Quick guide about SDI video
A brief explanation on SDI digital video transmission
What is SDI ?
SDI stands for Serial Digital Interface and is a transmission protocol for sending uncompressed digital video signals, using a 75 Ohm coaxial cable. The first SDI standard (SD-SDI) came into life in 1989. Since then, several additional standards have been introduced: HD-SDI, 3G-SDI, 6G-SDI and 12G-SDI.
What cables are used for SDI transmission ?
The cables used for SDI transmission are 75 Ohm coaxial cables, rated for digital video signals. These come in a wide variety of diameters, conductor cores, and shielding. Notice that any digital coax will have at least double shielding
Installation cables have a solid copper wire, flexible cables are made with stranded copper cores, meaning multiple small copper wires, improving on the flexibility of the cable. The disadvantage is that signals can be sent less far as the internal resistance between the wires causes more attenuation of the video signal. A solid copper wire coax will have a longer maximum transmission distance as this attenuation is significantly lower.
For installation cables such as the Belden 1505A, we have a foil shield around the dielectric (the white insulator around the conductor) and a braided copper shield on top of that. These cables tend to be less flexible and are suited for fixed or semi-mobile applications.
Flexible coax cables (such as the Sommer Cable Vector+ 1.2L/4.8) have a double braided shield in order to improve flexibility of the cable. Very thin solid core coax cables (like the Belden 1855A or Belden 4855R) can also be used as patch cords in mobile environments.
Sometimes, coax cables need to be bundled for sending multiple signals on one run (for example Quad 3G for transmitting 12G-SDI over four separate lines). This can be done with multi-channel coax cables. These are comprised of multiple (often 2 to 10) separate coaxial cables. Although the internal coaxes have solid copper conductor, the fact that they are internally twisted around each other gives enough flexibility to be used on cable drum or in any mobile environment.
What connectors are used for SDI ?
The most common used connectors used for SDI coax cables are BNC connectors. These are bayonet style connectors, locked by twisting them into the counterpart. You will find these for 3G, 6G and 12G transmission.
Another connector is the HD BNC (also known as micro BNC) which has the exact same construction as the classic BNC but is much smaller, which gives users the opportunity to place a greater number of connections on the same amount of space.
We also have what's called the DIN connectors, such as the DIN 1.0/2.3: this type has about the same size of the HD BNC but features a sligtly different design. The DIN connector has a maximum frequency of 4Ghz.
All of these connectors are a well established standard in the broadcasting and entertainment industry.
What's the difference between 1.5G, 3G, 6G and 12G ?
These are the bandwidths used (frequencies) for sending signals ranging from 720p (HD-SDI) all the way up to 2160p or 4K UHD (12G-SDI) Most digital coax cables you will find can handle both 3G and 6G transmission and even 12G-SDI. Of course, a small diameter 3G/6G coax may be able to handle 12G but be it on (much) shorter distances than it would on 1,5G, 3G or 6G transmission.
|Data Rate||SMPTE Standard||Maximum Resolution||Maximum Refresh Rate||1,5 Gbps||ST 292M||720p / 1080i||60 Hz|
|3 Gbps||ST 424M||1080p||60 Hz|
|6 Gbps||ST 2081||2160p||30 Hz|
|12 Gbps||ST 2082||2160p||60 Hz|