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Cyclic redundancy check in video applications

02 Oct 2013  | Michael Corrigan, Joe Triggs

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Modern video signal chains can be comprised of a host of different devices. For example, the bill of materials of a Audio Video Receiver (AVR) can include a HDMI buffers, a HDMI mux, a HDMI and analogue video receiver, a HDMI transmitter and a video signal processors integrating scaling, de-interlacing and on-screen display functions. To add further complication, these devices can also often be sourced from a broad range of semiconductor vendors.

Developing a reliable video signal chain incorporating all of these devices, supporting video formats with such high data rates, is becoming a significant challenge for video product design and manufacturers. Cable quality, power supply design, signal integrity, PCB quality, and silicon settings need to be at their absolute optimum to successfully support such video formats. But how can a video product design and manufacturer easily evaluate the impact of the tweaks to any of the previously mentioned system elements?

Cyclic redundancy checks
The Cyclic Redundancy Check (CRC) is a redundancy check which was invented by W. Wesley Patterson in 1961.1 It can be employed to detect errors in digital data and is used primarily in data transmission, e.g., a 32bit CRC is employed in the transmission of data over Ethernet. Limitations of the CRC are that a CRC can only detect errors in digital data; it cannot correct them once detected; this capability is confined to more complicated algorithms such as the Error Correcting Code (ECC) or the Forward Error Correction (FEC); and that a CRC cannot identify the number of errors in the received data.

Many different CRC implementations exist but the same basic premise persists; the data transmitter calculates and appends a number of check bits (often referred to as a checksum) to the data before it is transmitted. This is often implemented by dividing the data to be transmitted by a fixed binary number; the remainder of the division then forms the checksum. The receiver can determine whether or not the check bits agree with the data using a reverse of the transmitter side calculation. If the checksum does not agree with the data received, the receiver can conclude that an error occurred in the data transmission and request a retransmission of the data.

A video signal chain does not mimic the data transmitter and receiver pair outlined above; it is unidirectional and so it is not feasible for a video display (e.g. television) to request a video source (e.g. a Bluray player) to re-transmit an incorrectly received data frame. To account for this asymmetry, a CRC must be implemented in a slightly different manner to suit a video signal chain. The obvious location in the video signal chain to perform the analysis is in the video receiver given the limitation already outlined. The video receiver can apply a CRC to subsequent frames of incoming video data with the only caveat being that the incoming video data must be static in its content, e.g., a SMPTE video test pattern or DVD player menu screen.

The CRC is constructed using the known polynomial (e.g. x16 + x12 + x5 + 1) as the divisor, the video data for the selected frame or number of frames as the numerator and the remainder as the means of testing whether the video data has changed. The known polynomial never changes; If the incoming video doesn't change (i.e., a static pattern with no bit errors), then the remainder should always be constant (equation 1).

Equation 1: Video CRC calculation.

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