You need to test four key regions of an analog video signal: horizontal sync, front porch, back porch, and the active picture region. A few measurements in each of these regions provide a sufficient understanding of the analog signal quality. Figure 1 shows the four key parts to a video signal.
Figure 1. Parts of a Video Signal
1. HSync, VSync, Front Porch, and Back Porch Measurements
Two important parts of any video signal are the vertical sync (VSync) and the horizontal sync (HSync). They are used to ensure that each horizontal line is synchronized (HSync) and each frame begins and ends correctly (VSync). Figure 2 shows a frame with distorted horizontal and vertical syncs.
Figure 2. The photo on the left is a depiction of a normal video signal. The center photo depicts a video signal with a poor vertical sync. Notice that the depiction does not know where one frame ends and the other begins. The photo on the right depicts a video signal with a poor horizontal sync. Notice that the vertical edge of the letters “H” and “L” slant at an angle instead of being synchronized vertically.
The horizontal sync signals the beginning of a new line, so it is important to ensure that elements such as pulse width, amplitude, and rise and fall times are accurate and meet internal or industry standards. The same is true for the vertical sync, which is used to signal the beginning and end of a frame. Because you need to complete these tests before the active picture region test, you can perform them on any video signal.
Used as a reference level to remove any DC offset from the video signal, the back porch contains the color burst. Back porch levels and color burst information such as burst width, delay, and amplitude should be tested to ensure the color is decoded correctly from the video source.
NI Video Measurement Suite (VMS) Horizontal Timing Measurement Group
To implement these tests, the NI Video Measurement Suite (VMS) software was used to take measurements on an NTSC signal containing luma steps. Because the horizontal sync and color burst occur before the active picture region, no specific pattern is necessary to perform these tests. Figure 3 shows the measurement results taken with NI VMS for the horizontal sync, color burst, and blanking intervals.
Figure 3. The generated video pattern (left top) consists of 10 luma steps. The frame was captured and a single line was analyzed (left bottom). The measurement results (right) represent one test line.
By comparing the results obtained to an expected value, you can quickly implement a pass/fail for the timing parameters of the device being tested. These are common tests when evaluating the output of DVD players, Blu-ray players, and set-top boxes.
2. Active Picture Region
The active picture region of a video signal is the part of the video signal that a consumer sees. It also is where most video tests are performed. Video patterns like the one in Figure 4 are often generated because each part of the pattern provides the ability to identify different video defects, so you can test for several defects within a single frame.
Figure 4. Custom video matrices like this one are commonly used in video test. You can choose from several unique lines to make a variety of measurements.
The rest of this tutorial explores various aspects of the active picture region that should be tested to ensure a video signal meets requirements. It covers the following common tests:
- Luminance Levels
- Chrominance Levels
- Frequency Response
- System Noise
- Response Time
- Chrominance Nonlinearity
- Differential Gain and Phase
Each of these tests is discussed in more detail below.
The luminance levels of a video signal display the brightness of the image. Poor luminance levels can result in an output signal that is too dark or too bright, or a fluctuation between the two. One common way to measure luminance levels is to measure a luminance bar test pattern.
The NTC-7 is a common test signal that contains a luminance bar. The NTC-7 video signal is used to test insertion gain, time distortions, luminance-chrominance delays, and differential gain and phase defects. Figure 5 shows the components of an NTC-7 signal.
Figure 5. The components of an NTC-7 signal include the luminance bar (B2), the 2T pulse (B1), and the 12.5T pulse (F). D2 is a modulated staircase.
NI VMS Bar Line Time Measurement Group
To implement the luminance tests, NI VMS was used to measure the NTC-7 video signal. Figure 6 shows the measurement results for the luminance bar being generated.
Figure 6. The top-left picture is a representation of what the consumer sees in the active picture region, and the video signal in the bottom-left picture is one test line of that frame. The pattern tested for luminance levels is the luminance bar. Notice the measurement results for bar rise time, bar width, and bar amplitude on the right.
The chrominance levels of a video signal contain the color of the video image. Poor chrominance levels can result in an output signal that has color saturation or looks black and white. One common way to measure chrominance levels is to use a color bar test pattern. By evaluating each color in the color bar, you can ensure that the chroma signals are performing as expected for various color levels.
NI VMS Color Bar Measurement Group
To implement the chrominance tests, NI VMS was used to measure a color bar video signal. Figure 7 shows the chrominance measurement results for the color bars being generated.
