does mpeg capture degrade during edit?

raygo · Post by **raygo** » Fri Sep 08, 2006 12:00 am

hi,
I have a pretty fast pc [ 3.0 G processor/ 1gig ram].

I have completed a few projects with Video 9 and although I have plenty of resources and the rule of thumb is that I am equipped to capture in mpeg format for render speed sake,

I always captured my home video in AVI format because I have been of the impression that if you are going to do a lot of editing, MPEG will degrade somewhat the more u edit. same as a .jpeg picture will degrade vrs a bmp or Tif?

[I played around with my last project for a few months, changing video clips many, many times before I was satisfied enough to convert to mpeg then burn. I use TMPGEnc for the encoding part then back to 9 for the rest]
so...... am I correct to capture and edit as AVI even though my pc is more that enough for mpeg capture?

'Best looking final video for buring to dvd is my whole motive here' Thanks...

jchunter · Post by **jchunter** » Fri Sep 08, 2006 12:38 am

Mpeg2 editing has come a long way in the last several years, from the bad old days of buggy mpeg2 codecs and slow computers.

My tests with 10+ show no measurable loss of resolution after editing and smart-rendering Mpeg2 with a video bitrate of at least 8Mbps. These tests repeated the edit/smart-render sequence five times on the same clip and measured resolution with an ISO12233 resolution chart after each render. These edits were typical cuts (scissors control) and deletes. Smart rendering avoids re-encoding unedited video and only affects the cut/join junctions. I also ran similar tests with smart-render turned off and did not observe reduction in resolution.

This covers 99% of my video editing tasks.

I have not tested extreme editing that globally alters, say, color balance for a large section of video, where smart rendering will not be possible and the entire video may have to be decompressed, altered, and recompressed. De-shakers will have a similar effect. If you intend to do such editing, you may find that DV offers better edited quality.

DVDDoug · Post by **DVDDoug** » Fri Sep 08, 2006 12:38 am

There is some controversy here. Just a fair warning that I’m biased!

I’m on the anti-MPEG side, because I’ve experienced the infamous “lip-sync” problem as well as occasional crashing with edited MPEGs.

although I have plenty of power and the rule of thumb is that you are ok to capture in mpeg format

Even a fast computer can’t perform 2-pass encoding on the fly. And, “real time” processing on a PC makes be nervous. The computer is fast enough on average, but the multitasking operating system is always doing stuff in the background. This really isn’t really a big deal since there’s always a memory-buffer to allow the computer to deal with these little interrupts… It’s just something that makes me nervous.

MPEG will degrade somewhat same as a .jpeg picture will degrade vrs a bmp or Tif?

Yes. Like JPEG, MPEG is lossy compression. If you decode an MPEG to edit it, and then re-encode it, you’ve gone through an extra (lossy) encode step. And, most “real editing” requires the MPEG to be decoded first. You can hard-cut & hard-splice without decoding. Again, there are others who will tell you there is no degradation, or at least no detectable degradation. DV (AVI/DV) is lossy compression too, but it’s not as compressed.

ghoofie · Post by **ghoofie** » Fri Sep 08, 2006 3:50 pm

What am I not understanding in this thread ? Why would editing an mpeg file cause it to lose quality ? If I'm working on a project and deleting parts of the video, etc. I'm only saving the .VSP file but that doesn't REALLY do anything to the mpeg file. Once I burn a dvd or file, it creates an entire new file. The original file doesn't really get cut.
So are we saying that merely opening a mpeg file will deteriorate it ?

I was thinking if that's case...I'm working on a project right now and I have a copy of the mpeg file saved off. So when I'm actually ready to finally burn the project...if the mpeg file I've been using is now deteriorated, then I'll delete THAT mpeg and bring the saved mpeg file which has just been sitting out on another drive as backup .

Trevor Andrew · Post by **Trevor Andrew** » Fri Sep 08, 2006 4:27 pm

ghoofie wrote:What am I not understanding in this thread ? Why would editing an mpeg file cause it to lose quality ? If I'm working on a project and deleting parts of the video, etc. I'm only saving the .VSP file but that doesn't REALLY do anything to the mpeg file. Once I burn a dvd or file, it creates an entire new file. The original file doesn't really get cut. Correct

So are we saying that merely opening a mpeg file will deteriorate it ? No it will not.

I was thinking if that's case...I'm working on a project right now and I have a copy of the mpeg file saved off. So when I'm actually ready to finally burn the project...if the mpeg file I've been using is now deteriorated, then I'll delete THAT mpeg and bring the saved mpeg file which has just been sitting out on another drive as backup Do not do that, you will cause a lot of problems with re-linking.

When working on a project, editing/cutting the clips adding filters, you are not altering the original video file.
Video Studio creates links/pointers to the file.

When you Share-Create Video File you create a new file from the project, using the data from the original.
The process of putting all the info together is called rendering.

The new file may not be as good a quality as the original.
Repeating this process on the new file may produce a further lower quality video file.

Many believe that there is no quality loss, whilst others frown on rendering Mpegs.
The subject has been discussed many times.

Ok
You never change the quality of the original file.
Video studio creates a new file and does not overwrite the original.

