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BACKGROUND OF THE INVENTION
The present invention relates, generally, to a network or system for
transmitting both data and television signals between a plurality of
digital computer ("PC") workstations and/or "stand-alone" stations. More
particularly, the invention relates to two-way multimedia communications
in a local area network (LAN) environment.
Multimedia communications concern the transfer of digital data along with
video and/or audio information. Multimedia applications software is
available that permits digital computer workstations, particularly desktop
personal computers or "PCs", to access multimedia program sources. One-way
multimedia communications are particularly well-known in instructional
programs.
Live two-way multimedia communications, generally referred to as
"videoconferencing", have heretofore required a major investment,
involving a dedicated room, expensive single-purpose hardware and
specially-trained operators. Less ponderous conferencing hardware,
conferencing hardware that is more user-friendly, preferably including
multiple-use components, is needed.
As multimedia applications are incorporated into digital computer ("PC")
workstations it becomes possible to use such workstations for
videoconferencing with the users of one or more similar workstations at
either nearby or remote locations. To achieve this capability, such a
workstation must include, as a minimum:
(1) an image display (e.g., CRT display);
(2) a sound transducer (e.g., loudspeaker);
(3) a digital computer for processing data, connected to supply image and
control signals to the digital display for imaging data;
(4) a circuit for converting the video portion of a television signal into
image and control signals compatible with the image display for displaying
a picture on this imaging device; and
(5) an analog amplifier or the like which provides the proper gain and
impedance for driving the sound transducer with the audio portion of the
TV signal.
A conversion circuit of the aforementioned type, which digitizes an analog
video signal and formats this digitized signal in such a way as to permit
display of the video image on a digital computer display, is well known.
Such a circuit, called a "video window controller", is available for both
the DOS-based Personal Computer and the PS/2 computer of International
Business Machines Corp., as well as for the Macintosh computers of Apple
Computer Corp. This circuit is used to view standard recorded video, such
as VCR program material, or to view live images from a standard video
camera, on the monitor screens of the workstations.
If two-way communication with a digital computer workstation is desired, it
is necessary also to provide the workstation with a video camera, aimed
appropriately at the workstation user, for producing a video signal
representing the image of the workstation user, and a microphone arranged
to pick up the voice of the workstation user and to generate an analog
audio signal in response thereto.
If computer circuitry for controlling an image display is not present, a
display can be driven by another circuit, called a "video display
processor", which converts video signals into the standard or protocol
required to operate the display. Such circuits are conventionally employed
in the art of video games.
The term "information signal", as used herein, is intended to mean any
signal, analog or digital, which conveys information such as data or
graphics. An "information signal" may thus include a "data signal" and/or
a "television signal", depending upon the type of information transmitted,
and/or some other kind of information signal.
The term "television signal" or "TV signal", as used herein, is intended to
mean a conventional (NTSC or other standard) analog signal, which includes
both a video and an audio portion, and/or any other standard or
non-standard representation of video and/or audio information including
digitally encoded information (compressed or uncompressed). The terms
"video signal" and "audio signal" will be used to separately denote only
the video portion and audio portion, respectively, of the television
signal. As is well known, for NTSC standards, the video portion lies
within a frequency range of 0 to 4.75 MHz whereas the audio portion lies
within a frequency band of 0 to 15 KHz. As desired, these video and audio
signals are typically combined and modulated upward from baseband to a 6
MHz wide (in Europe, a 7 MHz wide) frequency channel within a broadband
spectrum of 30 to 800 MHz.
Similarly, broadband (30-800 MHz) television signals containing one or more
active frequency channels can be tuned and demodulated to produce the
video and audio portions (signals) of a single television signal at
baseband.
