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What Is an SDI Output?

SDI stands for serial digital interface, and it is the name of a group of standards used for transmitting professional quality video signals. SDI output from video equipment is of broadcast or high definition quality. There are several standards for various applications, and the output signals typically travel from the equipment outputs to broadcast facilities or recording systems via coaxial cables. Key characteristics of the SDI signals are high bit rates and low data loss.

General Characteristics
- The SDI standards all specify digital peak-to-peak signals of about 800 millivolts at the output, and they use normal, 75 ohm coaxial cables with standard connectors. The standard cables can be up to 300 feet long, and transmission up to 1000 feet is possible with high quality cables. Television stations use SDI to transmit television signals internally within the station in a lossless, uncompressed way. The signal includes noise suppression, data loss recovery and timing code so that it is reliable, high-quality and self-synchronizing.
Standard SDI
- The standard SDI specification accommodates bit transmission rates of 143, 177, 270 and 360 megabits per second. The two lowest rates were used for the older NTSC North American and PAL European television signals with 4:3 aspect ratios. These are now obsolete. The 270 Mbps rate can transmit signals with either 4:3 or 16:9 aspect ratios at a maximum of 720 pixels. The 360 Mbps rate is only for the 16:9 aspect ratio with 960 pixels. These two rates are used for most non-high-definition television signals.
Enhanced SDI
- While the standard SDI specification can transmit the signal for the normal 480-line, 60 Hz North American or 525-line, 50 Hz European television picture, these pictures are interlaced. The signal only transmits every other line each second, and so transmits only 60 or 50 half-pictures per second. To get a higher quality, the enhanced SDI standard doubles the bit rate so the signal can transmit twice the picture information. At 540 Mbps, the SDI output signal can transmit all the lines of each picture every cycle.
High Definition SDI
- The enhanced SDI output signal is not capable of transmitting true high definition or 1080i images. To reach that level of transmission quality, the standard must accommodate a transmission capacity of at least 1485 Mbps. The HD-SDI output gives 1080i high definition television at that bit rate and specifies even higher transmission rates for very high resolution applications, reaching 2970 Mbps, or the 3-gigabit level. The HD-SDI output achieves this latter capacity by connecting two coaxial cables in parallel.

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