Abstract:
A system capable of transmitting and receiving high frequency video signals across various lengths of a twisted pair cable while maintaining video quality is presented. The system includes a transmitter and a receiver tandem coupled together over twisted pair cable. Each video component is mixed with a reference signal in the transmitter and driven differentially onto the twisted pair cable. Upon detection of a signal in the twisted pair cable, the receiver adjusts its internal gains until the known characteristic of the reference signal is achieved. The receiver than automatically adjusts the skew &amp; DC offset. Thus, the receiver is able to automatically measure the degradation in video quality and appropriately compensate the video signals for the accumulated degradation caused primarily by the transmission between the transmitter and the receiver. The compensated video may subsequently be provided to a video display device.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to the field of video transmission. More specifically the invention relates to transmission of video over long distances using twisted pair cables. 
       BACKGROUND OF INVENTION 
       [0002]    Cables are one method commonly used to convey electronic video signals from a source device (e.g., a video camera or a DVD player) to a destination device (e.g., a video display screen). Two types of cable commonly used for video transmission are coaxial cable and twisted pair cable. It is desirable for the video signal at the destination device to correspond accurately to the original video signal transmitted by the source device. “Insertion loss” is a term used to describe signal degradation that occurs when a video or other signal is transmitted over a transmission medium such as a cable. Insertion loss is typically caused by the physical characteristics of the transmission cable. 
         [0003]    Typically, insertion loss is proportional to the cable length: longer length transmission cables will exhibit greater loss than shorter length cables. Coaxial cables typically exhibit less insertion loss than twisted pair cables. However, coaxial cables are more expensive and difficult to install than twisted pair cables. Twisted pair cables typically are manufactured as bundles of several twisted pairs. For example, a common form of twisted pair cable known as “Category 5” or “CAT5” cable comprises four separate twisted pairs encased in a single cable. CAT5 cable is typically terminated with an eight-pin RJ45 connector. 
         [0004]    Insertion loss is typically caused by the physical characteristics of the transmission cable. Insertion loss includes resistive losses (also sometimes referred to as DC losses) as well as inductive, capacitive and skin effect losses (also sometimes referred to as AC losses). The AC insertion loss exhibited by a cable is frequency dependent. For example, the insertion loss for a 1500 foot length of CAT5 cable as a function of frequency is shown in  FIG. 11 . In the example of  FIG. 11 , the insertion loss generally increases with increasing frequency, with the insertion loss for high frequency signals being significantly greater (−70 dB at 50 MHz for a 1500 feet CAT-5 cable) than the DC insertion loss of 2.6 dB for 1500 Feet (e.g. the loss at a frequency value of zero). 
         [0005]    Video signals come in a variety of formats. Examples are Composite Video, S-Video, and YUV. Each format uses a color model for representing color information and a signal specification defining characteristics of the signals used to transmit the video information. For example, the “RGB” color model divides a color into red (R), green (G) and blue (B) components and transmits a separate signal for each color component. 
         [0006]    In addition to color information, the video signal may also comprise horizontal and vertical sync information needed at the destination device to properly display the transmitted video signal. The horizontal and vertical sync signals may be carried over separate conductors from the video component signals. Alternatively, they may be added to one or more of the video signal components and transmitted along with those components. 
         [0007]    For RGB video, several different formats exist for conveying horizontal and vertical sync information. These include RGBHV, RGBS, RGsB, and RsGsBs. In RGBHV, the horizontal and vertical sync signals are each carried on separate conductors. Thus, five conductors are used: one for each of the red component, the green component, the blue component, the horizontal sync signal, and the vertical sync signal. In RGBS, the horizontal and vertical sync signals are combined into a composite sync signal and sent on a single conductor. In RGsB, the composite sync signal is combined with the green component. This combination is possible because the sync signals comprise pulses that are sent during a blanking interval, when no video signals are present. In RsGsBs, the composite sync signal is combined with each of the red, green and blue components. Prior art devices exist for converting from one format of RGB to another. To reduce cabling requirements, for transmission of RGB video over anything other than short distances, a format in which the sync signals are combined with one or more of the color component signals are commonly used. 
         [0008]    Thus, an RGB signal typically requires at least three separate cables for transmission of each of the red, green, and blue components and the combined horizontal and vertical sync information. If coaxial cable is used, three separate cables are required. If twisted pair conductors are used, three twisted pairs are also required, but a single CAT5 cable (which comprises four twisted pairs) can be used. Three of the four pairs may be used for the red, green, and blue components, respectively. The fourth pair is available for transmission of other signals (e.g., digital data, composite sync, and/or power).  FIGS. 2 and 3  illustrate examples of how video signals may be allocated to the four pairs of twisted conductors in a CAT5 or similar cable. 
