Thread: Re-editing an MPG, quality degradation?

I put together some MPGs consisting of film clips from videos using a simple freeware video cutter. Now I want to re-edit the MPGs to make them shorter. But I no longer have the original source CDs. So if I re-edit the MPG files, the software has to reprocess the scenes into a new MPG file. Does this cause the video to lose quality? Is there a way to do it with no quality loss?

Thanks.

A way to do it with less Image quality loss would be to render the finished file into a format that is not MPEG.

The main problem with editting MPGs is the block edges. MPG compression (warning - gross oversimplification here) divides each frame into small blocks of pixels then calculates how to best compress each block while preserving quality (keeping the viewer from noticing) and achieving the desired bit rate. This results in the block edges not quite matching with adjacent blocks. From frame to frame this error averages out in the human visual process. But if MPG compression is reapplied to the decompressed video, this edge causes the process to create really gross errors because the sharp block edges skew the amount of compression work towards preserving their edge sharpness and not the rest of the frame. This means all the bit rate is working to preserve this distortion, not a good thing.

I've found applying a slight blur to the decompressed MPG video takes the sharpness off the block edges which allows the subsequent recompression to spread the bitrate across more of the frame, producing a better looking result. Still not as good as the original clips, but much better than not blurring.

Thanks for the replies. So you both say some image degradation is inevitable. Yet I'm sure I read a few years ago that some programs can do this without any degradation but I can't find that info now.

It is possible to edit MPEG using Direct stream copy, this will lose no quality however you'll only be able to join clips and cut at key frames.

Thanks, BestOutRunner,

Well, I don't know what Direct Stream Copy is so I googled it and it seems to be something you do in a program called VirtualDub, is that right?

I should point out that these are just plain MPEG-1 low resolution videos. I just want to cut out some scenes without losing quality.

I am not sure what you mean when you say I will only be able to join and cut at key frames. I need to be able to cut wherever it's needed. I don't know what "key frames" means.

I am not a sophisticated video user, you see.

So is Virtual Dub the program I need?