Termination
Trying to describe problems simply when talking about termination is a problem in its self
One simply way is with the comparison of electricity and water. If you consider a sink which has to be kept at a constant water level from a tap while there is no plug in the drain away. There is the mains water pressure into the tap which can be imagined to be the flow of electricity. Then there is the tap which regulates the water flow this is the termination in the electrical world. To make sure that the water flow out of the sink is the same as that entering the sink, the tap is adjusted to match the sink output, at this point there is an exact match and the water level will appear constant this is a properly terminated system. Video is usually specified as 1 volt into a 75 ohm load. If the tap is turned off no water would flow and the sink would drain out this in the electrical world is a short circuit. In the electrical world you get no video if you short the output. If the tap is turned full on the sink would fill up and over flow, because there is too much water entering the system, in the electrical term too much. In video, an un-terminated line will show much more video level than expected or desired. Finally if the tap is again operated to the correct point to keep the sink at a constant level but now the mains water pressure reduces this has the effect of causing to restrictions in the flow or in other words double terminating the system so a lower flow than expected is seen. The video result of double termination is a signal of much less level than expected or desired.
When to Terminate
Every video line needs to be terminated into 75ohms ONCE , preferably at the end. A line or cable which is attached to a monitor should be terminated, at the monitor (sometimes the monitor has also a loop out but normally has any internal termination which is switchable potential for double termination error). A composite video feed to a genlock input of a TimeBase Corrector (TBC) should be terminated, at the TBC. The simple way to identify a self terminating instrument or device in both the digital and analogue video world, if the is only one connector for the input (no loop through) then this is a self terminating device. If there are 2 connectors (loop through) then this device does not terminate and if on the end of a video line would need a termination. Test and measurement instruments have normally loop through inputs to allow them to look at the signal between devices for monitoring purposes. Unused outputs from devices like distribution amplifiers or generators do not have to be terminated.
Another problem
The bandwidth of the circuit used in the actual terminator was originally designed for the analogue system of video with a required signal bandwidth of a few 10's of Megahertz but now the digital world is on us where the signal transmittion is now at 270Mbits/sec thus new terminators are required with different characteristics than the old analogue versions otherwise the flow of electricity through the circuit will not be exactly matched thus causing errors in the signal path which if the signal was very close to the digital cliff could be enough to kill the signal totally
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© Hamlet Video International 10/02/2005 |
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