As used herein, the term "baseband signals" is intended to define
information signals within the frequency range of 0 to 30 MHz, or perhaps
0 to 50 MHz. Computer data is normally transmitted over a network by
baseband signals. The term "broadband signals" is thus intended to mean
information signals at a frequency higher than the highest baseband
frequency--typically in the frequency range of 40 to 600 MHz. If the upper
limit of the baseband range terminates at a lower frequency, for example,
25 MHz, the lower limit of the broadband range can commence at this lower
frequency. The highest broadband frequency is determined only by the
limitations of the transmission technology (e.g. CATV cable) and the
applicable government standards, if any.
It is known in the television industry to transmit the audio portion of a
television signal--i.e., the "audio signal"--with frequency modulation
(FM) on a carrier at 41/2 MHz. Among television industry standards, only
the French standard SECAM transmits sound with amplitude modulation (AM).
The video portion of the television signal ("video signal") with most TV
standards is transmitted in AM with a given polarity.
It is known to transmit both baseband and broadband signals on a common
"backbone communication network" such as a local area network (LAN) which
is connected to a plurality of user workstations. The U.S. Pat. No.
4,885,747 to Foglia discloses a so-called "filter coupler" or "F-coupler"
by which (1) baseband signals (data) are transmitted between a backbone
network and a given workstation via a twisted pair shielded cable in a
balanced mode, and (2) broadband (television) signals are transmitted
between the same backbone network and the same workstation via the same
shielded cable in an unbalanced mode. The disclosure of this U.S. patent
is incorporated herein by reference. A coupler which enables the
transmission of both baseband and broadband signals on a LAN twisted pair
cable is termed a "video coupler" or "V-coupler"
With the Foglia system, a number of television programs (satellite
television, VCR or a "live" broadcast from a television camera) is
"broadcast" to all workstations connected to the LAN (an IBM Token Ring,
in this case) from a coaxial video cable through a so-called
"tap/combiner". While each workstation can select one from a number of
television channels that are broadcast on the broadband frequencies, and
while it is even possible for a workstation to broadcast to all other
workstations by means of a TV camera or some other program source, it is
not possible for any particular workstation to transmit television signals
to any other particular workstation or workstations on the LAN, or to
transmit to any remote workstation not connected to the LAN, thereby to
provide true videoconferencing capability.
The U.S. Pat. No. 4,564,940 to Yahata discloses a so-called "broadband
network system" which includes a private branch exchange (PBX) for
interconnecting a plurality of workstations. However, this system is
intended to replace an industry standard local area network (LAN) for the
multiplex communication of voice and data. No consideration is given to
the special problems encountered by the transmission of television
signals.
The U.S. Pat. No. 4,675,866 to Takumi et al. discloses a transmission
system between workstations that provides both a baseband and broadband
capability. One or more channels in the broadband network are used for
effecting transmission of signals of the baseband network. A central
re-transmission facility serves to provide videotext to a plurality of
workstations. No videoconferencing capability is contemplated or
disclosed.
The U.S. Pat. No. 4,814,869 to Oliver, Jr. discloses a "video surveillance
system" in which modulated signals from a plurality of video cameras are
multiplexed onto a single path capable of carrying, for example, up to 36
video channels. One or more such communication paths are provided to a
signal splitter which provides the paths to one or more video screens and
tuners. The tuners are operated under control of a computer so as to
sequence the display of information from the different video cameras onto
the video screens.
The U.S. Pat. No. 4,977,449 to Morgan discloses a similar video
surveillance system in which both the modulators at the video cameras and
the demodulators at the monitors are "frequency agile". In this case, the
control computer controls the channel selection of both the modulators and
the demodulators. In both the Oliver Jr. and Morgan patents, however, no
videoconferencing capability is contemplated or disclosed.
The U.S. Pat. No. 4,935,924 to Baxter discloses a signal distribution cable
network in which information signals from different signal sources, such
as cable television, FM radio, videotape recorder, video camera and
compact disk player, are transmitted on a common cable at different
frequency channels. A single channel allocation "controller" is connected
on the cable and transmits channel selection signals on the cable to both
the information sources and the information users (receivers) to control
the channel allocation. Again, no videoconferencing capability is
contemplated or disclosed.
