Best practices for compression settings

There is a balance between a number of compression settings. The encoder includes many predefined destination, video, and audio settings (profiles) that have already taken this balance into consideration. You can easily set up an encoding session using a predefined profile. However, if you want to customize the settings, then the following paragraphs provide information about the settings, including some of the tradeoffs to consider.

• Buffer. A larger buffer can accommodate a better image quality, but the user must wait longer for the video to start, as a player waits to play until its buffer is filled. Typically, the buffer delay matches the time, in seconds, of the buffer size set in the encoder. However, if you plan to stream from a Windows Media server, you can set a larger buffer size in the encoder. A feature in Windows Media Services, called Fast Start, enables a player to fill its buffer faster than real time, assuming there is sufficient bandwidth. This means that the initial buffer delay may be much shorter. For example, when a user attempts to play back 56-Kbps encoded content on a DSL or cable modem, the start-up delay may be only a second or two, even if the buffer size set during encoding was much longer. For more information about Fast Start, see Windows Media Services Help.

  If you have set up a session that uses peak bit rate-based VBR encoding for audio, the peak buffer size for audio should be lower than the buffer size for video (if you are using CBR encoding for video) or peak buffer size for video (if you are using peak bit rate-based encoding for video).

• Bit rate. A higher bit rate achieves a better image quality. If the bit rate setting is too low, the codec will drop frames to produce a higher quality image, although this may not be visible to the viewer. (Note that if you are encoding low-motion video, the actual bit rate may be significantly lower than the expected bit rate.)
• Frame rate. There is a tradeoff between image quality and frame rate, in that as the image quality increases, the frame rate decreases. If maintaining the frame rate is more important, you can decrease the image quality setting.
• Image quality (smoothness). You can adjust the image quality to be smoother or more clear. Increasing the clarity can affect the frame rate, depending on the video. For example, if your video contains a lot of motion, increasing the clarity may decrease the frames rate. In addition, increasing the clarity results in a higher bit rate requirement and may also necessitate increasing the buffer size.
• Frame size. The larger the frame size, the higher the bit rate requirement.
• Key frames. If you use a longer key frame distance, additional key frames are inserted when necessary; for example, when a scene changes. For low motion content, a setting of 20 seconds is typically appropriate. For higher bit rate content, you should lower the key frame distance.
• Codecs. Use the appropriate codec for the session. For example, when your audio source is voice or primarily voice, use the Windows Media Audio 9 Voice codec. For a screen capture session, use the Windows Media Video 9 Screen codec.
• Two-pass encoding. You will typically achieve higher quality with two-pass encoding, because the encoder has the opportunity to analyze the content during the first pass.
• Video resizing. Set up the output frame size to be the same as the input frame size. Do not use the encoder to resize the source video.

Related topics

• System configuration options
• Achieving optimal system performance
• Optimizing audio for voice
• Improving performance when encoding screens
• Understanding codecs
• About one- or two-pass encoding
• To match the frame size of the source video