Television – Special applications – Manufacturing
Reexamination Certificate
1996-06-03
2001-02-06
Faile, Andrew I. (Department: 2771)
Television
Special applications
Manufacturing
C333S025000
Reexamination Certificate
active
06184919
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to extended-in-length analog video communications links, and particularly to such a link wherein twisted pair conductors of an unshielded cable are differentially driven at the video source, and converted to a plurality of single ended signals prior to being provided to a monitor. Significantly, there is no direct ground conductor between the transmitter portions and receiver portions of the communications link, with signal returns being provided through low impedance outputs of signal transmitter amplifiers.
BACKGROUND OF THE INVENTION
There are many occasions where it is necessary to process and transmit color computer video signals over communications links. In one such instance, when analog VGA video and keyboard signals are routed between one of a plurality of computers and a single monitor and keyboard by switching equipment, such as the COMMANDER module, manufactured by Cybex Corporation located in Huntsville, Alabama, analog VGA video levels routed thereby may be reduced and high frequency components of the signal attenuated by switching circuitry inside the COMMANDER module. However, signal degradation may occur due to signal interaction in the cable between the COMMANDER module and the monitor.
In another instance, where analog video and other data signals are coupled to or between workstations via relatively long conductors, similar signal losses occur. In order to compensate for these signal losses, it is generally necessary to regenerate voltage amplitude of the video and data signals prior to inputting them to a monitor and other peripheral devices. Typically such regeneration is done by a pair of transistors coupled in a common emitter configuration, with the first transistor inverting the signal and the second transistor reinverting the signal in order to provide noninverted voltage amplification. While this type of amplifier requires power of a single polarity and ground, DC coupling of the amplifiers is generally difficult to implement and not particularly stable. In any case, some type of amplifier circuitry is required at the monitor end of the cable to regenerate the video signals.
There are also instances where it is desirable to distribute a video signal from a single video source to a plurality of monitors remotely located from the source. One example of these situations include airport terminals, where a single video source, such as a computer, provides scheduling information related to incoming/outgoing flights to monitors located at ticket sales desks or other locations where it is desirable to display such information.
While there are a number of extended video communications links presently available, several of which manufactured by CYBEX corporation, and as far as Applicants are aware, none utilize inexpensive, unshielded twisted pair conductors bundled in a cable to transmit analog computer video signals and separate sync signals.
Accordingly, it is an object of this invention to provide an extended analog video communications link having an unpowered passive receiver network and which may be used to distribute such a video signal from a single source to a plurality of monitors.
SUMMARY OF THE INVENTION
A video communications link is disclosed wherein a transmitter is provided with analog video signals, each of the video signals applied as an input to a respective one of a plurality of differential amplifiers. Each differential amplifier provides as outputs a positive video signal and a negative video signal, with these positive and negative signals coupled to conductors of respective twisted pairs of conductors in a cable absent an electromagnetic shield. A plurality of adapters, one for each twisted pair, are coupled to an opposite end of the twisted pair conductors, and function to provide the positive video signal as a single ended output and the negative video signal as a video return signal. As a result, current flow in each of the twisted pairs is equal and opposed, greatly reducing electromagnetic interference radiated from the cable.
REFERENCES:
patent: 3845240 (1974-10-01), Alaily
patent: 4054910 (1977-10-01), Chou et al.
patent: 5193200 (1993-03-01), Asprey et al.
patent: 5208560 (1993-05-01), Yasutake
patent: 5257390 (1993-10-01), Asprey et al.
patent: 5283789 (1994-02-01), Gunnarsson et al.
patent: 5321372 (1994-06-01), Smith
patent: 5386518 (1995-01-01), Reagle et al.
Asprey Robert R.
Kirshtein Philip M.
Lusk Thomas V.
Clodfelter Mark
Faile Andrew I.
Le Uyen
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