         [0009]    In a CAT5 or similar cable, each end of each conductor is typically connected to one of eight pins of a standard male RJ-45 connector. In  FIGS. 2 and 3 , the first conductor pair corresponds to Pins  1  and  2 ; the second conductor pair corresponds to Pins  4  and  5 ; the third conductor pair corresponds to Pins  7  and  8 ; and the fourth conductor pair corresponds to Pins  3  and  6 . For video signal configurations in which three or fewer conductor pairs are used for the transmission of the video signal, the remaining conductor pair or pairs (for example, the pair corresponding to Pins  3  and  6 ), may be used for communication of other signals, and/or for power transfer. Power transfer may be desirable if one of the devices is located remote from an external power source. For example, a source device may comprise a self powered laptop computer located at a distance from an external power source, such as a power outlet, while the destination device comprises a video projector display unit located in the ceiling of a room with a readily available AC power source. In such a configuration, the power needed to operate the transmitter may be conveyed from the receiver located near an AC power source via the twisted conductor pair not allocated for transmission of video signals. In such a configuration, the transmitter may be located within a wall or podium (e.g. in the vicinity of the laptop computer) without a nearby power source thus the transmitter can get its power from the receiver which is more likely to have a power source nearby. 
         [0010]      FIG. 2  shows example pin configurations for a number of video signal formats. For example, with RGBHV video, as shown in the column headed “RGBHV” of  FIG. 2 , the twisted pair corresponding to Pins  1  and  2  carries the differential Red signals (i.e. Red+ and Red−) and the differential vertical sync signal (i.e. V Sync+ and V Sync−), the pair corresponding to Pins  4  and  5  carries the differential green signals (i.e. Green+ and Green−), and the pair corresponding to Pins  7  and  8  carries the differential Blue signals (i.e. Blue+ and Blue−) and the differential horizontal sync signal (i.e. H Sync+ and H Sync−). In  FIG. 2 , the conductor pair corresponding to pins  3  and  6  is allocated to carrying a digital signal and power. 
         [0011]    For RGBS (i.e. RGB with one composite sync signal), in the example of  FIG. 2 , as shown in the column headed “RGBS,” the same pin assignments are used for the red, green and blue components as for RGBHV, with the composite sync signal combined with the Blue signal (i.e. Blue/C Sync+ and Blue/C Sync−). The composite sync signal could alternatively be combined with the Red component signal, or the Green component signal (as is done in the RGsB format, as shown in the column headed “RGsB” in  FIG. 2 ). When the format to be transmitted is RsGsBs (i.e. composite sync signal added to each color component), as shown in the column headed “RsGsBs” in  FIG. 2 , the same pin assignments are used for each of the red, green and blue components as for RGBHV, except in this case the composite sync signal is added to each of the three color components. 
         [0012]    In addition to showing example pin assignments for RGB signals,  FIG. 2  also shows example pin assignments for component video, S-Video, and composite video.  FIG. 3  shows an example of pin assignments that allow Composite video and S Video signals to share the same four-twisted pair cable. 
         [0013]    Whenever multiple cables are used to transmit different components of a video signal, they must be properly combined at the destination to reproduce the transmitted video signal. For example, the components must be synchronized at the receiving station to prevent distortion in the video reproduction. Differences in arrival time of the various signal components may become an issue if the transmission distance is long and there are differences in length among the multiple conductors. Such differences in arrival time are referred to as “skew.” CAT5 or similar twisted pair cables are particularly prone to skew the twist rate of each cable pair is different (to reduce cross-talk between the adjacent cables). Over long distances, this difference in twist rate can result in significant differences in conductor path length of the different pairs. 
         [0014]    Although twisted pair cables are convenient and economical for transmission of video signals, signal degradation (skew between video signal components and insertion loss) limits the distance over which satisfactory quality video signals can be transmitted via twisted pair cables. Video transmitter/receiver systems exist that amplify video signals transmitted over twisted-pair cables. In such systems, a transmitter amplifies the video source signal prior to being transmitted over twisted pair cable, and a receiver amplifies the received signal. These transmitter/receiver systems allow longer transmission distances over twisted-pair cable than are possible for unamplified signals. However, to prevent signal distortion, the amount of gain (amplification) supplied by the transmitter and receiver must be properly matched to the amount of insertion loss that occurs in the length of the twisted-pair cable over which the video signal is transmitted. Ideally the system gain should be flat across the frequency spectrum. If the resulting video signal is not flat across the frequency spectrum a smearing of the video image across the display will occur. 
