Using CBR encoding
With CBR encoding, you specify the average bit rate that you want to maintain and then set the size of the buffer. The bit rate will fluctuate across the stream; however, the fluctuations are constrained by the buffer size. The content quality also fluctuates to ensure that the buffer does not overflow or underflow. (If you want to guarantee the quality of the encoded output, use quality-based VBR encoding.) The size of the buffer determines the amount of initial delay when the content is played, but using CBR encoding ensures that the content is streamed smoothly, assuming that the bit rate is compatible with the client connection speed. If possible, use two-pass CBR encoding to reach a higher quality without losing the advantages of CBR encoding. Two-pass encoding is not supported in all scenarios.
Keep the following in mind when doing CBR encoding:
- A higher bit rate achieves a better image quality. If you are encoding low-motion video, the actual bit rate may be significantly lower than the expected bit rate.
- As the image quality increases, the frame rate decreases so you might have to experiment to achieve a balance between these two settings.
- A larger buffer achieves 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.
- Consider increasing the key frame distance. A longer key frame distance usually results in better video quality because key frames are only inserted when it is necessary, for example when a scene changes. However, key frames are synchronization points for streaming (if the user temporarily loses the connection), and earlier players seek only to key frames within a file, so you should not place key frames too far apart. (Adjusting the key frame distance also applies to VBR encoding.)
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