On a VGA connector, there are numerous 'ground' pins. Three of these are Red return, Green return and Blue return; But why does each color component have its own ground pin?
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2 Answers
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Probably so that the voltages induced by the ground currents don't interfere with each other. Copper is not a perfect conductor, so a current transmitted through copper results in a small voltage change. If they all returned through the same wire, the ground voltage would be affected by all three signals, and allow the colors to bleed into each other.
Also this would allow each pair of wires to be tightly twisted, so that the magnetic fields created from the currents going up one and down the other cancel each other out, reducing emissions. If they all used a common ground wire, there would be more loop space between them?
Oh yeah. Impedance is important, too, at video frequencies. The pairs need to have a given characteristic impedance so that there are no reflections when the signal gets to the monitor.
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4\$\begingroup\$ the cable is almost always using twisted pair in this situation. The returns are referencing which other wire they are twisted with. This is for EMI and a set input impedance. \$\endgroup\$– KortukCommented Apr 30, 2010 at 20:59
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2\$\begingroup\$ It has very little to do with resistance, it's all about EMI and impedance. \$\endgroup\$– starblueCommented May 1, 2010 at 14:13
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\$\begingroup\$ That is what I said in the comment previous starblue. Read the last sentence. \$\endgroup\$– KortukCommented May 3, 2010 at 21:10
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The colour channels are sent over 75Ω coax (it's an RF signal running with a high bandwidth - up to 250MHz - so it goes over coax). The ground pin next to each colour pin is the coax ground. It needs to be close to the centre pin to minimise impedance mismatch in the DB-15H connector (which is NOT a great connector for RF - it does not provide a coaxial join for the three colour channels).
Long runs of video cable (~30 metres) require low loss coax to function correctly. Poor cable quality results in degraded images (smearing and ghosting,etc).
Regards, Tony Barry