Visual impact of video parameters: effect of chroma to luminance delay

The ultimate measure of video system quality depends heavily on visually viewing the resulting image. Nevertheless, engineers must make objective measurements to determine video quality. The delay mismatch between chromaticity and luminance, the color and brightness of the optical signal, is an important video parameter that is both difficult to measure and needs to be measured by objective specifications. This paper, the second in a series of application notes discussing electrical parameters affecting video systems, will help designers understand the relationship between chromaticity and luminance delay mismatch, i.e., the color and luminance mismatch of an electrical signal. Methods for measuring these mismatch values and their impact on video image quality will be discussed.

Introduction

Designers must pay attention to several important parameters to maintain the image quality of processing composite video signals (CVBS). Chrominance and luminance, the color and brightness of the optical signal, are two such parameters. The chrominance to luminance delay mismatch parameter is related to the group delay deviation specification, which defines the electrical delays in the different frequency bands throughout the system.

A composite video signal is basically a combination of chromaticity (color) and luminance (brightness) information. This composite video signal is used to modulate the RF signal distributed as a common broadcast TV, which is important to reduce the required transmission bandwidth and allows a simple single line connection for baseband video.

This paper will illustrate the effects of chromaticity to luminance delay mismatch and describe methods for measuring the mismatch values.

Chroma to Luminance Delay

This parameter specifies the time difference it takes for the chrominance portion of the video signal to pass through the system, relative to the time difference it takes for the luminance portion. This time difference, delay error, can cause bleeding, especially at the edges of objects in the picture, and can make the picture look trailing. 12.5T sine square pulse and 3.58MHz (4.43MHz for PAL) modulation was used to test this specification (Figure 1). The delay mismatch was measured by analyzing the baseline of this signal. A straight line in the baseline indicates no delay error. If the chroma is leading or trailing luminance, the baseline will no longer be straight, but will waver. The shape of the baseline will indicate whether the chromaticity is leading or lagging luminance (Figure 2).

Figure 1.12.5T test signal.

Figure 2. Graphical representation of the effect of delay mismatch on modulated 12.5 T pulses. The difference between the chromaticity trailing luminance and leading luminance is clearly visible.

Visual effect of video system with large chromatic luminance delay error

Figure 3 shows the original image without chromaticity to luminance delay error. Figure 4 shows the visual effect of the luminance to chromaticity delay error. Note that the image in Figure 4 looks less clear; bleeding can be seen at the edges of the boxes and letters. Figure 5 shows a zoomed-in view to see the effect of the error more clearly.

Figure 3. Original image with no chroma to luminance delay.

Figure 4. Visual effect of luminance to chroma delay mismatch on the video electrical signal of the same image.

Figure 5. Close-up of the delayed mismatch error in Figure 4.

Conclusion

The above example is the result of a 300ns luminance to chroma delay error. It vividly illustrates the visual effect of this error on the displayed picture. For most video systems, the delay mismatch error is usually less than this value. Delay errors of less than 20ns usually result in and are considered high quality. Delay mismatch errors over 20ns can significantly degrade image quality. Therefore, video system designers must pay close attention to the chromaticity to luminance delay mismatch to ensure the vividness and clarity of the reproduced image.

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