NTSC half line

Spork Frog

I'm trying to do a few experiments in NTSC, and have so far been using the book as a reference. I understand the part that's there about the video lines and vertical sync pulses, but I know that an NTSC frame is 262.5 lines, and the book makes no mention of this.

I have seen in other drivers that it's not completely necessary, but can someone please explain what the half-line should be? I'm assuming it's the usual HSYNC timing followed by not all of a line of a active video, is this correct?

ericball

The half line changes the display from non-interlaced to interlaced. TVs originally would draw each raster as a diagonal line so the end of the line (on the right of the screen) would be slightly lower than the start of the line (on the left). The horizontal retrace would pull the electron beam back across the screen with little change in vertical position, so the start of the next line would be at the same horizontal level as the end of the previous line. The half line would therefore cause the next frame to draw lines in between (interlaced with) the lines of the previous frame. If you are generating a non-interlaced frame then you don't want the half lines.

I'd recommend looking at my video driver templates (linked below) for the actual timing as it's a little difficult to explain in words.

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Composite NTSC sprite driver: Forum
NTSC & PAL driver templates: ObEx Forum
OnePinTVText driver: ObEx Forum