compression is the process of reducing the number of bits and
bytes needed to represent a given set of data. Compression takes
advantage of redundancies or similarities in the data file.
By reducing the number of bits and bytes used to store a set of
data, we not only reduce the space required to store it, we also
reduce the bandwidth needed to transmit it.
simplest example: if we have a media file of nothing, the file
would be composed of binary zeros. Obviously we do not need to
store the whole file, instead we could store a
representation of the number of zero bits. This example can
be expanded to a more complex, and realistic file. Let's say
we have a video file with lots of blue sky and little
variation. The areas of blue sky can be represented in a
reduced number of bytes because that data does not change between
frames or within the same frame.
So why is
compression so important for audio and video files? Because
digital audio and video are data intensive. Audio files can be
large, up to 10 MB for ten minutes of plain speech. Music
files are larger. Digital video files can be enormous. An
uncompressed video file with lots of motion and changes in camera
angles can be 1 Gigabyte for 5 minutes of video. Even
videos with less motion ( five minutes 300 MB) are well beyond the
computational and storage capabilities of most computers.
Therefore, compression is required simply to make computer
handling of video economically feasible.
All digital compression
is a gamble. You are betting on redundancies and similarities in
the data that can be "compressed out." Fortunately,
video usually has a
significant amount of such similarities. However, it is sometimes
possible for a video clip, realistic or not, to overwhelm the
compression software so that
it cannot maintain a quality image. Although this is infrequent,
and most often caused by inexperience with the variables,
these failures cause many to give up compression (and video)
Because compression is
not much use without a way to decompress and recover the original
data, we normally speak of compressor-decompressor pairs, or
Which codec you choose
depends on several variables:
number of frames per second (fps) and quality are inter-related.
More frames per second generates a higher quality video if there
is high motion content but only if a corresponding key frame rate
is chosen. Quality will not degrade as quickly with
fewer frames if there is low motion. You should always reduce the
final file size by tailoring the key frame and frame rate not
limiting the data rate. Limiting the data rate creates an upper
limit which the compression settings cannot exceed and the
compression will fail to produce a quality image.
connection speed of the client matters because it dictates the
acceptable final file size. You can think of client
connections as a spectrum from the lowest to the highest speeds:
end 56 Kbps...... T1/LAN or Cable DSL ..... Gigabit Ethernet
connections can span anywhere from 56Kbps to 11 Mbps depending on
the number of users.)
In a college setting,
many non-traditional students may have slower connections speeds
but students in the dorms may be on the campus ethernet.
This is further complicated by the processing speed of the
client's machine. Sorenson 3 codecs
will bring better video quality to the low end but obviously
processes better on faster machines.
There are two kinds of
Codecs: Lossy and Lossless
Some codecs use lossless
compression, which ensures
that all of the information in the original clip is preserved
after compression. This maintains the full quality of the
original, which makes lossless compression
useful for final-cut editing or moving clips between systems.
However, preserving the original level of quality limits the
degree to which you can lower the data rate and file size, and the
resulting data rate may be too high for smooth playback on many
systems. Unfortunately, lossless compressors
cannot achieve the level of compression we need for video. Lossless
are hard pressed to deliver even a 2:1 compression ratio.
This is because lossless compression
is intraframe--every frame is a key
frame. The data is compared to itself within the frame. For
example, one whole section of the frame may be orange. This
information is represented by fewer bits/bytes than if the color
varied. DV (digital video) and
MPG 2 compressions are intraframe
Other compression methods
discard some of the original data during compression. This is
called lossy compression.
For example, if the pixels making up an leaf actually contain 90
shades of green, a lossy codec set
for less-than-best quality may record only 30 shades of green. Lossy
codecs also compare information between frames (interframe).
If data does not change between frames, the information is not
encoded. For example, a video clip of a tree in a field with
a plane flying in the distance would require only the data for the
plane's movement to be encoded in every frame.
compression allows you to trade off picture quality and
file size to lower the data rate for your audience. Lossy
compression allows much lower data rates and file sizes
than lossless compression, so lossy codecs are commonly used for
final production of video delivered using CD-ROM or the
Some codecs are always lossy,
such as JPEG, or always lossless,
such as Planar RGB. Other codecs may or may not be lossy,
usually depending on the settings you specify for the Quality and
Data Rate options–lowering the value for these options saves
more space by discarding more data. Many lossy
codecs can achieve 10:1 compression or more without visible
degradation of the image.
MPG 2 (16:1 compression), Sorenson 3
Apple photo jpeg file, can be used with PowerPoint
JPEG- three qualty settings,
including enhanced still from movie
BMP - not truly cross-platform for
QTIF - QuickTime Image Format
used for audio CDs
Wav- Not truly cross-platform for
Qualcomm Pure Voice - the most
flexible of the voice compressors, has variable settings. Best choice
for normal speech.
MP3 - for high qualty music files.
Best choice for slurred or guttural voices or highly intonated speech.
QuickTime audio- uses othe codecs
but does not produce the quality/size ratio available with Qualcomm
Provides high saturation color, no anti-aliasing (smooth lines and
curves). Use for Flash files.
Apple Compression Video- Digital
Video (DV) with no compression (27 Mbps QuickTime File)
AVI - creates large but good
quality files, useful for CD-ROMs or very short files
Cinepak - Older compression,
creates large files. Can be used for CD-ROM files. Does have
variable data rate for playback. Picture quality drops noticeable at
data rates below 30 MBps.
DV - (NTSC or PAL) Digital video
formats used by PAL (European and Asian video standard) and NTSC
(North American video standard) hardware.Lossless. Produces AVI or
QTIF - QuickTime Image Format
Within each video CODEC,
the compression can be fine-tuned by adjusting the number of key
frames and the number of frames
per second. This process is trial and
error. Reducing the number of frames per second produces
jerkier motion but much much smaller files. If your video
has only a small amount of motion this may be the best bet.
If you reduce the number of key frames you will also have a
smaller file but sometimes the video looks like the audio is
no longer synchronized. (The number of key frames is set by
picking a key frame every # of frames, so reducing the
number of key frames is performed by increasing the number.
For example, a key frame every 20
frames has fewer key frames than a key frame every 10