The U.S. Pat. Nos. 4,686,698; 4,710,917; 4,716,585; 4,847,829 and 5,014,267
of Tompkins et al. disclose a videoconferencing network in which a
plurality of video terminals are connected in a star configuration to a
"MIX" switching network. The MIX switching network operates in the manner
of a "telephone switch" to connect one of the video terminals to one or
more of the other video terminals. Each cable connecting a video terminal
to the MIX carries baseband frequencies and two channels above baseband
for the two-way transmission of audio and video information. MIX switches
can be interconnected but terminals can only be interconnected via one or
more MIX switches.
The French Patent No. 2,590,429 discloses a video terminal having both a
camera and an image display. Broadband television signals are received on
one cable and transmitted on another, in digital form. Time division
multiplexing is used to transmit plural signals. As in Tompkins et al.,
the video terminals are arranged in a star configuration.
The article "Video-und Datenkommunikation im VBN" by Pernsteiner et al.,
NTZ Nachrichtentechnische Zeitschrift (Vol. 42, No. 8, August 1989) pp.
486-493 discloses a "video workstation" (that is, a PC workstation adapted
to transmit and receive both video and audio) which is connected to a
glass fiber communication network established by the German Telephone and
Postal Authority (Bundespost) known as the "Preliminary Broadband Network"
(VBN). It is contemplated that each video workstation be either coupled
directly to the VBN, or coupled thereto via a "mega-switch". In the latter
instance, the video workstations are connected to the mega-switch in a
star configuration.
The European Patent Publication No. 0,119,588 discloses an "integrated
information system" in which a videoconferencing station (so-called
"BIGFON Island") sends digitized video, audio and data signals via an
optical fiber transmission cable to a video/telephone switching network.
All of the video stations are connected to this telephone switch in a star
configuration.
Similarly, the article "Software Architecture for Integration of Video
Services in the Etherphone System" by Rangan et al., IEEE Journal on
Selected Areas in Communication, Vol. 9, No. 9, December 1991, pp.
1395-1404 discloses a videoconferencing system having a plurality of
videoconferencing workstations all connected in a star configuration to a
matrix switch. The matrix switch is controlled by a computer called a
"central connection manager".
All of these known videoconferencing systems are relatively complex,
require a dedicated computer for controlling the interconnection of video
workstations and/or the selection of transmission channels on a
multichannel cable. As a consequence, these systems are relatively
expensive to implement on a per/workstation basis.
SUMMARY OF THE INVENTION
A principal object of the present invention, therefore, is to provide a
videoconferencing network for digital computer ("PC") workstations and/or
other "stand-alone" stations which enables each workstation and/or station
to enter into a videoconference with any other selected workstation and/or
station without the intervention of a central control computer or the
provision of a central switch analogous to a telephone exchange.
Another principal object of the present invention is to provide a
multimedia communications facility for a digital computer workstation
which enables the workstation to participate flexibly in multimedia
exchanges with media terminals on a network.
It is a further object of the present invention to provide full-motion, 30
frames per second, and high resolution videoconferencing between and among
a plurality of PC workstations or stand-alone stations, without
degradation as the number of simultaneous videoconferences taking place on
the network increases.
It is a further object of the present invention to provide a
videoconferencing system of the type just described at a minimal cost per
station.
It is a further object of the present invention to provide
videoconferencing capability among a plurality of workstations which are
all connected on a common LAN.
It is a further object of the present invention to provide a
videoconferencing capability among a plurality of workstations which are
not connected together on a LAN.
It is a further object of the present invention to enable a plurality of
workstations to conduct videoconferences between selected workstations
even though the videoconferencing workstations are located within
different "work groups" of a LAN or on different networks.
It is a further object of the present invention to enable one or more of a
plurality of workstations at a local site to conduct a videoconference
with a workstation or videoconferencing site at a remote location.
