I have been very active in video encoding for several years now and
have a good deal of experience with various file formats and encoding
methods.
AVI is a video format that can use any number of codecs for both audio
and video. A codec is a piece of software that takes one type of video
and encodes it to another type, and later decodes it back to a raw
format for display on a computer. Most codecs that have been made
compatible with the avi specification are not well suited to high
performance, high quality, low filesize videos. The big exception is
Divx, which can be downloaded from divx.com, and has become the file
format of choice for pirating movies on peer to peer networks due to
its high quality and low filesize. A good amount of useful information
on how to create divx format videos is available at the following
website: http://www.divx-digest.com/
Divx files can be encoded at whatever resolutions you may want, but it
generally is not useful to encode them at a higher resolution than
your camcorder supports. For digital camcorders this resolution is
720*480, which looks great in full screen mode with the divx codec,
even on a computer running at 1024*768. If you have an analog
camcorder, 720*480 is actually better resolution than what the
camcorder is capable of storing on its cassette.
Almost all video players can bring a video into full screen mode that
is at a lower resolution than the screen. There are actually drawbacks
to having the video itself be encoded at full 1024*768 resolution.
Namely, filesize will be increased, and frame rate will be decreased.
Many computers that were built more than 1 or 2 years ago, especially
laptops or computers with cheap graphics processors, would have a hard
time playing back a full 1024*768 video without skipping and jerking.
For these reasons, I would not recommend encoding at anything greater
than 720*480 resolution. Incidentally, this is the resolution used for
DVD video.
On to sizes:
Filesizes of videos are indeed linear. A 30 second clip will take up
about 1/6th as much space as a 3 minute clip. Action and such does
play a big role in the ability of a compression format to maintain
adaquate quality at a given bitrate, so less action typically equates
to a more crisp picture and a slightly smaller filesize. With the divx
codec and most others, you can specify the bitrate you would like to
encode at or to encode between a range of bitrates while attempting to
keep quality unifrom throughout the video. Since it sounds like you
are looking for the highest quality possible, I would tell you that a
very excellent (Nearly DVD quality) Divx file typically takes 1000
megabytes for a full length feature film, or 600 megs for a slightly
lower quality one. This amounts to about 5-8 megabytes per minute of
video. A 30 second video clip then should take up about 3 megabytes of
space if it is encoded at very high quality using the divx format.
If the divx format is unsuitable, there are three other kinds of
formats that lend themselves to high quality.
One is MPEG1, also known as VCD (video cd), and these videos typically
take up about 10 megabytes per minute of video. The maximum resolution
of this format is 352*240 and it's quality is akin to VHS tape.
The next higher up format is known as SVCD (or super video cd), and
increases both the resolution and the bitrate. Svcds are typically
encoded at 480 by 480 resolution and takes up about 20 megabytes per
minute of video. The quality of this is better than vhs, and is
slightly lower quality video than direc tv satellite service. Svcds
are encoded in a format known as MPEG2, which is the same encoding as
DVDs, except that svcds are lower resolution and lower bitrate.
The last format to speak of is DVD video, which currently is the best
format you might use in terms of image quality. Filesize for DVDs is
about 40 megabytes per minute of video. Image resolution is 720*480
and you can fit up to 120 minutes of this video on a recordable dvd
(dvd-r). This file format is not recommended for transfer over the
internet due to its large file size.
To fully answer your first question, size range, the size will not
vary too much for different types of things. For a completly
non-action video like a powerpoint presentation, expect the filesize
to be about half that for an "action packed thriller" so to speak. For
the kind of variations you might see in home movies, a reletivly
inactive scene should not vary by more than 15% from that of a
somewhat more active scene. However, all of this is configurable for
divx. The various mpeg formats (vcd, svcd, dvd) may impose a fixed
bitrate to be compatible with the widest variety of hardware playback
devices, and thus the amount of action will only affect image quality,
not file size.
Finally, to answer your third question, I have never come accross any
video encoded at 1024*768 resolution other than some small experiments
with 3d rendering. In any case, 1024*768 resolution is not going to
improve video quality in any case due to there not being available any
camcorders that can encode at this resolution. However, a video
encoded at 720*480 is available at the following website:
http://asuaf.org/~funkywizard/tourneydivx.avi . It is 22 megs large,
2.5 minutes long, and was encoded with the divx codec. I applied some
filters to the video to compensate for low lighting quality. The audio
was compressed in the mp3 format.
It should be noted that in order to play back this video skip free, a
computer with either a decent graphics card (nvidia or ati
recommended) or a 1ghz+ processor is recommended. It does not play
well for instance, on the laptop I am using to write this answer,
which has a 700 mhz celeron processor and an intergrated graphics
chip. It does however, play fine on a Pentium II-400mhz laptop with a
better (ati rage 128) video chip. To play videos wihtout skipping on
computers such as the celeron I used as an example, VCD video (MPEG1)
or lower resolution divx video, is recommended.
Some jagginess and overcompression can be noted in the video I
provided due to using an older version of the divx codec and less
pre-processing. If I had enabled some of the more advanced encodings
available in the newewst versions of divx (which was not available at
the time), the example video at that filesize would look much more
professional. Indeed, I have seen many DVD rips of movies that look
just as good as DVD quality at the same filesize as the provided
video.
Search terms:
dvd video bitrate spec
svcd bitrate
dvd bitrate |
