Re: How much is lost when exporting to a QuickTime movie?
- From: dempson@xxxxxxxxxxxxx (David Empson)
- Date: Wed, 12 Mar 2008 23:34:39 +1300
sbt <dogbreath@xxxxxxxxxxxxxxxxxxxxxx> wrote:
In article <47d6f977$0$9032$5402220f@xxxxxxxxxxxxxxx>, Anic297
<here@xxxxxxxxxx> wrote:
I have several mp4 files which I would like to export to ".mov" files
(QuickTime movie) (a ".mov" file seems so much smaller and QuickTime
doesn't seem to handle mp4 files properly (for example, when setting the
position earlier, QuickTime player freezes to about 1 minute or so)).
If a ".mov" file is smaller, I expect something to be lost in the
process, but it does not seem to be (visually).
Also, I wonder what format is a ".mov" file. Does it still have
something to do with the original format from which it has been exported?
Thanks for any advice.
mov is a container format, just as avi is a container format -- this
means that any of a number of codecs can be used to create them and
that the size will vary with the codec, framesize, framerate, bitrate,
and so forth.
mp4 files can use a number of different codecs, framesizes, framerates,
and bitrates and this can result in different sizes as well.
As a result, the question you're asking doesn't really have "an answer"
-- in fact, the mov file can be larger than the mp4 if you choose the
right (or wrong :) ) settings.
Agreed.
The original poster might be able to ask a more useful question if they
open the problematic MP4 files with QuickTime Player and do a Get Info.
Tell us what it says for the format. For example, a file I created
earlier says the following:
Format: H.264 Decoder, 720 x 540, Millions
AAC, Stereo (L R), 48.000 kHz
Chapter Track
The first line is describing the video format used within the MP4 file.
The second line is decribing the audio format.
The other possibly useful piece of information is "FPS" (Frames Per
Second).
The issue with freezing when you try to move back is likely to be a side
effect of the way the video has been encoded. Typically a compressed
video will start with a complete frame, and then some formats use a
"difference" system where each subsequent frame is described in terms of
what changed from the previous frame. This is commonly used with DVDs,
for example (which use MPEG-2 encoding). The video is organised into
"groups of pictures" consisting of a single complete frame of video and
a few difference frames. The pattern then repeats, starting with another
complete video frame.
If your particular video files don't have regularly spaced complete
video frames, then in order to move backwards, QuickTime Player has to
go right back to the beginning and then process each frame in order to
get to the point you selected. This will take an increasing amount of
time depending on how far through the movie you are clicking.
To go forwards, it needs to start from the current frame and scan
through the video, so a large forward jump might also be very slow.
You could correct this by re-encoding the video to make it use more
complete frames and fewer difference frames (if you have the right
software, and the video format is able to support this), hopefully with
very little loss in video quality. The catch is that the file produced
will be larger than the original, because complete video frames take up
more space than difference frames (unless you reduce the quality).
This sort of trick doesn't work for a medium like DVD, where you have a
limit on the rate at which data can be read from the media (for full
compatibility with all DVD players). You can only replace the difference
frames with complete frames if you reduce the quality.
--
David Empson
dempson@xxxxxxxxxxxxx
.
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