With custom speech, you can evaluate and improve the accuracy of speech recognition for your applications and products. A custom speech model can be used for real-time speech to text, speech translation, and batch transcription. Cited from: https://learn.microsoft.com/en-us/azure/ai-services/speech-service/custom-speech-overview
Standard offers pre-built models without customization while custom allows you to create and use custom speech models tailored to your specific needs.
Standard is also best for general use cases where pre-built models are sufficient while custom is best for specialized use cases requiring tailored models. Higher cost too.
I will recommend you check the links below to see the pricing details:
https://azure.microsoft.com/en-us/pricing/details/cognitive-services/speech-services/
https://azure.microsoft.com/en-us/pricing/calculator/
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Regards,
Abiola