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Will DisplayPort get a second chance?

Posted: 26 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:digital interface? DisplayPort? HDMI? validation test?

By Randy White
Tektronix Inc.

The standards "war" between HDMI and DisplayPort was short-lived. HDMI quickly and ubiquitously emerged as the champion of next-generation audio/video connection interfaces, becoming a standard feature of modern TV, media players, gaming consoles and cameras.

On a single cable, HDMI can support multichannel digital audio as well as any TV or PC video format, whether it is standard, enhanced or high-definition. This flexibility spurred its swift global adoption, and HDMI looks poised to become an effective and widespread connection interface for years to come

In light of these developments, why are more than 180 computer and consumer electronics companies actively supporting DisplayPort, another audio/video interface standard? This roster of companies, including industry titans such as Hewlett-Packard, Dell, Intel and Advanced Micro Devices, has grown by more than 20 percent annually for the past four years. And despite HDMI's wave of popularity, DisplayPort support and development continues to gain momentum.

Three possibilities exist for this set of circumstances: the "war" is not over; there was never a competitive struggle in the first place; or perhaps each standard just needed to find its own niche.

Different paradigms, apps
What many are now realizing is while DisplayPort and HDMI provide advanced audio/video interfaces, they possess key differences that are pushing them in somewhat divergent directions.

With its ability to support a variety of video and audio formats, HDMI has been broadly adopted for high-definition TV (HDTV) applications. This adoption has spawned HDMI support in the countless devices that connect to HDTVs. HDMI is not a panacea for all A/V connections, however. Its use in PC technologies!high-performance PC displays, in particular!has increasingly been scrutinized due to three unequivocal factors: cost, bandwidth, and internal connectivity.

The licensing and royalty fees associated with HDMI make it prohibitive for many low-cost, high-volume technologies, such as PC displays. HDMI's external clock limits its bandwidth and performance scalability. And its primary focus on consumer electronics box-to-box connectivity is devoid of internal, chip-to-chip connections that reduce design complexity and cost.

As displays increasingly transition to higher performance flat panel and microelectronic technologies, an affordable, extensible, open industry standard digital interface is needed that can scale in performance as well.

DisplayPort offering
The DisplayPort specification defines a scalable digital display interface with optional audio- and content-protection capability for broad usage within business, enterprise, and consumer applications. The interface is designed to support both internal chip-to-chip and external box-to-box digital display connections.

As shown in Figure 1 the DisplayPort interface consists of a main link for transporting high-bandwidth data, an auxiliary channel for link and device management, and a hot plug detect line for interrupt requests initiated by the sink device. For more information about the DisplayPort interface see the sidebar.

Figure 1: This image shows an overview of a DisplayPort link.

Validation, compliance testing
With the performance, cost and design benefits of DisplayPort come many device specification conformance requirements. These requirements include explicit, multifaceted transmitter (source), receiver (sink), and cable tests.

Fortunately, DisplayPort shares many similarities with entrenched serial data technologies, such as PCIe, serial ATA and the like. DisplayPort test and compliance procedures will therefore be familiar to those in the computer electronics industry who have worked with popular serial data standards.

Nevertheless, each subsection within a DisplayPort interface has its own unique test challenges. And because DisplayPort features a dynamic operating model, hundreds of test conditions must be examined. Like many other standards, test requirements are published in a Compliance Test Standard.

Leading test and measurement providers have responded in kind with tools and methodologies tuned specifically for DisplayPort. These include detailed step-by-step Methods of Implementation (MOI) for DisplayPort source, cable and sink validation under a variety of test conditions using specific test equipment.

There are 17 source tests for DisplayPort, 12 of which are required for compliance. These include amplitude, data rate, skew, spread spectrum clocking, and eye diagram tests among others. The challenge for developers is the sheer number of unique operating conditions that must be investigated during DisplayPort source testing!28 in total.

DisplayPort has seven cable tests, five of which are required for compliance. These include skew, noise, impedance, insertion and return loss, and other measurements. Developers must work diligently to ensure measurement accuracy, especially when employing de-embedding techniques. And depending on the test equipment used, they must determine whether their instrumentation is affecting measurement results.

While there is only a single sink test for DisplayPort, focused on jitter tolerance, it's very involved. Developers must confirm, given the worst possible yet compliant signal, that their receiver will still recover data with an acceptable BER.

They need to create seven types of jitter profiles!including random, sinusoidal, and inter-symbol interference patterns!characterize the jitter and test the tolerance of the device under test (DUT) as shown in Table 1. An example of a calibrated jitter pattern for the 2MHz jitter profile is shown in Figure 2.

Table 1: DisplayPort jitter tolerance requirements for high bit rate.

Figure 2: Jitter tolerance pattern validation showing the stressed eye with specific signal impairments.

First a PRBS7 pattern is used as the compliance pattern. Jitter is added in precise amounts and then the jitter is measured with advanced jitter analysis software tools.") Other high performance display standards typically perform sink testing with jittered patterns like DisplayPort but require the user to visually verify whether the jittered test pattern displays pixel errors. DisplayPort, however, uses a built-in error detector facilitating a more automated test process.

All told, the DisplayPort tests themselves are not difficult or complex. They are, however, time consuming and therefore costly. Manually testing many test states across multiple data lanes is tedious at best. This approach also hinders repeatability, human error is commonplace and development costs can quickly escalate.

By working with select test and measurement vendors, developers can dramatically increase their productivity and reduce the time and cost of DisplayPort compliance testing. These vendors can offer detailed, step-by-step MOIs and DisplayPort-specific test software to eliminate manual measurements and automate the analysis of DisplayPort test data. Once the DUT has been configured, these software tools automate all DisplayPort test procedures, including data processing, measurement, and analysis.

In addition, all test data is automatically saved and archived when using these software tools. This means tests can easily be rerun, even without the presence of the DUT, thereby improving the repeatability of debug, troubleshooting, and compliance procedures. And reporting is improved through common detailed test reports that include graphics, screenshots, and pass/fail results.

While HDMI is well-suited for its application to TVs, DisplayPort will continue to be an attractive option for those seeking to reduce the cost and improve the bandwidth and scalability of A/V interface connections.

Members of the computer electronics industry, in particular, are increasingly adopting this open industry standard that consolidates internal and external display signaling and shares similarities with other widespread serial data technologies.

- Randy White is a serial applications technical marketing manager at Tektronix Inc.

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