         [0015]    However, amplification of the video signal to compensate for insertion loss may result in unacceptably magnifying the noise accumulated over the transmission lines. This is because the signal to noise ratio decreases as the cable length increases. Thus, although a flat frequency response is ideal over a desired frequency spectrum, signal amplification may need to be tempered by noise considerations. 
         [0016]    It is not uncommon to find video signals with a DC offset, i.e., steady state signal component that is floating or biased with respect to ground. There are several potential culprits for existence of DC bias in a video signal, e.g., the DC bias may be directly from the video source, AC coupling through a capacitor from the source, or due to processing circuit elements in the receiving device. In order for the receiver to properly detect the synchronization signals and restore the video, the incoming video signal is DC restored. 
         [0017]    Therefore, there exists a need for a video transmission system that automatically compensates for signal losses, skew, DC offset, and other unacceptable characteristics of transmission of video signals over appreciable distances via conductors, including twisted pair cables. 
       SUMMARY OF THE INVENTION 
       [0018]    The invention comprises a transmitter and a receiver tandem coupled together over twisted pair cables for communication of high resolution video signals to greater distances than currently possible with prior art systems. The present invention extends the transmission capabilities of twisted pair video systems by several multiple times the distance of prior art video over twisted pair systems. 
         [0019]    One embodiment of the present invention is configured to automatically detect the presence of a signal between the transmitter and the receiver and adjust the video signals accordingly to correct for any losses in the video quality. For instance, when a twisted pair cable is connected between the transmitter and the receiver of the present invention, the receiver detects the presence of video signal in the line and automatically adjusts for DC loss, AC loss, Skew, and offset. 
         [0020]    Signal adjustment is done primarily with the synchronization signal. When the receiver is first coupled to the line, it sets the loop gains to maximum in order to facilitate recovery of the synchronization signal. After the synchronization signal is established, the receiver adjusts the DC and/or AC signal amplitude and peaking until the synchronization signal is restored to its proper level. 
         [0021]    Once the synchronization signal is restored to the proper level the skew is measured and signals are adjusted to compensate for any skew differences between the conductors in the cable and the receiver. 
         [0022]    One or more embodiments of the present invention may also include an appropriate amount of noise filtering for high fidelity restoration of the video signal at the receiver. 
         [0023]    Further objects, features, and advantages of the present invention over the prior art will become apparent from the detailed description of the drawings which follows, when considered with the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is an illustration of long distance twisted pair transmission apparatus in accordance with an embodiment of the present invention. 
           [0025]      FIG. 2  is an illustration of allocation of the conductors of a twisted pair cable for various video formats in accordance with an embodiment of the present invention. 
           [0026]      FIG. 3  is an illustration of allocation of the conductors of a twisted pair cable for video signals in accordance with an embodiment of the present invention. 
           [0027]      FIG. 4  is a block diagram illustration of architecture of a transmitter in accordance with an embodiment of the present invention. 
           [0028]      FIG. 5  is an illustration of a polarity converter in accordance with an embodiment of the present invention. 
           [0029]      FIG. 6  is a block diagram illustration of architecture of a receiver in accordance with an embodiment of the present invention. 
           [0030]      FIG. 7  is an illustration of a sync stripper circuit in accordance with an embodiment of the present invention. 
           [0031]      FIG. 8  is an illustration of insertion loss compensation circuit in accordance with an embodiment of the present invention. 
           [0032]      FIG. 9  is an illustration of the skew compensation circuit in accordance with an embodiment of the present invention. 
           [0033]      FIG. 10  is an illustration of the DC offset correction circuit in accordance with an embodiment of the present invention. 
           [0034]      FIG. 11  is a frequency response plot of an example 1500 feet length CAT5 cable. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    The invention comprises a method and apparatus for transmission of video over long distances using twisted pair conductors. In the following description, numerous specific details are set forth in order to provide a more thorough description of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the invention. 
         [0036]    In general, the invention comprises a transmitter and a receiver tandem coupled together over a twisted pair cable for communication of video signals, e.g. composite video, S-Video, Component video, computer-video, and other high resolution video, over long distances. Embodiments of the present invention extend the transmission capabilities of twisted pair video systems over long distances of twisted pair cable. 
         [0037]    Embodiments of the present invention are preferably configured for Plug and Play operation. Thus, when a twisted pair cable is connected between the transmitter and the receiver, with a video signal present, the system detects the presence of the video signals and automatically adjusts for DC loss, AC loss, Skew, and DC offset. 
         [0038]    In one or more embodiments, the transmitter is configured to transmit video signals over multiple conductor pairs to a receiver. Each conductor pair carries a component of the video signal. The transmitter obtains input video signals from a video source device (e.g. a video camera or a DVD player). In one or more embodiments, the transmitter modifies the input video signal by removing any DC offset present from the video source. The transmitter may also have a local buffered video output for local monitoring. 
