The ultimate video-connector guide

Ok. After throwing this information out on several occasions in other threads, I thought it was time I put it all in one place, ordered correctly, and with interesting pictures.

This will consist of the following `chapters':

• Digital connectors
• Analog connectors
• TV connectors

In `Digital connectors' I'll explain the DVI connector, and several other digital connectors that are less common.
In `Analog connectors', I'll explain the good old 15-pin D-Sub connector we all know and love, BNC coaxial connectors and the DVI-A connector.
In `TV connectors', I'll explain the different ways computers (and devices like consoles) can be connected to TVs.



Of course, if someone has interesting additions, or if I've made mistakes anywhere, feel free to PM me!

Digital connectors

First of all, the DVI (Digital Video Interface) connector. DVI connectors are by far the most common connectors found on modern graphics cards.
There are five flavours to choose from:

• DVI-D single link
• DVI-D dual link
• DVI-I single link
• DVI-I dual link
• DVI-A

DVI-A is in reality not a digital connector, and will be discussed in the `analog connectors' chapter.

DVI-D vs DVI-I
Now, what's the difference between DVI-D and DVI-I? Very simple: DVI-D only supports digital signals, while DVI-I is a hybrid of the DVI-D and the DVI-A connector.
To explain this further, let's have a look at the different connectors:



If you look closely at the two images, you'll notice the DVI-I connector has 4 pins in a square arrangement around the flat connector on the side. The DVI-D connector doesn't.

These pins carry the analog signal. The 3x8 pins on the other side are for the digital signal. The analog pins will be discussed together with the DVI-A connector.

Single-link vs Dual-link
You'll notice several columns of the digital pins are missing on the connectors in the images above. That's because these are single-link connectors. If we look at their dual-link counterparts, we'll see these pins are present:



What exactly is the difference between dual and single link? To explain that, we first must explain a bit more on how a digital link works.

A digital video link, in most cases, uses Transition Minimized Differential Signaling (TMDS) to transfer the signal from source to display. This is a serial protocol, where the bits are transferred after each other at a high speed, 165 MHz for DVI connectors.
To obtain a minimum 60Hz refresh rate for the display, this means a single frame can exist of maximum 2.750.000 pixels. For a `normal' 4:3 aspect ratio, this is roughly equivalent to 1914x1436 pixels.

To achieve higher resolutions, higher refresh rates, or for very special screens more bits-per-pixel, a second TMDS transmitter can be added to the video-card. This means two lines are going from the card to the screen, each carrying the aforementioned 2.62 megapixels.
This doubles the bandwidth, allowing for higher resolutions, refresh rates, etc.

As you can see in images 1.1 and 1.2, the (female) single-link socket on a graphics card usually still has the `receptacles' for the dual link pins, so that a dual-link cable can be used, even though no signal will be transmitted there.

Now, how do you know if your card has a dual-link connector?

There isn't a guaranteed-to-work method. A lot of manufacturers don't even advertise these things in the card's specification, as the number of screens that require dual-link are so small, it's very much a niche-market. However, saying a specific card is NOT single link will lower its value, even though for 99% of the customers, this really isn't that important.
The Geforce 7900 is the first card I know of that is actually advertised as having dual dual-link connectors (so you can attach two insanely large screens).

One thing you must also remember: It is not possible to attach two `small' screens to a single dual-link connector. Aside from the TMDS signal, there are several other signals in the connector, such as timing signals, that are only there once. Splitting these up won't work.

Other digital connectors
Before DVI became the official standard, several others hit the market. Two of these are most notable, and still in use: P&D (also known as M1DA) and ADC (Apple Display/Digital Connector). The most notable difference between these and DVI is the inclusion of an USB signal in the connector.



As you can see, P&D is nearly identical to DVI, only the 3x8 array of digital pins are now 3x10 pins. ADC is pin-compatible with P&D, but shaped differently (rounded sides instead of a trapezium).

P&D is sometimes used on beamers.

ADC was used on the first generation of Apple's cinema displays. The newer (aluminium) versions use `normal' DVI connectors. This is partly because people wanted to use them on PC's, while ADC connectors were only available on Mac graphics cards, and partly because the new 30" screen requires a dual-link connector, and both P&D and ADC are single-link only.

Analog connectors

There are three main types of analog connectors: The classic VGA connector, BNC coax cables and the DVI-A connector.

VGA connector
This is by and large still the most common video connector around. 95% of all CRT screens ship with it, as do a lot of the older and/or TFT screens.


As you can see, the connector as 3x5 rows of pins. These carry red, green, blue, vertical sync, horizontal sync and identification signals. Most of the other pins are grounds signals for these.
Several of the 15 pins are not connected, though I've seen monitors where these were used to transfer command signals back to the PC for volume controls build into the screen.

DVI-A connector
The DVI-A connector is the class of DVI connector where only the analog signal is passed through. As a socket on video-cards these are never used, as it is much more practical to use a VGA connector when only the analog signal is supplied. As a connector it can be found on DVI-to-VGA converters however.

