"standard ModePool format that can be used to easily create a screen mode without the need of a modeline"
It would be nice to reference where these modepools are located, I haven't been able to figure it out with google.
These test images might be useful for verifying a VGA -> RGB/PAL converter -> TV setup. Note that most boards are restricted to divisible-by-8 timings so horizontal centering is not guaranteed.
# 1408x576 @ 25Hz, 15.6kHz hsync, itu-r bt.470 PAL ModeLine "1408x576@25i" 27 1408 1446 1572 1728 576 581 586 625 interlace composite +hsync -vsync
Upgraded analog cable TV channels use the itu-r bt.601 720x576 aperture with an image aspect ratio of 4/3. The test image grid vertical period of 42 lines then yields a horizontal period of 39.375 dots thus requiring an oversampling factor of 8 for an integer period.