Setting a maximum duration for synthesized speech - Amazon Polly

Setting a maximum duration for synthesized speech

<prosody amazon:max-duration>

This tag is currently supported only by the standard TTS format.

To control how long you want a speech to take when it is synthesized, use the <prosody> tag with the amazon:max-duration attribute.

The duration of synthesized speech varies slightly, depending on the voice you select. This can make it difficult to match synthesized speech with visuals or other activities that require precise timing. This issue is magnified for translation applications because the time it takes to say particular phrases can vary widely with different languages.

The <prosody amazon:max-duration> tag matches synthesized speech to the amount of time you want it to take (the duration).

This tag uses the following syntax:

<prosody amazon:max-duration="time duration">

With the <prosody amazon:max-duration> tag, you can specify duration in either seconds or milliseconds:

  • ns: the maximum duration in seconds

  • nms: the maximum duration in milliseconds

For example, the following spoken text has a maximum duration of 2 seconds: 

<speak>
     <prosody amazon:max-duration="2s">
          Human speech is a powerful way to communicate. 
     </prosody>
</speak>

Text placed within the tag, it doesn't exceed the specified duration. If the chosen voice or language would normally take longer than that duration, Amazon Polly speeds up the speech so that it fits into the specified duration.

If the specified duration is longer than it takes to read the text at a normal rate, Amazon Polly reads the speech normally. It doesn't slow down the speech or add silence, so the resulting audio is shorter than requested.

Note

Amazon Polly increases the speed no more than 5 times the normal rate. If text is spoken faster than this, it usually doesn't make sense. If a speech cannot fit within your specified duration even when speeded up to the maximum, the audio will be speeded up but will last longer than the specified duration.

You can include a single sentence or multiple sentences within a <prosody amazon:max-duration> tag, and you can use multiple <prosody amazon:max-duration> tags within your text.

For example:

<speak>
     <prosody amazon:max-duration="2400ms">
        Human speech is a powerful way to communicate.
     </prosody>
     <break strength="strong"/>
     <prosody amazon:max-duration="5100ms">
        Even a simple ‘Hello’ can convey a lot of information depending on the pitch, intonation, and tempo.
     </prosody>
     <break strength="strong"/>
     <prosody amazon:max-duration="8900ms">
        We naturally understand this information, which is why speech is ideal for creating applications where 
        a screen isn’t practical or possible, or simply isn’t convenient.
     </prosody>
</speak>

Using the <prosody amazon:max-duration> tag can increase latency when Amazon Polly is returns synthesized speech. The degree of latency depends on the passage and its length. We recommend using text comprised of relatively short text passages.

Limitations

There are limitations both in how you use <prosody amazon:max-duration> tag and in how it works with other SSML tags:

  • The text inside a <prosody amazon:max-duration> tag can't be longer than 1500 characters.

  • You can't nest <prosody amazon:max-duration> tags. If you put one <prosody amazon:max-duration> tag inside another, Amazon Polly ignores the inner tag.

    For example, in the following, the <prosody amazon:max-duration="5s"> tag is ignored:

    <speak>
     <prosody amazon:max-duration="16s">
          Human speech is a powerful way to communicate.
        
          <prosody amazon:max-duration="5s">
               Even a simple ‘Hello’ can convey a lot of information depending on the pitch, intonation, and tempo.
          </prosody>

          We naturally understand this information, which is why speech is ideal for creating applications where a screen isn’t practical or possible, or simply isn’t convenient.
     </prosody>
</speak>

  • You can't use the <prosody> tags with the rate attribute within a <prosody amazon:max-duration> tag. This is because both affect the speed at which text is spoken.

    In the following example, Amazon Polly ignores the <prosody rate="2"> tag:

    <speak>
     <prosody amazon:max-duration="7500ms">
          Human speech is a powerful way to communicate.
      
          <prosody rate="2">
               Even a simple ‘Hello’ can convey a lot of information depending on the pitch, intonation, and tempo.
          </prosody>
     </prosody>
</speak>

Pauses and max-duration

When using max-duration tag, you can still insert pauses within your text. However, Amazon Polly includes the length of the pause when calculating the maximum duration for speech. Additionally, Amazon Polly preserves the short pauses that occur where commas and periods are placed within a passage and includes in the maximum duration.

For example, in the following block, the 600 millisecond break and the breaks caused by the commas and periods occur within the 8-second speech:

<speak>
     <prosody amazon:max-duration="8s">
          Human speech is a powerful way to communicate.
          <break time="600ms"/>
          Even a simple ‘Hello’ can convey a lot of information depending on the pitch, intonation, and tempo.
     </prosody>
</speak>