         [0039]    Subsequently, the transmitter adds a reference signal having a predetermined form to each component of the input video signal, preferably during the blanking period. The transmitter transmits the modified input video signal over the multiple conductor pairs to the receiver. The receiver processes the modified input video signal and provides a reprocessed video signal to a destination device (e.g. a video recorder or video display). 
         [0040]    Processing of each component of the modified video signal at the receiver is done based on the reference signals. In one embodiment, when the receiver is coupled to the transmitter via the conductor pairs, the receiver recognizes that a signal is present at its input terminals and begins processing of the input signals. The receiver attempts to detect the reference signal in each signal component. In one or more embodiments, the receiver comprises a closed loop signal amplifier for each signal component. The receiver initially sets the loop gains of the amplifiers to maximum for purposes of detecting the reference signal. In one or more embodiments, once the reference signal is detected in a particular signal component, the receiver adjusts the DC and/or AC signal amplitude and peaking for that signal component until the reference signal is restored to its original form. 
         [0041]    Once the reference signal for each signal component has been restored, skew between the different video signal components is measured. Delay is added to the earliest arriving signal component(s) such that they arrive at the same time as the slowest arriving signal component. 
         [0042]    An embodiment of a video transmission system comprising the present invention is illustrated in  FIG. 1 . As illustrated, the video transmission system comprises video source  102 , cable  103 , transmitter  104 ; twisted pair cable  106 ; receiver  108 , cable  109  and destination device  110 . Cable  103  couples the video (and audio, if applicable) signals from source  102  to transmitter  104 . Cable  103  may comprise any suitable conductors known in the art for coupling the type of video signal generated by video source  102  to transmitter  104 . Transmitter  104  comprises multiple input terminals for accepting different input signal formats. For example, transmitter  104  may comprise connectors for accepting a composite video signal, an S-Video signal, a digital video signal, an RGB component video signal, etc. Transmitter  104  may also comprise standard audio connectors such as, for example RCA input jacks. 
         [0043]    In one or more embodiments, cable  106  comprises a cable bundle of multiple twisted pair conductors. For example, cable  106  may comprise a CAT5 or similar cable comprising four pairs of twisted conductors and terminated with standard male RJ-45 connectors that mate with matching female RJ-45 connectors on the transmitter and receiver. The pairs of twisted conductors may, for example, be allocated as shown in  FIGS. 2 and 3 . 
         [0044]    Example embodiments of the present invention are described using RGBHV as an example video input signal format. However, it will be clear to those of skill in the art that the invention is not limited to RGBHV and that other video formats may be used in which the video signal is transmitted over more than one conductor pair. 
         [0045]      FIG. 4  is a block diagram showing the architecture of transmitter  104  of  FIG. 1  in an embodiment of the present invention. In the embodiment shown in  FIG. 4 , transmitter  104  receives a video source signal comprising separate video input signals and sync input signals. For example, if the video input source signal is in RGBHV format, video input signals comprise the R, G and B signals, while the sync input signals comprise the H and V sync signals. In other embodiments, the sync signals may be combined with one or more of the video component signals. 
         [0046]    In embodiments configured for S-Video; Component video; Composite video; or some forms of RGB video with a combined synchronization signal, the synchronization signals may be detected and extracted from the video information and then re-combined, after conditioning, with the video to provide the appropriate reference signals for compensation and skew measurements. In such embodiments, the synchronization signals are stripped from the incoming video signals, conditioned, and then recombined with the appropriate video data, in the transmitter. Thus configured, the input signal at the receiver provides the necessary information for the receiver to detect the insertion loss, compensate for skew, and also re-generate the appropriate synchronization signals for these video formats. 
         [0047]    In the RGBHV embodiment of  FIG. 4 , transmitter  104  comprises horizontal and vertical sync input terminals  431 H and  431 V, red, green and blue video input terminals  401 R,  401 G and  401 B, input amplifiers  410 R,  410 G, and  410 B, back porch clamp (BPC) generator  430 , offset correction circuits  440 R,  440 G, and  440 B, uni-polar pulse converters  450 H and  450 V, differential output amplifiers  460 R,  460 G and  460 B, and differential output terminals  402 R,  402 G and  402 B. Transmitter  104  may also contain local output amplifiers for each input signal (not shown) that provide a local video monitor output signal. 
         [0048]    Input amplifiers  410  receive the input video signal from video input terminals  401 , and uni-polar pulse converters  450  receive the sync input signals from sync input terminals  431 . In one or more embodiments, separate amplifiers are utilized for each video component signal. For example, in an embodiment for an RGBHV input signal, three input amplifiers  410  for the video components (one each for the R, G, and B components) and two uni-polar pulse converters  450  for the sync signals (one each for the H and V sync signals) are used. 
