Visual Impact of Video Parameters in Video Systems: Chroma-to-Luma Delay

Introduction

When processing composite video signals (CVBS), designers must pay attention to several important parameters to ensure image quality: chroma and brightness, that is, the color and brightness in the image signal; the delay error of chroma and brightness, which is related to the group delay deviation specification, which defines the electrical delay of different frequency bands in the whole system.

Composite video signals are basically chroma (color) and brightness (grayscale) information, usually modulated into RF signals for ordinary broadcast television. In broadcast television systems, it is critical to reduce the transmission bandwidth so that a simple single-wire solution can be used to connect baseband video.

This article will describe the impact of chroma/luminance delay error on the system and the existing detection methods. This article is the second part of the application note: Impact of electrical parameters in video systems. Before reading this article, you should read the first part: Visual Impact of Video Parameters in Video Systems: Part 1-Differential Gain and Differential Phase.

Chroma to Luma Delay

This parameter refers to the difference between the time when the chrominance component of the video signal passes through the system and the time when the corresponding luminance component passes through the system. This time difference, or delay deviation, will produce "stains" at the edges of objects in the image, making the image blurred. This indicator can usually be tested with a 12.5T sine-square wave pulse with 3.58MHz (4.43MHz for PAL format) modulation (Figure 1). The delay error is measured by analyzing the baseline of the signal. If the baseline is a straight line, there is no delay error; if the chrominance leads or lags behind the luminance, the baseline will fluctuate (no longer a straight line). At the same time, the relationship between chrominance and luminance can also be seen from the fluctuation of the baseline (Figure 2).


Figure 1. 12.5T test signal


Figure 2. Example of delay error in 12.5T pulse modulation test (you can clearly see the difference between chrominance leading luminance or luminance leading chrominance)

The visual impact of large chroma/luminance delay deviation on video systems

Figure 3 is the original image without chroma/luminance delay error, and Figure 4 is the visual effect after the chroma/luminance delay error. Obviously, Figure 4 does not look clear enough; and there are "stains" on the edges of the long blocks and letters. In order to see the impact of the delay deviation more clearly, the enlarged view is given in Figure 5.


Figure 3. Original image without chroma/luminance delay


Figure 4. The visual effect of the same image with luminance and chroma delay errors


Figure 5. A close-up view of the delay deviation in Figure 4

in conclusion

The above example is an image with a 300ns chroma/luminance delay difference, which clearly shows the visual impact of delay errors. For most video systems, the delay deviation is usually less than this value. Generally speaking, the image quality produced by a delay deviation of less than 20ns is good enough; when it is greater than 20ns, the image quality will be significantly reduced. Therefore, for video system designers, it is important to pay attention to the chroma/luminance delay deviation in order to ensure the output of vivid and clear images.