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Monitor Splitter

2015-01-24 14:53  
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A Simple Splitter Driving Two (or even more) PC Monitors

The signal sources in the analog output stage of the PC SVGA card see the right impedance - 75 ohms on each line (five resistors), and the input impedances of the transistors are so high that they do not make any significant additional load.

The three ID lines can be omitted. If used, they should be connected to the cheaper (less advanced) of the two monitors.

The transistors that form the ten emitter-followers should be as fast as possible. 2N3906 is not the best possible choice, but it does the job and is unusually cheap (for such a versatile component).

All connections should be as short and straight as possible. Monitor signals are RF, so the splitter should be put in a metal case which is connected to the circuit ground. It should be supplied with well-filtered 5 volts and some 600 mA.

There is some DC component in the ouput signals, but it should not be a problem. The splitter works fine at 800x600 and 1024x768 on 15" monitors. For higher pixel-rates maybe it would be preferable to use faster transistors (haven't tried). High gain is also a good thing, because it gives emitter-followers higher input resistances.

Alternative Transistors
A quick note regarding building this circuit with long cables and alternate transistors. The choice of transistors is critical to the operation of this circuit.
With a monitor splitter the video signal is analogue. Its quality therefore depends on good quality cables and connectors and also the switching speed of the transistors. One unavoidable problems with cables is capacitance. This limits high frequency response, and will reduce overall quality and force you to use lower video resolutions.

I am especially grateful to Stanislavs from Latvia who has built the above circuit and found the problem. At first Russian KT361 transistors were tried, with collector capacitance (Cc) = 10pF. The picture was not good quality and could not handle a video resolution of 1280x1024, the image was blurry. He then replaced them with A1318 (Cc=3pF) and results were great. Stanislavs also split the picture on two Sun monitors working ath 1600x1200 resultion with 60Hz refresh rate with just perfect quality.

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