         [0049]    Input amplifiers  410  are used in conjunction with horizontal sync BPC generator  430  and offset correction circuits  440  to detect and compensate for any DC offset in the source video signal. In the embodiment of  FIG. 4 , offset correction circuits  440  determine the DC offset for each video component using the back porch clamp signal from the BPC generator  430 , and the amplified video source signal from input amplifiers  410 . Offset correction circuits  440  apply compensation to each video component via a feedback loop comprising the respective input amplifier  410  for that component. 
         [0050]    The vertical and horizontal synchronization signals  431 H and  431 V are coupled to uni-polar pulse converters  450 . Uni-polar pulse converters  450  assure that output sync signals from transmitter  104  are always the same polarity regardless of the polarity of the input. An embodiment of a uni-polar pulse converter  450  is illustrated in  FIG. 5 . 
         [0051]    In the embodiment of  FIG. 5 , pulse converter  450  comprises two exclusive-OR gates (e.g.  510  and  520 ) that process the received sync input signal. Initially, the sync input signal  501  (e.g.  431 H and  431 V) is exclusive-ORed with ground in gate  510  and then the output of gate  510  is filtered in low-pass filter  530  (which in one or more embodiments comprises a resistor and capacitor circuit) and exclusive-ORed with itself (i.e. unfiltered output of gate  510 ) in gate  520  to generate the polarity-corrected sync output signal  502 . 
         [0052]    In one or more embodiments, the horizontal sync signal H SYNCP  is used as both the horizontal sync signal and as the reference pulse signal. H SYNCP  is therefore added to each of the video signal component signals. In addition, in one or more embodiments, the vertical sync signal V SYNCP  is added to one or more of the video components to provide vertical sync information to the receiver. 
         [0053]    As illustrated in the embodiment of  FIG. 4 , only the red video component signal is used to convey the vertical sync information. Thus, both the vertical and horizontal sync signals are added to the red video component signal, while only the horizontal sync signal is added to the blue and green component signals. H SYNCP  is summed with V SYNCP  at node  452  and subtracted from the red video component signal (i.e. differentially added) at differential amplifier  460 R. H SYNCP  is subtracted from the green video component at differential amplifier  460 G; and H SYNCP  is subtracted from the blue video component at differential amplifier  460 B. In this way, a negative reference pulse (i.e. H SYNCP ) is simultaneously added to all three differential video output signals. 
         [0054]    Differential output amplifiers  460  receive the reference, sync (if applicable) and video signals and provide corresponding amplified differential driver signals to differential output terminals  402 . In one or more embodiments, differential output terminals  402  comprise a female RJ-45 connector using pin assignments such as those shown in  FIG. 2  (pins  3  and  6  may be used for transmission of power, digital signals, and/or audio signals). Differential output terminals  402  may be connected via twisted pair cable  106  of  FIG. 1  to receiver  108 . 
         [0055]    Receiver  108  receives the differential video signals from transmitter  104  via twisted pair cable  106 . Receiver  108  processes the differential video signals to compensate for skew and signal degradation and then outputs the compensated video signals to a destination device such as projector  110 .  FIG. 6  is a block diagram of receiver  108  in accordance with an embodiment of the present invention. 
         [0056]    In the embodiment of  FIG. 6 , Receiver  108  comprises variable gain amplifiers  610 R,  610 G and  610 B, discrete gain amplifiers  620 R,  620 G and  620 B, skew adjustment circuit  630 ; output stages  640 R,  640 G and  640 B, DC offset compensation circuits  622 R,  622 B and  622 G, and sync detectors  650 H and  650 V. Receiver  108  may also include differential output terminals (not shown) that output a buffered and/or amplified version of the input signals for daisy chaining to other receivers. 
         [0057]    The differential video input signals  601  (e.g.  601 R,  601 G and  601 B) are coupled to the respective variable gain amplifiers  610  and discrete gain amplifiers  620 . Each variable gain amplifier  610  works together with the corresponding discrete gain amplifier  620  to compensate a respective one of the differential input video signals for insertion losses resulting from communication of the signal from transmitter  104  to receiver  108  over twisted pair cable  106 . In one or more embodiments, each variable gain amplifier  610  is capable of providing a controllable, variable amount of gain over a range from zero (0) to a maximum value (K), and each discrete gain amplifier  620  provides amplification in controllable, discrete multiples of K (e.g. 0K, 1K, 2K, etc). Together, variable gain amplifiers  610  and discrete gain amplifiers  620  provide controllable amounts of variable gain over an amplification range equal to the sum of the maximum gain of variable gain amplifiers  610  and the maximum gain of discrete gain amplifiers  620 . In one or more embodiments, K represents the amount of gain typically required to compensate for signal losses over a known length of cable (e.g. 300 feet). 
