You now know what
broadband is, and the ways streaming content can be delivered, but what
methods are there to encode the information? In Part II of the Digital
WebCast Primer we look at the codecs and formats to get your media over
the net.
Codecs
H.263 (also known
as I263) is a standard video conferencing codec that can be used on both
the Windows and MacOS platforms. It is optimized for low data rates and
thus low motion video (talking head type stuff). H.263 may not work well
on low-end machines, and does not support variable-length frames.
RealVideoG2 (.ra
or .rm) is a good streaming codec as movies encoded in this format can
vary according to the user's connection to the net. This is also a scalable
format so movies played back will be smoother on a wide variety of machines,
however it plays best on sysems with a fast CPU. Compared to all of the
other codecs discussed here, it encodes faster than the rest. As far as
drawbacks go, the end user does need to have the most recent version of
RealPlayer installed to view streaming content, as it is not backwards
compatible.
The Cinepak codec
was originally designed to run on low end computer systems and single
speed CD-ROM drives. The image size of Cinepak movies was originally very
small (120x90), but as computer speeds have increased, so have the sizes
of movies produced with this codec. While many people still use Cinepak
for compression, there are many newer codecs that can out perform it.
Intel Indeo 4 and 5 (IVI) provides really good image quality, but
can only run on high powered PC's or Macs. Version 5 of the codec is similar
to 4, but is optimized to run on MMX Pentium machines (it will work on
non MMX machines). Version 5 also allows for progressive downloads. While
versions 4 & 5 have a better picture quality than Cinepak, the quality
is not better than the Sorenson Video Codec.
The Sorenson Video
Codec provides high quality video using a small amount of bandwidth
for streaming over the Internet. Sorenson video looks great at 320x240,
but only if streamed at rates higher than 100kbs, which is only available
on fast machines or fast internet connections (faster than 56k, see Part
I of primer). The Developer Edition of the Sorenson Codec allows for
temperol scalability in which video to plays at a high frame rate on fast
connections and lower frame rates on slower connections. One of the biggest
drawbacks to using this codec is it takes a long time to encode media.
In addition to long compession times, areas of highly saturated color
tend to look blocky in the end product.
While they are not true streaming formats, it is important to touch on
MPEG-1 and MPEG-2 encoding. MPEG-1 (Motion Picture
Experts Group) is primarily used for CD-ROM and VCD (Video
CD) video and audio. MPEG (.mpg) video usually requires a fast computer,
and most units sold today will support video encoded in the MPEG-1 format
at full frame rate, but quarter screen size (350x240). MPEG-2 video
produces high date rate, broadcast quality video and is the format used
in DVD-Video. MPEG-2 supports full 30fps and full screen resolution (720x480).
Once the MPEG-4
format has been set, it will be availabe on all platforms. This codec
has an image quality better than the H.263, but because it is CPU intensive,
it can only be used on computers with fast CPU's. The MPEG-4 codec comes
standard in Microsoft's Netshow, and is based on the QuickTime format.
Formats
Video for Windows - (.avi) there are a variety of codecs available
in the video for windows format, but it's main advantage is that it comes
standard in the Windows OS. The drawback is that it can only be used in
the Windows environment. Video for Windows is aimed primarily at CD-ROM
video. Microsoft is no longer supporting Video for Windows in favor of
DirectShow (part of the DirectX family) and NetShow/Active Streaming Format
(.ASF).
Other disadvantages of the Video for Windows format is that audio/video
syncronization is not as reliable as other formats and it doesn't support
variable-length frames (a variable frame length movie is one that contains
frames that are not of equal duration). The codec most users of the Video
for Windows format will be familiar with is Cinepak. On the plus side
it is supported and plays on a large number of machines, on the negative
side, the quality is pretty low. For superior image quality in the Video
for Windows format Intel's Indeo 4 and 5 codecs do a good job.
Microsoft Windows Media (.asf) is designed to stream media over
a variety of speeds from 14.4 to DSL and higher. Netshow servers are designed
to support true streaming, so that as demand on a network increases, the
quality of the media delivered decreases, first video and then audio.
Primary codecs used in Netshow are H.263, MPEG-4 (primary), Indeo 3.2
and Indeo 4.1
QuickTime (.mov or .qt) is Apple's answer to getting media anywhere.
This format is ideally designed for use in kiosks, CD-ROM, DVD, and the
Internet. QuickTime is supported on both platforms and performs equally
well on each. QuickTime is used not only for delivery, but it can be used
for editing and production. QuickTime streaming uses RTSP (RealTime Streaming
Protocol) to deliver movies in realtime similar to Netshow and RealG2.
QuickTime supports Sorenson, Cinepak, Indeo 3.2 and 4.1, and MPEG-1 (Mac
only).
RealPlayer is the only format that concentrates on delivering media
to the internet using the RealSystem G2 engine. Content produced with
RealProducer can be streamed live from a server or through a progessive
download. Like Microsoft Windows Media, content produced with RealSystems
G2 is scalable and can instantly switch streams to ensure the highest
quality stream for the viewer.
In Part III, DigitalWebcast will look at programs to assist in the encoding
method.