loss of video quality converting vob to dv for iMovie/iDVD projects

[gspanos@comcast.net]

I've been using iMovie/iDVD to edit/author unauthored DVDs. In some instances there is noticeable degradation of video quality when the unauthored DVD is compared to my authored products. I'm wondering why. My standard procedure is to demux the unauthored vob files to m2v, then drop these files on to iMovie, which converts them to dv in preparation for editing and eventual iDVD authoring.

Is there a better way to do this in terms of not losing video quality? And, why would some projects suffer more degradation than others?

[terryj]

WELCOME TO THE WACKY WONDERFUL WORLD OF COMPRESSED VIDEO!!!

Seriously, let's take two examples in your scenario to get you
onto track.

First, the Already authored DVD:

The DVD started out footage that was in, let's say for argument's
sake, DV format. It was captured in as such, edited, and then
Compressed to fit to size of the Target DVD media. If it
was say, 3hrs of footage, and the target was a Standard DVD-5
(single Layer) then it was compressed to say about 3.5MBPS
Variable Rate. Then of course, muxed to VOB with audio,
and then burned.

The variable rate allows for the DVD to be high quality in say,
when there's a momentous Lightsaber Battle, but to be in
less than stellar when it is talking heads seated at the table
in a low level lighted room.
This compression going on is what makes some DVDs better
or worse than others when demuxed...Target size of DISC,
divided by length of footage equals the compression settings,
with slight variances ( Variable vs Constant) depending on
type of footage ( Action versus Narrative, Animation vs Live Action)

You are basically, in trying to get this back into iMovie, undoing
all those steps mentioned. Some of them cannot be done,
and thus as you....sidestep...to get the film into a suitable
Quicktime compatible format that your NLE likes, you make
some sacrifices.

Think of it like a piece of string.
Say the string was originally 3 ft long. ( original footage)
You needed it to be cut down to a foot and half to tie around
something ( compressing down to make the DVD)

Now you want to recreate it, but all you have is the foot and half
of string ( DVD VOB--> demuxed into m2v)

So now, you try and stretch it to get it back to where you can recreate it, but since you don't have the original 3ft piece of string, you try and do "magic math" to recreate it, but
it sill comes out not quite as good ( M2v to Dv Stream
for iMovie, but without it being the original UNCOMPRESSED
source, it creates MORE artifacts trying to re-invent a fascimile
of the wheel).

If you don't want to loose ANY quality period, don't use
video from compressed sources, period. You cannot re-create
the quality that existed before hand, and the quality you see
in DVD playback is due to the laser mechanism in your DVD
Player interperting the playback stream, at a certain bit rate.


In the case of "unauthored DVDs", which I assume you are
talking about DVD-VR? as what i would term "unauthored DVDs"
to be DVD Data discs containing either MPEG-2 files, Raw Video,
or something of that nature where only playback is achieved
by a computer using VLC or Quicktime.

If you mean DVD-VR, well..the VR streams are still somewhat
compressed, but they have not been muxed into traditional
VOB files. So they are more like...well, m2v files, in that
they have been set to a compression algorithim, but
are not muxed,which is a finalizing option for playback.

Here is a suggestion for these files: Edit them in the DVD
Recorder! Cut out what you need to cut, then finalize
the disc.

Then take ffmegx, MPEGStreamclip, Cinematize or Toast7,
and extract and demux the vobs from the disc.
The create your new discs from either Toast 7,
or step up into DVD Studio Pro, where you can drag and drop
the m2v files straight into the program, author, and go.

If however you are stuck with just iLife programs, then
you will need to look into a capture card option, which
will let you send the cable signal directly into the mac
to maintain the highest quality. Canopus' ADVC-100 is
a great capture solution, and I used it for many years.
STF for answers.

[gspanos@comcast.net]

Thanks for that excellent and informative response.

I guess i'm stuck because my sources are usually unauthored "blue screen" DVDs that were recorded in DVD-VR mode from videotapes that I don't have access to. Where I have tapes, I use my "pass-through" camcorder to send the files directly to iMovie, and those results have been pretty good.

Perhaps the reason why some of my projects are more degraded than others is that the tapes might have gone through 2-3 generations before being recorded to DVD, meaning that the "string" you aptly refer to has been stretched mighty thin even before reaching the m2v stage.

The DVDSP solution would be great except for the fact that I'm also sound upgrading the video portion with superior audio sources. I use iMovie to achieve the a/v sync.

Anyway, thanks for your advice and help in navigating this Wacky World of Compressed Video!

[rumplestiltskin]

WELCOME TO THE WACKY WONDERFUL WORLD OF COMPRESSED VIDEO!!!
...{snip}...

Excellent post. Very clear explanations of the wacky world...

[terryj]

U R both welcome!
...and Good Night!