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Analog video signal requirements: Similarities and differences (Part 1)

Posted: 19 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:analog video? I/O circuit design? CVBS? video transmission systems?

By Randy Stephens
Texas Instruments Inc.

Analog video has been around for decades and is still in use today. The original and most common video standards include NTSC and PAL. Other modern consumer analog video transmission systems include S-Video, component video, professional G'B'R' video and computer R'G'B' systems. This article explores some of these analog video signal requirements and how they are similar yet different from each other, and how to simplify the analog I/O design in these video systems.

Here to stay
In modern video systems, it is common to utilize digital processing on both the front end and back end. This is due to the fact that the flexibility of these systems, coupled with their relatively low costs and integration of features, makes them very attractive to consumers. So why not simply keep video in the digital domain for all interconnections?

Digital video transmission has been around for years in the form of serial digital video, but this was primarily limited to professional broadcast systems due to the costs involved. Newer digital transmission systems include digital video interface and HDMI. These systems are gaining ground in popularity, especially with high-end systems.

At the 2006 Consumer Electronics Show, the "old" analog video transmission systems, namely component video, are still considered the primary interconnection method between STBs and displays. Also, many "low-cost" video systems lacking high-end features are still widely used throughout the world. Where price is one of the most important buying factors for consumers, most people simply cannot afford HDTVs. Therefore, traditional CVBS and S-Video will be around for many years to come.

Video nomenclature, background
Color information in video is derived from linear (tristimulus) red, green and blue (RGB) components. Due to CRT display technology limitations, these fundamental colors need to be manipulated in a non-linear form known as gamma-corrected colors. To distinguish between true RGB and non-linear gamma-corrected RGB, R'G'B' will be used.

Traditional consumer video does not use R'G'B' due to high bandwidth requirements on all three signals. To reduce bandwidth, costs, delay issues and modern-day processing power, R'G'B' signals are mathematically manipulated to create other forms of video signals.

The most important signal is brightness, or luminance. The International Commission on Illumination (CIE) stipulates the true definition of luminance (Y) shall be derived from linear RGB. Due to the non-linearity (gamma) added to RGB, luminance no longer holds true. Thus luma (Y') is utilized and is technically the proper terminology for all consumer video systems.

Similarly, chroma (C'), aka color information consisting of hue and saturation, is utilized rather than chrominance (C) due to the non-linear R'G'B' terms being utilized. The color difference signals P'B, P'R, R'-Y, and B'-Y' are also referenced this way to denote the non-linear gamma-corrected signals. Other terms including U, V, I and Q generally do not have the tick mark because these are mathematical equation elements and not used in the CIE color space. However, an argument could be made that these terms should have tick marks as they are based on elements that are non-linear. Figure 1 illustrates a simplified RGB signal flow showing how these video terms are created, ignoring elements such as sync information.

This simplified RGB signal flow shows how the different video terms are created, ignoring elements such as sync information.

Analog video transmission
The original NTSC and PAL systems utilize a single wire transmission system, commonly called composite video baseband signals or CVBS. Generally, these systems are limited in bandwidth to less than 6MHz. Note that the Society of Motion Picture and Television Engineers' 170M standard technically does not limit the luma channel bandwidth in any way, only the chroma or color difference signals. However, most systems will limit the signal to 4.2MHz due to RF transmission requirements.

The most significant voltage amplitude requirements of CVBS signals include a -40IRE (-286mV for NTSC and -300mV for PAL) sync signal combined with a +100IRE (714mV for NTSC and 700mV for PAL) video signal. These levels can vary slightly between standards, but the values shown represent the general voltages of these signals.

The 140IRE peak-to-peak levels are true for a CVBS signal with 75 percent color saturation. However, many people forget that color can have 100 percent saturation. This leads to the CVBS signal having a possible voltage of 286mV + 935mV = 1.221Vpp for NTSC and 300mV + 933.5mV = 1.2335Vpp for PAL. This is higher than any of the other standard video signals and, if forgotten, could lead to potential clipping of the video signal.

One of the biggest problems with CVBS signals is that luma is combined with the chroma signal. Because these signals can occupy the same frequency spectra, it is very difficult to separate the signals from each other without causing artifacts to appear. This is why there are many different video decoders with terms such as 2D, 3D and 3D adaptive comb filtering. Even with such extravagant techniques, artifacts can and do exist when trying to separate the two video signals.

The best way to eliminate this problem is not to combine the luma and chroma signals in the first place. S-Video does exactly this and produces an improved picture over CVBS. The bandwidth of S-Video, just like CVBS, is generally less than 6MHz. The only drawback to using S-Video is that two transmission wires must be used.

About the author
Randy Stephens is a member of the technical staff at Texas Instruments Inc.




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