If you look at the pinout for VGA, there are several ground pins:

VGA Pinout

I was curious as to why, and I found this answer. To sum it up, the extra ground pins are so that each pin has its own ground in order to prevent interference in the analog signal.

But here's a DVI-I connector that supports analog signals:

DVI Pinout

The analog pins are on the right side. The big cross is ground, and the four smaller pins surrounding it are for the red, green, blue, and horizontal sync. What is interesting here is that the ground is shared by all three color channels, unlike VGA where each has its own.

Why are the additional ground pins necessary to prevent signal interference when using VGA but not DVI-I? They're the same pins that send the same data, just with a different physical connector, so it doesn't really make much sense as to why the number of ground connectors are different.

  • 2
    Dsub miniatures were common when VGA was being developed and the cost difference of a smaller connector was not worth the expense of trying to share a common ground pin with coaxial connections. Physically sharing the same pin while keeping the lead wire short to reduce the inductance of the 75 Ohm ground connection could be a problem. – Tony EE rocketscientist Feb 26 '17 at 3:45
up vote 38 down vote accepted

First: What's critical isn't so much that there's a ground pin for each signal as much as that there's a ground pin near each color signal. The cross-shaped ground pin largely satisfies that requirement.

Second: DVI doesn't prioritize high-quality analog video -- it's a Digital Video Interface, after all. The small loss of quality incurred by using a single analog ground pin was probably considered acceptable by the designers.

The HD-15 (aka DE-15) connector for VGA connection was compatible with hand-crimping of pins to conductors of coaxial elements of the multiwire connecting cable. That practically requires a two-pin set for each of R, G, B video signals, to accommodate a signal pin and ground (coaxial shield) pin. Those signals were not logic level, and lacked the noise immunity conferred by the logic margin.

The DVI-I analog compatible signals may use the same wiring, but hand-assembling of crimp connections is no longer how the cables are constructed. As for the digital signals, in DVI-I and DVI-D cables those are twisted pair with shield, so require three wires each for up to seven fast digital signals. To my knowledge, there aren't any hand-assembly crimp pin options, the cable connectors are intended for machine wiring. In any case, twisted pair digital signals are noise-insensitive (because the digital signals have a significant logic margin). The shielding of those digital pairs prevents crosstalk, but the shields carry very small currents. Four pins (shaded yellow in the above question) are shared for up to 7 twisted-pair shields.

  • Not just hand crimping; hand soldering of custom VGA cables is possible (though you can't fit the cables for a VGA--5*BNC adaptor inside an HD-15 backshell). – Chris H Feb 27 '17 at 10:26

You are confusing the DVI connector with the cable. That "single" analog ground is a big honking ground, but what goes into a cable? The simplest is a DVI-A cable, which usually has a VGA connector on the other end. Internally, it will carry 3 color channels (red, green and blue) and 2 sync: vertical and horizontal. And guess what? A DVI-A cable will typically have 10 conductors. That is, 5 twisted pairs, 3 for color and 2 for sync, each with a ground line. The 3 analog color grounds will be connected to the cruciform analog ground on the DVI connector and the sync grounds will be connected to another pin.

So, just because a DVI analog section only appears to have a single ground, that does not mean that the cable does as well. In fact, the cable and the VGA connector will have the same number of ground pins (5) as a standard VGA.

  • 3
    In good quality VGA displays, the three colour inputs are differential, the three separate ground pins are connected to the ADC/scaler chip. – TEMLIB Feb 26 '17 at 17:35
  • Sure? Using a coax shield as part of a differential system sounds like no good idea at all. – rackandboneman Feb 27 '17 at 14:03
  • @rackandboneman On the receiving end, the display. Twisted pair, not coax. Think of it as a separate ground for each signal. Each capacitively couples to its partner, and avoids "talking" to anything else. – Potatoswatter Feb 27 '17 at 14:21
  • A VGA cable is 3x75Ohm coax plus sundry bell wires, isn't it? – rackandboneman Feb 27 '17 at 14:55
  • @Potatoswatter I thought a typical VGA cable used coax elements for the color inputs though. Or are these custom cables permanently attached to the display? – Random832 Feb 27 '17 at 14:56

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