As you can see from figure 2.2, it only uses part of the 3x8 array, and the 4 `analog' pins on the side.
The 4 pins on the side are red, green, blue and horizontal sync. The flat, wide pin in between them is a ground signal. The pins used from the 3x8 array are vertical sync, the DDC(*) lines and additional ground.

Coaxial BNC connectors
Another way by which monitors are sometimes connected, is by component cables. The most common standard for this is using coaxial cable with BNC connectors.The version seen in almost all cases is 5 signals: Red, Green, Blue, Vertical Sync and Horizontal Sync. However, 4 signal versions, and even 3 signal versions also exist. In these cases the sync signals are mixed in with one of the color signals.

The big advantage of this way of connecting a screen, is the high quality of coaxial cables. Where a normal VGA cable is limited to several meters, coaxial cables can easily extend much further than that. They are therefore of course much more expensive .

It is very rare to find a graphics card with BNC connectors, the most common way to attach a screen that has BNC connectors is by means of a VGA to BNC cable, as shown in figure 2.3.



(*)DDC: Display Data Channel, a protocol by which a monitor can tell a computer more about itself, such as maximum resolution, refresh rates, etc.

TV connectors

Composite
By far one of the most popular ways to connect TV's to videocards is by means of an RCA composite video connector. RCA (Radio Corporation of America) connectors are used in many applications for both video and audio. Even digital S/PDIF signals are often transmitted over RCA cables.
RCA cables are coaxial, and therefore relatively interference-proof. Since it is used for so many different applications, different colours are used for each type. Figure 3.1 shows several of the most common types of sockets you may find around a computer


On the top row, you see a composite video socket (yellow), and left and right audio channels (white and red). On the bottom row, you can see a set of component video connectors. These will be discussed later.

As said, composite is used widely. In a composite video signal, the chrominance (colour) and luminance (brightness) signals are mixed together onto a single carrier signal by means of frequency multiplexing. This is the same way video is transmitted over antenna and cable networks, though there the carrier frequency varies (different channels), while with a composite video signal, the carrier frequency is fixed, and only one signal is transmitted. In addition to this, the bandwidth used with a TV signal is smaller (less amplification). This is also why antenna cable is always thicker and better shielded than composite video cables: Less bandwidth means a higher signal-to-noise ratio is required.


S-Video
S-Video, fully Seperate Video (not Super-Video as is sometimes wrongly used), is very similar to composite. However, the chrominance and luminance signals are transmitted seperately, and normal cable is used instead of coaxial cable.


S-Video usually uses a mini-DIN connector, as shown in figure 3.2.

The main advantage of S-Video over composite is that because of the fact the signals are split up, the total signal is less affected by noise.


Component video
A more recent development is the use of component video when connecting TV's, which before was limited to computer monitors only.
However, with the advance of High Definition televisions, the market is looking for ways to actually get HD content to the HD screens.

A set of component sockets can be seen in figure 3.1. As you can see, each has a different color, to indicate which `component' is carried on that line. In addition to this, below each socket a secondary function can be seen: Cr, Cb and Y. These are again the chrominance and luminance signals we saw earlier. Cr is Red, Cb is Blue, and Y is brightness. Green, being the brightest colour, can be mathematically reconstructed from the brightness signal and the two colour signals. These two systems differ little, and usually both are supported at the same time.

You won't find component RCA connectors on a graphics card though, usually these are on a break-out box, connected by a mini-DIN cable.

SCART
One of the most common connectors found on modern TVs is a SCART plug. This is not a seperate way of connecting video devices, but more an `all of the above' solution.
In the 21 pins of a SCART connector, both composite, S-Video, component and audio signals are transmitted. If a sending device cannot deliver one of these, it doesn't put anything on that wire. If a receiving device cannot read a certain signal, it ignores those wires.

Now, you'll obviously won't find a (big) SCART connector on your graphics card. There are however very simple converters available to convert composite or S-Video to SCART, usually with audio connectors as well. See figure 3.3 for an example.


Not however that on these, you can't use composite and S-Video at the same time, as these use partly the same pins. The same goes for several other combinations of signals. However, with normal use, this shouldn't be a problem, as you will only output video on one signal at the same time.

HDMI
A new standard, High-Definition Multimedia Interface is breaking ground fast in the world of HD TV. Since it has support for copy-protection systems, it has the backing of the MIAA and RIAA, and therefore a powerful push for manufacturers to include it in their hardware.


A HDMI cable carries in it a single TMDS channel, as we already saw on DVI connectors. However, here it also contains a full set of (surround) audio channels. In addition to this it has a DDC channel, as seen on both DVI and VGA connectors and a CEC channel, which can be used be the receiving device to control the sending device.

Because HDMI uses the same TMDS system as DVI, it is very easy to make a convertor cable. Of course, if this is done, audio has to be carried on a seperate channel.

The common HDMI connector is single-link. A dual-link version has been defined, but is not in use yet, since TV's commonly don't require resolutions so big as to need the added bandwidth.

The copy-protection system of HDMI has been subject to much controversy, and is the main reason why the HDMI connector is not yet a common sight on graphics cards. This would require the GPU to have DRM capabilities, which in turn would require a tighter cooperation between for instance the GPU and the DVD player. Something which in the current PC architecture is very unpractical, if not impossible.