VGA的行场时序
之前碰到接收VGA时有的电脑可以有的电脑会出现画面偏移。
先来了解下数字显示器时序（DMT）
DMT视频时序有四种：
（1）Positive H & Positive V Syncs
行同步为正极性，场同步为正极性
（2）Negative H & Negative V Syncs
行同步为正极性，场同步为负极性
（3）Negative H & Negative V Syncs
行同步为负极性，场同步为负极性
（4）Negative H & Positive V Syncs
行同步为负极性，场同步为正极性
整帧的时序如下：
注意以下几点：
（1）边界都是有效视频区域。
（2）该图是以行场同步极性为正为例
场的回扫间隔是这样的：
第二步：了解时序与像素之间的关系
在VESA Display Monitor Timing Standard 定义了显示器的不同分辨率的时序。这里我们以分辨率为1920x1080@60Hz为例。
详细时序参数如下：
Timing Name = 1920 x 1080 @ 60Hz;
Hor Pixels = 1920; //Pixels
Ver Pixels = 1080; //Lines
Hor Frequency = 67.500; //kHz = 14.8 usec/line
Ver Frequency = 60.000; //Hz = 16.7 msec/frame
Pixel Clock = 148.500; //MHz = 6.7 nsec
Character Width = 4; //Pixels = 26.9 nsec ± 0.5%
Scan Type = NONINTERLACED; // H Phase = 1.4 %
Hor Sync Polarity = POSITIVE // HBlank = 12.7% of HTotal
Ver Sync Polarity = POSITIVE // VBlank = 4.0% of VTotal
Hor Total Time = 14.815; // (usec) = 550 chars = 2200 Pixels
Hor Addr Time = 12.929; // (usec) = 480 chars = 1920 Pixels
Hor Blank Start = 12.929; // (usec) = 480 chars = 1920 Pixels
Hor Blank Time = 1.886; // (usec) = 70 chars = 280 Pixels
Hor Sync Start = 13.522; // (usec) = 502 chars = 2008 Pixels
// H Right Border = 0.000; // (usec) = 0 chars = 0 Pixels
// H Front Porch = 0.593; // (usec) = 22 chars = 88 Pixels
Hor Sync Time = 0.296; // (usec) = 11 chars = 44 Pixels
// H Back Porch = 0.997; // (usec) = 37 chars = 148 Pixels
// H Left Border = 0.000; // (usec) = 0 chars = 0 Pixels
Ver Total Time = 16.667; // (msec) = 1125 lines HT–(1.06xHA)
Ver Addr Time = 16.000; // (msec) = 1080 lines = 1.11
Ver Blank Start = 16.000; // (msec) = 1080 lines
Ver Blank Time = 0.667; // (msec) = 45 lines
Ver Sync Start = 16.059; // (msec) = 1084 lines
// V Bottom Border = 0.000; // (msec) = 0 lines
// V Front Porch = 0.059; // (msec) = 4 lines
Ver Sync Time = 0.074; // (msec) = 5 lines
// V Back Porch = 0.533; // (msec) = 36 lines
// V Top Border = 0.000; // (msec) = 0 lines
