Speech service quotas and limits

This article contains a quick reference and a detailed description of the quotas and limits for the Speech service in Azure AI services. The information applies to all pricing tiers of the service. It also contains some best practices to avoid request throttling.

For the free (F0) pricing tier, see also the monthly allowances at the pricing page.

Quotas and limits reference

The following sections provide you with a quick guide to the quotas and limits that apply to the Speech service.

For information about adjustable quotas for Standard (S0) Speech resources, see more explanations, best practices, and adjustment instructions. The quotas and limits for Free (F0) Speech resources aren't adjustable.

Important

If you switch an AI Services resource for Speech from Free (F0) to Standard (S0) pricing tier, the change of the corresponding quotas may take up to several hours.

Speech to text quotas and limits per resource

This section describes speech to text quotas and limits per Speech resource. For information about adjustable quotas, see Detailed description, quota adjustment, and best practices.

Real-time speech to text and speech translation

You can use real-time speech to text with the Speech SDK or the Speech to text REST API for short audio.

Important

These limits apply to concurrent real-time speech to text requests and speech translation requests combined. For example, if you have 60 concurrent speech to text requests and 40 concurrent speech translation requests, you reach the limit of 100 concurrent requests.

Quota Free (F0) Standard (S0)
Concurrent request limit - base model endpoint 1

This limit isn't adjustable.		 100 (default value)

The rate is adjustable for Standard (S0) resources. See more explanations, best practices, and adjustment instructions.
Concurrent request limit - custom endpoint 1

This limit isn't adjustable.		 100 (default value)

The rate is adjustable for Standard (S0) resources. See more explanations, best practices, and adjustment instructions.
Max audio length for real-time diarization. N/A 240 minutes per file

Fast transcription

Quota Free (F0) Standard (S0)
Maximum audio input file size N/A < 300 MB
Maximum audio length N/A < 120 minutes per file
Maximum requests per minute N/A 600

Batch transcription

Quota Free (F0) Standard (S0)
Speech to text REST API limit Not available for F0 100 requests per 10 seconds (600 requests per minute)
Max audio input file size N/A 1 GB
Max number of blobs per container N/A 10000
Max number of files per transcription request (when you're using multiple content URLs as input). N/A 1000
Max audio length for transcriptions with diarization enabled. N/A 240 minutes per file

Model customization

The limits in this table apply per Speech resource when you create a custom speech model.

Quota Free (F0) Standard (S0)
REST API limit 100 requests per 10 seconds (600 requests per minute) 100 requests per 10 seconds (600 requests per minute)
Max number of custom model deployments per Speech resource 1 50
Max number of speech datasets 2 500
Max acoustic dataset file size for data import 2 GB 2 GB
Max language dataset file size for data import 200 MB 1.5 GB
Max pronunciation dataset file size for data import 1 KB 1 MB
Max text size when you're using the text parameter in the Models_Create API request 200 KB 500 KB

Text to speech quotas and limits per resource

This section describes text to speech quotas and limits per Speech resource. For information about adjustable quotas, see Detailed description, quota adjustment, and best practices.

Real-time text to speech

You can use real-time text to speech with the Speech SDK or the Text to speech REST API. Unless otherwise specified, the limits aren't adjustable.

Quota Free (F0) Standard (S0)
Maximum number of transactions per time period for standard voices. 20 transactions per 60 seconds

This limit isn't adjustable.		 200 transactions per second (TPS) (default value)

The rate is adjustable up to 1000 TPS for Standard (S0) resources. See more explanations, best practices, and adjustment instructions.
Max audio length produced per request 10 min 10 min
Max total number of distinct <voice> and <audio> tags in SSML 50 50
Max SSML message size per turn for websocket 64 KB 64 KB

Note

Most HTTP 429 errors with Text-to-Speech Standard Voice are caused by limited backend service capacity for a specific voice in the selected region, not by quota limits. Increasing your quota won't resolve these errors. For best results, use the voice in its native region or select a more popular voice in your current region.

Audio Content Creation tool

Quota Free (F0) Standard (S0)
File size (plain text in SSML)1 3,000 characters per file 20,000 characters per file
File size (lexicon file)2 30 KB per file 100 KB per file
Billable characters in SSML 15,000 characters per file 100,000 characters per file
Export to audio library 1 concurrent task N/A

1 The limit only applies to plain text in SSML and doesn't include tags.

2 The characters of lexicon file aren't charged. Only the lexicon elements in SSML are counted as billable characters. Refer to billable characters to learn more.

Detailed description, quota adjustment, and best practices

Some Speech service quotas are adjustable. This section provides more explanations, best practices, and adjustment instructions.

The following quotas are adjustable for Standard (S0) resources. The Free (F0) request limits aren't adjustable.

Before requesting a quota increase (where applicable), check your current TPS (transactions per second) or TPM (tokens per minute) and ensure that you need to increase the quota.

Note

Batch transcription is an asynchronous process, and jobs are processed one-by-one in a queue. So, increasing the quota won't improve transcription performance. For performance improvements, see Batch transcription best practices.

Speech service uses autoscaling technologies to bring the required computational resources in on-demand mode. At the same time, Speech service tries to keep your costs low by not maintaining an excessive amount of hardware capacity.

Let's look at an example. Suppose that your application receives response code 429, which indicates that there are too many requests. Your application receives this response even though your workload is within the limits defined by the Quotas and limits reference. The most likely explanation is that Speech service is scaling up to your demand and didn't reach the required scale yet. Therefore the service doesn't immediately have enough resources to serve the request. In such cases, increasing the quota won’t help. In most cases, the Speech service will scale up soon, and the issue causing response code 429 will be resolved.

