Speech-to-Speech vs. Text-to-Speech: When to Use Which

Ever since the 2023 strikes, every mention of AI voice comes with a lawyer attached. And for good reason.

On a professional set, there's no room for uncertainty. The money is real, the deadlines are real, the potential for a legal fight to derail everything is all too real. If a lawsuit is the most horrendous outcome, misusing the technology is a close second.

The first step to using AI responsibly is to be crystal clear on the difference between Text-to-Speech vs Speech-to-Speech. To clear it up, they're not in competition. They're built for completely different jobs.

Key Takeaways

• One follows a script, the other saves a performance. TTS creates a new voice performance from scratch based on your text. STS starts with a real actor's recording and preserves that performance in a different voice.
• TTS is for the functional jobs — prototypes, scratch tracks, and interactive AI voice agents. STS makes the final cut — when human performance is the one thing you can't afford to lose.
• The tech is a distraction if the voice isn't legally cleared. A professional technology requires a signed contract with the actor; no paperwork, no successful project.

Input and Output: How TTS and STS Technologies Differ

Every creator is, by necessity, a control freak — and that's with only good intentions. We need to know the final product reflects our vision, exactly how it came to us.

However, all the new AI voice tools can make that feel difficult — so many of them sound painfully similar. Many people easily confuse text-to-speech (TTS) and speech-to-speech (STS), thinking they must choose a single winner. 

That “winner” depends on where you, the creator, want to have more control: over the script or over the performance.

Text-to-Speech (TTS): You Control the Script

You start with the written word, and the AI does the performing for you. Your creative control is entirely in the script you write and how you format it.

• Input: A text file [.txt, .doc].
• Process: The AI interprets the words on the page and generates a voice performance from scratch. It’s making its own choices about tone and rhythm based on your text.
• Creative Output: A clean and consistent audio file. It's the perfect solution for animatics, tutorials, or powering the voice of a real-time AI agent — anywhere where clarity matters most.

Speech-to-Speech (STS): You Control the Performance

Here, you start with an actor. The AI won’t create a performance, but will preserve one you’ve already directed.

• Input: An audio file  [.wav, .mp3] with recorded human performance.
• Process: The technology analyzes the source recording (all the timing, the emotion, the unique cadence of the actor) and maps that entire act onto a different voice.
• Creative Output: The exact performance captured in the studio delivered in a new voice. This is for high-stakes work, like final-cut dialogue, dubbing that keeps the original emotion, or creating new character voices.

Where Speech-to-Speech (STS) Works

You use STS when the human performance is the one thing you can't leave to chance.

• In Film and TV. No more dubbings where the emotional weight of a scene is lost in translation. You take an original performance and give it a new voice in any language. STS also solves audio problems, like de-aging an actor’s voice or cleaning up a perfect line reading from a noisy set.
• In Gaming. You use STS to build worlds. Direct one actor to create a master performance and use the tech to create a whole cast that shares that same emotional core.
• In Marketing & Advertising. Your brand's voice is a trusted asset. STS lets you take a CEO or ambassador's authentic performance and adapt it for any market, so the trust won’t be lost in a new voice.

Where Text-to-Speech (TTS) Makes Sense

Not every piece of content requires an Oscar-worthy performance. Sometimes, you just need functional audio, and you need it now. That’s the job for Text-to-Speech.

• For prototyping and animatics. With TTS, you can create an instant table read, where you’d actually hear the script's pacing. This will help you spot awkward lines and fix the rhythm of the dialogue before you bring in an actor.
• For content at the speed of now. To get through that mountain of quick explainer videos or social posts, you just need a clean, understandable, good-quality voiceover. TTS is ideal for this.
• For real-time interactive audio. This is the tech that powers AI voice agents, chatbots, and in-game NPCs. When you need a voice that can respond instantly and naturally in a conversation, TTS can generate that dynamic audio on the fly.

Who to Call: Top TTS and STS Providers

In a real production, your choice of tools for Speech-to-Speech and Text-to-Speech has consequences. The right partner can save you time and legal headaches; the wrong one can create them. 

The market has settled into a few clear categories of specialists, so let's look at who they are and what they offer.

#1. Respeecher

The ones who show up when the studio needs a voice that simply cannot sound synthetic, robotic, or ambiguous. If you need to preserve every ounce of performance, our team is all yours. 

We use Speech-to-Speech technology that is transformative, yet respects the original human art:

• Our STS isn’t purely generative. We take the audio of a source actor (with all its subtle nuances, the inhaled gasp, the genuine strain of emotion) and transform it into your target voice.
• In Hollywood, AAA gaming, or your next indie project, "is it cleared?" is the first and last question that matters. Our answer is always yes — every voice in our system is there because we have a signed legal agreement with the actor.

But when you need instant, dynamic, perfect audio (real-time NPC chatter in a game, AI agents or a prompt for an IVR system) you need speed. And we’ve got that covered:

• Our Real-Time TTS API delivers audio in under 200ms, perfect for any application that needs natural dialogue, especially interactive voice agents. It’s a simple API call: send text, get a voice stream back. Matter of fact, you can start building with it today.
• That speed is useless if it introduces legal risk. Our API voices are ethically sourced and fully licensed, and the same rules apply, whether it’s for an NPC, a chatbot, or the star of your film.

#2. Resemble AI

The ones you go to if you need a custom engine built for fast iteration. Resemble AI is brilliant at blending TTS and STS, which means you can clone a voice quickly and use it for almost anything.

Their best feature is providing real-time generative features — the kind of technical versatility that lets you clone a voice in seconds. They also give you advanced security tools like deepfake detection and watermarking. 

This is a high-spec, flexible choice that works great when identity security is as important as the audio quality itself.

#3. Google Cloud TTS

The engine you plug in when you need to power a global system. Though not a creative partner, Google Cloud TTS is a piece of core infrastructure created by developers for developers.

It’s a Text-to-Speech utility, a foundational piece of the cloud you integrate into your application. Its strength is its sheer breadth — hundreds of voices across dozens of languages with a 99.9% uptime guarantee (SLA), so you know it won't fail on you.

You choose this for functional audio used to power call center menus, read web pages for accessibility, or give you navigation prompts. Scale and reliability are the most important metrics here.

How to Use TTS and STS Without Legal Risk

You wouldn't use a photo in an ad without a model release. You wouldn't use a song in your film without a sync license. So why are we even having a conversation about using someone's voice without a contract?

The first wave of AI tools, for both Text-to-Speech and Speech-to-Speech, treated the internet like a free-for-all sample library. Today, professional technology works the way the rest of our industry works. It’s built on a simple, boring, and absolutely critical foundation: paperwork.

• We talk to the actor. 
• We draw up a clear agreement that says exactly what their voice can be used for. 
• They get paid. 

That's all, folks. The voice in your project is a fully cleared asset, not a future lawsuit, and you get to work with an actor who’s a willing partner — which is always how the best work gets done.

Final Thoughts

Whether Text-to-Speech vs Speech-to-Speech, it all comes back to one thing: trust. Trust that the technology will deliver, trust that it was built the right way — with permission.

That’s our entire approach: solving high-stakes audio problems for partners who can't afford mistakes. Our Real-Time TTS API delivers a high-quality voice that's fast and, just as importantly, already legally cleared. And if you already have a performance that’s too valuable to lose, our Speech-to-Speech team is here to preserve it.

The tech is just tech. What matters is your unique vision. Bring us your impossible audio challenge, the one you’re not sure can be solved, — that's the work we love.