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H264 to MPEG-2 - very different transcode times

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Raven Plenty
H264 to MPEG-2 - very different transcode times
on Sep 25, 2008 at 4:36:50 pm

I have two different H.264 quicktime movies that vary widely in their processing times to convert to MPEG-2 using Compressor (DVD Best Quality 4:3).

Source 1 - 20-minute video (DVD sourced live action video, extracted using Handbrake), 640x480, 2000 kb/s - about 8 hours conversion time (video only, no audio)
Source 2 - 2-minute video (animation sequence created in Combustion, 30 fps), 1280x1024, 4700 kb/s - about 6 minutes conversion time (video only)

Extrapolating the 2 minute video to a 20 minute video puts it up to about 1 hour conversion time. I would have thought it would take longer to convert the 1280x1024 video. Any insights into why there's such a huge difference in conversion times?



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Daniel Low
Re: H264 to MPEG-2 - very different transcode times
on Sep 25, 2008 at 9:17:21 pm

A very good question, did you try just encoding a 2min section of 'source 1' clip to see how long that takes?

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Raven Plenty
Re: H264 to MPEG-2 - very different transcode times
on Sep 26, 2008 at 3:34:12 pm

I'm trying it now. I've made a 2-minute portion in Quicktime by using "trim to selection" then save as... to a new clip. Sure enough, Compressor is estimating about 1 hour to convert to mpeg-2.

Interesting though, I noticed that scrolling around the problem video in Quicktime is very jumpy with lots of lag, whereas the other file (the higher res animation clip that only takes 6 minutes to convert) scrolls much smoother.

So clearly the "problem" video has some issues. I just don't know if this is a normal range of performance with H.264. I would have thought the performance would be proportionate to the files respective bitrates and resolutions. Could Handbrake be producing "poorly encoded" H.264 that is harder to decode? I was kind of thinking that H.264 is H.264.

Learning is fun...



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Daniel Low
Re: H264 to MPEG-2 - very different transcode times
on Sep 26, 2008 at 4:15:05 pm

Handbrake is a very efficient transcoder, it's actually great at producing very high quality pictures at low bit rates. It's designed to rip a 4GB movie and fit it into 650MB without any loss in quality.

What you have found by skipping through the Handbrake clip is that there are far greater distances between H.264's equivalent of I-frames. I-frames contain a full frame of information, all the other frames need to calculate themselves based on that I-frame.

So when compressor starts to process it, it has to look ahead at each frame and each time it lands on anything other than an i-frame it needs to calculate the result and this takes time. The greater the distance between i-frames the longer the calculation.

Look under the 'Options explained' (Frames) section here to see how complex this process can be
http://trac.handbrake.fr/wiki/x264Options

Combustion won't have anything like as complex an encoder and as such it's easier to read from.

If you converted both clips to an I-frame only format you'll find that they both transcode at the same rate.

__________________________________________________________________
Please post back saying what solved your problem. It could help others, and saying 'thanks' is free!


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Raven Plenty
Re: H264 to MPEG-2 - very different transcode times
on Sep 26, 2008 at 4:28:05 pm

Great info Daniel, that makes sense, thanks!

The animation was actually exported to an Animation MOV, then exported to H.264 by Quicktime. Just the same, the Quicktime settings and the Handbrake keyframe settings must have been different.






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