HDMI - Testing for interoperability
17 September 2008
High-definition multimedia interface (HDMI) is a digital standard that delivers superior quality video and audio content that does not require a conversion from analogue to digital signals
The current standard, HDMI 1.3B, is rapidly becoming the interface of choice for devices such as highdefinition televisions, set-top-boxes, DVD and Blu-ray players, video games and personal computers.
The interface creates sharper digital, defines contrast details such as text and speeds up transmission rates. In addition, HDMI features two-way communication between a video source and a device, creating consumer ease-of use by decreasing complex technology processes and reducing the cost of having multiple cables. HDMI 1.3B supports 8bit, 10bit, 12bit and 16bit color depths (previous specifications supported 8bit depths), and also has a built-in foundation that will enable future versions of HDMI to reach higher speeds.
HDMI supports standard, enhanced, or high-definition video, plus multi-channel digital audio on a single cable. It is independent of the various DTV standards such as ATSC, DVB (-T,-S,-C), as these are encapsulations of the MPEG data streams, which are passed to a decoder, and sent to the output as
uncompressed video data, which can be highdefinition.
The video data is then encoded with a ‘TMDS’ (transition minimised differential signalling), for digital transmission over HDMI. HDMI also includes support for eight-channel uncompressed digital audio.
The standard Type A HDMI connector has 19 pins. The higher resolution Type B with 29 pins has been
defined to support resolutions higher than 1080p, although it is not yet in common use. Type A HDMI is
backward-compatible with the single-link DVI (digital visual interface) used in modern computer monitors and graphics cards. This means that a DVI source can drive an HDMI monitor, or vice versa, by means of a suitable adapter or cable, but the audio and remote control features of HDMI will not be available.
According to In-Stat, nearly 200million consumer devices featuring HDMI are expected to ship in 2008, with an installed base of nearly 1 billion products by 2010. HDMI was developed when a group of leading
technology companies started a consortium, HDMI.org, to address the growing needs of the consumer
electronics industry to conduct physical, data link and protocol test layer testing before delivering products.
Interoperability test
Flawless interoperability between consumer devices designed by hundreds of manufacturers is a key requirement for the success of the HDMI standard. A key concern is the physical layer signal integrity
guaranteeing that digital content is driven from one instrument, like a Blu-ray player or a receiver, via a cable to a receiving device such as a television. Therefore, testing of the devices is necessary for companies to
help meet specification. Test standards are defined in the HDMI CTS (compliance test specification) set by the ATCs (authorised test centres), which are managed by HDMI.org, and conduct compliance testing for consumer devices.
HDMI characterisation can be segmented into four areas: source testing, sink testing, media physical layer evaluation, and protocol test. In each of these areas, the HDMI CTS has recommended procedures, methods and equipment to perform these tests. In many cases, there are multiple methods, tools or processes to get the job done.
There are many tests stipulated by the CTS for HDMI sources. These include measuring standard waveform parameters such as rise times, over and undershoot, but also eye diagrams and jitter analysis where the clock recovery method is stipulated by the HDMI standard. Further, new specified filtering algorithms require equalising the cable and eye and must be performed to correlate receiver performance.
Whether troubleshooting, capturing contiguous waveforms, ensuring correct operation, or providing compliance, a realtime oscilloscope with active differential probes and test point access fixtures are critical. Noise floor, jitter noise floor, probe accuracy and test fixture quality all affect measurement quality. Poor noise performance in the oscilloscope may deceive the test by indicating a lesser level is setup than really is, thereby overestimating receiver performance.
Sink test using a pattern generator.
TMDS min/max differential swing tolerance, intra pair skew, and jitter tolerance are some of the key HDMI sink tests. They are elementary to ensure interoperability.
HDMI uses three data channels D0, D1 and D2 to cover the three colours green, red, blue and a fourth channel for the forwarded clock. The clock runs at 1/10 of the data rate.
A TMDS generator therefore requires to have four synchronised channels and the capability to insert precisely specified jitter to test the sink for jitter tolerance and a video frame generator with powerful sequencer capabilities to produce the required test pattern.
In the example shown in figure 4, a 740 x 480 p video frame is generated and repeated infinitely. The software structures the video data in blocks for efficient use of the TMDS generator memory. It creates the proper bit flow by providing the bit flow control.
Leading semiconductor suppliers confirm that thorough, in-depth HDMI characterisation and margin test requires test modes to modulate jitter for clock and data in a synchronous and asynchronous test modes.
The TMDS signal generator needs to provide two individual clock groups to allow for freely adjustable nonsynchronous clock versus data jitter modulation capabilities.
Calibration
Obviously the inserted jitter needs to be calibrated. The calibration software uses the latest software tools and the Microsoft.NET environment. The C# language avoids overhead which, for example, graphical
development environments usually impose. The calibration time which competitive solutions require are
often reduced by half. The same applies to the test execution.
Considering the large number of test and the many test points per test, test automation software is needed for efficient compliance test.
Automated HDMI compliance and in-depth characterisation measurements are enabled by the Agilent N5990A Test Automation Software Platform. This software configures and controls instruments, such as the
TMDS signal generator for the sink tests or the oscilloscope for the source tests, for the selected video format and colour depth. System calibration is configured, too. System calibration ensures calibrated
amounts of jitter are applied to stress the device under test.
HDMI compliance test and characterisation used to be challenging but with the proper test solutions, comprising hardware, software and accessories, HDMI test can now be run reliably and convenient.
KURT HELLINGA is HDMI program manager, Digital Test Division, Agilent Technologies
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