Video Quality

  • Thread starter Thread starter Marshall Karp
  • Start date Start date
M

Marshall Karp

Hi Papajohn,

Let me see if I have this straight.

As one uses a digital video camera, the video is being saved in an avi
format.

When I capture the video to an avi format using MM, it is the original avi
file? In other words, there is no loss of quality as it is the same 1's and
0's as what is on the digital video camera cassette?

When I create and import video clips into a MM and onto the project
timeline, it is still in the original AVI format? There is still no loss in
quality as it is the same 1's and 0's as what was on the digital video
camera cassette?

When finished, I save the movie as an avi and it is still the same quality
as the original AVI on the digital video camera cassette?

When I use Nero 5 to make a video cd, then I am finally getting some quality
loss as I am going from a gigs big avi file format to the hundreds mb VCD
format?

Or am I experiencing degradation (copy of a copy of copy) all along the way?

Thank you.

Marshall Karp
 
Or am I experiencing degradation (copy of a copy of copy) all along the
way?
Click to expand...

Almost!

DV AVI itself is a lossy compression format, so saving multiple times, using
the last output as input would increase the loss exponentially.

On top of that internal working of Movie Maker is such that it also incurs
some loss when you apply any transition or effect.

However the losses in these steps are marginal and not really noticeable.

Conversion from AVI to MPEG2 for SVCD or VCD is the place where you can
incur most notable loss of quality since it changes the size of frame as
well as shape and aspect ratio of the encoded pixels. Hence you must use a
high quality encoder to achieve this step if preserving quality is an issue.
(like TMPGenc)

Rehan
 
Hello there,

AVI is a wrapper for a variety of other compression algorithms that
might range from MJPeg to DivX. I have today finished writing a program that
automatically creates a DivX file out of any other format....the resulting
file has an AVI extension and is importable to Movie Maker etc.....

I have no experience of this my self, I am told that you cannot always
take a tape from one camera and place it in another make of camera and get
it to work. A reason for that would be the data stored on the tape could be
in some proprietary format (I "do" know that this happens with digital still
images) The file would be converted as it was output to whatever device
(computer etc) If you are transferring data to and from the same camera this
may not of course affect you. It would be an interesting experiment, if some
one wants a way to wear their camera out, to repeatedly copy a video to
camera and back to PC a large number of times to see if any degradation does
occur.

Converting the video to VCD is almost the equivalent of using two tins
and a length of string....SVCD would be quite a bit better if you have that
option.

Best Wishes.....John Kelly
www.the-kellys.org
www.the-kellys.co.uk
 
DV AVI itself is a lossy compression format, so saving multiple times, using
the last output as input would increase the loss exponentially.


Rehan
Click to expand...

I have done the above and have not noticed any visible differences in the
video (or audio), but I did come up with a slight difference in file size.
I imported into Movie Maker a small test file, test_0000.avi, saved it as
DV-AVI, then imported that one and saved, etc. Each test file got a bit
smaller (see below) but I can not see or hear any differences playing them
back on my computer.

test_0000.avi 25,215,488
test_0001.avi 25,095,168
test_0002.avi 24,974,848
test_0003.avi 24,854,528

So I now wonder if I continued when would the file size stop decreasing?

I opened the files in VirtualDub and found that the number of frames
decreased by one frame each time.

test_0000.avi 209 frames
test_0001.avi 208 frames
test_0002.avi 207 frames
test_0003.avi 206 frames

Go figure, at this point I stopped my testing...

Rich
 
Rich

This is really interesting.

It seems to suggest that my theoretical assertion that repeated saving would
increase loss may not be correct (or needs some qualification). As mentioned
by Papajohn the one frame loss you are noticing is caused by an unrelated
bug in Movie Maker where by it drops every 27th frame. So if that bug gets
fixed then in principle we should be able to maintain the video quality
during cyclic saving.

This suggests that dv avi codec is smart enough to sense and identify bits
that are already compressed and does not compress them again. Now I think of
it, I can recall that same thing happens with JPEG which is also lossy
format but cyclic saving of a jpeg image wont increase the quality loss,
because the algorithm used by most packages (I tested with an old version of
painstshop pro) is smart and does not recompress already compressed data.

However I would guess it wont be able to use this trick if you modify the
video in some manner like add titles, effects or transitions.
 
Lot's of possibilities, it may vary from editor to editor (and which codecs
are used). Perhaps movie maker just resaves the file with no change if no
changes were made to the file. (Other than the 27th frame). Is there a bit
for each frame to indcate the frame has been manipulated?

I have not gotten around to doing the same test and comparing one frame bit
by bit to see if there are any changes. I just compared them visually.

Rich
 
Rich,

The summary results of testing last summer is on the Problem Solving > Video
Issues page of my site..... the Generational Loss section.

Movies rendered to DV-AVI dropped the 27th frame of each clip on the
timeline for a few generations before stabilizing..... Those rendered to WMV
dropped the last frame each time through all generations until reaching the
total size of a single clip.

I made the test clip with the number on each frame.... using Photoshop....
so I could go through each frame of the rendered movies to see if any
numbers were missing.... the clue that something might be missing was the
slight reduction in total file size.
--
PapaJohn
Movie Maker 2: www.papajohn.org
PhotoStory 2: www.photostory.papajohn.org
..
..
 
Thanks PapaJohn. As I went back and started to read it, I vaguely recalled
having read it in the past. At times I have this brain/RAM problem...

Rich
 
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