A Summary of NetShow Codecs and Video Content
When authoring NetShow video content, you must consider many factors. Bit rate, frame rate, color depth, key frames, and the audio and video codecs that you use all impact how the video looks and sounds. Just as you can change a video's bit rate or number of key frames, you can also determine the codecs used to compress the audio and visual portion of the video. The codecs you choose are important, because they determine how much bandwidth is needed to transport the audio and video. In order to give you greater control of how the ASF stream looks and sounds, NetShow offers a full suite of audio and video codecs. When choosing a codec, consider the following information.

Total Stream Bandwidth

When creating an ASF stream, consider all of the data that will travel across the network bandwidth. Audio, video, built-in error correction, and possibly script commands, can all be part of an ASF stream, but the ASF stream also has to share the bandwidth with other types of network communication. When using a 28.8 Kbps modem connection, Microsoft recommends that all ASF information be at approximately 22 Kbps, leaving the rest of the bandwidth free for network communication. Keeping in mind that all components of your ASF stream must fit within 22 Kbps, choose the codecs that can compress the source files enough to fit in the destination bandwidth. Use the following information to determine the codecs you should use.

Voxware MetaVoice (RT24)

RT24 is an extremely low-bit-rate codec (2.4 Kbps) appropriate for voice-only audio. Because this codec compresses the audio so much, there is a lot of bandwidth left for the video portion of the ASF stream. This codec works best for compressing the audio of a person talking. Of all the NetShow audio codecs, the Voxware RT24 compresses audio the most, leaving the most room for video.

Lernout & Hauspie 4.8 Kbps CELP

This is also a low-bit-rate codec at 4.8 Kbps. L&H 4.8 has a slightly broader range than the Voxware RT24 codec, which means that it sounds better and but it is geared primarily for voice-only audio, and but uses up twice as much bandwidth as the Voxware RT24.

FhG MPEG Layer-3 and Voxware

Both of these codecs offer excellent high-fidelity compression for many different types of audio. The minimum bit rate for both of these codecs is 8 Kbps. At this bit-rate, these codecs provide excellent quality for voice and music. It is important to understand that at 8 Kbps these codecs require more bandwidth at a 28.8 Kbps connection than the codecs mentioned above, and therefore will detract from the bandwidth and quality of the video that is available.

Microsoft MPEG-4

Microsoft's MPEG-4 video codec is an extremely high-quality, scalable codec for Internet and intranet content. It is the video codec of choice when authoring content using the audio codecs listed above.

Vivo G.723 and H.263

The Vivo G.723 audio and H.263 video codecs are standards-based technologies that offer high-quality solutions for network video. In addition, Vivo has one of the simplest tools for authoring Internet video content. For more information about the VivoActive Producer, see http://www.vivo.com/netshow.

Additional Resources

Finally, for more current information about codecs please see
http://www.microsoft.com/netshow/codecs.htm. In addition, NetShow content authoring information is available in the documentation installed with the
NetShow Tools install and in the NetShow Content Creation Authoring Guide, also available from the NetShow download page on http://www.microsoft.com/netshow/download.htm.
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