Serial Digital Interface

SDI is a unidirectional 270 Mbps transport service that is offered by most local telephone companies. It is designed to transport NTSC System M (525 lines and 60 fields per second) digital television signals. Compliant end equipment conforms to SMPTE 259M, which describes 10-bit 4:2:2 component video.

The 270 Mbps data rate is derived from the standard sampling requirements for component video:

Luminance (Y) - 13.5 MHz
Color Difference (B-Y) - 6.75 MHz
Color Difference (R-Y) - 6.75 MHz

The total sampling frequency is 27.0 MHz. Each sample is a 10-bit word, so the serial output is clocked at 10 times the sampling frequency -- 10 bits per Hz gives us a data rate of 270 Mbps.

The SDI service is also compatible with DV-ASI equipment, allowing broadcasters to transport multiprogram MPEG2, and ancillary data streams.

Source: Qwest Technical Publication - Serial Digital Video Service at 270MBIT/S (SDVS-270)

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