AI Voice & Speech Comparison

Amazon Polly vs. AssemblyAI

At a glance

Amazon Polly

Best for

Best for developers already on AWS who need speech synthesis inside an application, billed per character.

Freelancers
Small and mid-sized
Enterprise

AssemblyAI

Best for

Best for developers who need accurate transcription, speech insights or voice agents behind a pay-as-you-go API.

Small and mid-sized
Enterprise

Cataleo does not name a winner. Both statements come from the vendors themselves.

Full Comparison

Criterion	Amazon Polly	AssemblyAI
Starting price	Free tier, then from $4 per 1 million characters Pay-as-you-go by characters sent for synthesis, including spaces and most SSML tags. Standard $4, Neural $16, Generative $30 and Long-Form $100 per 1 million characters. Speech Marks requests are charged at the same rates.	lower value From $0.15 / hour of audio Pay-as-you-go, no subscription tiers. Pre-recorded speech-to-text is $0.15 / hour on Universal-2 and $0.21 / hour on Universal-3.5 Pro; streaming is $0.15 / hour on Universal-Streaming and $0.45 / hour on Universal-3.5 Pro Realtime; the Voice Agent API is $4.50 / hour, billed per second of connected conversation. Add-ons such as speaker diarization ($0.02 / hour) and medical mode ($0.15 / hour) are charged on top.
Free trial	AWS Free Tier: 5 million Standard characters a month, plus 1 million Neural, 500,000 Long-Form and 100,000 Generative characters a month for the first 12 months	$50 in free credits, no credit card required, covering up to 185 hours of pre-recorded or 333 hours of streaming transcription
Commercial use	Not stated	Yes, usage-based
Output	Synthesised speech as MP3, Ogg Vorbis or raw PCM, plus Speech Marks metadata	Transcripts, structured speech insights and live voice agent conversations
API	Yes, the Amazon Polly API through the AWS SDKs and the AWS CLI	Yes, REST APIs and SDKs
Languages	Voices in 42 languages and language variants	99 languages on Universal-2 and 18 on Universal-3.5 Pro for pre-recorded audio; translation across 100+ languages
Hosting	Cloud (AWS)	Not stated
Voice cloning	Brand Voice only, and not as a self-service feature. It is a custom engagement in which the Amazon Polly team builds a Neural voice for the exclusive use of one organisation, covering persona, casting an actor, recording their speech and training the model, after which the voice is released to that customer's AWS account. It starts with an AWS account manager rather than an upload	No, the platform covers speech recognition and voice agents rather than voice creation
Real-time use	Yes, the API returns an audio stream that an application can begin playing as it arrives, in near real time, with a choice of sampling rates to trade bandwidth against audio quality	Yes, streaming speech-to-text and a Voice Agent API, alongside batch transcription of pre-recorded files
Speech-to-speech and dubbing	No. Polly synthesises speech from text and nothing else. Transcription and translation are separate AWS services rather than features of this one	Yes, the Voice Agent API runs speech-to-speech conversations by orchestrating speech-to-text, an LLM and text-to-speech at one all-inclusive rate
Editing and controls	SSML for phrasing, emphasis, intonation, pitch, rate and volume, custom Amazon SSML tags such as the Newscaster speaking style, custom lexicons that set the pronunciation of particular words through IPA-style phonemes, and Speech Marks metadata for speech-synchronised facial animation or word highlighting	Speaker identification, diarization, custom formatting, key phrase extraction and guardrails for PII redaction, profanity filtering and content moderation
Consent and trust	Brand Voice is a managed engagement rather than an upload, so a custom voice exists only where an actor has been cast and recorded for that purpose, and the finished voice is restricted to the commissioning organisation's AWS account IDs. There is no self-service cloning path that could be pointed at a voice without its owner taking part	SOC 2 Type 2, ISO 27001, PCI-DSS and a HIPAA BAA included at no premium; EU region available at identical US pricing for GDPR
MCP-ready	No	stated Yes
Deployment	Cloud (SaaS), Browser-based	Cloud (SaaS), Browser-based
Onboarding	Documentation and knowledge base	Documentation and knowledge base
Company size	more entries Freelancers, Small and mid-sized, Enterprise	Small and mid-sized, Enterprise

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Row where the values differ The small markers point at the lower number, the longer list or the stated feature. They describe the data, they are not a verdict.