         [0058]    In one or more embodiments, the total amount of gain provided by variable gain amplifiers  610  and discrete gain amplifiers  620  may be selected based on the length of cable  106 , or may be automatically controlled, as described in more detail in co-pending U.S. patent application Ser. No. 11/309,122, entitled “Method And Apparatus For Automatic Compensation Of Video Signal Losses From Transmission Over Conductors”, specification of which is herein incorporated by reference. 
         [0059]      FIG. 8  is an illustration of a variable gain amplifier  610  and a discrete gain amplifier  620  in one embodiment of the invention.  FIG. 8  shows a variable gain amplifier  610  and discrete gain amplifier  620  for a single video signal component, namely the red color component of an RGB signal (designated R X  in  FIG. 8 ). However, it will be understood that in one or more embodiments each color component is provided with its own variable gain amplifier  610  and discrete gain amplifier  620 , as shown, for example, in  FIG. 6 . 
         [0060]    In the embodiment of  FIG. 8 , variable gain amplifier  610  provides amplification over an initial amplification range of zero up to a maximum gain (represented herein by the letter “K”). Discrete gain amplifier  620  provides selectable, discrete amounts of frequency dependent gain in multiples of K. For example, in the embodiment of  FIG. 8 , discrete gain amplifier  620  provides selectable gain in the amounts of 0K, 1K, 2K, 3K or 4K. Together, variable gain amplifier  610  and discrete gain amplifier  620  provide continuously variable gain with values from 0 to 5K over a desired frequency range. The frequency range may be determined based on noise considerations. 
         [0061]    In the embodiment of  FIG. 8 , variable gain amplifier  610  includes a fixed gain amplifier circuit (FGA)  850 , a variable gain amplifier circuit (VGA)  840 , and a compensation circuit  842 . VGA  840  and FGA  850  are both coupled to the differential input signals R X (+ve)  801 P and R X (−ve)  801 N. The coupling may be via a differential line buffer, e.g.  810 , to prevent unbalancing of the transmission line. FGA  850  converts the differential video input signal to a single ended output with fixed gain. VGA  840  adds a controllable amount of variable (DC and AC Compensation) gain to the differential video input signal. The outputs of FGA  850  and VGA  840  are summed at node  843 . The resulting summed signal is provided to the input of discrete gain amplifier  620  from node  845 . 
         [0062]    The amount of gain supplied by VGA  840  is controlled by Fine Gain Control Signal  805  supplied, for example, by a microcontroller. Compensator circuit  842  is used to set the desired frequency response of VGA  840 . The fine gain control of VGA  840  compensates for both DC and AC signal losses in cable lengths of 0 feet to N feet (e.g. 300 feet). 
         [0063]    If the maximum gain “K” provided by variable gain amplifier  610  corresponds to the insertion loss exhibited by 300 feet of CAT5 cable, then variable gain amplifier  610  can provide variable signal compensation for zero (0) to 300 feet of CAT5 cable. In the illustration of  FIG. 8 , the amount of gain between 0 and K (e.g. for between 0 and 300 foot lengths of CAT5 cable) provided by variable gain amplifier  610  is controlled by fine gain control signal  805 . For longer lengths of cable, additional signal amplification is required. In the embodiment of  FIG. 8 , that additional signal amplification is provided by discrete gain amplifier  620 . 
         [0064]    Discrete gain amplifier  620  provides additional compensation for longer line lengths in discrete amounts of “K”. For example, for a cable length of 450 feet, 1.5K of total compensation is required. In this case, discrete gain amplifier  620  provides 1K (300 feet) of compensation, while variable gain amplifier  610  provides the remaining 0.5K (150 feet) of compensation. 
         [0065]    In the embodiment of  FIG. 8 , discrete gain amplifier  620  comprises a multiplexer  820 , a zero-gain buffer  803 , and a plurality of fixed gain compensation circuits  806 ,  809 ,  812  and  815 . Each fixed compensation circuit provides an amount of gain that is approximately equal to the maximum amount of gain provided by variable gain amplifier  610  (e.g. “K”). However, each fixed compensation circuit may include noise compensation circuits to compensate for noise in the longer cable lengths. 
