OpenAI TTS-1 HD

Name: OpenAI TTS-1 HD
Brand: OpenAI
SKU: openai-tts-1-hd
Price: 0.036 USD
Availability: InStock

Text-to-speechAvailable

by OpenAIopenai-tts-1-hd

OpenAI's high-definition TTS model. Better quality for production use cases.

Try it Get API key Compare

Price: $0.036/1k chars
Input → output: Text → Audio
Developer: OpenAI
Updated: September 23, 2026

Playground

Try OpenAI TTS-1 HD

Input & output

$0.036/1k chars

Text

Voice(optional)

Advanced settings (1)

Format(optional)

⌘/Ctrl + Enter

Output

The generated speech appears here.

This run

$0.0001 · 0.01 credits

New here?

10 free credits ($0.10) when you sign up with Google

Usable 24 hours after sign-up, up to 5 runs per day and at most 2 credits per run. Other sign-in methods start without credits. Enough for 1,000 runs of this model.

About OpenAI TTS-1 HD

TL;DRAs of September 23, 2026

OpenAI TTS-1 HD is a model by OpenAI in the Text-to-speech category. On Railwail, OpenAI TTS-1 HD costs $0.036 per 1,000 characters.

Background

About OpenAI

Founded 2015 · San Francisco, California, USA

OpenAI was founded in December 2015 by Sam Altman, Elon Musk, Greg Brockman, Ilya Sutskever, Wojciech Zaremba and John Schulman as a non-profit AI research lab, restructured to capped-profit OpenAI LP in 2019. The company is best known for the GPT, DALL-E and Whisper model families and has raised more than $13B from Microsoft and billions more from Thrive Capital, Sequoia, Khosla and SoftBank, with a 2024 valuation above $157B. The TTS-1 HD model launched together with TTS-1 in November 2023 as the high-quality sibling, intended for offline audiobook, podcast and content-production workflows where fidelity matters more than latency. It powers the higher-quality option in the OpenAI audio endpoint and is exposed in tools like ChatGPT's Read Aloud feature on premium tiers.

Visit OpenAI

Architecture

Proprietary Transformer text-to-speech with neural codec (high-fidelity variant)

OpenAI TTS-1 HD is the high-fidelity variant of the TTS-1 family. It uses the same architectural family (Transformer-based neural-codec language-model TTS) and the same six voices (alloy, echo, fable, onyx, nova, shimmer) as TTS-1, but with a larger model and additional fine-tuning on long-form narration. The result is more natural prosody, clearer diction and better handling of long sentences, punctuation and emotional pacing, at roughly 2x the price ($0.030 per 1,000 characters) and noticeably higher latency. Output is 24 kHz in six formats (MP3, Opus, AAC, FLAC, WAV, PCM). Text input is capped at 4,096 characters per request, which is recommended to be split into paragraph chunks for very long narration. The model targets offline production: audiobooks, podcasts, training videos, accessibility narration. It does not support voice cloning.

Parameters: Undisclosed (larger than TTS-1)
Context: 4,096 tokens

Capabilities

High-fidelity narration with natural prosody and emotional pacing
Six recorded voices: alloy, echo, fable, onyx, nova, shimmer
Multilingual: 50+ languages including English, German, Spanish, French, Italian, Japanese, Mandarin
Six output formats including FLAC for lossless production
Long-sentence handling tuned for audiobook-style narration
Drop-in API-compatible with TTS-1 (just change the model name)
Best for: audiobooks, podcasts, training content, voiceover, accessibility narration

Training & license

Not disclosed. OpenAI states voices were recorded with paid professional actors; training corpus is a curated mix of recorded speech and licensed text-audio pairs.

License: Proprietary commercial API. Generated audio may be used commercially under the OpenAI Usage Policy; the six stock voices cannot be impersonated outside the API.

Safety testing: Same Preparedness red-team coverage as TTS-1; voice cloning is intentionally not exposed; outputs may carry provenance metadata.

Known limitations

No voice cloning
2x price of TTS-1 ($0.030 vs $0.015 per 1k chars)
Higher latency unsuitable for real-time voice agents
Hard cap of 4,096 characters per request
Limited explicit prosody / emotion controls

Research papers

Pricing

Prices in US dollars. Usage is charged from prepaid credits.

1,000 characters	$0.036 per 1,000 characters

1 credit = $0.01

Cost calculator

Price calculator

Characters per run

/ run

Runs

Total

$3.60

360 credits

Per run

$0.036 · 3.6 credits

Fixed price per run, known before the run starts.

