A video interface is a way for two devices to send video to one another. For example, HDMI or HD-SDI. In order for video data to get from a camera to a monitor, both devices have to agree on the format of the data that is going to be exchanged. Video interfaces aren't actually video, they're just a set of rules that defines how video data can be sent between devices.
The only way we can use a Sony monitor with a Grass Valley switcher with Hitachi cameras is because all of those devices implement the same SDI interface. The role of the electronics in the device is to take pixel data and convert it into the SDI format and transmit it along to the receiving device. Then, the receiving device will take the data coming to it from the input and turn it into a series of pixels that it understands.
In today's world, only digitial interfaces are used. Analog interfaces, like composite or VGA aren't used anymore. In digital, there are two main types, serial interfaces and parallel interfaces. Serial Digital Interface (SDI) is, as its name tells us, a serial interface. High-Definition Multimedia Interface (HDMI) is a parallel interface.
Today, we only have one main serial interface, SDI. The SDI specification is developed by SMPTE, the Society of Motion Picture and Television Engineers. SMPTE is a standards body that is made up of representatives from various companies. These engineers get together and define how the new stardards are going to work. For instance, 3G-SDI is really SMPTE 424M, an interface capable of transmitting 1080 59.94P video. Each different resolution of SDI video has a different SMPTE number associated with it.
SDI is the defacto standard way of transmitting video signals between professional video equipment. It's important to know that SDI is not a codec. It is an interface. SDI also is not a cable. SDI is just a format for a series of bits transmitting between two pieces of gear. There are different ways this is done, but commonly it is over a coaxial cable with BNC connectors on it. There are also optical systems that user fiber optic cable for transmission. There are also wireless systems that use RF atenna systems.
The distance limitation of a coaxial cable being able to transmit SDI video is based on the amount of impedance the cable has. Impedance is a lot like resistance. Resistance refers to voltage loss across two points of a DC signal. Impedance refers to the resistance to change of an AC signal. To transmit a bit in SDI, the voltage on the cable has to change from its 1 state or its 0 state. If the signal changes, the bit is a 1. If the signal does not change, the bit is a 0.
A way to gauge the integrity of an SDI signal on a cable is by using a testing device to look at the 'eye pattern'. The eye pattern chart is a composited image of a few hundred samples of the voltage state of the cable over time. Meaning, the test device reads the voltage on the cable extremely quickly and remembers the time that it took the sample. With these samples, it overlays a number of them on top of each other on a single chart. This allows the engineer to have a visual representation of the integrity of the signal. An ideal eye pattern is like the one above, if the signal clearly avoids the grey box, then the device recieving the signal should have no trouble reading it.
The 'eye pattern' in a high quality signal.
If the signal stays the same from the 'A' line to the 'B' line, then the bit is a 0. If the signal moves from the top of the voltage chart to the bottom, or the reverse, then the bit is a 1.
As a cable gets longer, it is more difficult to apply voltage to the cable or take it away due to the increased resistance of the cable. The longer the cable, the more attenuation the signal under goes. The device receiving the signal needs to be able to distinguish if the signal on the cable changed or not. If the signal on the cable is being attenuated too much, the receiver won't be able to distinguish between a change or not. In the figure below, the signal has increased skew, which is preventing the receiving signal from being able to correctly interpret it.
A degraded eye pattern, the signal is not clearly transitioning between voltages.
Today, the most common parallel video interfaces are DVI and HDMI. They are relatively similar in how they work. Both rely on a set of pairs of signals that transmit together, in parallel. Also, a big difference with parallel interfaces is that they are able to send data in both directions. This allows the devices being plugged into one another to perform a handshake upon the start of their connection. Doing this handshake allows for different parameters of the video to be adjusted to make sure that things are going to work.
Prallel video interfaces use differential signaling pairs for transmitting bits between devices. A DVI or HDMI cable is a bundle of smaller cables. For instance in DVI, there are two cables that transmit the data for the red component of a pixel. The pair of cables have voltage between them for a 1 and no voltage between them for a 0.
In parallel video interfaces, the components of an individual pixel are broken up and transmitted across multiple pairs of cables. Because of the nature of this, parallel video signals transmit video in RGB color space. There is a pair of cables for each of the red, green and blue components of the pixel. This maps cleanly to how a display presents the video to the viewer. A single pixel on a display is made up of a red, green and blue component that, to the eye, blend together to form the needed color.
Because parallel video signals use differential signaling pairs to transmit video, they are limited in their effective transmission distance. SDI video is able to travel hundreds of feet over coaxial cable, whereas DVI and HDMI video signals only go about 20 feet. SDI is able to go much further because it uses a coaxial cable.
Video is transmitted at very high data rates. Because video is just made up of 1’s and 0’s, voltage on and off, cables emit electromagnetic interference. A coaxial cable is designed to absorb the electromagnetic interference that radiates off of the signal pin in the middle of the cable. HDMI and DVI, using differential signaling, prevents them from using high current or voltage in their transmission. To go a longer distance requires more electrical current, and if more current were used on a DVI or HDMI cable, it would spit out harmful interference that would affect the cables and other electronic components near it. Because of this, DVI and HDMI cables have fairly short working distances when compared to SDI.