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Serial Digital Interface

From Academic Kids

Serial Digital Interface (SDI), standardized in ITU-R 656, is a digitized video format used for broadcast grade video. It typically uses 75 Ohm BNC coaxial cables (which makes it easily upgradeable from analog video setups, which use the same cables). Using equalisation at the receiver, it is possible to send SDI over 300 metres, but shorter lengths are preferred.

Uncompressed digital component signals are transmitted. The SDI signal is self-synchronizing, uses 8 bit or 10 bit data words, and has a data rate of 270 Mbit/s.

The SDI signal starts with a Timing Reference Signal (TRS) which is a 4 word sequence which contains the unique bit pattern 3FF, 000, 000. The last word is called the Timing Reference Word (TRW), which contains information such as start/end of active video, and the current field. The last digits of the last word are a Hamming Code to protect the TRW. A Hamming Code is a error recovery code, meaning that if any of the bits of the TRW are corrupted, the Hamming code can be used to detect the error and recover the data.

The order of the data samples sent down the line is Cb, Y, Cr, Y, and 720 Luminance samples are taken across a line of video data, with 360 Cb and 360 Cr samples (also known as YUV 4:2:2). These samples can be at either 8 or 10 bit depth. See Component Video for definitions of Cb & Cr.

The luminance samples may take any value between 64 (black) and 940 (white), and the Cb and Cr samples may take values between 64 and 960. Since the Cb & Cr samples can take positive or negative values, the zero value is 448. These values are true for 10-bit SDI, for 8-bit, simply divide the values by 4.

A SDI signal may also contain up to sixteen (8 AES3 channels) embedded 48kHz, 24bit audio channels along with the video. These are placed in the (horizontal) blanking periods, when the SDI signal carries nothing useful, since the receiver generates its own blanking signals from the TRS.

There is an expanded specification called SDTI (Serial Digital Transport Interface), which allows compressed (i.e. DV, MPEG and others) video streams to be transported over a SDI line. This allows for multiple video streams in one cable or faster-than-realtime (2x, 4x,...) video transmission.

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