Video speed change may be achieved by varying the video frame rate. Video editors such as
VirtualDubMod can do the job.
I just tried this on the
Uta Doki! #082 clip using VirtualDubMod. The original clip has the following properties:
File : 61 MB, duration:
0:03:59, type: AVI, 1 audio stream(s), quality: 84 %, VirtualDubMod 1.5.4.1 (build 2066/release)
Video : 57 MB, 2015 Kbps,
29.970 fps, 640*480 (4:3), DX50 = DivXNetworks Divx v5
Audio : 3.64 MB, 128 Kbps, 44100 Hz, 2 channels, 0x55 = Lame MP3
Load it in VirtualDubMod and click on
Frame Rate... under the
Video menu. Under
Source rate adjustment, specify to change to, say, 36 frames per second, while leaving all other options untouched. Then save the video to a new avi file using
direct stream copy. Since the same number of frames in the new file are now displayed at a higher rate, the playback time is shortened. The audio stream, however, is unchanged (i.e. the audio will be played at the normal rate,) hence the audio and video won't synchronize in the new file, and basically the audio stream is cut short:
File : 60 MB, duration:
0:03:19, type: AVI, 1 audio stream(s), quality: 86 %, VirtualDubMod 1.5.10.2 (build 2540/release)
Video : 57 MB, 2420 Kbps,
35.999 fps, 640*480 (4:3), DX50 = DivXNetworks Divx v5
Audio : 3.03 MB, 128 Kbps, 44100 Hz, 2 channels, 0x55 = Lame MP3
Now, the frame rate in the above file is a non-standard 36fps, which might cause playback compatibility problem or might cause trouble during further processing. But no worry, we can actually force the frame rate to a regular one. Again, open the original file in VirtualDubMod and bring up the
Video frame rate control box. This time, in addition to specifying a source rate adjustment to 36fps, we also highlight the
Convert to fps radio button under
Frame rate conversion and give it a regular rate, say, 29.97. This way, the editor will automatically
drop certain number of frames at a certain rate so that the end product will have the same duration as the higher-frame-rate video. Again, audio and video are desynchronized, and the audio stream is cut short. Save it as another avi file using direct stream copy, and we get this:
File : 50 MB, duration:
0:03:19, type: AVI, 1 audio stream(s), quality: 84 %, VirtualDubMod 1.5.10.2 (build 2540/release)
Video : 47 MB, 2016 Kbps,
29.970 fps, 640*480 (4:3), DX50 = DivXNetworks Divx v5
Audio : 3.03 MB, 128 Kbps, 44100 Hz, 2 channels, 0x55 = Lame MP3
If you want to alter the audio speed to sync, then there'll be a little more work involved. First, demux the MP3 audio stream using VirtualDubMod to a standalone audio file, then use one of many audio editors, say,
GoldWave, to speed up the audio. (In GoldWave it's under
Effects ->
Time warp, where you can specify the exact length you want, which should be the length of the new video file.) Then save the modified audio to a new MP3 file. The last step is to use something like VirtualDubMod to remux the sped-up audio and video files. In VirtualDubMod, open the new video file, then, under
Streams ->
Stream list, add the new audio file and disable the old audio stream. Make sure the earlier video frame rate changes are cleared in the settings. Save it to a new avi file using direct stream copy, and we have the following, where audio and video are both sped up and synchronized*, and the audio is not cut short:
i) if the higher frame rate has been used:
File : 60 MB, duration:
0:03:19, type: AVI, 1 audio stream(s), quality: 86 %, VirtualDubMod 1.5.10.2 (build 2540/release)
Video : 57 MB, 2420 Kbps,
35.999 fps, 640*480 (4:3), DX50 = DivXNetworks Divx v5
Audio : 3.03 MB, 128 Kbps, 44100 Hz, 2 channels, 0x55 = Lame MP3
ii) if frames have been dropped to maintain the frame rate:
File : 50 MB, duration:
0:03:19, type: AVI, 1 audio stream(s), quality: 84 %, VirtualDubMod 1.5.10.2 (build 2540/release)
Video : 47 MB, 2016 Kbps,
29.970 fps, 640*480 (4:3), DX50 = DivXNetworks Divx v5
Audio : 3.03 MB, 128 Kbps, 44100 Hz, 2 channels, 0x55 = Lame MP3
* This is true if you use the higher frame rate approach. If, instead, frames are dropped during video speeding-up, one may notice that A/V is out of sync from time to time when playing back the final product.
