CRC testing in video applications: The stages of CRC
[Part 1 discusses using cyclic redundancy checks for video and looks at CRC alternatives.]
The Stages of CRC
CRCs can be employed across many facets of the development phase of a video product to gauge the performance of the video signal chain; during thermal testing; during power supply testing; during the assessment of layout changes, during final software configuration changes; even during cable selection if a cable is to be supplied with the product.
Before a video product can be marketed as HDMI compliant and carry the HDMI logo, the product must undergo a series of stringent test at an officially licensed HDMI approved test centre (ATC). These tests ensure that the product meets all of the requirements set out in the HDMI compliance test specification (released in alliance with the main HDMI specification). One of the toughest tests conducted as part of this suite of testing is an analysis of how robust the video receiver is to jitter on both the clock and data channels.
Meeting the criteria outlined in this test are quite often challenging and video product design and manufacturers frequently send their prototype systems to for expensive pre-compliance checks if they do not have access to the ultra expensive equipment specified in the official tests.
The frame checker in the ADV7850 can be used as a low-cost substitute for early iterations of pre-compliance testing if the specified equipment is not available; it provides an insight into whether the receiver is correctly receiving and decoding the HDMI data (factors which can influence this range from whether the correct configuration writes are being employed, to power supply design).
If the specified equipment is available, the frame checker can still be employed as it provides a definitive insight into whether the receiver is correctly receiving and decoding the data. This level of analysis goes beyond that mandated by the CTS which mandates only visual checking.
HDMI Cable Selection
Many video product design and manufacturers, especially in the professional audio/video market, depend on HDMI cables to route video between system components. An HDMI cable is constructed using 19 conductors; the HDMI specification outlines five different categories of HDMI cable for varying speed grades.
Cables, due to their limited bandwidth, typically introduce a particular type of noise or jitter into the video stream; inter-symbol interference (ISI) jitter. ISI jitter is the interference between current and subsequent symbols. This "blurring" of symbols makes it more difficult for a receiver to decode and interpret the data.
For example, in a video conferencing system, video may be routed around a room from a central console to multiple monitors or projectors via a series of HDMI cables of up to 30 metres in length (see Figure 6). HDMI cables at such lengths however can be a significant component of the system cost with prices ranging from tens of dollars to hundreds of dollars; video product design and manufacturers may choose to evaluate cables from low, medium and high cost suppliers. Cables suppliers can often justify the costs of the cables which they manufacture through the quality of those cables; video product design and manufacturers however must balance the quality and cost of the cables which they choose to supply with their products.
When evaluating such different cables, a CRC test can be employed to great effect; starting with the benchmark of a known good, trusted cable which provides stable system results, an evaluation engineer can compare how cheaper or longer cables impact on the CRC results - gaining an interesting data-point in determining the suitability of such cables.