Understanding
Compression in AVI
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.
Compression can be
broadly classified into
two types-
- Lossless
- Lossy
Lossless
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.
Lossy
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.
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.
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.
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.
When making your choice
of compression methods,
consider these key
points about the
following codecs:
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.
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.
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.
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.
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.
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.
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.