[MEncoder-users] Usage of -ofps

[=?utf-8?Q?Bj=F6rn?= Nadrowski]

Hello, 
I would like to ask a question concerning the use of -ofps

Let's suppose I want to prepare a movie for my nokia e90's
internal realplayer.
In order to do this, I use the following commands:

V_BITRATE=800
X_SCALE=640
Y_SCALE=352
V_NAME=film.avi
BASENAME=`echo ${V_NAME} | cut --delimiter='.' --fields=1`

mencoder -ofps $FPS -o temppi.m4v -of rawvideo \
		    -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=${V_BITRATE}:threads=2 \
		    -vf lavcdeint,scale=${X_SCALE}:${Y_SCALE} -lavdopts threads=2 \
		    ${V_NAME}

mplayer -vo null -vc null -ao pcm:fast ${V_NAME}
faac --mpeg-vers 4 audiodump.wav

MP4Box -new -add temppi.m4v -add audiodump.aac -fps $FPS \
   -brand mp42 -isma ${BASENAME}${1}-${V_BITRATE}-${FPS}-${X_SCALE}.mp4

Audio is always fine and faac results into an audio track which has a duration of
2597.2 seconds, which is only 0.1 seconds longer than the film
(deduced using mplayer -v:)
==> Found video stream: 0
====== STREAM Header =====
Type: vids   FCC: xvid (64697678)
Flags: 0
Priority: 0   Language: 0
InitialFrames: 0
Rate: 24000/1001 = 23.976
Start: 0   Len: 62269
Suggested BufferSize: 90898
Quality 10000
Sample size: 0
==========================
and 
62269/23.976=2597.13880547214



Now, above commands seem ok for values of FPS < 23;
for this range of values, the length of the video matches closely the length 
of the audio track. 
However, when approaching FPS=24, the video becomes shorter and shorter,
leading to very substantial audio/video desync, for extreme values, the video 
even plays back in a visibly accelerated speed. 

Is this the expected behavior?
I have no idea where this comes from , and would like to know the commands which 
allow me to specify the FPS value I want (well, it would be sufficient to
know the commands which allow encoding without audio/video desyncing; but not using
ofps and FPS at all does not resolve the problems, in this case, the video is 10 minutes
shorter than the original one...)

I attach a list of FPS - Duration values, 
which I obtained for a series of encodings using different values of FPS and
I extracted the duration of the movie using
MP4Box -info film.mp4

I also attach a figure which displays this curious behavior. 

THanks for any help, Bjoern

#FPS   Duration
10.0	2597.2
10.5	2597.14
11.0	2597.18
11.5	2597.22
12.0	2597.17
12.5	2597.2
13.0	2597.15
13.5	2597.18
14.0	2597.14
14.5	2597.17
15.0	2597.13
15.5	2597.1
16.0	2597.06
16.5	2596.97
17.0	2596.82
17.5	2596.8
18.0	2596.5
18.5	2596.43
19.0	2596.26
19.5	2596.26
20.0	2596.1
20.5	2595.85
21.0	2595.71
21.5	2595.63
22.0	2595.59
22.5	2595.33
23.0	2595.48
23.5	2595.23
24.0	2592.08
24.5	2539.18
25.0	2488.4
25.5	2439.61
26.0	2392.69
26.5	2347.55
27.0	2304.07
27.5	2262.18
28.0	2221.78
28.5	2182.81
29.0	2145.17
29.5	2108.81
30.0	2073.67
30.5	2039.67
31.0	2006.77
31.5	1974.92
32.0	1944.06
32.5	1914.15
33.0	1885.15
33.5	1857.01
34.0	1829.7
34.5	1803.19
35.0	1777.43
35.5	1752.39
36.0	1728.06
36.5	1704.38
37.0	1681.35
37.5	1658.93
38.0	1637.11
38.5	1615.84
39.0	1595.13
39.5	1574.94

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