Using Frame Rate to reduce file size in Compressor

Hi there,

I'm looking to optimize my file size on some H.264 files that we stream online. Does anyone know if the frame rate will influence the end file size? i.e. right now everything I have is 29.97 fps. However, I can reduce it down to 22 fps when I encode this video to H.264 and the change is almost unnoticeable. What I'm wondering is, if I leave my bitrate where it is at (600KB/Sec) will the frame rate itself make any difference.

I can't find an calculator anywhere that will let me specify the frame rate so I can calculate out the difference. Anyone got any input on this?

Thanks

It kinda depends on what you plan to do with these once they're encoded. I would strongly recommend against ever changing the frame rate, both for technical and aesthetic reasons, but that's a personal thing and not objective advice.

But remember, if you're encoding to a fixed bit rate (or, given this is H.264, an average bit rate) absolutely nothing you do will change the size of the encoded file except changing that bit rate. Once you tell your encoder "make this XXXX bits per second," the encoder goes "Okay" and proceeds to parcel out that number of bits per second, either at a fixed or average rate. Reducing the frame size, for example, won't make the finished file any smaller; it'll just give the encoder more bits to use per square pixel of frame, resulting in a cleaner image overall. Changing the frame rate is the same, more or less, except you're screwing with your motion quality in the process. I wouldn't call that a good trade-off, but again, that's me being me, not me trying to be objective.

>Does anyone know if the frame rate will influence the end file size?

I don't think you will see the a smaller resultant file size. On the older codecs, adding more frames usually result in more information (larger file size), but today, all those frames are extremely efficiently compressed such that most of the frames are no longer full picture frames, but rather calculations derived from moving blocks and vectors, which are a lot smaller than storing a full frame.

Reducing the frame rate on the other hand, results in a loss of temporal resolution, but does not necessarily reduce the size of the file.



www.strypesinpost.com

Back when I used to use it (which has been a while), a lower frame rate can be less taxing to play back for a computer. But I'd agree with Jeff Harrell and strypes -- the resulting choppiness usually isn't worth it.

But, in the spirit of experimentation, you could try some tests. Make one movie file with a high frame rate and lowered bit rate, then one with a lower frame rate and higher bit rate. See if you're happy with the playback, quality, and file size. What looks right for one editor/client may not look right for another.


www.derekmok.com

This is great input. Thanks guys. I've experimented with taking our original 29.97 footage and am able to drop it down to 22 fps without any noticeable change. I fired two comparison clips (before and after) off to our broadcast dept here and some other folks who would have a critical eye, and they actually said they like the lower frame-rate footage better.

I'm guessing since I left my bandwidth at 600k/Sec, it was able to leave more detail in each frame, since there were only 22 frame in each second as opposed to 29.97 frames fighting over that bandwidth.

Does that logic stand to reason or can anyone shoot holes in it? That's what I post it for.

Thanks again for the input.

>since there were only 22 frame in each second as opposed to 29.97 frames fighting over that
>bandwidth.

the key to that answer with a temporally compressed codec (h.264 or mpeg2), isn't how many frames you have, but rather how many I frames you have in there. Removing a few B frames/slices won't really result in a much smaller file size.

For example, if you are encoding a talking head, basically not much of the picture is changing from frame to frame, you are unlikely to notice a smaller file size even if you drop the framerate to 12 fps. On the other hand, you are more likely to notice the missing frames when the guy moves his head slightly.



www.strypesinpost.com

Ahaaa. Good point, so my "Keyframe" setting or Iframe is going to be more important than my actual frame rate. Did I understand that correctly?

For H.264, i usually leave it as "automatic". On the other formats (can't remember which it is), usually about 5 seconds for every enforced keyframe.



www.strypesinpost.com