Using National Instruments for Efficient Analog Video Test
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Overview
As electronics technology continues to develop, improvements in both analog and digital video are providing consumers with higher quality pictures. Although more development is occurring on digital video interfaces, there are still many devices that include analog video interfaces, and these devices still need to be tested. Analog video encompasses a wide range of signal formats from Composite (PAL and NTSC) video to high-resolution Component HDTV. While normally associated with television broadcasting, analog video products also include CCD video cameras, video recorders, computer graphics cards, DVD players, and set-top boxes, or any equipment that connects to an analog television set or video monitor.
This paper examines how you can perform functional tests on products that generate analog video signals while reducing your test time and cost. Discover how you can integrate audio and digital video testing for HDMI or DVI within the same platform to gain further efficiencies. Finally, see an example of a complete video test solution and view a comprehensive demonstration. You can also read the “Fundamentals of Analog Video” white paper to gain a deeper understanding of analog video signals before learning how to test those signals in this paper.
Table of Contents
Analog Video Functional Test
Functional testing on a product focuses on shorter test times to lower test cost per unit. These tests are 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 an analog video product can be broken into two parts: signal acquisition and video quality measurements. This paper examines these pieces individually and shows how the PXI platform and modular instrumentation from National Instruments offer benefits for both validation and manufacturing test of analog 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.
Functional Test - Signal Acquisition
The first part of analog video test is acquisition and extraction of the desired information from the Composite, S-Video, or Component video signal that you are testing. There are several important aspects of the acquisition hardware that you must consider when testing and acquiring a video signal. The NI 5122 100 MS/s, 14-bit digitizer includes features such as video triggering, input termination, and DC restoration to help with the testing of video signals.
Video Triggering
Acquisition of video signals, with the intention of locating specific line numbers for measurement, requires a compatible hardware sync separator and trigger capable of synchronizing to the requested field (odd or even) or frame, and the requested line number. Acquisition should then start prior to the horizontal sync pulse on the requested line. A system used for testing video should have the capability to synchronize to interlaced and progressive video signals containing bi-level and tri-level sync pulses. Sync information is sometimes provided on separate channels, as with RGBHV signals, so test equipment must also have the flexibility to accommodate Composite and separate syncs in order to be able to analyze all analog formats.
Input Termination
Since the characteristic impedance of all analog video signals is 75 Ohms, the input impedance to video test equipment must provide the same impedance. If a precise 75 Ohm input impedance cannot be provided (as is the case with most digital oscilloscopes), then a high impedance input can be used in conjunction with a 75 Ohm terminator. The terminator often takes the form of a 75 Ohm BNC end-terminator fitted to a BNC T-piece, but an even better solution is a short-bodied BNC feed-through type.
DC Restoration
Analog video signals are defined to have a nominal luminance voltage range of 1 Vpk-pk, but the DC offset of the signal can range over several volts, depending on the grounding in the source equipment. The level of floating is typically worse in consumer products than in professional products. It is common for video generation equipment to have the sync signal minimum at zero volts, so that their digital-to-analog converter (DAC) circuitry only needs a positive supply rail. In HDTV signals, the DC offset should be within ±1 V.
Luminance signal amplitudes are typically measured relative to the blanking level, and though it is not essential, it is often useful for display purposes to restore the blanking level in a floating video signal to its nominal level of 0 V. This process is called DC restoration, or sometimes DC clamping. While usually performed in hardware, it can also be done in software if the input range of the digitizer is not exceeded. If the input signal does fall outside the range of digitizer, unwanted distortion of the signal can occur due to clipping, which can lead to incorrect measurements of the blanking level variations.
Digitization
Analog video signals must be digitized prior to sending the data to software for analysis. Prior to sampling, the video signal must be low-pass filtered to prevent aliasing and to remove unwanted out-of-band signals (those not in the signal of interest). In acquiring the video signal, it is generally recommended to sample at a rate greater than twice the analog bandwidth, and with at least 10 bits of resolution (over the 1 Vpk-pk range). However, the full dynamic range of a digitizer usually cannot be utilized, due to DC offsets or a wider signal range. In order to compensate for this, a higher resolution (12- or 14-bit) should be used. For example, a 14-bit digitizer with the input range configured to 2 Vpk-pk will have a least significant bit (LSB) resolution of 0.12 mV, comfortably exceeding the requirements of resolving video signals to within 7.14 mV, or one IRE (Institute of Radio Engineers) unit.
Oversampling the signal will significantly improve accuracy in the measurement of signal transitions and high-frequency components. The full benefits of oversampling can be achieved for Composite and SDTV signals with a sample rate of 50 MS/s, while 100 MS/s is sufficient for HDTV. However, oversampling also necessitates deep onboard memory for the digitizer to capture a sufficient sample of the video signal. For example, a 1920x1080p 23.98 Hz HDTV signal, which has a frame interval of 41.7 ms will require approximately 8 MB of memory per channel while acquiring at 100 MS/s to store 1 frame. If multi-channel signals are being acquired, memory needs will scale accordingly.
Figure 1 - The PXIe-5122 100 MS/s, 14-bit digitizer (module in slot 3 with the BNC inputs) can be used to acquire Composite, S-Video, or Component Video data for frame analysis. In addition, because it leverages the high-speed PCI Express bus, video data can be streamed to NI RAID hard disks for minutes or hours for playback or further analysis.
Functional Test - Video Quality Measurements
Once you have acquired the analog 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 K-factor, multiburst, and 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 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 NI TestStand software 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.
Video Test Challenges
Hopefully, you now have a high-level perspective of the requirements for Composite, S-Video, and Component Video 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). Additional data such as closed captioning can be transmitted outside the active video region. 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 often overlooked; 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.
NI VideoMASTER - a Complete Video Test Solution
The NI VideoMASTER analog video analysis suite is built on the PXI platform. It uses the PXIe-5122 100 MS/s, 14-bit digitizer for acquisition of Composite, S-Video or Component Video signals. 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 architecture of PXI allows you to test not only analog and digital video, but audio as well in the same platform. You can even add video pattern generation to a test system
NI VideoMASTER lowers the cost of both analog and digital video test. In addition to a lower initial investment than many video test solutions, NI VideoMASTER 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; since NI VideoMASTER is built on the PXI platform, you can expand your test capabilities to include audio in addition to analog and digital video analysis. You can even add video generation functionality if the application requires it. To view a comprehensive demonstration of the NI VideoMASTER analog and digital video analysis platform, watch the “Analog and Digital Video Measurements” webcast.
Additional Resources
Analog and Digital Video Test Portal
Fundamentals of Analog Video white paper
Full Demonstration of the VideoMASTER Analog and Digital Video Test Platform
Video Test Using LabVIEW and PXI Article
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