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# ACE-Step 1.5

ACE-Step 1.5 was introduced in [ACE-Step 1.5: Pushing the Boundaries of Open-Source Music Generation](https://arxiv.org/abs/2602.00744) by the ACE-Step Team (ACE Studio and StepFun). It is an open-source music foundation model that generates commercial-grade stereo music with lyrics from text prompts.

ACE-Step 1.5 generates variable-length stereo audio at 48 kHz (10 seconds to 10 minutes) from text prompts and optional lyrics. The full system pairs a Language Model planner with a Diffusion Transformer (DiT) synthesizer; this pipeline wraps the DiT half of that stack, and consists of three components: an [`AutoencoderOobleck`] VAE that compresses waveforms into 25 Hz stereo latents, a Qwen3-based text encoder for prompt and lyric conditioning, and an [`AceStepTransformer1DModel`] DiT that operates in the VAE latent space using flow matching.

The model supports 50+ languages for lyrics — including English, Chinese, Japanese, Korean, French, German, Spanish, Italian, Portuguese, and Russian — and runs on consumer GPUs (under 4 GB of VRAM when offloaded).

This pipeline was contributed by the [ACE-Step Team](https://github.com/ace-step). The original codebase can be found at [ace-step/ACE-Step-1.5](https://github.com/ace-step/ACE-Step-1.5).

## Variants

ACE-Step 1.5 ships three DiT checkpoints that share the same transformer architecture but differ in guidance behavior; the pipeline auto-detects turbo checkpoints from the loaded transformer config and ignores CFG guidance for those guidance-distilled weights.

| Variant | CFG | Default steps | Default `guidance_scale` | Default `shift` | HF repo |
|---------|:---:|:-------------:|:------------------------:|:---------------:|---------|
| `turbo` (guidance-distilled) | off | 8 | ignored | 3.0 | [`ACE-Step/Ace-Step1.5`](https://huggingface.co/ACE-Step/Ace-Step1.5) |
| `base` | on | 8 | 7.0 | 3.0 | [`ACE-Step/acestep-v15-base`](https://huggingface.co/ACE-Step/acestep-v15-base) |
| `sft` | on | 8 | 7.0 | 3.0 | [`ACE-Step/acestep-v15-sft`](https://huggingface.co/ACE-Step/acestep-v15-sft) |

Base and SFT use the learned `null_condition_emb` for classifier-free guidance (APG, not vanilla CFG). Users commonly override `num_inference_steps` to 30–60 on base/sft for higher quality.

## Tips

When constructing a prompt, keep in mind:

* Descriptive prompt inputs work best; use adjectives to describe the music style, instruments, mood, and tempo.
* The prompt should describe the overall musical characteristics (e.g., "upbeat pop song with electric guitar and drums").
* Lyrics should be structured with tags like `[verse]`, `[chorus]`, `[bridge]`, etc.

During inference:

* `num_inference_steps`, `guidance_scale`, and `shift` default to the values shown above. For turbo checkpoints, `guidance_scale > 1.0` is ignored with a warning because guidance is distilled into the weights.
* The `audio_duration` parameter controls the length of the generated music in seconds.
* The `vocal_language` parameter should match the language of the lyrics.
* `pipe.sample_rate` and `pipe.latents_per_second` are sourced from the VAE config (48000 Hz and 25 fps for the released checkpoints).
* For audio-to-audio tasks, pass `src_audio` and `reference_audio` as preprocessed stereo tensors at `pipe.sample_rate`.
* `flash` and `flash_hub` use FlashAttention's native sliding-window support for ACE-Step's self-attention and expect unpadded text batches. If a batched prompt contains padding, use `flash_varlen` or `flash_varlen_hub` instead. Single-prompt inference with `padding="longest"` is normally unpadded.

```python
import torch
import soundfile as sf
from diffusers import AceStepPipeline

pipe = AceStepPipeline.from_pretrained("ACE-Step/Ace-Step1.5", torch_dtype=torch.bfloat16)
pipe = pipe.to("cuda")

audio = pipe(
    prompt="A beautiful piano piece with soft melodies and gentle rhythm",
    lyrics="[verse]\nSoft notes in the morning light\nDancing through the air so bright\n[chorus]\nMusic fills the air tonight\nEvery note feels just right",
    audio_duration=30.0,
).audios

sf.write("output.wav", audio[0].T.cpu().float().numpy(), pipe.sample_rate)
```

## AceStepPipeline
[[autodoc]] AceStepPipeline
	- all
	- __call__