As you are working with mpeg after editing you should create a new Mpeg video file.
Share Create Video File- Same as first clip, should create a file using the same properties as the original.
You then use this file in a new project Share Create Disc-Add Video (using the new file.)

Trevor

jchunter · Post by **jchunter** » Fri Sep 08, 2006 4:29 pm

Ghoofie,

You have it right - opening a video file does not cause it to lose quality.

Video files are not immediately modified when you perform virtual cuts (e.g., with the scissors control) because these are simply pointers, stored in the project file, to locations in the video file where actual cuts will be made when the project is rendered into a video file.

You can think of this as a "potential" edit until the Video file creation.

Not so, with the "Save Trimmed Video" control, which immediately renders a new video file at the Marked cut locations.

In any case, my tests actually forced the creation of a video file and then re-edited that and smart rendered again, etc.. The level of detioration was not measurable when this sequence was repeated five times.

Trevor Andrew · Post by **Trevor Andrew** » Fri Sep 08, 2006 4:33 pm

Hi
A bit more

Your Mpeg video file should be Dvd compliant.

Its properties should be compatible for making a dvd

Odd sizes and non-compatible properties may give problems.

Trevor

jchunter · Post by **jchunter** » Fri Sep 08, 2006 4:37 pm

Adding to Trevor's comment, if you change the video field order from Lower Field First to Upper Field First (or vice versa) you will cause fairly serious deterioration in the picture quality.

raygo · Post by **raygo** » Fri Sep 08, 2006 11:04 pm

[quote]When you Share-Create Video File you create a new file from the project, using the data from the original.
The process of putting all the info together is called rendering.

The new file may not be as good a quality as the original.
Repeating this process on the new file may produce a further lower quality video file.

since I started this thread, I think the above is what I was getting at assuming u you start your project off with an mpeg capture and continue on [repeating the process]
Thanks for all the input.. very informative

ghoofie · Post by **ghoofie** » Fri Sep 08, 2006 11:46 pm

I was thinking if that's case...I'm working on a project right now and I have a copy of the mpeg file saved off. So when I'm actually ready to finally burn the project...if the mpeg file I've been using is now deteriorated, then I'll delete THAT mpeg and bring the saved mpeg file which has just been sitting out on another drive as backup Do not do that, you will cause a lot of problems with re-linking.

Well allright !! It all basically works the way I thought..but I wonder why I would have a re-linking problem in this scenario:

I captured a video.....it's on my hard drive and is named AAA.mpg.

I then did a copy/paste of AAA.mpg putting it on a backup external drive.

I'm working on the project...and have my VSP file saved with all my added audio, etc. No work at all done to the actual video I captured, just adding audio tracks. So if I accidently delete AAA.mpg on my hard drive in my library directory.... and then I copy/paste my backup of AAA.mpg into the library directory. When I open VS9 and open the project, you mean that VS9 will not see the AAA.mpg the same way it saw the original one. It's just a copy of the original. I thought VS9 would not even 'know' the difference.

Anyway...I agree with the other poster....very informative information here.

raygo · Post by **raygo** » Sat Sep 09, 2006 2:48 pm

As a foot note here,

The bottom line I am assuming is: If u capture as mpeg, you can cut,snip, remove,add,save, open up, do it again, do whatever to the VSP[mpeg] project as many times as needed and the project footage will not degrade as long as you have not rendered it, and it is the Re-rendering that will cause a project to degrade a bit. correct?

Trevor Andrew · Post by **Trevor Andrew** » Sat Sep 09, 2006 7:23 pm

ghoofie wrote:
I'm working on the project...and have my VSP file saved with all my added audio, etc. No work at all done to the actual video I captured, just adding audio tracks. So if I accidently delete AAA.mpg on my hard drive in my library directory.... and then I copy/paste my backup of AAA.mpg into the library directory. When I open VS9 and open the project, you mean that VS9 will not see the AAA.mpg the same way it saw the original one. It's just a copy of the original. I thought VS9 would not even 'know' the difference.

Anyway...I agree with the other poster....very informative information here.

Instead of deleting your Mpeg just rename it.

Now copy and paste the back-up file to your hard drive.
This is as though you have replaced the files.
When you open your project VS will ask for a re-link. this process is normaly quiite straight forward, but sometimes can be a bit of a pain.

As you have only the one video file the process will be easy.

To make things worse you may be able to link to the wrong file provided the new file is longer than the first.

Trevor

jchunter · Post by **jchunter** » Sat Sep 09, 2006 8:22 pm

raygo wrote:As a foot note here,
The bottom line I am assuming is: If u capture as mpeg, you can cut,snip, remove,add,save, open up, do it again, do whatever to the VSP[mpeg] project as many times as needed and the project footage will not degrade as long as you have not rendered it, and it is the Re-rendering that will cause a project to degrade a bit. correct?

My tests showed that "degredation" is not detectable even if you edit, then render, then re-edit that rendered clip, then re-render, then re-re-edit that re-rendered clip again, ad nauseum...

This should demonstrate to everyone that mpeg2 is fully editable in VS10+.