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 Cognitive Services. The information applies to all pricing tiers of the service. It also contains some best practices to avoid request throttling.

Quotas and limits reference

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

Speech-to-text quotas and limits per resource

In the following tables, the parameters without the Adjustable row aren't adjustable for all price tiers.

Online transcription

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

Quota Free (F0)1 Standard (S0)
Concurrent request limit - base model endpoint 1 100 (default value)
Adjustable No2 Yes2
Concurrent request limit - custom endpoint 1 100 (default value)
Adjustable No2 Yes2

Batch transcription

Quota Free (F0)1 Standard (S0)
Speech-to-text REST API V2.0 and v3.0 limit Not available for F0 300 requests per minute
Max audio input file size N/A 1 GB
Max input blob size (for example, can contain more than one file in a zip archive). Note the file size limit from the preceding row. N/A 2.5 GB
Max blob container size N/A 5 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

Model customization

Quota Free (F0)1 Standard (S0)
REST API limit 300 requests per minute 300 requests per minute
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 Create Model API request 200 KB 500 KB

1 For the free (F0) pricing tier, see also the monthly allowances at the pricing page.
2 See additional explanations, best practices, and adjustment instructions.

Text-to-speech quotas and limits per resource

In the following tables, the parameters without the Adjustable row aren't adjustable for all price tiers.

General

Quota Free (F0)3 Standard (S0)
Max number of transactions per certain time period per Speech service resource
Real-time API. Prebuilt neural voices and custom neural voices. 20 transactions per 60 seconds 200 transactions per second (TPS)
Adjustable No4 Yes5
HTTP-specific quotas
Max audio length produced per request 10 min 10 min
Max total number of distinct <voice> and <audio> tags in SSML 50 50
Websocket specific quotas
Max audio length produced per turn 10 min 10 min
Max total number of distinct <voice> and <audio> tags in SSML 50 50
Max SSML message size per turn 64 KB 64 KB

Long Audio API

Quota Free (F0)3 Standard (S0)
Min text length N/A 400 characters for plain text; 400 billable characters for SSML
Max text length N/A 10000 paragraphs
Start time N/A 10 tasks or 10000 characters accumulated

Custom Neural Voice

Quota Free (F0)3 Standard (S0)
Max number of transactions per second (TPS) per Speech service resource Not available for F0 See General
Max number of datasets per Speech service resource N/A 500
Max number of simultaneous dataset uploads per Speech service resource N/A 5
Max data file size for data import per dataset N/A 2 GB
Upload of long audios or audios without script N/A Yes
Max number of simultaneous model trainings per Speech service resource N/A 3
Max number of custom endpoints per Speech service resource N/A 50
Concurrent request limit for Custom Neural Voice
Default value N/A 10
Adjustable N/A Yes5

Audio Content Creation tool

Quota Free (F0) Standard (S0)
File size 3,000 characters per file 20,000 characters per file
Export to audio library 1 concurrent task N/A

3 For the free (F0) pricing tier, see also the monthly allowances at the pricing page.
4 See additional explanations and best practices.
5 See additional explanations, best practices, and adjustment instructions.

Detailed description, quota adjustment, and best practices

Before requesting a quota increase (where applicable), ensure that it's necessary. 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 most cases, this throttled state is transient.

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.
  • 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 will get response code 429 (too many requests).
  • Test different load increase patterns. For more information, see the workload pattern example.
  • Create additional Speech service resources in the same or different regions, and distribute the workload among them. This is especially important for the text-to-speech TPS) parameter, which is set to 200 per resource, and can't be adjusted.

The next sections describe specific cases of adjusting quotas.

Speech-to-text: increase online transcription concurrent request limit

By default, the number of concurrent requests 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

If you use custom models, be aware that one Speech service resource might be associated with many custom endpoints hosting many custom model deployments. Each custom endpoint has the default limit of concurrent requests (100) set by creation. If you need to adjust it, you need to make the adjustment of each custom endpoint separately. Note also that the value of the limit of concurrent requests for the base model of a resource has no effect to the custom endpoints associated with this resource.

Increasing the limit of concurrent requests doesn't directly affect your costs. 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.

Concurrent request limits for base and custom models need to be adjusted separately.

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.

Note

Speech containers don't require increases of the concurrent request limit, because containers are constrained only by the CPUs of the hardware they are hosted on. Speech containers do, however, have their own capacity limitations that should be taken into account. For more information, see the Speech containers FAQ.

Have the required information ready

  • For the base model:
    • Speech resource ID
    • Region
  • For the custom model:
    • Region
    • Custom endpoint ID

How to get information for the base model:

  1. Go to the Azure portal.
  2. Select the Speech service 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:
    • Resource ID
    • Location (your endpoint region)

How to get information for the custom model:

  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:
    • Service Region (your endpoint region)
    • Endpoint ID

Create and submit a support request

Initiate the increase of the limit for concurrent requests for your resource, or if necessary check the current limit, by submitting a support request. Here's how:

  1. Ensure you have the required information listed in the previous section.
  2. Go to the Azure portal.
  3. Select the Speech service resource for which you would like to increase (or to check) the concurrency request limit.
  4. In the Support + troubleshooting group, select New support request. A new window will appear, with auto-populated information about your Azure subscription and Azure resource.
  5. In Summary, describe what you want (for example, "Increase speech-to-text concurrency request limit").
  6. In Problem type, select Quota or Subscription issues.
  7. In Problem subtype, select either:
    • Quota or concurrent requests increase for an increase request.
    • Quota or usage validation to check the existing limit.
  8. Select Next: Solutions. Proceed further with the request creation.
  9. On the Details tab, in the Description field, enter the following:
    • A note that the request is about the speech-to-text quota.
    • Choose either the base or custom model.
    • The Azure resource information you collected previously.
    • Any other required information.
  10. On the Review + create tab, select Create.
  11. Note the support request number in Azure portal notifications. You'll be contacted shortly about your request.

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 very good idea to test the workload and the workload patterns before going to production.

Text-to-speech: increase concurrent request 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.

Increasing the limit of concurrent requests doesn't directly affect your costs. 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.

Note

Speech containers don't require increases of the concurrent request limit, because containers are constrained only by the CPUs of the hardware they are hosted on.

Prepare the required information

To create an increase request, you provide your deployment region and the custom endpoint ID. To get it, perform the following actions:

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

Create and submit a support request

Initiate the increase of the limit for concurrent requests for your resource, or if necessary check the current limit, by submitting a support request. Here's how:

  1. Ensure you have the required information listed in the previous section.
  2. Go to the Azure portal.
  3. Select the Speech service resource for which you would like to increase (or to check) the concurrency request limit.
  4. In the Support + troubleshooting group, select New support request. A new window will appear, with auto-populated information about your Azure subscription and Azure resource.
  5. In Summary, describe what you want (for example, "Increase text-to-speech concurrency request limit").
  6. In Problem type, select Quota or Subscription issues.
  7. In Problem subtype, select either:
    • Quota or concurrent requests increase for an increase request.
    • Quota or usage validation to check the existing limit.
  8. Select Next: Solutions. Proceed further with the request creation.
  9. On the Details tab, in the Description field, enter the following:
    • A note that the request is about the text-to-speech quota.
    • Choose either the base or custom model.
    • The Azure resource information you collected previously.
    • Any other required information.
  10. On the Review + create tab, select Create.
  11. Note the support request number in Azure portal notifications. You'll be contacted shortly about your request.