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Analog video signal requirements: Similarities and differences (Part 2)

Posted: 26 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:analog video signal? analog video transmission? SD video? ED video? high-definition video?

By Randy Stephens
Texas Instruments

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 will explore 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.

Analog video transmission: Component video
To improve upon S-Video, component video was created to eliminate modulation of the Chroma signal that reduces errors. Component analog video maintains essentially the same Luma (Y') information, but keeps the color difference information separated. P'B, not C'B which is used for the digital domain color difference, is the blue color difference signal. Likewise, P'R is the red color difference signal, while C'R is the digital domain red color difference.

The 1Vpp voltage amplitude requirement of component Luma is essentially the same as CVBS Luma. The sync is -300mV and the video information is 700mV on top of the sync level. The color difference signals, which the sync information is at the midpoint rather than the bottom-like Luma, allows for a 700mVpp signal.

Component analog video (Y'P'BP'R) includes several different formats. These formats include standard definition (SD), enhanced definition (ED) and high-definition (HD) video. SD video includes NTSC based 480i (aka 525i) and PAL based 576i (aka 625i) where the 'i' term indicates interlaced video. These video systems have a video bandwidth up to 6.75MHz for Luma and up to 3.375MHz for P'B and P'R signals. The sync information is just like the CVBS signal except for the midpoint level for the color difference signals.

ED video includes NTSC-based 480p (aka 525p) and PAL-based 576p (aka 625p). The 'p' indicates progressive scan and, thus, requires more bandwidth. The Luma signal is limited to 12MHz while the color difference signals are limited to 6MHz. The sync levels are just like the 480i requirements, but have a shorter (2.33?s vs. 4.7?s s) width and refresh rate.

HD video includes 720p, 1,080i and 1,080p. The Luma signal 720p and 1,080i flavors have a bandwidth limited to 30MHz, while the color difference signals are limited to 15MHz. The Luma signal 1,080p flavor is limited to 60MHz and the color difference signals to 30MHz. The SMPTE standards, 274M and 296M, allow for varying frame rates and sampling rates that can alter these analog bandwidths, but most systems use the above numbers. Note that the bandwidths and sync widths can, and do, vary for each respective profile as there are a considerable number of options (as much as 8 for 720p and 11 for 1,080i/1,080p) for each signal.

The voltage requirements for these HD video signals follow the same form as the 480i and 480p requirements. The Luma channel requires 1Vpp while the color difference channels require 700mVpp. The sync information is different in the respect that the HD signals utilize tri-level syncs. However, the bottom-level voltage excursions still follow the traditional 300mV single-sync pulse excursion that CVBS, 480i and 480p follow. Due to the tri-level sync and faster signal rates, the 720p sync width is as short as 0.54?s, 1,080i is as short as 0.59?s and 1,080p can be as short as 0.296?s.

Analog video transmission: computer and professional interfaces
Going into computer R'G'B' video signals opens up a very large matrix of signal requirements based on pixel resolutions and refresh rates based in large part on Video Electronics Standards Association (VESA) standards. Note that in the computer world, RGB is commonly used, although in reality these are also non-linear gamma corrected signals and should realistically be R'G'B'. The biggest difference between the consumer video signals and R'G'B' is that all three R'G'B' signals require the exact same signal bandwidth.

R'G'B' signals have the highest frequency requirements that can reach beyond 148.5MHz (1,920 x 1,440 at 75Hz) and undoubtedly will be going even higher. Thus, processing power is substantially increased to handle three signals with very wide bandwidths. Systems such as Y'P'BP'R have much less processing power requirements (due to 4:2:2 processing) resulting in a cheaper system. Timing of all three signals is critical or else color shifting can occur. To produce pure white, all three signals must be 100 percent while only one signal, Luma, is required for the consumer signal. Generally, this allows for easier calibrations as brightness or Luma is separated from the hue/saturation, chroma or color difference, information. Timing is also not as critical with the consumer video signals like R'G'B'.

Voltage requirements for computer R'G'B' is just like the Luma signal requirements of 700mVpp. The only difference is that the sync information may or may not be included with the signals. Sync information can be completely separate and in this case requires two separate signals: Horizontal and Vertical Sync lines.

Sometimes these are also combined into one signal: H + V Sync. Many times the sync information is included on the Green signal: R'G'sB'. Other times the sync information is included on all three signals: R'sG'sB's or sR'G'B'. When sync is embedded with the video signal, the amplitude is 300mVpp, just like the traditional Luma signal. Obviously the sync duration depends on the resolution and refresh rate of the signal. This can vary from 3.8?s (640 x 480 at 60Hz) to as short as 0.74?s (1,920 X 1,200 at 85Hz) or further such as the reduced blanking 1,920 x 1,200 at 60Hz requirement of 0.208?s.

Table 1 lists some of the analog requirements for video signals.(Click image for larger view)

Lastly, in professional and broadcast systems, G'B'R' is utilized. The SMPTE component standard stipulates that the Luma information is placed on the first channel, blue color difference is placed on the second, and red color difference is placed on the third, which is consistent with the Y'P'BP'R nomenclature. Because the Luma channel (Y') carries the sync information and the green channel (G') also carries the sync information, it makes sense that G' be first. Since the blue color difference channel (P'B) is next and the red color difference channel (P'R) is last, it also makes sense to place the B' signal on the second channel and the R' signal on the third channel. Thus, hardware compatibility is better achieved by using G'B'R' rather than R'G'B'.

Video and sync amplitudes for each G'B'R' signal are the same as the respective Y' requirement for each of the following standards respectively480i/525i, 576i/625i, 480p/525p, 576p/625p, 720p, 1,080i and 1,080p. Note that for many G'B'R' systems sync is embedded on all three channels but may not always be the case in all systems. The bandwidths required for these systems are essentially the same as the Luma bandwidth requirements previously mentioned for each respective SD, ED and HD standard. Just like the computer R'G'B' signals, timing of each signal is critical to the video signal along with the calibration of the display.

Table 1 lists some of the analog requirements for video signals. Because there are many other variations for each video system, these numbers represent a good starting point. For ideal minimum slew rate numbers, the general formula SR = (2 Pi F 0.707 Vpeak * 2) was used with Vpeak being 1V (2Vpp) assuming an output buffer under worst case conditions, 0.707 used as the -3dB amplitude at a given -3dB frequency point and a factor of 2 multiplier buffer just to be safe. In a real system, the video signal does not need to go from 0V to 1V (1Vpp or 0.5Vpeak) but more like 0V to 700mV (350mVpeak) to go from black to white. For an input buffer, the amplitude is one-half the output buffer requirements. So, use information in Table 1 only as a starting point.

End notes
* Generally used limit but not required.

For Part 1, please click here.

About the author
Randy Stephens
is a Member Group Technical Staff at Texas Instruments.




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