Reduced Blanking和CVT介绍

 

如下部分参考网络。

1、         What is RB and CVT。

The symbol “(RB)” means that this DMT includes Reduced Blanking.

Reduced Blanking timings can only be generated using the Coordinated Video Timing (CVT) Formula

2、         CVT

The VESA Standards Body has, as of early 2003, replaced the GTF (General Timing Formula) video timings standard with the CVT (Coordinated Video Timings) standard.

You can't get the standards documents for free, but you can download the GTF Spreadsheet and the CVT Spreadsheet from THe VESA site, which are the reference calculations and also explain the timing standards reasonably well. Note that, as of 2005, many monitors don't implement new CVT timings, more are starting to, and CVT gives several benefits: 
      Reduced Blanking - Timings for LCD flat panels and other monitors that do not require long blanking intervals as a retrace period. CRTs require this because the electron beam takes time to horizontally resteer to the other side of the monitor. Reduced Blanking is very valuable in reducing the total video bandwidth for the otherwise same video mode. When possible, always use a Reduced Blanking video mode for an LCD, certainly when using DVI-Digital! It even works for Analog modes on some newer LCD monitors.
      Encoded Aspect Ratio in VSync - Previously, with GTF and other generic RGB scan methods, a digital monitor or framegrabber could not tell what the horizontal resolution of a scan mode was, and it had to guess based on known 'Standard' modes, which frankly weren't a very large list in many monitors. Now common aspect rations such as 4:3, 16:9, 16:10, and a few others are defined to be encoded into the VSync length. So, in those cases, a monitor detecting a CVT mode can now know exactly what the horizontal resolution is.
       Standardized Pixel Clocks - Required pixel clock rates are now rounded to 0.25 MHz, which is closest to what most real video cards can deliver.

3、         Reduced blanking

        blanking time最初是由CRT显示器引入的。

To understand blanking time it helps to understand the basics of how CRTs display images. As a result, a CRT spends a substantial portion of its time doing retraces and other things which are not part of drawing the visible screen image. The timing can vary quite a bit among different screen modes, but for standard CRT timing about 70% of the time is spent drawing the screen image and the other 30% is invisible overhead. This overhead is often called 'blanking time' because the signal is blank (shut off) while not drawing the visible screen image.

        LCD显示器不需要blanking time，但因为DVI涉及去兼容CRT，所以有附带blanking time。DVI单个link最大的pixel clock为165MHZ.因为blanking time导致timing的pixel clock超出165MHZ,

LCDs don't need that extra overhead but it's in the DVI video signal anyway. Recall from the earlier single link and dual link section that a single DVI link has a maximum pixel clock of 165 MHz. For an LCD, the blanking time's portion of that 165 MHz is wasted due to using CRT timing. When connected to an LCD it would be awfully nice to be able to get rid of that overhead because you could display the same screen resolution and refresh rate with a lower pixel clock. A lower pixel clock means the data is less likely to be corrupted going through the cable. It's also easier on the DVI transmitter which can be useful if you have one of the NVIDIA cards with a slow transmitter. You could also get away with using a longer cable before the screen gets corrupted. With the blanking time overhead removed, you can use a higher screen resolution and refresh rate before having to resort to using a double link setup. Lots of things get better for DVI if you can get rid of the CRT overhead when using an LCD. Fortunately, there's a fairly well standardized reduced blanking video format which reduces the overhead to about 5%.

     降低blanking 5%。可以看到1920 * 1080 60HZ之前要用dual link来传输的数据，现在只需要用single link来传输。 

  LCD液晶显示器在2004年前尚未支持reduced blanking，后面才开始引用。

Most LCD monitors made before 2004 don't support reduced blanking. Most LCDs made since then support reduced blanking in the helpful cases mentioned above. The monitor manual often includes a list of the supported display modes. Any reduced blanking modes will normally mention that they have reduce blanking or they'll mention the term CVT-RB (coordinated video timing- reduced blanking).

        显卡驱动一般能够自动选择reduced blanking模式（该模式为显示屏幕自带EDID来配置），例：一个显示屏上的EDID中有支持1920 * 1080 @60 HZ RB的timing，则显卡驱动输出时为reduced blanking模式。当我们的DVI连接线为single link的，我们需要传输的pixel clock不能超过165MHZ。

Display drivers automatically choose when to use a reduced blanking screen mode. The normal Windows methods of setting a screen resolution and refresh rate don't allow you to manually select reduced blanking. If you have a single link setup and are displaying a mode like 1920 X 1200 at 60 Hz then the video card must have selected reduced blanking. But in other cases it may not be obvious. Some drivers seem to be aware of the limitations of their DVI transmitters (those that don't work well close to 165 MHz) and automatically select reduced blanking screen modes to reduce the DVI pixel clock to an acceptable value. I don't know of any standard way to tell if your DVI setup is using reduced blanking. The on screen displays of many monitors can display the DVI pixel clock. It's usually in the information screen and is most commonly called 'pixel clock' or just 'clock'. If your monitor displays the pixel clock then you can compare with the table above to determine whether it's using reduced blanking. Keep in mind that the numbers may be slightly different than the table values. But if you need to try out reduced blanking to solve a problem, both ATI and NVIDIA have provided ways to do it.