It is a further object of the present invention to provide a
videoconferencing system for digital computer workstations and/or
stand-alone stations which is secure against the unauthorized monitoring
of videoconferences by the connection of a conventional television set, or
the like, to the system.
It is a further object of the present invention to enable any one of a
plurality of digital computer workstations to selectively obtain
television programming from any one of a plurality of programming sources.
It is a further object of the present invention to provide a stand-alone
videoconferencing "telephone" station having a dedicated digital computer
as a control element.
These objects, as well as other objects which will become apparent from the
discussion that follows, are achieved, by providing a videoconferencing
network for a digital computer ("PC") workstation and/or a "stand-alone"
station wherein each station is connected on (1) a signalling local area
network (A-LAN) for transmitting and receiving data signals between the
stations and (2) a broadband local area network (B-LAN) for transmitting
and receiving television signals between the stations. Each television
signal is transmitted at a selected frequency channel within the broadband
spectrum, in any standard or custom signal format. In addition, a software
program, stored in and operable on the internal computer of each station,
generates and receives data messages transmitted via the A-LAN to and from
the computers of other stations, respectively, which initiate and control
the transmission of the television signals on the B-LAN. A plurality of
television signals can thus be transmitted simultaneously on the B-LAN,
with each signal assigned to a separate frequency channel. The software
program in each computer monitors the status of the channel allocations
and generates channel selecting control signals for the modulator and
demodulator at the respective station.
With this network arrangement, every station in the system can be a
transmitter, and every station can be a receiver. Multiple stations can
act as transmitters and receivers simultaneously with the television
signals being transmitted on different channels.
The B-LAN can be implemented by providing an RF or CATV cable which
interconnects all computer stations in the network. Alternatively, the
aforementioned V-coupler (or equivalent) can be provided so that the
broadband television information can be transmitted over the same physical
communication layer as the baseband data. Hereinafter this physical layer,
which can be a shielded or unshielded twisted pair wire, for example, will
be designated as the "LAN cable".
According to a another feature of the present invention, the data messages
which control the transmission and receipt of television signals are
transmitted over either a conventional local area network, at baseband
frequencies, or via a dedicated frequency channel on the broadband
network. In either case, these "signalling messages" are transmitted
separately from the television signals on the so-called "A-LAN".
According to another particular feature of the present invention, the
television signals may be transmitted to or received from the outside
world, for example using the ISDN digital network service.
According to another feature of the present invention, individual work
groups (for example, work groups on each floor of a building) of a local
area network may be coupled together via "bridges" for transmission of
television signals from one work group to another.
According to another feature of the present invention, system security may
be provided by the simple expedient of inverting the video signal that is
transmitted on the B-LAN and/or utilizing a SECAM audio signal standard
(i.e., AM) where SECAM is not normally used, and an NTSC audio signal
standard (i.e., FM) where SECAM is used. Such simple security measures
would prevent an unauthorized person from simply attaching a conventional
television receiver to the B-LAN cable and tuning to the various frequency
channels to extract confidential transmissions.
Alternatively, security may be provided by means of conventional CATV
scrambling techniques or by other suitable circuits which will be
described hereinbelow.
According to another feature of the present invention, a dedicated "video
server" computer is coupled to the A-LAN and B-LAN to effect special
software control of the system. For example, this video server may serve
to combine the television signals on several channels to produce another
television signal representing a composite image.
According to another feature of the present invention, audio messages
and/or video frames may be stored at a workstation in digital form for
subsequent playback or transmission.
According to further features of the invention, the software in each
workstation can implement telephone style functions, such as "call
waiting", "call forwarding", "answering machine", etc., and can control
the access to features of the network, for example, in a "conference call"
between more than two parties.
For a full understanding of the present invention, reference should now be
made to the following detailed description of the preferred embodiment of
the invention as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a preferred embodiment of the
videoconferencing system, according to the present invention, comprising a
plurality of PC workstations arranged on a common LAN.