Figure 7. The color bar video pattern consists of yellow, cyan, green, magenta, red, and blue colors. Measuring the amplitude and phase of these colors helps to ensure that the chrominance levels of each device in production meets requirements.
It is important to measure the frequency response of a signal to ensure that the resolution of a signal is intact. Devices with a poor frequency response typically attenuate at higher frequencies, causing them to look blurry to the customer. Poor frequency response makes an HD video look like it is being played in standard definition. Frequency response testing is commonly performed by using a multiburst signal that resonates at multiple frequencies between different luminance values. The test frequencies in a multiburst pattern are 0.5, 1.25, 2.0, 3.0, 3.58, and 4.1 MHz.
NI VMS Multiburst Measurement Group
To implement the frequency response tests, NI VMS was used to measure a multiburst video signal. Figure 8 shows the amplitude and frequency results for each burst being generated.
Figure 8. The video pattern being generated consists of luminance bursts at different frequencies. Notice the measurement amplitude and frequency results for each burst.
Noise in a video system can hurt the quality of the picture in the active picture region. Signal-to-noise ratio (SNR) is a common measurement that helps ensure the video source does not introduce excess noise. Testing for noise in a video signal is typically done by testing a signal that has constant chrominance values with a steadily increasing luminance signal.
NI VMS Noise Spectrum Measurement Group
To test for system noise, NI VMS was used to measure a luminance ramp video signal. Figure 9 shows the noise levels and locations for the signal being generated.
Figure 9. The video pattern being generated is a luminance ramp that goes from black to white. Notice the peak noise level and the peak noise frequency.
Testing the response time of a video signal (transient response) ensures that the video signal can span the range of colors from black to white within the pixel clock rate. Poor response time means the video signal is not capable of changing colors quickly from pixel to pixel, so edges look blurred. The response time is commonly tested using a 2T pulse along with a luminance bar as a reference value. The 2T pulse is a sine wave that goes from black to white and has a pulse width that is a function of the bandwidth. The common measurement expression is K2T, which represents the ratio of the maximum pulse voltage versus the luminance bar voltage.
NI VMS K-Factor Measurement Group
To implement the response time tests, NI VMS was used to measure a luminance bar and a 2T pulse from the NTC-7 pattern. Figure 10 shows the response time and ratio of pulse to luminance bar (P/B).
Figure 10. The video pattern being generated is the NTC-7 pattern. This measurement makes use of the 2T pulse and the luminance bar sections of the signal to test the response time of the video signal.
Evaluation of the chrominance gain versus the chrominance voltage level is another important test for video signals. This test is called chrominance nonlinearity. A signal with incorrect chrominance linearity can have unwanted color changes when the chrominance amplitude changes and vice versa. Chrominance nonlinearity is commonly tested with multiple chrominance packets. You can measure the amplitude and phase of each packet to ensure the chrominance amplitude changes linearly with phase changes.
NI VMS Chrominance Nonlinearity Measurement Group
To implement the chrominance nonlinearity tests, NI VMS was used to measure a three-step modulated bar. Figure 11 shows the response time and ratio of pulse to luminance bar (P/B).
Figure 11. The video pattern being generated is a three-step modulated bar consisting of three modulated packets. The measurement bases the gain and phase changes of the first and third packets off the middle. Each packet is evaluated to ensure that as chrominance values change, chrominance gain and phase are proportional.
Differential Gain and Phase
The differential gain and phase measurement evaluates the luminance gain versus the luminance voltage level. Differential gain and phase is similar to the chrominance nonlinearity test, but, instead of chrominance, it measures the luminance amplitude and phase over sudden changes to ensure linearity. A signal with incorrect differential gain can cause color saturation in the video signal as the luminance values vary. Differential phase errors can cause color changes in the video signal as the luminance values vary.
NI VMS Differential Gain and Phase Measurement Group
To implement the differential gain and phase tests, NI VMS was used to measure a modulated staircase. Figure 12 shows the differential gain and phase of each step in the staircase.
Figure 12. The video pattern being generated is a modulated staircase. The luminance gain and phase are evaluated at each step to ensure that as luminance values change, the color amplitude and phase do not.
The measurements covered in this tutorial are some of the most common analog video measurements. With the NI Analog Video Analyzer platform, you can combine these measurements with several other measurements to fit your specific application needs. For multimedia devices that require digital video testing, NI also provides a digital video test platform.