Using low-cost CAT5 network cables for video transmission
There are inherent problems in transmitting a video signal over hundreds of meters of CAT5 cable. Using equalization, as well as a differential transmitter and receiver, can help.
By Robert Demattei and Jurica Ivisic, Maxim Integrated Products -- EDN, August 31, 2008
For short distances, a single-ended video-transmission system can easily supply high-quality video. However, over hundreds of meters, such a system is susceptible to voltage differences in transmitter versus receiver grounds or radiated noise injected into the cables. Such noise appears as a single-ended signal and can degrade video significantly. The video degradation is much worse in hostile environments, such as automobiles and industrial buildings.
A differential video-transmission system is relatively immune to these noise sources because the noise sources appear as common-mode to both signal lines. This ability gives a differential pair a unique advantage over a conventional single-ended approach when transmitting over hundreds of meters, where injected noise is more likely to occur. The end result is superior picture quality when transmitting video through especially hostile environments.
The disadvantage of differential video transmission is cost. Differential signaling is best when used with differential coaxial cables, which are relatively expensive. In industrial environments, where distances are often in hundreds of meters, installing dedicated differential coaxial cable is cost prohibitive for all but the most demanding applications. However, due to the widespread use of computer networking, CAT5 cables are much less expensive than even ordinary coaxial cables and are often already installed, making the incremental installation cost very low.
When using CAT5 cable for video transmission, there are two parameters to consider. First, CAT5 cable has four twisted pairs, so if more than one pair is to carry video at the same time some amount of crosstalk will occur between the channels. Crosstalk can be reduced to an acceptable level if the CMB (common-mode balance) of the differential driver and the CMRR (common-mode rejection ratio), of the receiver is 30 dB or more within the video bandwidth (0 to 5 MHz). In this case, the resulting picture quality remains intact.
The second problem with CAT5 cable is high frequency loss, which is significantly higher than in 
coaxial video cable. This requires the designer to add some type of cable equalization when the length exceeds 10 ft. Equalization, which is a means of compensating for loss, will increase the gain at the frequencies that are reduced due to the loss in the CAT5 cable. Therefore, since equalization has made the signal larger at the frequencies where the loss occurs, the higher gain will cancel the effect of the loss from the cable.
Because the inherent design of a differential pair minimizes crosstalk, we will focus on equalization using the NTSC Multiburst signal. is a photo of this signal as it appears on a television monitor and is the same signal as it appears on an oscilloscope.
The white bar on the left of is the same bar shown in on the left. This bar is followed by six sets of small, decreasing width, white sine-wave bars. If the image is displayed correctly, each of the bars will have clearly visible white and dark areas. When phase and bandwidth distortions occur, such as those due to long lengths of CAT5 cable, the progressively smaller white bars become altered; they may appear to have color or simply become less defined. shows an example of this distortion. The white bars are not very 
well defined, the image has less contrast, and the smaller sets of bars are highly distorted. The faint, diagonal white bars are artifacts due to the interaction of the camera's focal plane shutter and the horizontal retrace of the TV; they are not visible to the human eye. The remaining distortions are quite visible.
The graph in clearly illustrates the loss in Vpp of the NTSC Multiburst signal over various lengths of CAT5 cable. This signal loss clearly affects the video quality, as the scope trace captures in demonstrate. Referring back to and , the waveform is very close to perfect when the CAT5 cable length is only 1m (3 ft). Note the evenly matched Vpp of the six frequencies in the test, the leading white bar is sharp and well defined, and the gray areas between the bars are also well defined.
Things begin to change in . Here the cable length is increased to 150m (500 ft). Note the Vpp of the six frequencies are no longer matched in amplitude and the edges of the white pulse are softened and no longer sharp.
In the length of CAT5 is increased to 300m (1000 ft). The differences between the six frequencies are much more dramatic and the white bar is very soft 
and poorly defined. At this length, the cable loss is starting to distort the luma signal, which is evident in the figure.
In the CAT5 cable length is increased to 450m (1500 ft). Now the six frequencies are highly distorted, the Luma for each frequency is obviously altered, and the white bar doesn't reach full brilliance.
