11. Black Magic and Sync Pulses

OK, now you've computed HFL/VFL numbers for your chosen dot clock, found the refresh rate acceptable, and checked that you have enough VRAM. Now for the real black magic -- you need to know when and where to place synchronization pulses.

The sync pulses actually control the horizontal and vertical scan frequencies of the monitor. The HSF and VSF you've pulled off the spec sheet are nominal, approximate maximum sync frequencies. The sync pulse in the signal from the adapter card tells the monitor how fast to actually run.

Recall the two pictures above? Only part of the time required for raster-scanning a frame is used for displaying viewable image (ie. your resolution).

11.1. 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:

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

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 concretness 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 vendors 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 time units (usually microseconds). HAT * DCF = HR.

blanking time (HBT)

Corresponds to (HFL - HR), but in time units (usually microseconds). HBT * DCF = (HFL - HR).

front porch (HFP)

This is just HGT1.

sync time

This is just HSP.

back porch (HBP)

This is just HGT2.

11.2. Vertical Sync:

Going back to the picture above, how do we place the 247 clock ticks as shown in the picture?

Using our example, HR is 944 and HFL is 1176. The difference between the two is 1176 - 944=232 < 247! Obviously we have to do some adjustment here. What can we do?

The first thing is to raise 1176 to 1184, and lower 944 to 936. Now the difference = 1184-936= 248. Hmm, closer.

Next, instead using 3.8, we use 3.5 for calculating HSP; then, we have 65*3.5=227. Looks better. But 248 is not much higher than 227. It's normally necessary to have 30 or so clock ticks between HR and the start of SP, and the same for the end of SP and HFL. AND they have to be multiple of eight! Are we stuck?

No. Let's do this, 936 % 8 = 0, (936 + 32) % 8 = 0 too. But 936 + 32 = 968, 968 + 227 = 1195, 1195 + 32 = 1227. Hmm.. this looks not too bad. But it's not a multiple of 8, so let's round it up to 1232.

But now we have potential trouble, the sync pulse is no longer placed right in the middle between h and H any more. Happily, using our calculator we find 1232 - 32 = 1200 is also a multiple of 8 and (1232 - 32) - 968 = 232 corresponding using a sync pulse of 3.57 microseconds long, still reasonable.

In addition, 936/1232 ~ 0.76 or 76%, still not far from 80%, so it should be all right.

Furthermore, using the current horizontal frame length, we basically ask our monitor to sync at 52.7khz (= 65Mhz/1232) which is within its capability. No problems.

Using rules of thumb we mentioned before, 936*75%=702, This is our new vertical resolution. 702 * 1.05 = 737, our new vertical frame length.

Screen refresh rate = 65Mhz/(737*1232)=71.6 Hz. This is still excellent.

Figuring the vertical sync pulse layout is similar:

   |___ __ __ __ __ __ __ __ __ __ __ __ __
   |_ _ _ _ _ _ _ _ _ _ _ _                |
   0                      VR              VFL     unit: ticks
                           ^   ^       ^
                           |   |       |
                            VGT    VSP

We start the sync pulse just past the end of the vertical display data ticks. VGT is the vertical guard time required for the sync pulse. Most monitors are comfortable with a VGT of 0 (no guard time) and we'll use that in this example. A few need two or three ticks of guard time, and it usually doesn't hurt to add that.

Returning to the example: since by the defintion of frame length, a vertical tick is the time for tracing a complete HORIZONTAL frame, therefore in our example, it is 1232/65Mhz=18.95us.

Experience shows that a vertical sync pulse should be in the range of 50us and 300us. As an example let's use 150us, which translates into 8 vertical clock ticks (150us/18.95us~8).

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

active time (VAT)

Corresponds to VR, but in milliseconds. VAT * VSF = VR.

blanking time (VBT)

Corresponds to (VFL - VR), but in milliseconds. VBT * VSF = (VFL - VR).

front porch (VFP)

This is just VGT.

sync time

This is just VSP.

back porch (VBP)

This is like a second guard time after the vertical sync pulse. It is often zero.