The NTSC VIC-I has the peculiarity though, that changes of $9004 (and the LSB of $9003) do not happen with HSync, rather they happen somewhere in the middle of a raster (please note the screen caption of the first test program here).JonBrawn wrote:There's a bit about NTSC interlaced mode in my notes at HTTP://www.brawn.org/VIC-20/Video
However, there are some bits in there that don't make sense to me yet.
When in interlaced mode the odd and even frames each have about the same number of lines as a non-interlaced frame, which is not how interlaced video is supposed to work. I measured that an interlaced field has 1.5 lines more than a non-interlaced frame.
By convention, the field that starts with a full line is the odd field, its last line will be a half-line. The next field is the even field and will start with a half-line.
The PAL VIC-I shows more reasonable behaviour here, $9004/$9003 change during HSync. VICE gets both cases right.