Computer signal transmission system

Electrical computers and digital data processing systems: input/ – Input/output data processing – Peripheral adapting

Reexamination Certificate

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Details

C710S072000, C725S040000, C174S019000, C174S027000

Reexamination Certificate

active

06618774

ABSTRACT:

The present invention relates to a computer signal transmission system, and in particular to a system for transmitting a group of analogue and digital signals between a computer and peripheral devices using a twisted pair data cable.
For convenience, operational and security reasons it is often desirable to locate video screens some distance away from the device that is generating the video signal. If a cable is used to transfer the signal between the source and screen then the quality of the resulting video picture will depend upon the type and length of the cable used. As the cable length is increased the quality of the picture will degrade. To maintain reasonable video quality. coaxial cable is commonly used.
In many practical applications, video information is generated by personal computers. These personal computers typically output their video as red, green and blue (RGB) analogue signals together with horizontal and vertical picture synchronisation signals. This type of video signal can usually be successfully transmitted over several meters using multi-core cable that includes three coaxial cores for the red, green and blue signals and other wires for each of the horizontal and vertical synchronisation signals. This ‘tri-coax’ (three coaxial core) cable is typically used for personal computer monitor leads. Such cable performs well when used over short distances but has several disadvantages when longer cable runs are required. ‘Tri-coax’ cable is relatively bulky, stiff and costly and typical PC video signals start to noticeably fade and smudge when transmitted over distances of ‘tri-coax’ cable exceeding about 30 meters. The smudging effect becomes worse as the frequency of the video signal increases (for example when setting a higher screen resolution on a personal computer).
Twisted pair data cable is popular for use in computer networks and other data communications applications and consequently is commonly installed within the structure of buildings. This cable is cheaper, lighter and more flexible than ‘tri-coax’ cable and can consequently be installed more easily. There are therefore cost and convenience advantages in using twisted pair data cable to carry RGB video.
There are technical advantages and disadvantages in using twisted pair cable as a medium for carrying RGB video. This type of cable is typically constructed using eight wires configured as four twisted pairs within a common outer sheath. Each twisted pair can carry a balanced electrical signal in such a way that the effect of noise voltages induced equally onto both wires is removed by receiving circuitry that detects the voltage difference between the two wires. This enables the effect of electrical noise on the transmitted signals to be minimised.
Unfortunately, RGB signals transmitted over twisted pair cable are attenuated more rapidly than with ‘tri-coax’ cable causing the picture to degrade more quickly. Further, the higher frequency components of the video signal are attenuated by the cable more than the lower frequency components causing smudging at colour transitions on the picture. The attenuation and smudging gets worse as the cable distance increases.
The four twisted pairs contained within a typical data cable are not sufficient to allow a pair to be used for each of the red, green, blue, horizontal synchronisation and vertical synchronisation signals. In practical applications it is frequently desirable to transfer other digital and analogue signals over the same wire as the RGB video. These signals can include those that flow to and from human interface devices (HIDs) such as keyboards, mice, speakers and microphones.
Although twisted pair cable is relatively inexpensive and widely used within buildings, installation of new cables can be inconvenient and costly. Expanding computer networks and telephone systems that compete for connection sockets can often make the installed building cabling a valuable resource. Consequently, products that use the minimum number of twisted pair cables are more desirable due to their lower cost and greater convenience.
According to an aspect of the invention, there is provided a system for transmitting electrical signals between a computer and peripherals along a twisted pair cable. The system can comprise a computer interface, a peripheral interface and a twisted pair cable in communication between the computer interface and the peripheral interface. A red, a green and a blue video colour signal, a horizontal and a vertical video synchronisation signal and at least one audio signal from the computer are transmitted via three twisted pairs of the cable to the peripherals.
According to a further aspect of the invention, there is provided apparatus for transmitting a multiplexed video synchronisation signal and an audio signal from a computer to a video display unit and an audio output. The system can include a signal combiner, which receives a video synchronisation signal from a video output of a computer, a sampler which samples the audio signal and communicates the sampled audio signal to the signal combiner, a twisted pair cable connected at a first end to the signal combiner and a signal separator connected to a second end of the twisted pair cable. The signal combiner can modulate the amplitude of the video synchronisation signal with the sampled audio signal and the signal separator can provide separate transmitted video synchronisation and audio signals to the video display unit and the audio output respectively.
According to a further aspect of the invention, there is provided a system for transmitting electrical signals between a computer and peripherals along a twisted pair cable. The system can comprise a computer interface, a peripheral interface and a twisted pair cable in communication between the computer interface and the peripheral interface. A video colour signal, a video synchronisation signal and an audio signal from the computer can be transmitted via a single twisted pair of the cable to the peripherals.
According to a yet further aspect of the invention, there is provided a method for transmitting electrical signals between a computer interface and a peripheral interface. The method can comprise providing a twisted pair cable connected between the computer interface and the peripheral interface. A red, a green and a blue video colour signal, a horizontal and a vertical video synchronisation signal and at least one audio signal can be transmitted from the computer interface to the peripheral interface via three twisted pairs of the cable.
According to a further aspect of the invention, there is provided a method for transmitting electrical signals between a computer interface and a peripheral interface. The method can comprise providing a twisted pair cable connected between the computer interface and the peripheral interface. A video colour signal, a video synchronisation signal and audio signal can be transmitted from the computer interface to the peripheral interface via a single twisted pair of the cable.
The invention maximises its desirability and applications by allowing the use of just one standard 8-wire twisted pair cable and by supporting long cable lengths. This functionality is achieved by a carefully designed signal multiplexing strategy and the use of signal compensation circuits that boost the signals to compensate for the loss introduced by long cables. By multiplexing peripheral signals together with the RGB video onto a single twisted pair data cable the invention enables a keyboard, monitor, mouse, microphone and set of stereo speakers to be located at some distance away from the computer that they are interacting with.
The ability to locate a keyboard, monitor, mouse, microphone, set of speakers and an RS232 device at some distance from a computer using a single inexpensive twisted pair data cable provides an answer to many practical requirements. In areas of high security or high risk of theft it is undesirable to locate high value computers in accessible areas. The invention enables high value computers to b

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