How to determine the horizontal pixel count out of a HSync and VSync signal

Question

I have a project where I want to read the HSync and VSync signal from a VGA cable into an microcontroller and calculate the pixel clock.

Calculation: Pixel clock = Horizontal Pixel Count * Vertical Line Count * Refresh rate

I'm currently stuck by the question of how I can get the horizonal pixel count.

If we assume that my VGA cable caries a 640x480 (800x525 with the porches) @ 60Hz signal, like shown here:

http://tinyvga.com/vga-timing/640x480@60Hz.

I can get the refresh rate by timing the vertical sync pulses to each other (~16.68ms > 1/16.68ms = 60Hz.) I can get the vertical pixel count by dividing the time of every frame and the time of every HSync pulse to each other (16.68ms / 31.77µs = ~525 lines).

How do I get the horizontal pixel count?

For 640x480 @ 60Hz it should be 800 pixels.

Really important is that the resolution and framerate of the VGA cable is not fixed, so I can not use any definitions for timings.

For clarification: I know that a VGA cable sends analog signals and there aren't any pixels. But my source device have pixels and they convert them into analog video signals. So want to get the pixel clock from my source device that is connected via a VGA cable.

Answer 1

You could find out what is the display formats of the signal by analyzing the time between VSYNCs and between HSYNC.

I have found a table in the datasheet of the AD9888. At page 15 they give the expected timing on theses signals versus the display formats.

There you will also find the pixel clock rate for that given format.

Thus you need to implement a lookup table with these values in your code, measure the HSYNC and VSYNC timing to know the format and fetch the corresponding pixel clock from the table.

Answer 2

There are no horizontal pixels in VGA.

Remember VGA comes from the era of CRT (cathode ray tube) monitors. The electron beam (cathode ray) sweeps from left to right across the screen, and the VGA signal continuously adjusts the brightness of the beam. There are no pixels.

You already knew this.

Answer 3

There is no way you can calculate it with 100% certainty.

As is very rare that video cards are programmed to any arbitrary or non-standard format, not even monitors can lock on to any arbitrary format, or at least show them properly, since the exact parameters are not known.

You can only try to detect automatically what it might be, by trying to match the measured HS and VS signal parameters to known formats and allowing some tolerance.

Even with VGA, the video drivers read the monitor EDID for list of supported formats, and then simply use supported formats for which the monitor can know the parameters.

For example, if you measure 525 lines per frame, with sync on for 2 lines, and if VS rate is about 59.94 Hz and HS rate is about 31468.75 Hz with duty cycle matching the ratio of 96 active compared to 800 being length, then you can simply assume that you have a standard VGA format with 800 pixels, which means a standard pixel clock of 25175000 Hz.

Since you know what the standard values for a detected format are, you can then scale the calculated pixel clock accordingly, if the video card uses slightly non-standars pixel clock, e.g. if the frame rate is 60.00 Hz instead of 59.94 Hz.