Rethinking the Development of DisplayPort

The battle between High-Definition Multimedia Interface (HDMI) and DisplayPort has quickly died down, with HDMI quickly and undisputedly becoming the preferred protocol for the next generation of audio/video connection interfaces, becoming a standard feature of modern TVs, media players, game consoles and cameras.
HDMI can support multi-channel digital audio and any TV or personal computer (PC) video format, including standard, enhanced and high-definition formats, on a single cable. This flexibility has led to its rapid adoption around the world, and it is expected to become an efficient and universal connection interface in the next few years.

In the face of this development momentum, why are there still more than 180 computer and consumer electronics companies actively supporting another audio/video interface standard DisplayPort? Among these companies are industry giants such as HP, Dell, Intel and AMD, and in the past four years, this number has been growing at an annual rate of more than 20%; in the face of the mainstream trend of HDMI, the support and development of DisplayPort continues to flourish.

There are three possibilities: the "war" is not over yet; there was no so-called competition before; or each standard just needs to find its place.

Different Models, Different Applications
Many people now realize that while both DisplayPort and HDMI offer advanced audio/video interfaces, there are key differences between them that drive them in different directions.

HDMI has been widely used in high-definition television (HDTV) applications due to its ability to support multiple video and audio formats, and in turn, countless devices that connect to HDTVs also support HDMI. However, HDMI is not a panacea for all audio/video connections. Engineers are carefully examining its use in PC technology, especially in high-performance PC displays, based on three obvious factors: cost, bandwidth, and internal connections.
The licensing fees and royalties associated with HDMI have made it prohibitive for many low-cost, high-volume technologies such as PC displays. HDMI's external clock limits its bandwidth and performance scalability. Its focus is on connections between consumer electronic devices, and it lacks internal chip-to-chip connections that can reduce design complexity and cost.
As displays increasingly move to higher-performance flat-panel and microelectronic technologies, engineers need an economical, scalable, open, industry-standard digital interface that can scale in performance.

Understanding DisplayPort
The DisplayPort specification defines a scalable digital display interface with optional audio protection and content protection features for a wide range of commercial, enterprise, and consumer applications. This interface is designed to support both internal chip-to-chip connections and digital display connections between external devices.
As shown in Figure 1, the DisplayPort interface consists of a main link for transmitting high-bandwidth data, an auxiliary channel for link and device management, and a hot-plug detect line for receiving device-initiated interrupt requests.

Figure 1 DisplayPort link overview

DisplayPort Validation and Conformance Testing
While DisplayPort offers performance, cost and design advantages, it also brings with it many device specification conformance requirements, including explicit, multifaceted transmitter (source), receiver (sink) and cable testing.

Fortunately, DisplayPort has many similarities with established serial data technologies such as PCI-Express, Serial ATA, etc. Therefore, engineers in the computer electronics industry who deal with common serial data standards will be familiar with the DisplayPort testing and compliance testing process.

However, each subsection within the DisplayPort interface presents its own unique testing challenges. Because DisplayPort uses a dynamic operating model, hundreds of test conditions must be examined. Like many other standards, DisplayPort has published its own compliance test standard (CTS) requirements.

Leading test and measurement providers have developed tools and methodologies for DisplayPort, including detailed conformance test procedures (MOIs) that provide validation for DisplayPort sources, cables and sinks using specific test equipment under a variety of test conditions.

DisplayPort has 17 source tests, 12 of which require compliance testing, including amplitude, data rate, offset, spread spectrum clock, eye diagram testing, etc. The challenge facing developers is that they must examine a large number of unique operating conditions in the DisplayPort source test, a total of 28.

DisplayPort has seven cable tests, five of which require compliance testing, including skew, noise, impedance, insertion loss, and return loss, among other measurements. Developers must be diligent in ensuring measurement accuracy, especially when using de-embedding techniques. In addition, depending on the test equipment used, they must also determine if the instrument affects the measurement results.

DisplayPort has only one receiver test, which focuses on jitter tolerance, but this test is particularly tricky. For a "worst case" but still compliant signal, developers must confirm that the receiver can still recover the data with an acceptable bit error rate (BER).

They must create seven jitter curves—including random jitter, sinusoidal jitter, and intersymbol interference (ISI) patterns—to characterize jitter and test the tolerance of the device under test (DUT), as shown in Table 1. An example of the jitter pattern for 2MHz jitter curve calibration is shown in Figure 2.

Figure 2: Example of jitter pattern for 2MHz jitter curve calibration

Other high-performance display standards typically perform receiver testing using jittered patterns like DisplayPort, but require the user to visually check for pixel errors. DisplayPort, on the other hand, uses a built-in error detector to facilitate a more automated testing process.
As we all know, DisplayPort testing itself is not difficult or complicated. However, it is time-consuming and therefore expensive. Manually testing multiple test states in multiple data paths is tedious. This results in non-repeatable testing, frequent human errors, and rapid escalation of development costs.

By working with select test and measurement vendors, developers can significantly improve productivity, shorten and reduce the time and cost of DisplayPort compliance testing. These vendors can provide detailed step-by-step MOI and DisplayPort specific test software to eliminate manual measurements and automatically analyze DisplayPort test data. Once the DUT is set up, these software tools can automatically perform all DisplayPort test procedures, including data processing, measurement and analysis.

In addition, when using these software tools, all test data is automatically saved and archived. This means that even without the DUT, tests can be easily rerun, improving the repeatability of debug, troubleshooting and compliance test procedures. Frequently used detailed test reports include graphs, screen shots and pass/fail results, improving reporting capabilities.