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Is there a way to duplicate a wire from an RGB source without degrading the signal. I wish to take a wire and split it into two wires without the current halving. (edit) 75 ohms and one RGB signal wire i.e just the blue line.

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  • \$\begingroup\$ Is the RGB source a DC source or a variable digital one? \$\endgroup\$ – hcabral Aug 30 '16 at 19:38
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    \$\begingroup\$ Please define RGB source, and "degrading the signal". What is your bandwidth? What is the RGB source current? \$\endgroup\$ – Voltage Spike Aug 30 '16 at 19:38
  • \$\begingroup\$ This question is almost useless without a better definition of what "RGB source" means. It could have many definitions, each with different answers. \$\endgroup\$ – Richard Crowley Aug 30 '16 at 20:12
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It depends on what you want to do with the signals once you separate them. It also depends on the specific type of "RGB" interface you are dealing with: 3-wire, 4-wire, or 5-wire.

The 3-wire interface has only the R,G & B signals and carries the sync information (usually) on the G wire. Aka: sync-on-green. The 4-wire interface has an additional wire which carries both H-Sync (horizontal sync) and V-Sync (vertical sync) OR'ed together into one electrical signal. The 5-wire interface has two additional wires - one for H-Sync and one for V-Sync.

The R,G, & B signals are almost always "75 Ohm signals", meaning they have a source impedance of 75 Ohms and they must be terminated with 75 Ohms at the receiving end (generally a video monitor, but possibly a repeater input, splitter input, and similar). The connecting cable must have a characteristic impedance of 75 Ohms as well, and is usually co-axial cable.

On the other hand the sync signals in a 4 or 5-wire hook-up are typically NOT 75 Ohms, they are essentially TTL logic levels, most typically 0-5 volts. That's typical, there are systems where these signals are also "75 Ohm signals" and are therefore carried by coax cable. If you are dealing with a PC, laptop, TV-set with auxiliary "VGA" or "RGB" input, these are almost universally TTL signals. Sometimes these signals are terminated at the receiving end, sometimes not. If terminated you will find anywhere from 470 to 2,000 Ohms as terminators.

Now, to split the RGB signal passively (not using a powered splitter, but a simple "Y" connection) you have several problems to overcome. The first is not losing signal amplitude when you connect the receiving ends to two 75 Ohm loads simultaneously. This will halve the signal amplitude and cause various problems, primarily loss of contrast in the image. In a 3-wire system the sync-on-green signal may be so adversely affected that the connected monitor(s) will not be able to acquire sync. Depends on the actual monitors you are connecting and how much internal gain they use to recover the RGB signals. There is one cheap trick fix I describe below for this situation which will work in some instances.

The sync signals present less of a problem if they are the TTL type (very typical in PC and Laptop environments). These you can usually split with no ill effects because of the way they are usually lightly terminated at the receiving end. If the syncs are 75-Ohm, you have more of a problem. The cheap trick described next might even work for these.

Cheap Trick: Remove the 75-Ohm termination resistors from one of the monitors. If the connecting cables are "short enough" and the signals you are sending are low-resolution (e.g. 800x600 @ 60 Hz), you might get a suitable image. There are lots of variants possible with this trick. If you have two monitors, leave the termination resistors in the far monitor and remove them from the close monitor. Daisy chain the monitors together, remove the termination from the first monitor and run the signals to the second monitor right from the connector of the first monitor, but leave the terminators in the second monitor. Don't worry about the sync termination resistors unless all else fails. Leave them intact in both monitors.

Your best bet is to try both monitors first with their original 75-Ohm termination resistors intact, and adjust the contrast on each monitor to see if you can get a decent enough picture on the screen. The "Contrast" control is actually the video gain control. Depending on the specific monitor this gain might have enough high-end margin to squeak you by. (The "Brightness" control is actually video offset and will have much less of an effect on weak signals.)

If all else fails, buy a splitter. They are actually pretty cheap these days.

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VGA means you have a 75 Ohm source on each RGB signal that is ~1Vpp when loaded by a 75 Ohm load (monitor)

Loading with 1/2 * 75 Ohms implies you need the same source impedance to get the same voltage. for black and white levels.

It will work but only only at 50%luminance. (peak voltage =100% bright)

Then you can compensate on monitor or leave as is.

When you use long coax cables, if you have 75/2 Ohm load and source now any mismatch may result in ghosted edges with cable being 75 Ohms.

A video splitter amp uses separate 75 Ohm video buffers for each output to drive monitors directly near DC to xxx MHz .... unlike RF splitters which use hybrid transformers.

Here's a collection of suitable designs from LTI http://cds.linear.com/docs/en/application-note/an57fa.pdf

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