Handbrake encoding at Variable FPS when Constant is chosen

[ManiKz]

I've wondered about this for a while now but never got round to finding out why this happens.

Over time I've been backing-up my blu-rays and using handbrake for the encoding of the video. Now when I choose the FPS and the FPS mode, I choose constant framerate but when looking at the finished results with mediainfo, it shows that the FPS is variable and not constant. Why is this?


Results from mediainfo

GeneralFormat                                   : MPEG-4
Format profile                           : Base Media / Version 2
Codec ID                                 : mp42
File size                                : 51.4 MiB
Duration                                 : 1mn 0s
Overall bit rate mode                    : Variable
Overall bit rate                         : 7 189 Kbps
Encoded date                             : UTC 2036-02-06 06:28:16
Tagged date                              : UTC 2036-02-06 06:28:16
Writing application                      : HandBrake 0.10.2 2015060900


Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.1
Format settings, CABAC                   : Yes
Format settings, ReFrames                : 5 frames
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 1mn 0s
Bit rate                                 : 6 506 Kbps
Width                                    : 1 920 pixels
Height                                   : 800 pixels
Display aspect ratio                     : 2.40:1
Frame rate mode                          : Variable
Frame rate                               : 23.976 fps
Minimum frame rate                       : 23.974 fps
Maximum frame rate                       : 23.981 fps
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.177
Stream size                              : 46.8 MiB (91%)
Writing library                          : x264 core 142 r2479 dd79a61
Encoding settings                        : cabac=1 / ref=5 / deblock=1:-2:-2 / analyse=0x3:0x113 / me=umh / subme=8 / psy=1 / psy_rd=1.00:0.25 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=0 / deadzone=6,6 / fast_pskip=1 / chroma_qp_offset=-4 / threads=12 / lookahead_threads=2 / sliced_threads=0 / nr=0 / decimate=0 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=2 / b_adapt=2 / b_bias=0 / direct=3 / weightb=1 / open_gop=0 / weightp=2 / keyint=240 / keyint_min=24 / scenecut=40 / intra_refresh=0 / rc_lookahead=50 / rc=2pass / mbtree=1 / bitrate=6506 / ratetol=1.0 / qcomp=0.80 / qpmin=0 / qpmax=69 / qpstep=4 / cplxblur=20.0 / qblur=0.5 / vbv_maxrate=62500 / vbv_bufsize=78125 / nal_hrd=none / filler=0 / ip_ratio=1.10 / aq=1:0.50
Encoded date                             : UTC 2036-02-06 06:28:16
Tagged date                              : UTC 2036-02-06 06:28:16
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709

If this is in the wrong section then admins/mods, feel free to move this to the correct section.

[Dougster]

If you manually select frame rate instead of using 'Same As Source' you won't have that problem. Yes it is an annoyance in Handbrake.

[ManiKz]

If you manually select frame rate instead of using 'Same As Source' you won't have that problem. Yes it is an annoyance in Handbrake.

Forgot to mention that I do select the framerate manually. The settings I use for framerate are in the image below.

[Baldrick]

You could try latest handbrake beta( https://handbrake.fr/nightly.php ) or vidcoder beta see how they work.

[sneaker]

It's because of the timescale HandBrake uses. 23.976 is 24000/1001 fps rounded so for exact constant frame rate storing a muxer would have to use 24000 as a base for timings and then just stamps by 1001 for each frame. But HandBrake always uses 90000 for the timebase which means you cannot exactly represent 24000/1001 fps. You end up having to alternate between 3753 and for 3754 for the timestamp so you end up with a minimum and maximum of 90000/3753 ~= 23,981 and 90000/3754 ~= 23,974 fps, the average being ~23.976 fps.

[hello_hello]

For whatever reason, MediaInfo seems more likely to "correctly" interpret the frame rate type if the output is MKV rather than MP4, although I'm sure on occasion I've seen it label MKVs as variable frame rate which are supposed to be constant frame rate.

If it really bothers you, try opening the output video with a muxing program and remux while specifying the correct constant frame rate. It shouldn't have any noticeable effect aside from changing the way MediaInfo sees the video. It works when remuxing with MKVMergeGUI but for MP4's I'm not sure. You could try MyMP4BoxGUI to see if it does.

[sneaker]

For whatever reason, MediaInfo seems more likely to "correctly" interpret the frame rate type if the output is MKV rather than MP4, although I'm sure on occasion I've seen it label MKVs as variable frame rate which are supposed to be constant frame rate.

It's not possible to do exact 24000/1001 fps in mkv, you will always have a jitter similar to the one in HandBrake's MP4 files. MediaInfo treating them differently is kind of a double standard, if you will.

[tjack360]

It's because of the timescale HandBrake uses. 23.976 is 24000/1001 fps rounded so for exact constant frame rate storing a muxer would have to use 24000 as a base for timings and then just stamps by 1001 for each frame. But HandBrake always uses 90000 for the timebase which means you cannot exactly represent 24000/1001 fps. You end up having to alternate between 3753 and for 3754 for the timestamp so you end up with a minimum and maximum of 90000/3753 ~= 23,981 and 90000/3754 ~= 23,974 fps, the average being ~23.976 fps.

Fantastic explanation! I always have wondered why that happens with Handbrake but never looked for an answer before now. Thanks!