Ios 如何使用SSML降低文字到语音的速度-语音失真/扭曲/可怕

我正在为iPhone使用nuance dragon mobile sdk，用于将文本转换为语音。
读取的文本有点快，我想让它慢一点，这样用户可以学习单词。我的目标是把课文写慢一点。这对SSML和韵律标记非常有效，请参见以下代码：

<prosody rate="slow">This is the text which is spoken slow,
but the voice sounds distorted/warped/ghastly</prosody>

这是说得慢的文本，
但声音听起来扭曲/扭曲/可怕

声音听起来扭曲、扭曲、可怕
你明白我的意思吗？
我该怎么做才能获得一个清晰的声音但慢速的文本？

从这里开始：

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有没有一种没有“nuance dragon mobile sdk”的方法可以做到这一点？AVSpeechSynthesizer似乎不支持SSML。

rate: a change in the speaking rate for the contained text. Legal values are: 
a relative change or "x-slow", "slow", "medium", "fast", "x-fast", or "default". 
Labels "x-slow" through "x-fast" represent a sequence of monotonically non-decreasing
speaking rates. When a number is used to specify a relative change it acts as a 
multiplier of the default rate. For example, a value of 1 means no change in speaking 
rate, a value of 2 means a speaking rate twice the default rate, and a value of 0.5 
means a speaking rate of half the default rate. The default rate for a voice depends on 
the language and dialect and on the personality of the voice. The default rate for a 
voice should be such that it is experienced as a normal speaking rate for the voice when 
reading aloud text. Since voices are processor-specific, the default rate will be as 
well.