Gunstick wrote:What I discovered was 2 types of 0 byte lines. One which was displaying the background color, and so not increasing the screen counter. And another one was displaying nothing. A really black line (I think that's what you mean by changing sync signals) where even color 0 was not shown.
No, that's not what I meant, you can have a "black line" without altering sync. Furthermore, you can have a black line with or without increasing the screen counter (zero bytes line or not). They are not directly related.
By altering sync I mean that the timing of sync signals that go to the monitor are altered somehow. They can be altered in different ways, but the point is that they are altered or are not. Once sync is altered, you are at the mercy of the monitor/tv/capture/scan-doubler. And as long as you don't alter sync, all the tricks are kept "inside" the computer, and then you don't depend on any specific display device.
I have to say that the second VBL is a quite stable signal as long as it's not over too many lines. My old Thompson CRT was very useful for this. It shows (when you blow up brightness) all border switches in nice grey/black patterns on the right HBL. It also shows all distorted sync signals, and that 2nd VBL did nothing like this...
What this means is that this alteration of the sync signals is not fatal for most monitors, conceivable it works fine in all the CRTs at the time.