I also found thi son the web:
The problem with interlace and rolls is that as the text moves up the
screen, its position with respect to each field's line structure can either
change, or stay the same. If it stays the same, your text will look as good
moving as it does when it's still. If it changes, the text will lose
resolution and may flicker, distort, and crawl around as it rolls.
Imagine characters in a screen font with a height of 10 lines. When placed
on a page at its starting position, the even scanlines in the character all
fall on the even video field, while the odd scanlines fall on the odd field.
As the text sits there, the even fields show lines 2, 4, 6, 8, and 10 in the
text, while the odd fields show lines 1, 3, 5, 7, and 9. All ten scanlines
in the text are seen over the course of any two fields.
Now start a roll. Any decent CG updates the roll on a field basis; after one
field is displayed, the text is moved up a certain amount for the next
field, up the same amount for the next field, and so on.
Let's say that the director wants a nice, slow roll, to kill some time.
You've selected 60 lines/second (CGs that allow you to set the roll rate
usually use scanlines per second as the measure, and in NTSC-land the 59.94
Hz field rate is rounded to 60 Hz to keep operators from getting bogged down
in fractional math. If you're in PAL-land, assume you're rolling at 50
lines/second for this example), and pushed the "go" key on the CG.
Now in the first even field, character scanlines 2, 4, 6, 8, and 10 are
shown. Next the text is moved up one scanline because at a nominal field
rate of 60 Hz, one scanline per field results in 60 lines per second. So
when the odd field is displayed, the text, being up one line from its
even-field position, has lines 2, 4, 6, 8, and 10 displayed again - and
lines 1, 3, 5, 7, and 9 don't get put onscreen! The next vertical interval
comes along, and the text is moved up one line again, so that the even
scanlines once again appear in the even field. The next vertical interval
arrives, and up goes the text again - one line! - so that the next odd
field, like all the even and odd fields before it, shows the even scanlines
of the text. The odd scanlines in the text never appear onscreen!
The result is half-vertical-resolution text that looks awful. Thin
horizontal strokes will either appear about twice as thick as they should,
or twice as thin, depending on your luck (sometimes you get the evens,
sometimes the odd scanlines, depending on the CG you're using, the initial
position of the text, and the field timing when you press the "go" key).
Now go back and double the roll speed, to 120 lines/second (or 100
lines/second if using a 625/50 CG). Now, the first even field shows the even
scanlines. Come the vertical interval, the text is moved up two lines (two
lines per field times 60 fields per second gives 120 lines per second); the
relative positioning of the text with respect to the field structure remains
the same, since the even scanlines are moved up two lines to the next higher
even scanline, while the odds are moved up to the next higher odd line. When
the odd field is displayed, the odd scanlines in the text are shown, just as
they should be! The full vertical resolution of the text remains.
You may have noticed a pattern here. When the roll rate (in lines/second)
was the same as the field rate (in fields/second), the roll looked awful.
When the roll rate was twice the field rate, things looked fine. As it turns
out, this relationship holds for all integer multiples: roll rates that are
odd multiples of the field rate look awful. Even multiples look great. Thus
for 525/59.94 (or 525/60, more or less) video, good roll rates are 120, 240,
360, and the like. In 625/50, the good ones are 100, 200, 300, 400, and so
on.
Unfortunately, in 525/59.94 the only two decent rates that are slow enough
to be read are 120 and 240 (and the latter only on a good day!). 625/50
video is better - not only are the roll rates about 20% slower, there are
almost 20% more active scanlines in a frame, so in 625 you can roll at 100,
200, and 300 lines/second without straining any eyeballs.
What about roll rates that aren't integer multiples of the field rate? As
you might guess, as the text moves, its positional relationship to the field
structure no longer follows an integral structure, but changes on a
field-by-field basis. This leads to two things:
1.. Unless the CG you are using offers sub-pixel positioning, the roll
won't be able to execute smoothly, and the text will stutter or judder up
the screen.
2.. The roll motion will "beat" with the field structure, as the scanlines
themselves appear to roll through the text (at a rate proportional to the
difference between the roll rate and the nearest integral multiple of the
field rate), causing time-dependent rippling distortions of the text (the
"crawlies") that look really horrible.
What real CGs do to avoid this is to offer only the "good" rates by default;
extra work is required to set arbitrary roll rates. When you select timed
rolls (the total time is set, rather than the speed), better CGs will fiddle
things to wind up with a good rate. Some may just pick the rate that comes
closest to meeting the desired time; some Chyrons run part of the roll at
one rate, then "shift gears" to finish at a different rate to get the
desired total duration; the A72 (and probably the Texus) "adaptively spaces"
the text in the roll, stretching or compressing the vertical line spacing to
allow a good roll rate to be used and still meet the time target.
What crummy CGs offer in the way of speed control is none at all. For
example in Premiere 5.0, where they've finally understood that folks want to
roll credits, there are no tools for setting or even reporting roll rates in
the titler: one just has to adjust things by trial and error. This stinks:
if you're stuck with such a CG and need to produce for interlaced video,
complain loudly to the vendor about their lousy non-video-aware tools, and
then go look for a CG that does things right ( Inscriber Technology's CG is
one of the few that does; you might also check out Pinnacle's TypeDeko on
the PC, McRobert's Comet CG on the Mac, and Boris Graffiti on PC or Mac.
These may offer proper controls... and there may be others out there as
well. Let me know what you come up with and how well it works - or doesn't).
Post by J***@adobeforums.comI'm having trouble with my rolling titles. They seem to flicker and ripple
no matter which font or size I use. To further explain, if you looked at
the horizontal crossbar of the capital H, for example, you would see it's
thickness expand and contract as it rolled across the screen.
Having successfully created professional looking rolling titles in 6.5,
this is a new problem for me. Anyone have any ideas?