Note

As a best practice every implementation should gracefully handle 429 errors with retry logic to ensure best performance and to handle autoscaling. Please consider this before requesting additional quota. See general best practices for additional details.

General best practices to mitigate throttling during autoscaling

To minimize issues related to throttling, it's a good idea to use the following techniques:

  • Implement retry logic in your application to handle 429 errors.
  • Avoid sharp changes in the workload. Increase the workload gradually. For example, let's say your application is using text to speech, and your current workload is 5 TPS. The next second, you increase the load to 20 TPS (that is, four times more). Speech service immediately starts scaling up to fulfill the new load, but is unable to scale as needed within one second. Some of the requests get response code 429 (too many requests).
  • Test different load increase patterns. For more information, see the workload pattern example.
  • Create more Speech service resources in different regions, and distribute the workload among them. (Creating multiple Speech service resources in the same region won't affect the performance, because all resources are served by the same backend cluster).

The next sections describe specific cases of adjusting quotas.

Example of a workload pattern best practice

Here's a general example of a good approach to take. It's meant only as a template that you can adjust as necessary for your own use.

Suppose that a Speech service resource has the concurrent request limit set to 300. Start the workload from 20 concurrent connections, and increase the load by 20 concurrent connections every 90-120 seconds. Control the service responses, and implement the logic that falls back (reduces the load) if you get too many requests (response code 429). Then, retry the load increase in one minute, and if it still doesn't work, try again in two minutes. Use a pattern of 1-2-4-4 minutes for the intervals.

Generally, it's a good idea to test the workload and the workload patterns before going to production.

Speech to text: increase real-time speech to text concurrent request limit

By default, the number of concurrent real-time speech to text and speech translation requests combined is limited to 100 per resource in the base model, and 100 per custom endpoint in the custom model. For the standard pricing tier, you can increase this amount. Before submitting the request, ensure that you're familiar with the material discussed earlier in this article, such as the best practices to mitigate throttling.

Note

Concurrent request limits for base and custom models need to be adjusted separately. You can have a Speech service resource that's associated with many custom endpoints hosting many custom model deployments. As needed, the limit adjustments per custom endpoint must be requested separately.

Increasing the limit of concurrent requests doesn't directly affect your costs. The Speech service uses a payment model that requires that you pay only for what you use. The limit defines how high the service can scale before it starts throttle your requests.

You aren't able to see the existing value of the concurrent request limit parameter in the Azure portal, the command-line tools, or API requests. To verify the existing value, create an Azure support request.

Prepare the required information

Instructions on how to get the general resource information required is described in create and submit a quota increase request.

To create an increase request for custom speech, you need to provide the following additional information:

  • Custom endpoint ID

How to get information for the custom speech endpoint:

  1. Go to the Speech Studio portal.
  2. Sign in if necessary, and go to Custom speech.
  3. Select your project, and go to Deployment.
  4. Select the required endpoint.
  5. Copy and save the values of the following fields:
    • Endpoint ID

Create the quota increase request

Follow the steps described in create and submit a quota increase request with the information collected to create the request.

Fast transcription: increase maximum requests per minute limit

Prepare the required information

Instructions on how to get the general resource information required is described in create and submit a quota increase request.

To create an increase request for fast transcription, you need to provide the following additional information:

  • Average Audio Length per API request

The average audio length of each request sent to the API. Example: 5 minutes/request. Provide an estimate based on the workload you're aiming to process.

Create the quota increase request

Follow the steps described in create and submit a quota increase request with the information collected to create the request.

Text to speech: increase real-time TPS limit

For the standard pricing tier, you can increase this amount. Before submitting the request, ensure that you're familiar with the material discussed earlier in this article, such as the best practices to mitigate throttling.

Estimating Your Needs

  • Usage Under ￥10,000/month: Typically, 32 TPS is sufficient, assuming your peak usage is within 10x of your average.
  • Default Limit: 200 TPS is available by default, which exceeds most use cases.

Example: Call Center Scenario
If you're building a call center with 1,000 concurrent calls:

  • Assume agents speak half the time.
  • Average TTS response length is 5 seconds.

Required TPS: 1000 calls / (2×5 seconds) = 100 TPS

Information Required for TPS Increase Request
Please provide the following details:

  • Peak TPS:
  • Average TPS:
  • Average TTS Request Length (in characters):

With this data, you can estimate your monthly TTS usage with formula below:
Monthly Usage=Average TPS×Request Length×3600×24×30
Multiply the result with unit price ￥15 per million characters to estimate the monthly cost.

Note

If your estimated usage significantly exceeds your budget, you may be overestimating your needs.

Cost Considerations
Increasing the concurrent request limit does not directly affect your costs. You only pay for what you use. The limit simply defines how much the service can scale before throttling begins.

You aren't able to see the existing value of the concurrent request limit parameter in the Azure portal, the command-line tools, or API requests. To verify the existing value, create an Azure support request.

Prepare the required information

Instructions on how to get the general resource information required is described in create and submit a quota increase request.

To create an increase request for standard voice, you need to provide the following additional information:

  • Voice names you are requesting the increase for

You can find a list of all voice names here: Text to speech languages

Create the quota increase request

Follow the steps described in create and submit a quota increase request with the information collected to create the request.

Create and submit a quota increase request

To get the resource information needed for the quota increase request follow these steps:

  1. Go to the Azure portal.
  2. Select the resource for which you would like to increase the concurrency request limit.
  3. From the Resource Management group, select Properties.
  4. Copy and save the values of the following fields:
    • Subscription ID
    • Resource ID
    • Location (your endpoint region)
  • Last updated on