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What Is a Serial Digital Interface?

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  • Written By: Solomon Lander
  • Edited By: Susan Barwick
  • Last Modified Date: 03 September 2016
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Serial digital interface (SDI) is a video interface typically used in professional applications. It uses standard coaxial video cables with professional Bayonet Neill-Concelman (BNC) connectors to carry video that is encoded as a digital data stream. Since serial digital interface cables originally lacked the capacity to carry a full high definition (HD) video signal, the SDI standard has been expanded to include high-bit rate HD-SDI cables.

In its most basic standard form, referred to as SD-SDI, a serial digital interface carries 270 megabits per second (Mbits/sec) of uncompressed digital video data. This is enough to transmit a 480-line signal at 60 interlaced frames per second, which corresponds to the North American National Television System Committee (NTSC) standard resolution. It can also carry a 576-line signal at 50 interlaced frames per second, which corresponds to the European phase alternating line (PAL) standard. These interlaced standard definition NTSC and PAL signals contained 30 or 25 full frames, broken into halves, per second.

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As television resolutions have grown, the Society of Motion Picture and Television Engineers (SMPTE) has upgraded the serial digital interface standard to allow for higher data bandwidths. While ED-SDI, which stands for enhanced definition SDI, enabled the transmission of 480- or 576-line signals consisting of 60 or 50 full frames per second, this standard proved inadequate for high definition (HD) video. HD-SDI went through multiple forms to reach the third generation 3G-SDI standard which builds on the dual-link HD-SDI standard to allow for up to three gigabits per second of data transfer, more than enough to carry a full high-definition video signal at 1,080 lines of resolution with up to 60 full frames per second.

The serial digital interface has a number of benefits over consumer audiovisual connectors. SD-SDI allows for cable runs of up to 984 feet (300 meters) and even the high-bitrate high-definition serial digital interface cables operate well with 328-foot (100-meter) cable runs, much longer than what is reasonably possible with consumer standards like HDMI or component video. SDI also transmits an uncompressed digital video signal without any of the encryption and security that is built into the HDMI standard. It also uses standard 75-ohm coaxial cables and secure-fitting BNC connectors, increasing the likelihood that connections will remain tight, reducing both the risk of cables falling out and the risk of noise interfering with the digital signal.

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