Hi Frank.
The first thing I should point out is that the Export Movie>Settings
function is not where you want to be for creating web-directed output. You
want whatever choices you have under something like "Windows Media Export".
You can use the Export Movie choices to compress video using older codecs
like Indeo, used some years ago for CD play, but these days the bigger bang
for your buck is web compression algorithms, and depending on the tools
available these can include Windows Media Video (wmv), DivX, Real, Quick
Time, and MPEG-4 to name the major players.
Whatever codec you work in the parameters are the same. All web
compressions are known as lossy. Some data is thrown away to reduce the
file size, as it is with still image codecs like jpg. Reducing resolution
and frame rate are biggies, and frame rate reduction is common in web
video, although it will, depending on the source material, sometimes create
jerky video because it leaves out frames that would otherwise convey
smooth, continuous motion. It has no affect on duration, it just holds
longer on a single frame before changing to the next one. If there's a lot
of difference from frame to frame this can create that jerky look.
Keyframes: In DV25 which includes miniDV, DVCAM and DVCPRO, the file is
written at a constant bit rate of 3.6 MBps (MegaBytes per second). Each
frame is a complete picture. Export a few frames and you'll find they are
all the same size (about 126 kp if I recall correctly) which is why you
can't zoom them. There isn't much information there but it's enough to
convey a moving picture. When you want to make a file smaller something has
to give. Compression, in simple terms, works by taking a full frame of
information (keyframe) and then creating follow frames that record only
what has changed between the first and next keyframe. At 30 frames per
second a keyframe every half second might work fine for a talking head and
not so well for a pan following a race car.
Data rate: If you start with DV you're working with files that grow by 3.6
MBps. If you compress to DVD (MPEG-2), you're down to between 2000-9000
kpbs, roughly. This data rate can be handled on DVD Video disks and at the
mid-point by cable and satellite with a set top box to decompress it on the
fly. For high speed Internet, 300-700 kbps is the norm, and handled by the
receiving computer well or not so well depending on a variety of factors.
You get to a lower data rate by compressing your source material. You
select that data rate according to your target audience; DVD, CD, Internet.
I hope that moves toward answering your questions. Feel free to pursue the
subject off-line or on, if no one objects.
David Hurdon
At 08:34 AM 4/6/2005 -0700, you wrote:
Wow David,
That was a great explanation and I saw the differences in the examples at
the link. Of course now, you know some of us would some explanation of
some of the points that you made...
I should say that I was trying to shrink the overall size of a 6 min avi
file to something that would fit on cd, so a client could upload to a
server for streaming.. I was in the export setting box, but it sounds like
the same principles are involved...
Frame Rate: I thought if I changed the frame to something lower it have
the effect of changing the duration, and/or the playback speed. I mean, if
I taped at 30fps, what happens if I encode at 15fps? How will it affect
quality of playback, and how will it affect the final size of the file?
Keyframing: What's that all about ? I know how it works in editing, but
how does it work in encoding?
Data Rate: How does the data rate effect playback and size? Is higher
better? of lower? should I "recompress" always, or what? Maintain data
rate? LOL I'm totally lost in this area..
I appreciate any help you can give me on this. If there is some link you
know with this information, I would appreciate that as well.
Thanks a lot (I'll definitely be saving this answer you gave about encoding
for the net)
-Frank
www.Time-LineProductions.com
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