There are many line driver-receiver pairs available from various vendors, but for this example, consider Maxim's MAX9546 and MAX9547 to demonstrate the effects of equalization with real transceivers.
illustrates the schematic for this differential transceiver pair with a basic equalization circuit added across pins Zt+ and Zt–. The increased gain from the equalization circuit will bring these frequencies back into proper perspective. 
However, when losses are high—and losses may vary somewhat from one cable manufacturer to the next—equalization may be required on a cable-by-cable basis to obtain the best picture quality.
A typical equalization network schematic is shown in ; each branch of the network adjusts the gain at the frequency set by the resistor and capacitor pair. The resistor sets the gain and the capacitor sets the frequency where it begins to have an effect, although in the crossover region both parts have an effect on the gain. The crossover point where the resistor begins to become dominant is roughly determined by the reciprocal of the product of the two values times 2π.
Table 1 lists the compensation values for 150, 300, and 450m (500, 1000, and 1500 ft). Here the capacitors, C1, C2, and C3 select the frequency where the series gain resistors, R1, R2, and R3 will have an effect. Gain 
resistors R4 and R5 adjust the overall compensated gain along with R5. When R5 is used alone and equal to 75Ω, the gain is set to 2V/V. is a scope photo demonstrating the results of the 150m (500 ft) compensation network when applied to the waveform shown in . Compare the results shown in to the results shown in ; the equalization network effectively removed the effects of 147m of CAT5 cable. 
is a photo of the equalized waveform as it appears on a TV monitor.
demonstrates the results of the 300m (1000 ft) network. There is just a touch of excessive DC gain on the trailing edge of the white bar due to the higher frequency equalization required due to the loss of the CAT5 cable. However, the frequencies and the Luma are very uniform. The picture quality here is preserved and intact.
demonstrates the results of the 450m (1500 ft) network. The equalization circuit has boosted the gain of the IC to maximum and while it is not quite enough equalization at this distance, and the frequencies are not at full amplitude, the picture quality is preserved and intact, as shown in . Compare the image in 
to the image shown in , which is the same waveform without equalization. Contrast is substantially improved, the bars are sharp and well defined, and divisions within the smaller sets of bars are easily distinguished.
Higher gain devices can be used for equalization, and will compensate the waveform shown in even further. Using higher gain devices and excessive equalization will cause noise in the video. If the application is less demanding, the resultant picture from longer cable lengths with equalization can be very acceptable.
CAT5 cable is readily available and preinstalled in most cases, thus offering an inexpensive network for video. The balanced design of CAT5 cable requires a high-quality differential transmitter and receiver. The inherent CMB and CMRR of a good differential pair address the crosstalk problem in CAT5 cable. CAT5 cable is lossy, and thus a video signal requires equalization to compensate. Excessive equalization will eventually increase noise in the background of the video. For transmission distances of 450m (1500 ft) or less, low gain ICs provide an inexpensive means of transmitting video through CAT5 cable. For longer transmission distances where picture quality is less demanding, the some loss resulting from insufficient equalization will continue to provide effective video.
| References |
|
-
Also the scheme we had used a single pot to compensate from 0 - 400m in a linear fashion with a single switchable 400m fixed stage!
Bob Moreton - 2008-10-9 08:39:00 PDT -
Table 1 lists the component values for different cable lengths. The capacitor values are shown as "pFd". Can someone tell me what does the third letter "d" stand for?
Carl Kopin - 2008-10-9 03:55:00 PDT -
Bob's comment is further emphasised by the fact that the components are listed on Maxims website as no longer available!! A replacement is recommended :-)
Dave Burke - 2008-10-9 03:13:00 PDT -
Just shows there is nothing new. We were doing this 10 years ago, differential drive and receive on CAT5 to 800m (limited by running 2 video +SC signals down the cable) or 1200m (for 1 video +SC signal) or 600m for full duplex video (limitations due to crosstalk between signals)
Bob Moreton - 2008-10-9 01:06:00 PDT
