A S t r e a m i n g M e d i a P r i m e r :
H O W D O I M A K E ( G O O D ) S T R E A M I N G M E D I A ?
But even reducing both the frame size and the frame rate does not reduce the bit rate
of video fi les enough for effi cient streaming. So additional methods for compressing the
bloated streaming media
amount of data that needs to be transmitted are used. We haven't space here to go into
has a tendency to alienate your
too detailed a discussion, but the following general descriptions will give you a general
site visitors. Your clip may look
understanding of compression methods.
good off your LAN on your fast
Intra frame compression: Derived from still formats, intra frame compression, also
known as spatial compression, treats each frame individually. Some spatial compression
CPU. But if it looks like junk
schemes work by discarding much of the color information in the picture. As long as this
to your user, you have just told
type of compression is not too severe, it is generally unnoticeable. In fact, in even the
highest quality uncompressed video used by broadcasters, some of the original color
them that you do not need
information has been discarded. Avoiding hot colors and harsh contrast when shoot
their business... Do the band
ing helps ensure the best results from this type for compression. Spatial compression
works best on continuous tone images without sharp edges, such as photography and
width math. Test on a slower
videography (as opposed to animated graphics). The softer and smoother the imagery,
CPU. Take as many people along
the better results you'll achieve with systems that use spatial compression.
for the ride as you possibly can.
Inter frame compression: Inter frame compression, also known as temporal compres
sion, takes advantage of the fact that any given frame of video is probably very similar to
the frames around it. So, instead of storing all the information for each and every frame,
Streaming Media World
temporal compression schemes store keyframes in their entirety, while delta frames
(comprising the majority of frames in Web video) contain only the information that
is different from the keyframes that come before and/or after. You can usually specify
A codec (set of mathematical
how frequently keyframes occur less frequent (and, therefore fewer) keyframes mean
smaller fi les, but will also result in lower quality, since more frames will need to be
algorithms) is used by an encoder
inferred, or predicted, from keyframe information.
(component of an architecture that
What is a codec?
produces fi nal streaming fi les) to
Compression and decompression are accomplished by a set of mathematical algorithms
compress (reduce the data rate of)
called a codec short for compressor/decompressor. Codecs compress data as it is
digital video and/or audio. The player
encoded, and decompress it for playback. In order to be played, a fi le compressed with a
certain codec must be decompressed by the same codec.
(client component of an architecture
While the codec itself is comprised of software code, a codec may be a part of either a
that plays back media fi les) must use
hardware or a software solution. For example, an onboard codec may be found in a
the same codec to decompress the
video capture card, or in a specialized encoding workstation used for capture and encod
fi le that was used to compress it.
ing of real time (live live) streaming video. Encoding software, such as that included in
the three major streaming architectures, provides codecs, as well.
Different codecs for different tasks
the best way to learn to
Video codecs and audio codecs work differently. You'll need to choose a video codec to
compress your imagery, and an audio codec to compress your sound. And some codecs
take advantage of these [different
are more appropriate for certain kinds of work than others, since different codecs use
codecs] is to play around with
different compression strategies. The Adobe Premiere Technical Guides, which can be
found in the Support area of the Adobe Web site at www.adobe.com, offer some more
them. Take the same source clip,
detailed descriptions of different types of compression and codecs.
run it through different codecs,
Which streaming media codecs are available to you depends on which architecture(s)
data rates, and frame rates,
you choose. You'll fi nd some of the same codecs (MPEG 4, for example, for video; MP3,
for example, for audio) in different architectures. But remember, since encoding not
namic Medianamic Media
and carefully explore the
only compresses the fi le but also formats it for a specifi c architecture, even if the same
results. This is still more of
codec is used, streaming fi les are likely to play back only on the player that is part of
the encoding architecture.
an art than a science.
Currently, the three dominant architectures offer just a few options for streaming video
obe Dobe D
codecs. While QuickTime provides some variety, RealNetworks and Windows Media
DV Web Video
deliver little choice in codecs for streaming video. In terms of audio codecs, Real offers
a wide variety of choices, with QuickTime
47 Don't Be Scared of Bandwidth Math, by Tim Kennedy, Streaming Media World, May 26, 2000, http://smw.internet.com/symm/tutor/bandmath/index.html
48 Choosing a Streaming Video Technology, by Ben Waggoner, DV Web Video, May 2000