Editing MPG versus AVI/WMV

[dt126]

Hello - I am new to this forum. I have tried searching but have not been able to find an answer to this question, so I figured I would try this forum. I have been editing digital video for some time using Video ReDo Plus ("VRD") for MPG and Cut Assistant (using ASFBIN and VirtualDUB) for AVI and WMV files. The question I have is why am I able to scan back and forth through MPG files so much faster than with AVI or WMV files? I realize that there are many codec's used in AVI files, but it doesn't seem to matter which codec the file is encoded with. When I load an MPG file into VRD, I can literally "fly" through the file using whatever speed I set the hotkeys for (sort of like 10X or 30X FF on a DVR). If I go frame-by-frame (using the scroll wheel) I get each frame as fast as I can scroll the wheel with my finger. For example, if I set a hotkey for 3 second advance and hold down the hot-key it will go through a 60 minute video in less than 30 seconds and show a frame for every 3 second skip.

Using Cut Assistant, or ANY type of playback program for AVI or WMV files (and this includes VLC, MPC, and KMP) playback and higher speeds always has a delay associated with it (more like a slide show than the smooth super high-speed fast forward/reverse I see with VRD). I have gottent to the point that if I have a file where I plan to make more than 15 or 20 cuts, I convert to MPEG2 first just to save lots of time editing the files.

My question is, is this indicative of the different compression schemes? I have tried every utility I can find to re-sequence or otherwise add any missing time code info to the WMV or AVI files but have never found any way to get performance which approaches what I get with the same files converted to MPG compression. Any ideas?

[redwudz]

Probably the simple explanation has to do with keyframes. MPEG has them closely spaced, while highly compressed formats like Xvid/Divx and H.264 have widely spaced keyframes. Xvid uses about a 300 keyframe spacing and other AVI type compressed formats are similar. A keyframe is a complete video frame and the frames in between are just the differences to the next keyframe. That makes those type of AVIs difficult for making frame accurate edits and makes 'scrubbing' though the frames smoothly also a problem.

But others may have a better explanation.

And welcome to our forums.

[dt126]

Thank you. Your explanation makes alot of sense and I never really thought about it that way. Am I correct that I can use VirtualDub or similar to redo the files and choose a much more frequent keyframe rate?

I might experiement with this and see if I can make for faster edits. I realize this will balloon the file size, but I can re-encode after I have my finished product.

[redwudz]

You can adjust the keyframe (I frame) rate in the encoder configuration>Quality presets for Xvid. But there's not much use doing this as the file will be much larger and then you lose the benefit of using a high compression codec. And high compression codecs can have a fair amount of quality loss on a re-encode and you would be doing two of them.

I usually convert to a lossless codec like Lagarith or HuffyUV. Even DV-AVI has all keyframes, but then you would need to use DV resolution. Converting to a lossless codec will give you a huge filesize, but at least you could minimize the quality loss.