Quantcast Loss of Quality of MPEGS - digitalFAQ Forum
  #1  
03-01-2009, 02:17 PM
scogdell scogdell is offline
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Hi,

With the DVDs that DigitalFAQ made for me from my VHS tapes, there is a large MPEG file. What I am doing in Adobe Premiere 4 is bringing the individual files into the project. Then I am spliting the large single clip into numerous smaller clips based on the different scenes. Then I create a single MPEG for each of the many clips. This will give me a large library of all of my video clips. Then, I want to bring the individual MPEG clips into another adobe project and create DVDs. The purpose is to allow me to choose the clips I want for each DVD.

Question is: If I bring in a MPEG, break it down into numerous smaller MPEGS, then bring the smaller MPEGS into another project to make DVDs.............am I losing quality based on the compression each time I bring it in to a new project.

I read the guide but it doesn't specifically discuss loss of quality of MPEGS when they are used in this manner.

Thanks.
Steve
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  #2  
03-01-2009, 11:21 PM
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I think I understand what you're asking, and I think I'll be able to give an answer that makes sense. If not, ask for some more clarification...

Dogma states that "every time you re-encode a video, it loses quality".

However, I'm not a big believer in dogma, especially when my own common sense says otherwise. When we say "loses quality", you and I would probably agree that such a statement means the visual quality has obviously deteriorated to some degree.

Now then, I find that education is often made easy by relying on analogy. To that end, consider this:
  • Re-encoding a compressed file to another compression format is like walking across the living room carpet wearing shoes caked with mud and dog poop, after you put in a long sweaty day outside. That carpet is going to look pretty bad, and probably smell worse.
  • Re-encoding an uncompressed of low-compression file is like walking across the floor after you've had a shower (and completely dried off) and are walking across the floor in freshly laundered socks. Sure, the floor is technically getting "dirty", but there's a big different between clean person+socks and that other scenario!

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If I recall correctly, your video project resulted in both DVD-Video discs, as well as "source" copies (which contained MPEG video files, plus restored audio files).

NOTE: Your first batch of restored audio "source" files contained errors, if memory serves correctly, which was noticed after shipment. The restored audio can be ripped by the DVD-Video versions, and it will be fine. The second batch had no such source-file errors.

Your MPEG files specifically are what would be considered high-bitate (low compression) MPEG files. There is some leeway that would allow the MPEG files to be re-compressed ONE MORE TIME and still look quite decent. There would be virtually no detected quality loss, at least not from the conversion process itself. Only your choice in output settings could potentially harm the video quality, but that could happen with even an uncompressed studio film source, if the wrong encoder choices are made when exporting from Premiere.


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The idea behind editing is that you'll cut up the files, re-arrange, add effects, etc.

SCENARIO: Now let's pretend you're making a "best of" clip show from each disc worth of source material. Let's say there were 6 source discs. Each source video is an hour long. Now let's say your edited clip show was 5 minutes long each. So you've got 30 minutes of video edited into a new series of videos. Now let's say you're going to a family reunion, and that grouchy aunt is only allowing you 20 minutes to present a video. So you've got to cut out 10 minutes.

Now then, if you were to add the 6 edited videos to a new Premiere timeline, and edit out 10 minutes from there, you'd be making a mistake!

Each time you make a new video, it is best to return to the source files, even if you're re-using some of the same edits. One easy way to work with this is to save your project timelines, and then "save as" a copy of the timeline, which can open separately and edit in a different way.

For the best in quality, you'd want to always refer back to your 6 source or "parent" videos, never use a "child" video as the source. While you cannot easily see re-encoding errors along early generations, errors will compound as time goes by, and slowly become more and more visible on each successive re-encode.


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Now then, whether you choose to obey this line of thought largely depends on how lossy the re-encodes are getting. If your "child" videos are also encoded at a very high bitrate -- maybe even in a semi-compressed output format such as HuffYUV or MJPEG (not MPEG, note the "J" in there), or an uncompressed AVI format -- then you'll be fine. Save the real DVD-Video compression in MPEG-2 format for that very last DVD edit.

I do that myself, for complicated projects, where a middle-man encoding step saves headache. It's sometimes entirely necessary for a restoration scenario, where going from source to final edit is impossible, because the source had to be fixed first. Quality loss is mitigated and avoided by using those high bitrate or uncompressed formats.

A 1TB hard drive is very helpful here, be it SATA, USB2 or Firewire.


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Hopefully that answered your question AND it was easy to comprehend.

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  #3  
03-01-2009, 11:28 PM
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Given that your source files are indeed MPEG, there is another choice, a third option not mentioned. To "pre-edit" your videos into a library of source clips, you could splice the file apart using an MPEG editor. I would suggest Womble MPEG Video Wizard for this.These sort of MPEG editors do not re-encode the entire video stream, only the frame or GOP where the edit itself is made. Any quality loss is a blip of less than a second. The video stream is copied without decompression or re-compression.

The video-based guide for Womble was added just a few days ago, and is something available to Premium Members such as yourself: http://www.digitalfaq.com/forum/show...-mpeg-381.html

It's a minor investment, only $34: http://www.shareit.com/product.html?...80eb9dfb5998c7

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