         [0066]    The amount of gain required to compensate for insertion losses resulting from transmission of video signals over long cable lengths will tend to increase the noise in the video signal. For instance, as illustrated in  FIG. 11 , the gain required to compensate for insertion loss for a 40 MHz video signal transmitted over 1500 feet of CAT5 cable is approximately 62 dB, or a voltage gain of approximately 1,259. At such large amplification, the effect of amplified input noise becomes significant. Noise is not desirable and will show up as sparkles in the video display. To reduce the noise problem, noise filters may be incorporated in one or more discrete gain amplifier stages. Therefore, each fixed compensation circuit (e.g.  806 ,  809 ,  812 , and  815 ) may include an appropriate noise filter (e.g. low-pass filter to attenuate noise beyond a certain frequency) as well as the fixed gain “K”. Noise compensation is further described in co-pending U.S. patent application Ser. No. 11/309,123, entitled “Method And Apparatus For Automatic Reduction Of Noise In Video Transmitted Over Conductors”, specification of which is herein incorporated by reference. 
         [0067]    In the embodiment of  FIG. 8 , input  831  of multiplexer  820  is connected to the output of buffer  803  (i.e. the buffered output signal from variable gain amplifier  610 ). Input  832  is connected to the output of compensation circuit  806  (i.e. the output signal from variable gain amplifier  610  after it has been amplified by compensation circuit  806 ). Input  833  is connected to the output of compensation circuit  809  (i.e. the output signal from variable gain amplifier  610  after having been amplified by compensation circuits  806  and  809 ). Input  834  is connected to the output of compensation circuit  812  (i.e. the output signal from variable gain amplifier  610  after having been amplified by compensation circuits  806 ,  809  and  812 ). Input  835  is connected to the output of compensation circuit  815  (i.e. the output signal from variable gain amplifier  610  after having been amplified by compensation circuits  806 ,  809 ,  812  and  815 ). If K is the amount of gain provided by each compensation circuit, then the additional gain applied to the output signal from variable gain amplifier  610  is 0K, 1K, 2K, 3K or 4K, depending on which of inputs  831 ,  832 ,  833 ,  834  or  835  is selected. If the amount of gain supplied by variable gain amplifier  610  is “J” (i.e. a value between 0 and K), the total amount of gain provided by variable gain amplifier  610  and discrete gain amplifier  620  is J, J+K, J+2K, J+3K or J+4K, depending on which of inputs  831 ,  832 ,  833 ,  834  or  835  is selected. 
         [0068]    In the embodiment of  FIG. 8 , the fixed amount of compensation provided by each of compensation of circuits  806 ,  809 ,  812  and  815  is approximately equal to the maximum compensation provided by variable gain amplifier  610 . However, it will be obvious to those of skill in the art that the amount of compensation provided by each of the compensation circuits  806 ,  809 ,  812  and  815  may be greater or less than the maximum provided by variable gain amplifier  610 . Further, the discrete amount of compensation provided by each of compensation circuits  806 ,  809 ,  812  and  815  need not be the same. 
         [0069]    The connection of either of inputs  831 ,  832 ,  833 ,  834  or  835  to output  802  of multiplexer  820  is controlled by coarse gain selection signal  807 . In one or more embodiments, coarse gain selection signal  807  is generated by a micro-controller, which determines both the coarse gain selection signal  807  and the fine gain control signal  805  based on the actual loss in the reference signal as detected in the video signal received from the transmitter. 
         [0070]    Skew compensation is performed through Skew Adjustment circuit  630 . An embodiment of skew adjustment circuit  630  is illustrated in  FIG. 9 . As illustrated, skew adjustment is accomplished by first recovering the reference signal (H REF ) from each video component at the output of adjustable delay circuit  910 . Skew compensation is accomplished by measuring the skew (i.e. difference in arrival time) between the reference signals in the color component signals using the circuit comprising: reference signal detectors  920 , high speed sampler  930 , skew capture circuit  940 , and micro-controller  950 ; and then applying compensating delays to the fastest arriving signals with adjustable delay circuits  910 . In  FIG. 9 , subscripts “X” and “Y” for each of the R, G, and B video signals are used to refer to the input signal to the skew adjustment circuit and the output signals from the skew adjustment circuit, respectively. 
         [0071]    In one or more embodiments, each reference signal detector  920  comprises a comparator which compares the respective video signal to a negative reference voltage threshold, H REF , generating a pulse when the reference signal is detected in the video signal. For example, signal detector  920 R generates an output reference pulse signal R_ref corresponding to detection of the reference signal in the red component signal R Y . Similarly, signal detector  920 G generates an output reference pulse signal G_ref corresponding to detection of the reference signal in the green component signal G Y , and signal detector  920 B generates an output reference pulse signal B_ref corresponding to detection of the reference signal in the blue component signal B Y . 
