Using the Microsoft Speech SDK (Microsoft.CognitiveServices.Speech) 1.20.0 I am trying to implement the Azure Cognitive Services Text-to-speech as an upgrade from the Microsoft Speech Platform.
The documentation (https://learn.microsoft.com/en-us/azure/cognitive-services/speech-service/speech-services-quotas-and-limits#text-to-speech-quotas-and-limits-per-resource) says 200 transactions per second but every time I test it starts rejecting at ~20/minute (600 times slower). This is on a the F0 free tier, although the standard tier also starts at 200/s.
Basic testing by creating a new SpeechSynthesizer then looping calling SpeakSsmlAsync it reliably fails at 20/min. Waiting on each call so should only be one concurrent connection. On the 21st call in a minute the result is Canceled and the error is BadRequest "Connection was closed by the remote host. Error code: 1007. Error details: Throttled due to too many requests USP state: 3. Received audio size: 0 bytes."
I posted a query on the documentation here and it has now been updated to clarify the free (F0) service is limited to 20 per 60 seconds