[MPlayer-users] Framerate vs. monitor refresh rate

[Richard Bromley]

Hi, I have a question as to video framerate and display refresh rate
(actually, more of an exposition of a little theory of mine).

For a long time, I'd struggled to get perfectly smooth playback out of
progressive NTSC DVDs at 23.976 fps. I knew I had a perfectly capable
system and felt confident all of my I/O parameters, etc. were properly
configured. All in all, the video had a very uneven feeling to it,
with lots of what looked like frame drops, though not to such an
extent as to make it unwatchable. Recently, however, I discovered that
playing back the video at 25 fps using the -speed parameter resulted
in nearly perfectly smooth display.

Now, I realize that a higher framerate in general results perceptually
in smoother video, but I am certain this is not the full explanation
for the visual improvement. In particular, it does not explain the
disappearance of the *unevenness* of the playback at 23.976 fps. I
realized at one point that a possible explanation was that my LCD
refresh rate is a perfect multiple of 25 fps (75 Hz). Logically, it
makes sense that if one's refresh rate is not a perfect multiple of
the video rate, some unevenness should result.

However, I know little to nothing about LCDs, CRTs, or any other kind
of display device, and so I have no idea if my explanation has any
element of truth to it. However, I did conduct a quick test of my
theory. I switched my LCD to 72 Hz and played back the video at 24
fps. The result, again, was a drastic improvement in smoothness,
though occasional frame drops were still observed, which I believe to
be part of normal A-V sync.

So, the upshot is that smooth video (on an LCD display in particular)
is only possible if the refresh rate is a multiple of the framerate.
Is this correct? Commentary is requested and greatly appreciated.