XFree86 Video Timings HOWTO : Black Magic and Sync Pulses : Horizontal Sync:
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## 9.1. Horizontal Sync:

By previous definition, it takes HFL ticks to trace the a horizontal scan line. Let's call the visible tick count (your horizontal screen resolution) HR. Then Obviously, HR < HFL by definition. For concreteness, let's assume both start at the same instant as shown below: <hr>

      |___ __ __ __ __ __ __ __ __ __ __ __ __
      |_ _ _ _ _ _ _ _ _ _ _ _                |
      |_______________________|_______________|_____
      0                       ^               ^     unit: ticks
                              |   ^       ^   |
                              HR  |       |  HFL
                              |   |<----->|   |
                              |<->|  HSP  |<->|
                              HGT1         HGT2

<hr>

Now, we would like to place a sync pulse of length HSP as shown above, ie, between the end of clock ticks for display data and the end of clock ticks for the entire frame. Why so? because if we can achieve this, then your screen image won't shift to the right or to the left. It will be where it supposed to be on the screen, covering squarely the monitor's viewable area.

Furthermore, we want about 30 ticks of "guard time" on either side of the sync pulse. This is represented by HGT1 and HGT2. In a typical configuration HGT1 != HGT2, but if you're building a configuration from scratch, you want to start your experimentation with them equal (that is, with the sync pulse centered).

The symptom of a misplaced sync pulse is that the image is displaced on the screen, with one border excessively wide and the other side of the image wrapped around the screen edge, producing a white edge line and a band of "ghost image" on that side. A way-out-of-place vertical sync pulse can actually cause the image to roll like a TV with a mis-adjusted vertical hold (in fact, it's the same phenomenon at work).

If you're lucky, your monitor's sync pulse widths will be documented on its specification page. If not, here's where the real black magic starts...

You'll have to do a little trial and error for this part. But most of the time, we can safely assume that a sync pulse is about 3.5 to 4.0 microsecond in length.

For concreteness again, let's take HSP to be 3.8 microseconds (which btw, is not a bad value to start with when experimenting).

Now, using the 65Mhz clock timing above, we know HSP is equivalent to 247 clock ticks (= 65 * 10**6 * 3.8 * 10^-6) [recall M=10^6, micro=10^-6]

Some makers like to quote their horizontal framing parameters as timings rather than dot widths. You may see the following terms:

active time (HAT)

  Corresponds to HR, but in milliseconds. HAT * DCF = HR.

blanking time (HBT)

  Corresponds to (HFL - HR), but in milliseconds. HBT * DCF = (HFL - HR).

front porch (HFP)

  This is just HGT1.

sync time

  This is just HSP.

back porch (HBP)

  This is just HGT2.

XFree86 Video Timings HOWTO : Black Magic and Sync Pulses : Horizontal Sync:
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