关于windowsmedia压缩速率的问题
在实践中我们发现一个有意思的事情。那就是我们所设置的声音视频数据传输率相加的数字比实际获得的数据传输率要小。

例如 声音32kbps +视频168kbps 应等于200kbps 但是实际得到的流速为207kbps

再windowsmedia 9得enocder中我们终于看到了这个多出来的7k是什么东西。就是头部数据overhead ，为了适应网络传输而加的一些信息正是这些信息占用了7K的大小。当然overhead不是固定的7k随着视频数据的增加，数量也会增加。



在MSDN上搜索了一下发现了更加精确的数据，就不翻译了 直接paste过来

Selecting Bit Rates
For files that will be streamed over a network, you must consider carefully what bit rates you should use. Under most circumstances you can add the bit rates of all of the streams in a file together to get a general idea of the available bandwidth required to stream the file. However, a certain amount of overhead is also required for each stream. This overhead is summarized in the following table.

Bit-rate range (Kbps) Additional bandwidth required for overhead (Kbps)
10 – 16 3
17 – 30 4
31 – 45 5
46 – 70 6
71 – 225 7
>225 9


The normal overhead required for a stream does not take data unit extensions into account. Every data unit extension adds to the size of the sample to which it is attached. Depending upon the type of data unit extension, this can greatly increase the bit rate for the stream.

You must also consider that the theoretical maximum bandwidth available over a network connection is not a practical target bit rate. The average available bandwidth for any given connection falls well short of the bandwidth capacity of the connection, because of network traffic and many other factors.