FIG. 2 is a representational, perspective view of a digital computer
workstation of the type employed with the videoconferencing system
according to the invention.
FIG. 3 is a block diagram showing a number of circuit boards employed in a
workstation for implementation of the present invention.
FIG. 4 is a block diagram which illustrates the operation of a workstation
in the videoconferencing system.
FIG. 5, comprised of FIGS. 5A and 5B, is a block diagram showing an actual
implementation of a video window controller and modulator/demodulator
circuit board according to a preferred embodiment of the present
invention.
FIG. 6 is a block diagram of an RF modulator circuit according to the
preferred embodiment of the present invention.
FIG. 7 is a block diagram of a video window controller circuit board
according to the preferred embodiment of the present invention.
FIG. 8 is a representational diagram of the present invention employed in a
local area network that is divided into two separate work groups.
FIG. 9 is a representational diagram showing the number of "video bridges"
required in a LAN divided into three work groups.
FIG. 10 is a representational diagram of a LAN divided into three work
groups in a star configuration.
FIG. 11 is a block diagram of a video bridge comprising a plurality of
video switches.
FIG. 12 is a block diagram of a single switch employed in the video bridge
of FIG. 11.
FIG. 13, comprised of FIGS. 13A and 13B, is a flow chart of a software
program for a workstation, which controls the initiation of a video
conference between workstations.
FIG. 14, comprised of FIGS. 14A and 14B, is a flow chart of a software
program for a workstation, which controls the termination of a video
conference between workstations.
FIG. 15 is a representational diagram of a data message which is broadcast
on the A-LAN by one workstation to all other workstations on this LAN.
FIG. 16 is a flow chart of a workstation software program which adds
updates to the workstation network "state table".
FIG. 17 is a flow chart of a workstation software program which adds a
workstation "node" to the local "user's directory".
FIG. 18, comprised of FIGS. 18A and 18B, are diagrams of the frequency
spectrum utilized by the videoconferencing network according to the
invention in which the workstations are interconnected by both a LAN cable
and a CATV cable (FIG. 18A) or by a CATV cable alone (FIG. 18B).
FIG. 19 is a representational timing diagram showing how data messages are
used to establish and terminate a videoconference between two
workstations.
FIG. 20 is a block diagram showing how a plurality of workstations can be
connected via a plurality of CATV cables to provide as many frequency
channels as desired.
FIG. 21, comprised of FIGS. 21A and 21B, is a block diagram showing how
analog audio and video information can be transmitted on a selected
frequency channel in digital form (FIG. 21A) and then retrieved as analog
audio and video information at the receiving workstation (FIG. 21B).
FIG. 22 illustrates two groups of workstations, Group A and Group B,
interconnected by a "static multichannel bridge".
FIG. 23 is a block diagram of a static multichannel bridge of the type
employed in the system of FIG. 22.
FIG. 24, comprised of FIGS. 24A and 24B, are block diagrams of a video
scrambler circuit (FIG. 24A) and descrambler circuit (FIG. 24B) for
providing security in the videoconferencing network according to the
present invention.
FIG. 25 is a representational, perspective view of a "stand-alone" station
of the type employed with the videoconferencing network according to the
present invention.
FIG. 26 is a block diagram of an integrated stand-alone video station and a
telephone unit connectable to a conventional private automated branch
exchange (PABX).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments of the present invention will now be described
with reference to FIGS. 1-26 of the drawings. Identical elements shown in
the various figures are identified with the same reference numerals.
FIG. 1 illustrates the data and television network according to the present
invention. The system comprises a number of computer workstations PC 2 . .
. PC X which will be described in detail below in connection with FIG. 2.
