SDI

Serial Digital Interface.  A serial link standardized by ITU-R BT.656 and the Society of Motion Picture and Television Engineers (SMPTE).  SDI transmits uncompressed digital video over 75-ohm coaxial cable within studios, and is seen on most professional video infrastructure equipment.

The first revision of the standard, SMPTE 259M, was defined to carry digital representation of analog video such as NTSC and PAL over a serial interface and is more popularly known as standard-definition SD-SDI.  The data rate required to transmit SD SDI is 270 Mbps.

With the advent of high-definition (HD) video standards such as 1080i and 720p, the interface was scaled to handle higher data rates of 1.485 Gbps.  The 1.485-Gbps serial interface is commonly called the HD-SDI interface and is defined by SMPTE 292M, using the same 75-ohm coaxial cable.  Studios and other video production facilities have invested heavily on the hardware infrastructure for coaxial cable and have a vested interest in extending the life of their infrastructure.

Fortunately, SMPTE has ratified a new standard called SMPTE 424M that doubles the SDI data rates to 2.97 Gbps using the same 75-ohm coaxial cable.  This new standard, also called 3-Gbps or 3G-SDI, enables higher resolution of picture quality required for 1080p and digital cinema.