         [0072]    The three reference pulse signals generated by reference signal detectors  920  feed into high speed sampler  930  which takes digital measurements of the recovered reference pulse signals. The digital outputs of high speed sampler  930  (i.e. Sync_Red, Sync_Grn, and Sync_Blu) feed to skew capture circuit  940 , wherein the skew is determined and subsequently fed to micro-controller  950 . Micro-controller  950  determines the appropriate delay to be applied to each component signal to compensate for the measured skew, and commands adjustable delay circuits  910  to apply the appropriate delay to the two earliest arriving color component signals such that they will line up in time with the slowest arriving component signal. 
         [0073]    A skew adjustment circuit is described in more detail in co-pending U.S. patent application Ser. No. 11/309,120, entitled “Method And Apparatus For Automatic Compensation Of Skew In Video Transmitted Over Multiple Conductors”, the specification of which is incorporated by reference herein. 
         [0074]    DC Offset Compensation circuit  622  of  FIG. 6  and Offset Correction circuit  440  of  FIG. 4  (referred to collectively as “DC Offset Compensation”) may be configured as illustrated in  FIG. 10 . 
         [0075]    As illustrated, the DC restore circuit comprises: summing node  1010 ; amplifier  1012 ; Circuitry Causing Offset  1014 ; Sample &amp; Hold circuit  1016 ; and Clamp Pulse Generator circuit  1018 . The DC restore circuit operates on Input Signal  1001  to generate the clamped video signal, i.e., Offset Corrected Signal  1002 . The offset signal (i.e. output of Sample &amp; Hold circuit  1016 ) is generated when the clamp pulse is received from Clamp Pulse generator  1018 . 
         [0076]    Generally, clamping of the video signal with respect to ground involves detecting the offset voltage level. This may be accomplished in one or more embodiments of the present invention by sampling the back porch to obtain a reference for the video signal. This is because the voltage at the back porch of all video signals should be zero. Thus, measuring the voltage level at the back porch produces an offset voltage which may be applied to the video signal through a feedback path, continuously, until the back porch is restored (or clamped) to a ground level. 
         [0077]    Input Signal  1001 , e.g. video signal which includes the horizontal sync signal, is used by Clamp Pulse Generator  1018  to determine the back porch period (i.e. falling edge of the horizontal sync signal). The output of clamp pulse generator  1018  (i.e. clamp pulse) controls when Sample &amp; Hold circuit  1016  samples the output video signal  1002  to generate an offset voltage equivalent in magnitude to the back porch voltage level, but with opposite polarity. Thus, the offset voltage feeds back at node  1010  to remove the DC offset error in the video signal. 
         [0078]    DC offset correction circuits and methods are described in co-pending U.S. patent application Ser. No. 11/309,558, entitled “Method And Apparatus For DC Restoration Using Feedback”, specification of which is herein incorporated by reference. 
         [0079]    Referring back to  FIG. 6 , Sync Output signals  603 , which is output of Sync Detector  650 , comprises primarily of Horizontal Sync and the Vertical Sync signals. In one embodiment of the present invention, the Horizontal Sync and the Vertical Sync signals are generated by comparing the Red (i.e. R Y ) and the Blue (i.e. B Y ) outputs of Skew Adjustment circuit  630  against a negative voltage level. A comparator may be used for such comparison. Thus, the Vertical Sync signal is generated when the R Y  output of Skew Adjustment circuit  630  meets the negative voltage threshold level, V REF ; and the Horizontal Sync signal is generated when the B Y  output of Skew Adjustment circuit  630  meets the negative voltage threshold level, H REF . 
         [0080]    Video Output  602  may be generated by stripping the sync signals from the video signal components at Output Stage  640 . The sync stripping circuit may simply comprise a switch which grounds the video output during the sync period. For example, the circuit may be such that when either the Vertical Sync or the Horizontal Sync pulse is high, the video output (i.e.  602 ) is switched to ground; otherwise, the video output is switched to the corresponding video signal output of Skew Adjustment circuit  630 . This is illustrated in  FIG. 7 . 
         [0081]    As illustrated, R X    701  is the video source from the output of Skew Adjustment circuit  630 , and R Y    702  is the stripped video output. The Vertical Sync signal (i.e. V Sync ) is wired-ORed with the Horizontal Sync signal (i.e. H Sync ) to generate the Select signal. When the Select signal is true (“T”) the video output, R Y    702 , is coupled to ground through switch  710  to remove the sync pulse. Otherwise, i.e. when the Select signal is false (“F”), the video output R Y    702  is coupled to the input signal, R X    701 . 
         [0082]    Thus, a method and apparatus for automatic compensation of video transmitted over long distances using twisted pair cables have been presented. It will be understood that the above described arrangements of apparatus and the method therefrom are merely illustrative of applications of the principles of this invention and many other embodiments and modifications may be made without departing from the spirit and scope of the invention as defined in the claims.