[MPlayer-users] Framerate vs. monitor refresh rate

[Corey Hickey]

Richard Bromley wrote:
> 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.

Theoretically, yes, but I've never found the lack of synchronization to
cause any noticeable problems. The framerate looks low, of course,
because it is low, but it always looks even to me.

If I did the math right, you should be seeing each frame for either 3
refreshes or 4 refreshes, which means approximately 0.040 seconds or
0.053 seconds, repectively, versus a nominal 0.042 seconds per frame.
Can you really tell the difference? If you're seeing a very uneven
framerate, then it would seem to me that either you're quite sensitive
or something else is wrong, and probably the latter.

You might as well go ahead and post the full output of mplayer when
playing your DVD.

-Corey