Here are some captures of the video output at the monitor port:
Firstly normal.png is one-and-a-bit lines from a normal black pmode 4 screen. The hsync pulses are the lowest levels and the white borders are the highest levels. One hsync to the next is 64us as expected.
![normal.png](./download/file.php?id=2062)
- normal.png (1.18 KiB) Viewed 3833 times
The next set of captures is from a screen with the overscan routine running. There's a bit of distortion at the top where the border curves for a few lines before settling down for the rest of the screen. It is stable though, meaning the same thing is happening each frame.
overscan1.png is from a line with overscan. The left border is larger and the right border is smaller. The line is still 64us long. Most of the 192 lines look like this.
![overscan1.png](./download/file.php?id=2063)
- overscan1.png (1.18 KiB) Viewed 3833 times
overscan2.png was captured near the top of the screen where there was a bit of data from the text screen. This line is 65us long.
![overscan2.png](./download/file.php?id=2064)
- overscan2.png (1.27 KiB) Viewed 3833 times
overscan3.png is a black line, also longer than normal. I'm guessing this must have been from near the top of the screen as that is where the distortion is. I had to press the capture button a lot of times before this one appeared, so there can't be many lines like it. The second hsync looks long as well.
![overscan3.png](./download/file.php?id=2065)
- overscan3.png (1.17 KiB) Viewed 3833 times
overscan4.png is near the top of the screen (text screen data visible) but with normal timing. Not many of these on screen either.
![overscan4.png](./download/file.php?id=2066)
- overscan4.png (1.24 KiB) Viewed 3833 times
It looks like it's the change from 64us to 65us lines that is messing things up, but the vdg somehow gets back to a 64us line, even with the overscan. Figure that one out
![Smile :-)](./images/smilies/icon_e_smile.gif)