Each PC workstation 10 has an attached unit 11 comprising a video camera,
microphone and loudspeaker. Suffice it to say at this point that each
computer workstation has a first input/output port 12 for transmitting and
receiving data signals and a second input/output port 14 for transmitting
and receiving television signals. These ports can be physically separate,
or can be physically identical but connected to provide signals on
different frequency channels on a common cable.
The system further includes a signalling (e.g., baseband) local area
network (A-LAN) connected to the first port 12 of each workstation, for
transmitting and receiving data signals between selected ones of these
workstations. The A-LAN may comprise a separate LAN cable 16 such as a
shielded twisted pair wire, a coaxial cable or other suitable waveguide.
A broadband local area network (B-LAN) is connected to the second port 14
of each workstation for transmitting and receiving television signals
between selected ones of the workstations. The B-LAN network may comprise
a standard co-axial CATV cable 18. As will be further explained below, the
broadband television signals can also be transmitted on the LAN cable 16
with the aid of video couplers ("V-couplers"). The television signals are
transmitted from one workstation to another at a selected frequency
channel. Full duplex communication between two workstations--for example,
between PC 2 and PC X--requires the use of two channels--for example, one
for transmission from PC 2 to PC X and another for transmission from PC X
to PC 2.
Each workstation, PC 2 . . . PC X, has stored therein a software program
for generating and receiving data messages, transmitted via the A-LAN, to
and from another workstation, respectively, for initiating and terminating
a videoconference. The data messages initiate and control the transmission
of the television signals on the B-LAN such that a number of television
signals can be, and are transmitted simultaneously on the B-LAN with each
television signal assigned to a separate frequency channel.
With the system so configured, each workstation is capable of being both a
receiver and a transmitter of television signals simultaneously. The
television signals are generated at baseband as a standard (e.g., NTSC)
video signal and a separate audio signal. These signals are modulated into
a 6 MHz wide signal and then placed into one of the sixty-four channels in
the 50-475 MHz range. For example, the center frequency of the lowest
channel may be set at 55.25 MHz.
If a European television standard is used, the baseband television signal
may be either a SECAM or PAL color television standard which is modulated
into a 7 MHz wide channel. In practice, the videoconferencing system
according to the invention can support any recognized standard.
As shown further in FIG. 1, it is also possible to obtain television
signals from other sources. For example, a television camera or video
player may produce an NTSC video and audio signal which may be placed on
the B-LAN, under control of the A-LAN, for receipt by any and all of the
workstations. Alternatively, or in addition, a cable television signal
received on a coaxial cable 20 may be demodulated into a video and audio
signal for transmission on the B-LAN. The channel selected from among the
many channels delivered by the cable 20 is controlled by data messages
transmitted via the A-LAN cable 16 with a tuner and demodulator 22.
Whereas the video cable 18 is designed to serve the "B-LAN" or broadband
local area network, one frequency channel of this cable may be dedicated
to serve the "A-LAN" or baseband local area network. In this case, the LAN
cable 16, which serves to transmit data and signalling messages between
the workstations 10 may be eliminated. In this case, only one cable,
namely the video cable 18, is needed to transmit the signalling
information (A-LAN) and the broadband television information (B-LAN).
The television signals are modulated into the proper channel for
transmission on the B-LAN, under control of data signalling messages
received on the A-LAN, by an RF modulator within a television control
circuit board 24 located at each workstation. This same circuit board
contains a tuner and demodulator for receiving the television signal on a
selected channel on the B-LAN and demodulating this signal into the
separate baseband video and audio signals for displaying an image and
producing sound at the respective workstation. If the particular unit is
to serve only as a source, a separate circuit board 26 may be provided
which does not contain the tuning and demodulating capability. Similarly,
if the unit is to serve only as a receiver, the circuit board does not
require a modulator.
For a connection to the "outside world" beyond the LAN, a modulating and
demodulating circuit board 24 is required to convert to and from baseband
video signals. The signals to be transmitted outward are digitized and
compressed, and the signals received from beyond the LAN are decompressed
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