What are HSYNC and VSYNC?

[Teg-Men]

Hi, can someone explain me what they are doing?
1. How TV signal is managed by these two?
2. How a pixel is controlled by them?
3. Let pixel frequency be 20 Mhz. Then how much time I have to wait to control the pixel located 40th row and 80th column of the monitor that has 800x600 pixels. (Let the starting point be the upper-left side of monitor)

 

[betwixt]

1. Hsync is "Horizontal Sync", it is a pulse that synchronizes the start of the horizontal picture scan line in the monitor with the picture source that created it.
Vsync is the equivalent vertical synchronization, it ensures the monitor scan starts at the top of the picture at the right time.
2. Pixels are not controlled by them at all. A Pixel is the smallest picture unit that can be displayed. Imagine the screen is a grid (like a spreadsheet) and each cell can contain one shade or color, the picture is composed of many of them, each small enough that the human eye cannot distinguish them individually so it sees them 'blend' into a picture. In the spreadsheet analogy, each cell is one pixel, Hsync would mean start at column zero and Vsync would mean start at row zero.
3. In an 800x600 pixel screen there are 480000 pixels (multiply them) so the time to scan the whole picture would be the period of one pixel (1/f) multiplied by the number of pixels. That only gives the time to wait on the visible part of the picture though, it doesn't include the time spent in sync pulses. The total is the sum of 800 horizontal sync pulse durations plus the time of all the pixels. Depending on where you start your timing, you might also have to add one vertical sync pulse duration. Without knowing how long those sync pulses last, I can't tell you the total time.

Brian.

 

[Teg-Men]

Thanks first of all, Then
For question 3 what if we ignore front and back porch and negative pulse of VSYNC? Can we find it?
What if one of them goes to one to zero? Would the picture remain on the screen or they start to refresh the screen?
What information do I need to find period of VSYNC and HSYNC? Refresh rate? or what?

 

[betwixt]

The way the sync pulses control the scan oscillator or scan counter depends on the way the circuit is designed. In some cases, the difference between the monitor and the incoming sync is measured and a correction factor fed back to the monitors scan circuit, a bit like a PLL system. On other devices, the syncs may directly reset the scan oscillator or scan counter back to zero.
Isn't possible to calculate the sync periods, it's something decided at the design stage. If you have an existing video source, looking at it on an oscilloscope will tell you the exact timing.
All you can assume is:
1. if the scan is 800 pixels across, the time taken to scan the line is 800 * Time for each pixel (800 * (1 / 20MHz)) = 40uS
2. That is repeated 600 times, once for each line of 800 pixels so the total raster time is 40uS * 600 = 24mS.

The refresh rate cannot be quicker than the time it takes to complete one scan so it must be slower than 24mS ( = 41.6Hz).

Brian.

 

[Teg-Men]

Thank you, I got it