Multiple bit rate video

Whether you are creating content for live broadcast or on-demand distribution, you can use multiple bit rate video to make your content accessible by users that have a network connections that range from 28.8 Kbps dial-up modems to high bandwidth LANs. Encoding content with multiple bit rate video creates multiple video streams along with the audio steam and script command stream (if selected). The video streams are encoded at different target network bandwidths. For example, if you choose to use multiple bit rate video with a single target bandwidth of 56 Kbps, two video streams will be created that use greater and lesser amounts of that bandwidth, but it will not create a video stream that is low enough to make the content accessible by users with a 28.8 Kbps network connection. To supply content to users with 28.8 Kbps connections, choose a template that lists both target networks or specify both target audiences when you create a custom configuration.

When you create a custom configuration with multiple bit rate video, you first select whether to support target audiences with low bandwidth network connections (18 Kbps to 300 Kbps) or high bandwidth (81 Kbps to 10 Mbps) network connections. In each category, Low Bandwidth and High Bandwidth, you can select up to five target audiences for your content. Each target audience supports a different range of network bandwidths. You can also specify the same target audience multiple times within the category and then specify greater or lesser amounts of bandwidth for each. The following table details the bandwidth ranges available and the default category for each target audience.

Using multiple bit rate video allows Microsoft Windows Media Player to continue rendering content when network bandwidth is reduced. If the server detects a reduction in the amount of network bandwidth available during the playback, the lower bandwidth video stream will be sent to the player. The user will experience a slightly lower quality stream during the time when the bandwidth is reduced, but the stream will not be interrupted or need to be buffered to recover from the loss of bandwidth.

When you encode a multiple bit rate stream, make sure that there is enough bandwidth available to transfer the full ASF stream between the encoding computer and the Windows Media server. Because additional data is being encoded and all of the encoded video streams must be sent to the server, multiple bit rate streams have a higher aggregate bandwidth requirement for live stream distribution from an encoder to a Windows Media server. The Windows Media server determines the appropriate stream to send to each individual client.

The following table shows the aggregate requirements for the default multiple bit rate template stream formats. The target bandwidths column represents the bandwidths at which the client computer can connect to the server and receive the content. Aggregate bandwidth is the amount of bandwidth necessary for distributing the stream from the encoder to the server. The numbers provided are approximate; actual numbers vary slightly depending on your content.

The lowest bandwidth is an automatic "insurance" bandwidth that is created when multiple bit rate encoding is enabled. This insurance bandwidth is created every time you use multiple bit rate encoding, even when a custom configuration is created.

If you are using a custom encoding configuration you can estimate the aggregate bandwidth requirement by adding the following values:

• Bandwidths specified per target audience selected
• Audio bandwidth selected
• Insurance bandwidth

Calculate your insurance bandwidth as a percentage of the lowest target audience bandwidth you have selected. The following table provides the percentages used for each target audience.

Finally, be aware that only a single audio stream is encoded for all of the video bandwidths selected. So you must subtract the amount of audio bandwidth required for the audio stream from each of the target audience(s) you selected before performing the addition.