supervision of the communication
system. While this has certain advantages, particularly for mobile
subscriber terminals to minimize multipath problems, it is not a necessary
limitation in the present invention. In particular, one might choose to
have both the data and synchronization and supervision signals transmitted
through the television station using, for example, the TV Chroma
subcarrier as prescribed later, or any of a number of techniques. By way
of example, the video intensity portion of a conventional television
broadcast signal is placed on an RF carrier using amplitude modulation
(AM) techniques The "color" portion is placed on the Chroma subcarrier. In
particular each television channel is approximately 6 megahertz wide, with
a picture signal carrier being 1.25 megahertz above the lower end of the
band (in general, while an approximately 4 megahertz wide upper sideband
is transmitted, the lower sideband is truncated approximately 1 megahertz
below the carrier frequency). The Chroma subcarrier is at 3.579545 MHz.
Thus a black and white picture signal carrier may be phase modulated with
the synchronization and supervision signals using narrow band phase
modulation techniques as described. As disclosed in my prior U.S. Pat. No.
4,117,405 (see also my prior U.S. Pat. No. 4,208,630) by using small angle
low frequency phase modulation of an AM carrier, the carrier frequency
deviation may be maintained within predetermined limits, and conventional
AM detectors will be insensitive to the modulation. The technique of
course is exactly as described with respect to the modulation of the
carrier of an AM radio broadcast station in the previous described
embodiment of the present invention, though the parameters may be somewhat
different because of the higher frequencies of the video carrier and the
different characteristics and sensitivities of video receivers. In any
event the block diagrams such as those of FIGS. 2 through 5 are directly
applicable, whether the AM tuner is responding to the video carrier or the
AM broadcast station carrier.
Another approach, if the TV station is sending only a black and white
picture, is to use the RF carrier or Chroma subcarrier on the television
signal for synchronization and supervision purposes instead of or in
addition to using the AM broadcast station. (Syncing the AM carrier to the
Chroma signal and detecting both at subscriber terminals for
synchronization purposes, particularly for the return link transmissions
from mobile terminals, has the advantage of being able to maintain sync
even on dropout of one of the two reference signals, so that even if data
communication is temporarily interrupted, word sync, frame sync, etc.,
will not be lost and therefore need not be reestablished. Also syncing on
the stronger of the two references provides a better sync, allowing the
best data detection and recovery possible.) In accordance with this
technique a frequency (i.e., 3.579545 MHz) is selected within the picture
signal bandwidth, with that frequency being phase modulated at a
predetermined and limited extent with small angle phase modulation in
accordance with the synchronization and supervision signals to be
transmitted. Such modulation may be limited to only the TV vertical
blanking interval, or when sending only black and white TV signals.
Accordingly the bandwidth of the phase modulated subcarrier may be
appropriately controlled so as to not significantly reduce the bandwidth
available for data communication purposes in a manner similar to present
color TV transmission. In such a case, the AM tuner 76 of FIGS. 4 and 5
would also include a detector with a narrow band filter 78 of these
Figures being centered on the subcarrier frequency for detection both of
the frequency itself for use by other frequency synthesizers and for the
detection of the phase modulation thereof to strip out the synchronization
and supervision signals. As a further alternative, one might choose the
subcarrier frequency to be 4.5 megahertz, the frequency on which the audio
portion of a conventional television signal is frequency modulated. This
leaves the full picture frequency band available for data transmission and
relatively well isolates the carrier, making the audio carrier and phase
modulation detection thereof more easily detected. Yet another alternative
is to use a separate inaudible subcarrier on an audio transmitter, such as
an FM transmitter as described in my patent application entitled FM
Communication System, filed Mar. 29, 1982 as Ser. No. 362,888.
As previously mentioned, the system of the present invention may be used
for one way or two way communication during the non-viewable portion of a
conventional television transmission (the vertical blanking interval), or
may be used with a television channel devoted to such transmission or at
least available for such transmission during normally off hours. In such
event, it is desirable to provide a capability of receiving ordinary
television programming on a conventional television receiver and
simultaneously transmitting and receiving information through a radio
subscriber terminal (and perhaps a conventional television receiver for
display purposes) utilizing the subscriber's existing television antennae.
In particular, it is not uncommon in a private home to have two or more
television receivers connected to the same antennae, with the receivers
being used simultaneously to receive the same or different programming in
different rooms of the dwelling. If at certain times one of the two or
more receivers is to be used as part of a radio net terminal, it is
desirable to be able to do so (or to use a radio net terminal of any
configuration) on the antennae. A potential problem exists however, when
communicating from the radio net terminal to the radio net central
receiver 28. This problem could be overcome however, using duplexer 75, by
effectively duplexing the antennae of the subscriber so that simultaneous
transmission and reception on a single antennae may be achieved. Such
duplexing depends upon the frequency separation between the signals to be
received and the signals to be simultaneously transmitted. In particular,
the high frequency amplifiers 100 of FIGS. 4 and 5 may be made highly
frequency selective so as to provide a low impedance drive to the antennae
in the very limited frequency range within which transmission is desired
and to otherwise not significantly load the antennae line. As such,
signals of other well seperated frequencies such as | | |
