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VGA Video Signal Format and Timing Specifications

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Summary
This page explains VGA video signal format and its timing for the different video modes. I refer to the classic analog video signal, not the modern DVI (Digital Video Interface) signal in used today.

Table of Contents


VGA Video Signal Format

A color VGA video signal is composed by 5 different signals, two synchronization signals (HSYNC and VSYNC) and three video signals (R, G, B)

  • HSYNC Horizontal sync. Make electron beam restart at next screen's scanline (starts a new line)
  • VSYNC Vertical sync. Make electron beam restart at first screen's scanline (starts a new frame)
  • R Red intensity
  • G Green intensity
  • B Blue intensity

HSYNC and VSYNC signals determines the screen resolution (for example 640x480) whereas the colour of every pixel is determined by the value of R, G and B signal. Each color is a combination of the 3 primary colours R, G and B.

HSYNC and VSYNC signals are a train of squared pulses of +5V (+3.3V serves too) whereas RGB signals take values in a continuous (analog) voltage range from +0V (absolutely dark) to +0.7V (maximum brightness). Each of this 3 signals controls a electron gun that makes the screen's phosphor bright a basic colour (R, G or B) in a pixel. Any colour is the visual mixture of different levels of brightness of the 3 primary colours.

A single dot of colour on a video monitor doesn’t impart much information. A horizontal line of pixels carries a bit more information. But a frame composed of multiple lines can present an image on the monitor screen. In a 640x480 mode, for example, a frame of VGA video has 480 lines and each line usually contains 640 pixels (see later).

In order to paint a frame, there are deflection circuits in the monitor that move the electrons emitted from the guns both left-to-right and top-to-bottom across the screen. These deflection circuits require two synchronization signals in order to start and stop the deflection circuits at the right times so that a line of pixels is painted across the monitor and the lines stack up from the top to the bottom to form an image. The timing for the VGA synchronization signals is shown in Figure 2.

VGA signal format.png
  • Pulses on HSYNC signal mark the start and end of a line and ensure that the monitor displays the pixels between the left and right edges of the visible screen area.
  • Pulses on VSYNC signal mark the start and end of a frame made up of video lines and ensure that the monitor displays the lines between the top and bottom edges of the visible monitor screen.
  • As you may have guessed, the horizontal resolution of each line is not actually determined and could be anything, this resolution is determined typically by a pixel clock. Every rising edge of the pixel clock marks the start of a new pixel.

Video Modes and Their Signal Timings

The following table shows the time restrictions that video signal must obey in order to the monitor can synchronize and displays the image correctly (without blinks). The pixel clock frequency is only orientative, when designing a video hardware, you can use the pixel clock frequency that you want, the only important thing is that the video signal fits with the time restrictions (measures A, B, C, etc).

VGA Signal Timing.png
Analog Video Signal Timing Specifications
IBM VESA
Measure Unit 640x480
60Hz
720x400
70Hz
640x480
75Hz
640x480
85Hz
800x600
75Hz
800x600
85Hz
1024x768
75Hz
1024x768
85Hz
F_HSYNC kHz 31.469 31.469 37.500 43.269 46.875 53.674 60.023 68.677
A us 31.778 31.777 26.667 23.111 21.333 18.631 16.660 14.561
B us 3.813 3.813 2.032 1.556 1.616 1.138 1.219 1.016
C us 1.907 1.907 3.810 2.222 3.232 2.702 2.235 2.201
D us 25.422 25.422 20.317 17.778 16.162 14.222 13.003 10.836
E us 0.636 0.636 0.508 1.558 0.323 0.589 0.203 0.508
F_VSYNC Hz 59.940 70.087 75.000 85.008 75.000 85.061 75.029 84.997
O ms 16.683 14.268 13.333 11.764 13.333 11.758 13.328 11.765
P ms 0.064 0.064 0.080 0.671 0.064 0.056 0.050 0.044
Q ms 1.048 1.080 0.427 0.578 0.448 0.503 0.466 0.524
R ms 15.253 12.711 12.800 11.093 12.800 11.179 12.795 11.183
S ms 0.318 0.413 0.027 0.023 0.021 0.019 0.017 0.015
Pixel Clock MHz 25.175 28.322 31.500 36.000 49.500 56.250 78.750 94.500
Polarity HSYNC Neg Neg Neg Neg Pos Pos Pos Pos
Polarity VSYNC Neg Pos Neg Neg Pos Pos Pos Pos

VGA Connector

PC connector (DB15)
Macintosh connector
Pinout. NC means "No Connect"
Pin PC (DB15 connector) Macintosh
01 Red GND-R
02 Green Ref
03 Blue H/V-Sync (not separate sync)
04 NC Sense 0
05 DDC Return Green
06 GND-R GND-G
07 GND-G Sense 1
08 GND-B Reserved
09 NC Blue
10 GND-Sync/Self Raster Sense 2
11 NC GND
12 DDC Data V-Sync
13 H-Sync GND-B
14 V-Sync GND
15 DDC Clock H-Sync

References