The bit rate of a wma, or wmv stream depends on a number of factors including compression, frame size, frame rate, quality, codec, and type of content. When choosing a bit rate, find the rate at which bandwidth is kept as low as possible while at the same time offering the best user experience. The Windows Media Audio 8 and Windows Media Video 8 codecs are optimized to provide the highest quality sound and video at both low- and high-range data rates. It is important to keep in mind the target bandwidth for which you are authoring. For example, you can achieve CD-quality audio for music at 48 Kbps, but 28.8 kbps modem users will not be able to listen to the stream because it will require excessive buffering that entails a long wait.

If you are planning on supporting a wide range of bandwidths, such as those required by users with dial-up modem and LAN connections, multiple bit rate encoding is a good solution, but not a cure-all. When using multiple bit rates to encode at dial-up (28.8 Kbps) and LAN (300 Kbps) speeds simultaneously, you cannot increase the audio higher than the lowest target bit rate. For high quality encoding, this may not be the best solution. Therefore, it is suggested that you offer multiple versions of your media content, each with a range of connection speeds. For example, you could offer a stream targeted at dial-up modems encoded for 28.8-Kbps and 56-Kbps speeds with a small frame size (176 x 144 pixels) but good image quality and little pixel distortion; a second targeted at ISDN, WAN, and cable modems/DSL users (from 80 Kbps to 128 Kbps) with a larger frame size (320 x 240 pixels) but lower frame rate (15 frames per second [fps]) and some pixel distortion; and a third targeted at users with high-speed LAN connections (250 Kbps to 512 Kbps) at 320 x 240 or larger frame size with full motion (30 fps) and near-digital-quality video.

You should experiment with different types of content and encoding templates, using the Windows Media Tools or third-party tools. When deciding on an acceptable bit rate for your media, consider your network capacity. For help with network capacity planning, see the Network Capacity Planning section later in the document. In addition, see the Windows Media Audio and Video 8 page on the Microsoft Web site for samples of audio and video encoded at various bit rates. The audio comparison page includes comparisons with other common audio formats.