1. HDMI and DVI Overview
HDMI and DVI are used to transmit uncompressed digital video data. You can use the interfaces to connect a source (set-top box, high-definition DVD or Blu-ray Disc player, PC or laptop, video game console, and so on) to a compatible digital video display (digital TV, monitor, and so on). Developed before HDMI, DVI is used primarily as an interface to LCD computer displays and digital projectors. DVI uses a digital protocol that transmits binary video data in high-speed, serial low-voltage differential signaling (LVDS) streams corresponding to the desired red, green, and blue color values for the pixels on a display. While HDMI uses the same digital protocol as DVI for video transmission (HDMI is backward-compatible with single-link DVI), the HDMI specification adds embedded audio and digital encryption (High-bandwidth Digital Content Protection, or HDCP) to the video stream while using a single, small connector.
2. Physical Layer Test versus Functional Test
When considering HDMI or DVI testing, you need to distinguish between testing the physical characteristics of the signal during transmitter design and testing the functionality of the digital video output. For physical layer testing or for parametric test – tests used primarily during the initial design of a product – measurements like bit error rate (BER), rise/fall time, jitter, impedance, and common-mode rejection ensure that the timing and performance meet the protocol specifications. In contrast to exhaustive physical layer testing to verify the quality of a design, functional testing on a product focuses on shorter test times to lower test cost per unit.
Functional test is used during product validation or production to test the video signal for correct color and brightness levels as well as to verify the functionality of the signal processing used for scaling to different resolutions, improving the picture, reducing noise, or other video processing. In design validation, the need to run through numerous measurements means efficiency in automation is very important, while in manufacturing test, where measurements can be taken on thousands of units per day, shorter test times mean lower test costs per unit. Functional test of a digital video product can be broken into two parts: signal acquisition and video quality measurements. This paper examines each of these pieces individually and shows how the PXI platform and modular instrumentation from National Instruments offer benefits for both validation and manufacturing test of digital video products. NI provides efficient hardware for video acquisition and powerful software that you can use to develop video analysis algorithms and high-level test management.
3. Digital Video Functional Test - Signal Acquisition
The first part of digital video test is acquisition and extraction of the desired information from the DVI or HDMI signal that you are testing. Because the video signal you want to test is a high-speed serial LVDS stream, you must first develop hardware to convert the high-speed serial data to more useful RGB (Red, Green, Blue) or YCbCr parallel data. Then you can acquire and pass this digital data to software for analysis. The resolution of the signal you want to analyze determines the clock rate you use to capture the parallel data. For example, while you can acquire a 720p or 1080i digital video signal with a clock rate of 74.25 MHz, capturing 1080p digital video requires a much faster clock rate of 162 MHz. One implementation of this conversion architecture is the PXI SPX-I DVI/HDMI converter and the NI PXI-6542 high-speed digital interface, which together compose the acquisition portion of the NI Digital Video Analyzer, a full validation/production video test solution discussed in detail at the end of this paper. The PXI SPX-I module, shown in Figure 1, captures serial video data through an HDMI connector, converts the stream to parallel color data, and then sends that signal to a digital interface that you can cable to a module such as the PXI-6542 for acquisition of the now parallel color data. Because HDMI is backward-compatible with single-link DVI, you can use the same modules for testing a DVI output with the addition of an inexpensive DVI-to-HDMI converter. One other challenge with testing HDMI signals is the HDCP encryption that is required for HDMI-compliant devices. HDCP was developed as a way to make it difficult for a hacker to ever have access to a raw video stream of copyrighted media, but it also introduces additional testing challenges. One test option requires the manufacturer to turn off the HDCP encoding on its devices during test, but a better solution is for a test system to include the necessary licensed decoding IP to create a receiver that most closely resembles interaction with the released product.
Figure 1. The PXI SPX-I module (second module from the right) acquires video data through the HDMI connector, converts the serial data stream to parallel color data, and then outputs that signal, which can be input to a module such as the PXI-6542 100 MHz digital interface (third module from the right) for acquisition of the color data.
