英语翻译Only set this value if you know none of the individual s

英语翻译
Only set this value if you know none of the individual stream durations and also do not set any of them
// 这句怎么翻译呢?前面如果你知道单个流没有一个 和 也不能设置他们任何的一个?
//原出处
Decoding:duration of the stream,in AV_TIME_BASE fractional
* seconds.Only set this value if you know none of the individual stream
* durations and also do not set any of them.This is deduced from the
*AVStream values if not set.

Only set this value if you know none of the individual stream durations and also do not set any of them.
仅在以下情况下设置该值：你不知道任何一个单流持续时间,而且你也没有设置任何这些值.
This is deduced from the AVStream values if not set.
如未设置,该值从AVStream值推算.