The short version

Key Differences

Starting price Amazon Polly Free tier, then from $4 per 1 million characters AssemblyAI From $0.15 / hour of audio
Free trial Amazon Polly AWS Free Tier: 5 million Standard characters a month, plus 1 million Neural, 500,000 Long-Form and 100,000 Generative characters a month for the first 12 months AssemblyAI $50 in free credits, no credit card required, covering up to 185 hours of pre-recorded or 333 hours of streaming transcription
Company size Amazon Polly Freelancers, Small and mid-sized, Enterprise AssemblyAI Small and mid-sized, Enterprise
Languages Amazon Polly Voices in 42 languages and language variants AssemblyAI 99 languages on Universal-2 and 18 on Universal-3.5 Pro for pre-recorded audio; translation across 100+ languages
MCP-ready Amazon Polly No AssemblyAI Yes

Every line is one datapoint from the table above, picked automatically. Nothing here is written text.

The trade-offs

Strengths and Limitations

Amazon Polly

Strengths

Four voice engines let quality be traded against cost per character, from $4 to $100 per million
Generous free tier, with 5 million Standard characters a month that does not expire
Generated speech may be cached and replayed at no additional cost
Speech Marks and custom lexicons cover lip-sync and awkward pronunciations properly
Brand Voice cannot be used without the voice owner being cast and recorded

Limitations

Text to speech only: no transcription, translation or dubbing in this service
No self-service voice cloning, and Brand Voice starts with an AWS account manager
The Generative and Long-Form engines cover far fewer voices than Standard and Neural
Long-Form voices cost $100 per million characters, twenty-five times the Standard rate
Useful only from code, so it suits developers rather than anyone wanting an editor

AssemblyAI

Strengths

Pay-as-you-go with no subscription and $50 in free credits to start
99 languages for pre-recorded transcription on Universal-2
Compliance certifications and an EU region at no price premium
Official MCP server exposes transcription and transcript search to AI agents

Limitations

A developer API, not a ready-made studio or editor
No voice cloning or voice creation
Speech Understanding add-ons are billed on top of the base rate, so cost stacks per request
Streaming is billed for as long as the connection stays open, not for the audio sent, and multichannel files are billed per channel
Universal-3.5 Pro covers 18 languages against 99 on the older Universal-2

Plans

Pricing

Amazon Polly

AWS Free Tier Free 5 million Standard characters a month. Neural 1 million, Long-Form 500,000 and Generative 100,000 characters a month for the first 12 months.
Standard voices $4 Per 1 million characters for speech or Speech Marks requests, outside the free tier.
Neural voices $16 Per 1 million characters for speech or Speech Marks requests, outside the free tier.
Generative voices $30 Per 1 million characters for speech requests, outside the free tier.
Long-Form voices $100 Per 1 million characters for speech or Speech Marks requests, outside the free tier.

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AssemblyAI

Pay-as-you-go From $0.15 / hour No subscription. Pre-recorded transcription $0.15 / hour on Universal-2 and $0.21 / hour on Universal-3.5 Pro, streaming $0.15 / hour on Universal-Streaming and $0.45 / hour on Universal-3.5 Pro Realtime, Voice Agent API $4.50 / hour. Add-ons are billed on top: speaker diarization $0.02 / hour, medical mode $0.15 / hour, keyterms prompting $0.05 / hour on Universal-3.5 Pro and included on Universal-2. Free accounts get 5 new streams per minute, pay-as-you-go accounts 100.
Custom Not stated Contact sales for custom rate limits, higher concurrency and volume discounts. HIPAA BAA, PCI-DSS, ISO 27001 and SOC 2 Type 2 compliance at no premium, and an EU region at identical US pricing.

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What users say

Review Scores · opens after launch

Amazon Polly

Ease of use: Not rated yet
User interface: Not rated yet
Onboarding: Not rated yet
Support and service: Not rated yet
Value for money: Not rated yet
Features: Not rated yet

No reviews yet. Be the first to review this tool. Every review is checked for fairness before it appears, and the vendor cannot have one removed.

AssemblyAI

Ease of use: Not rated yet
User interface: Not rated yet
Onboarding: Not rated yet
Support and service: Not rated yet
Value for money: Not rated yet
Features: Not rated yet

No reviews yet. Be the first to review this tool. Every review is checked for fairness before it appears, and the vendor cannot have one removed.

Where this comes from

Amazon Polly https://aws.amazon.com/polly/ · checked against the official source on 2026-09-13

AssemblyAI https://www.assemblyai.com · checked against the official source on 2026-08-30

This table lists factual criteria taken from information the vendors publish themselves. For how we put it together, see the methodology.