4. Digital Video Functional Test - Video Quality Measurements
Once you have acquired the digital video data, you must develop software algorithms to analyze the video signal quality, ensuring the product is sending the correct brightness and color information to be displayed on a TV or other device. Common measurements include amplitude and timing parameters such as color bars, horizontal timing, and signal frequency; distortion measurements such as multiburst or nonlinearity; and noise spectrum analysis, but the required software development and complexity depends on the application. For example, testing in the production phase may require only a few carefully chosen measurements to verify the functionality of the output, but because those measurements could be repeated thousands of times per day, you should take extra care to develop efficient algorithms. On the other hand, video analysis in the validation phase of a product should be exhaustive to ensure thorough testing of the full functionality of a product, which requires ease of automation and may necessitate the development of many complex algorithms.
Further, both types of test can benefit from multiple measurements being computed in parallel, especially as multicore CPUs are becoming more prevalent. The NI LabVIEW graphical programming language makes it easy to interact with hardware, and you can use it to develop algorithms from simple-level measurements on a color bar test to more complex tasks like control channel information decoding or image processing. In addition, you can execute measurements in parallel, and LabVIEW automatically manages threads to maximize the use of multiple cores without any additional coding. Finally, you can use test executive software, such as NI TestStand, to manage test execution and create repeatable measurement sequences using LabVIEW or other coding languages, add quantitative limits to provide pass/fail criteria, store the test data to a file or database, and create comprehensive reports.
5. Video Test Challenges
Hopefully, you now have a high-level perspective of the requirements for HDMI and DVI signal testing at the hardware and software levels, but there is obviously much more depth to these tests than a short paper can cover. For example, consumer electronics devices have many different modes of operation. They can output in SDTV resolution of 480i or 480p all the way to HDTV resolutions of 720p, 1080i, or 1080p. Each of these resolutions can have different frame rates, from 24 to 30 to 60 frames per second (fps). HDMI and DVI product outputs can also include a Display Data Channel (DDC), control channels (CEC), and digital copyright protection (HDCP). Developing efficient software algorithms for video analysis can be challenging, and developing a whole suite of measurements can be time-consuming. Another part of testing that can significantly affect test system development time is if algorithms are well-designed, you can use them in both validation and production test. You can overcome all of these hardware and software challenges when developing test systems, but it is sometimes desirable to have a complete solution that integrates reliable acquisition hardware with powerful video analysis software.
6. NI VideoMASTER - A Complete Video Test Solution
The NI Video Measurement Suite Digital Video Analysis Suite is built on the PXI platform. It uses the PXI SPX-I module for serial-to-parallel conversion of HDMI or DVI signals and the PXI-6542 100 MHz high-speed digital interface for acquisition of the color data. Once you have acquired the data, you can use the configuration-based environment to easily select test steps from a comprehensive library of measurements, or you can create custom measurements for your signal. While you configure measurement steps in LabVIEW, you can create a test sequence using the NI TestStand test executive to manage all of the measurements for the test unit. You can also add quantitative limits to each step to provide pass/fail criteria. In addition, NI TestStand gives you easy access to test data and reporting.
Figure 2. You can use the PXI platform to lower the cost of video test. The modular PXI architecture helps you test not only analog and digital video but also audio in the same platform. You can even add video pattern generation to a test system.
The NI Video Measurement Suite lowers the cost of both analog and digital video test. In addition to a lower initial investment than many video test solutions, the NI Video Measurement Suite leverages the high throughput and low latency of the PXI bus to help reduce test times. For example, the time to acquire a signal from the composite video, component video, and HDMI outputs of a product (set-top box, DVD player, and so on) and perform an entire set of common measurements can be as short as six to eight seconds. Many consumer electronics products include both audio and video outputs; because the NI Video Measurement Suite is built on the PXI platform, you can expand your test capabilities to include analog and digital audio in addition to analog and digital video analysis. You can even add video generation functionality if the application requires it.
7. Additional Resources