API

Call OpenAI TTS-1 HD with your Railwail API key. Use this model ID in the request:

openai-tts-1-hdAPI documentation Get an API key

curl https://railwail.com/api/v1/audio/speech \
  -H "Authorization: Bearer $RAILWAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai-tts-1-hd",
    "input": "Hello from railwail. This sentence was spoken by a text-to-speech model.",
    "voice": "alloy"
  }' \
  --output speech.mp3

import os
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["RAILWAIL_API_KEY"],
    base_url="https://railwail.com/api/v1",
)

with client.audio.speech.with_streaming_response.create(
    model="openai-tts-1-hd",
    input="Hello from railwail. This sentence was spoken by a text-to-speech model.",
    voice="alloy",
) as response:
    response.stream_to_file("speech.mp3")

import fs from "node:fs";
import OpenAI from "openai";

const client = new OpenAI({
  apiKey: process.env.RAILWAIL_API_KEY,
  baseURL: "https://railwail.com/api/v1",
});

const speech = await client.audio.speech.create({
  model: "openai-tts-1-hd",
  input: "Hello from railwail. This sentence was spoken by a text-to-speech model.",
  voice: "alloy"
});

fs.writeFileSync("speech.mp3", Buffer.from(await speech.arrayBuffer()));

Set your key as RAILWAIL_API_KEYCreate API key

Specifications

Model ID: openai-tts-1-hd
Developer: OpenAI
Category: Text-to-speech
Input: Text
Output: Audio
Output formats: MP3, OPUS, AAC, FLAC
Billing: Fixed price, known before the run
Model size: Undisclosed (larger than TTS-1)
License: Proprietary commercial API. Generated audio may be used commercially under the OpenAI Usage Policy; the six stock voices cannot be impersonated outside the API.
Catalog entry updated: September 23, 2026

Input parameters

Inputs and settings from the model's input schema. The example in the API section shows which of them the API accepts.

ParameterTypeDefaultAllowed values

textrequired
Type: Text
Default: –
Allowed values: –
voice
Type: Choice
Default: –
Allowed values: alloy, echo, fable, onyx, nova, or shimmer
response_format
Type: Choice
Default: –
Allowed values: mp3, opus, aac, or flac

Example prompts

Examples from the Railwail catalog. They were not generated live on this page.

Audiobook Passage
The old lighthouse keeper climbed the spiral stairs for the last time. Forty years of storms, shipwrecks, and solitary nights had carved deep lines into his face. But tonight, as the automated beacon flickered to life without him, he felt not relief but an aching emptiness—the sea no longer needed his watchful eyes.
Product Demo
Introducing AuraSync, the smart home hub that learns your routines. It dims the lights when you start a movie, adjusts the thermostat when you fall asleep, and brews your coffee exactly six minutes before your alarm. Your home, finally as intelligent as you are.

Use cases

What it is used for

Audiobook production
High-quality podcast narration
Training and e-learning voiceover
Marketing videos and explainer content
Accessibility narration for premium products

Related guides

AI Voice & Text-to-Speech Models

All models in Text-to-speech

Frequently asked questions

What is OpenAI TTS-1 HD?

OpenAI TTS-1 HD is a model by OpenAI in the Text-to-speech category. On Railwail you can call it with an API key through the Railwail API.

How much does OpenAI TTS-1 HD cost on Railwail?

On Railwail, OpenAI TTS-1 HD costs $0.036 per 1,000 characters. The price is known before the run starts. Usage is paid from prepaid credits; 1 credit equals $0.01.

Which settings does OpenAI TTS-1 HD support?

According to its input schema, OpenAI TTS-1 HD knows these parameters: text, voice (alloy, echo, fable, onyx, nova, or shimmer), and response_format (mp3, opus, aac, or flac).

How fast is OpenAI TTS-1 HD?

There are not enough measured runs of OpenAI TTS-1 HD on Railwail yet to state a run time. It depends on the input, the settings and the load at the provider.

Is OpenAI TTS-1 HD better than AudioLDM 2?

That depends on the task. OpenAI TTS-1 HD (OpenAI) and AudioLDM 2 (Haohe Liu) are both models in the Text-to-speech category. The comparison page shows their prices and specifications side by side.

Compare OpenAI TTS-1 HD and AudioLDM 2

How do I use OpenAI TTS-1 HD through the API?

Create a Railwail API key and send your request with the model ID openai-tts-1-hd. Code examples for curl, Python and JavaScript are in the API section of this page.

Comparable models

All in this category

ChatterboxResemble AI
Resemble AI's open Chatterbox TTS. Zero-shot voice cloning from a short audio prompt with an exaggeration control for emotion intensity, plus CFG weight to balance pacing and fidelity.
$0.030/1k chars
17 % cheaper per unit
Compare OpenAI TTS-1 HD vs. Chatterbox
OpenAI TTS-1OpenAI
OpenAI's text-to-speech model. Six built-in voices with natural intonation.
$0.018/1k chars
50 % cheaper per unit
Compare OpenAI TTS-1 HD vs. OpenAI TTS-1
Qwen3 TTSAlibaba (Qwen)
A unified Text-to-Speech demo featuring three powerful modes: Voice, Clone and Design
$0.024/1k chars
33 % cheaper per unit
Compare OpenAI TTS-1 HD vs. Qwen3 TTS

Use OpenAI TTS-1 HD via the API

One API key for every model on Railwail. Usage is charged from prepaid credits, 1 credit = $0.01.

Get API key Browse all models