Oscilloscopes support the video craze
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2008 2:00:00 AM
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Flat-panel TVs now appear everywhere, from homes to supermarkets to airports. Appliances may soon have LCDs that replace knobs and buttons. All of this equipment needs a data interface, and the high-definition multimedia interface (HDMI) port is fast becoming the digital video interface of choice.
Market-research firm In-Stat reports that millions of HDMI ports have already shipped, and the number of shipments is expected to increase each year (figure). Because these ports are used in products such as TVs, DVD players, Blu-ray players, set-top boxes, and video games, interoperability among manufacturers is an issue. Consequently, the HDMI Consortium (www.hdmi.org) has developed compliance test specifications designed to minimize interoperability problems, and oscilloscope makers Agilent Technologies, LeCroy, and Tektronix have developed hardware and software to automate HDMI physical-layer compliance testing.
 Shipment of goods with HDMI ports will continue to grow. Source: In-Stat. |
HDMI signals now run at speeds up to 3.4 Gbps based on HDMI specification 1.3, which was released in June 2006. The HDMI Compliance Test Specification (CTS) version 1.3b specifies test equipment and physical-layer performance limits for sources (transmitters), sinks (receivers), and cables. “Because HDMI specification 1.3 increased the data rate, the CTS needed to specify higher bandwidth oscilloscopes for compliance testing,” said Faride Akretch, marketing manager for high-speed serial applications at Tektronix.
Because HDMI is a high-speed digital serial port, it falls victim to the usual set of signal-integrity problems: timing jitter, rise and fall time, noise, and signal loss. Source testing includes clock jitter, duty cycle, overshoot, rise and fall time, eye-mask tests, and inter-pair skew. For eye-diagram tests, you need to capture at least 2.6 million unit intervals when testing high-resolution devices.
Sink tests require a signal source such as a pattern generator and a jitter source. You must test for jitter tolerance, intra-pair skew, and differential voltage swing. Cable tests require a sampling oscilloscope with a time-domain reflectometer to measure S-parameters and cable loss.
Just because the HDMI specification version 1.3 specifies a data rate of 3.4 Gbps—double the rate specified by HDMI 1.2—doesn’t mean that a device actually pushes data at that rate. Because most products with HDMI ports are consumer devices, cost is always an issue. HDMI 1.3 was a technology disruption because of its jump in data rate, said Brian Fetz, high-performance oscilloscope product manager at Agilent Technologies. “Engineers must design with inexpensive materials. They are trying to push data across standard FR4 PCB material, and the highest data rate running today is about 2.6 Gbps.”
Even with compliance tests and approved test equipment, HDMI products may not interoperate. “Some interoperability problems occur even if a product is in compliance,” said Akretch. “The specification writers may have not foreseen some behavior, and they may have to adapt the specification.” Interoperability cases between compliant products, are rare, he added.
| For Further Information |
| “HDMI/DVI,” Tektronix, www.tek.com/Measurement/applications/serial_data/hdmidvi.html. |
| “HDMI Licensing Knowledge Base,” www.hdmi.org/learningcenter/kb.aspx. |
| For inter-skew and intra-skew waveforms, see “Protecting the HDMI interface,” by Jeff Donnihoo, Video Imaging Design Line, July 16, 2005. www.videsignline.com/165702853?pgno=2. |
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