What is the downside of using unspecified framerate?

[mrttmr]

Hi all,

I am in the process of converting an NTSC VOB file and to prevent skipping frames in the resulting XviD file I used unspecified framerate in FFmpegx which worked fine. (I also chose the bilinear scaling method.)
I noticed that the resulting file-size was much smaller than the one that I ended up with using the NTSC framerate but for the same bitrate. I realised that I could increase the bitrate up to 20% and would still be able to get the file to fit on a 700Mb CD.
I also had to export the movie from within Quicktime as a Save as AVI (provided in the MPEG streamclip package) to sort out the tracking issue in VLC.

All this is good news but is there a downside using unspecified framerate will there be an issue of compatibility with standalone DivX players (knowing that the file plays fine on my macbook pro)?

Many thanks for your reply.

[AlanHK]

I noticed that the resulting file-size was much smaller than the one that I ended up with using the NTSC framerate but for the same bitrate.

I don't use ffmpegX, but a couple of points might be relevant:

What do you mean by "NTSC framerate"?
A DVD VOB has a framerate of 29.97 fps. However, almost always this is achieved by a real framerate of 23.975 with pulldown -- so some frames are duplicated on playback.
You can easily see this by stepping through a sequence where something is moving across the screen and you will see a couple of duplicate frames in every five.

So when converting to other formats it's most efficient to discard these dupe frames and store it at 23.976 fps. This is known as "inverse telecine". There is probably an option to do this in ffmpegx.

If however it's animation, this often has an even lower real framerate, like 15 fps, and the rest are duplicates. Your program may be able to detect this and reduce the framerate to match the actual rate of new frames.

[mrttmr]

Many thanks for your reply.

It looks like a 23.975 VOB.
What is interesting is that by using unspecified framerate the 'show info' option in the encoding progress window shows endless duplicate frames that might have been discarded.
So I guess that we are saving space by discarding duplicate frames but at the same time this allows us to increase bitrates and potentially end up with a better looking conversion.

But is the resulting file compatible with standalone players? Will this encoding approach not cause any stuttering?

[jagabo]

True duplicate frames cost almost nothing in terms of bitrate with Xvid encoding. The encoder just says "show the last frame again".

A VOB encoded at 23.976 fps with pulldown flags will not show duplicate frames if the decoder performs the pulldown. It will show 29.97 fps with two combed frames out of every five frames. You never want to encode that with Xvid. If you deinterlace that before encoding you will get a 29.97 fps video with one duplicate frame in every five frames.

Players that can handle Xvid AVI are usually tolerant of a wide range of frame rates the player won't have problems playing back 29.97 fps or 23.976 fps . But 29.97 fps with duplicate frames will be obviously jerky. AVI files always have a frame rate in the header.

[mrttmr]

Thx Jagabo,

What I do not understand is why do I get smaller file size for the same bitrate just by selecting the Unspecified Framerate Option.
Which led me to significantly increase the bitrate just to fill a 700Mb CD. What I initially used was a 697kbit/s bitrate with a 23.976 Framerate but had some skipping frames and jerkiness in the encoded output so I re-encoded the video with the unspecified Framerate selected but ended up increasing the bitrate to 856kbit/s to have the same filesize. Isn't it weird???

[AlanHK]

What I do not understand is why do I get smaller file size for the same bitrate just by selecting the Unspecified Framerate Option.

What kind of video is this? If live action, almost always this is 23.976 with pulldown.
If animation, they may use different fps for different parts (all padded out to 29.97 with dupes or blends for broadcast/DVD).

[jagabo]

The frame rate should have no effect on bitrate or filesize when using bitrate based encoding.

[Case]

For 'NTSC Film' (23.976 fps with pulldown) VOB source files, ffmpegX sees 23.976 fps, not 29.97 fps. No need to set inverse telecine, as the telecine is "soft".

When using "framerate: undefined", ffmpegX just doesn't specify a target framerate when sending commands to ffmpeg or mencoder. It just leaves it out of the command line, and the encoder doesn't mind. It seems to encode every source frame to an output frame.

I couldn't replicate your results with my PAL test file.

[mrttmr]

@ all,

Many thanks for your comments.
This is a feature movie not an animation. Although it is 23.976 NTSC video, it also has quite a few 29.97 alternating sequences within, as shown by the progress Info window if the conversion is forced at 23.976 framerate, hence my unspecified framerate choice.

One more thing, the difference in size seems to happen mostly with NTSC videos and only if the Bilinear Scaling method is selected (XviD conversion).

[jagabo]

Given the ~25 percent difference in bitrates, and the fact that 29.97 is 25 percent more than 23.976, I suspect the program is getting confused. Many programs can't handle mixed frame rate sources properly.