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Digital Media Converter
v2.78
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Understanding
Compression in AVI |
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Even with the advent of
powerful processors,
bigger hard-drives and
previously unimaginable
Internet speeds, the
need for data compression
can't be
overstated. While text
files and documents take
little space to store
and little bandwidth to transmit;
image,
audio and video files
are a different ball game
altogether. Uncompressed
data, especially video
and audio content,
simply takes up a lot of
space. Compressed
files take up less
space, saving you money
on additional storage
and bandwidth charges. |
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Compression can be
broadly classified into
two types- |
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Lossless |
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2. |
Lossy |
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Lossless |
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In the lossless type of
compression, no data is
lost during the
compression process.
When the compressed file
is decompressed, the
result is identical to
the data before
compression. Computer programs,
software applications,
databases etc. are
compressed using
lossless techniques,
since a change in even
one bit of data can make
them useless or produce
incorrect results.
For most files, lossless
methods reduce the file
size by only 50-60%.
Well known lossless
compression methods
include Zip and RAR. |
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Lossy |
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Lossy compression
reduces file size by
permanently eliminating
redundant information,
so that only a part of
the original data is
retained and reproduced. Most
video and audio
compressors compress
data in ways that are lossy, but
produce very high
compression levels. Properly compressed
video and audio files
are nearly
indistinguishable from
the original to the
human eye or ear. In audio
compression, psychoacoustic
techniques are used
to remove non-audible
components of the audio
signal. Lossy
compression techniques
are known to produce
high compression. |
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Uncompressed AVI files
are huge. AVI codecs (COmpressor – DECompressor) greatly
reduce file sizes
while maintaining
optimum quality.
(Generally, a codec
is a program that
compresses or
decompresses audio or
video. There can
be several
codecs—programs—that can
compress using the same
technique. For
instance, there are
several codecs that
implement MPEG 4 video
compression.) AVI’s
popularity and
extensibility depend on the fact that
you can freely select
different video and
audio codecs.
Other file formats like
MPEG 2 or WMV are not as
flexible. Unlike AVI,
which is more of a
general
specification, MPEG 2 is a
compression
technique itself.
With MPEG, you do not
have the option of
substituting your own codec. This
makes it difficult to
adopt MPEG to use newer
and better compression
technologies. With AVI,
you can just switch to a
better codec. The Windows Media format
(WMV) also uses codecs,
but fewer of them, and
the selection process is
hidden from the user.
You just have to select
an appropriate profile
and the profile will
automatically select a
codec. This makes creating a WMV
file very simple.
Because the process of
making an AVI
file lets you choose
among up to dozens of different
codecs, you face the challenge of selecting
the right codec. |
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Many AVI compressors,
tailored for different
needs, have been
designed. For our
purpose, the key things
to know are the quality
of the video playback
and the compression
ratio for each codec. Which
compressor to choose depends on the
intended use of the file
and the desired
output quality. Many codecs can
be downloaded directly
from the Internet. |
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Digital Media Converter
enables you to control
the frame rate, the
video resolution and the key frame
rate, and to choose the
audio and video
compressors when
creating AVI files. Changing these
parameters is discussed
in
Reducing Video File
Sizes. |
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When making your choice
of compression methods,
consider these key
points about the
following codecs: |
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CINEPAK:
This is a
popular
codec
developed
by Super Mac
Inc and by
Radius. It
handles
Videos that
contain a
lot of
motion quite
well. It is
a good
choice for
distributing AVI files
because it
is included
with
Microsoft
Windows. |
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MICROSOFT
MPEG-4
VIDEO:
MPEG is a
standard
defined by
the Moving
Pictures
Experts
Group. There
are many codecs
complying
with the
MPEG 4
standard,
including
ones from
leading
manufacturers
like Apple,
Ligos, DivX,
etc. The
Microsoft
MPEG-4 codec
gives high
quality
compressions,
is easy to
use and
enjoys good
compatibility. |
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MICROSOFT
VIDEO 1:
This is
a popular
codec
bundled with
all versions
of Windows. Microsoft
Video 1
provides
excellent
video
quality and
reasonable
compression
ratio. |
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MJPEG
(Motion
JPEG):
This is a
video
adaptation
of the JPEG
standard for
images. It
is ideal for
editing and
video
capture.
Many
editing
systems are
built around MJPEG. One
major
advantage is
that
compression
is fast and
can be
carried out in
real time. However,
there are
many
implementations
of MJPEG,
raising
compatibility
issues among
videos
compressed
by different
systems. |
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DivX:
This MPEG-4
compliant
codec
provides
high quality
compression.
The codec
boasts
offers
support for
multiple
languages,
good visual
quality,
speed and
compression. However,
users have
reported
synchronization
issues. Users have
also
reported
compatibility
problems
amongst
files
compressed
using
different
versions of DivX. |
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XviD:
XviD is a
MPEG-4
compliant
video codec
designed to
compress and decompress
digital
video.
It is
designed as
an Open
Source
alternative
to DivX. |
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For almost
any
practical
use, you
will
compress the
audio and
video when
making AVI
files. Compression reduces the
file size,
but reduces
the quality
by an amount
that varies
depending
the settings
chosen. While
different codecs
provide
different
compression
ratios,
a lot
depends on
the specific
settings
that you
select when
creating
your files. In addition
to the codec
selected, output
file size
depends
on the
video dimensions,
the frame
rate, the
output quality desired, and
other
parameters. Overall,
we recommend Microsoft
Video 1 when
distributing
to a wide
audience,
since it has
the widest
compatibility. Xvid is a good
choice for
high quality
compressions. If you are
looking for
fast
compression
times, and have a
large number
of videos, MJPEG gets
the
compression
done very
quickly. |
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