Vllm omni tts integration

TTS Model Integration Workflow

Overview

HF Reference -> Stage Separation -> Online Serving -> Async Chunk -> CUDA Graph
   (Phase 1)      (Phase 2)          (Phase 3)        (Phase 4)     (Phase 5)

Phase 1: HuggingFace Reference

Goal: Understand the reference implementation and verify it produces correct audio.

Steps

1. Run the reference model end-to-end using the official HuggingFace / GitHub code
2. Document the architecture:
   • What are the sub-models? (AR decoder, codec decoder, vocoder, etc.)
   • What is the token vocabulary? (semantic codes, RVQ codebooks, special tokens)
   • What is the output format? (sample rate, channels, codec type)
3. Capture reference outputs for comparison during integration
4. Identify the config structure: config.json fields, model_type, sub-model configs

Key Questions

• How many codebooks? What are the codebook sizes?
• What special tokens exist? (<|voice|>, <|audio_start|>, <|im_end|>, etc.)
• What is the token-to-ID mapping for codec codes?
• What is the hop length / frame rate of the codec?
• Does the model support voice cloning? How? (reference audio encoding, speaker embeddings, etc.)

Deliverables

• Working reference script that produces audio
• Architecture diagram / notes
• Token vocabulary mapping
• Reference audio samples for regression testing

Phase 2: Stage Separation (Offline Inference)

Goal: Split the model into vLLM-Omni stages and get offline inference working.

Steps

1. Register the model in vllm_omni/model_executor/models/registry.py
2. Create config classes (configuration_<model>.py) with model_type registration
3. Implement Stage 0 (AR model):
   • Subclass appropriate base (e.g., wrap Qwen3 decoder layers)
   • Implement forward() for autoregressive token generation
   • Handle special token logic (start/stop tokens, codec token mapping)
   • If dual-AR (like Fish Speech), implement Fast AR as a nested module
4. Implement Stage 1 (Decoder):
   • Load codec weights (may need lazy loading from separate checkpoint)
   • Implement forward(): codec codes -> audio waveform
   • Return OmniOutput with multimodal_outputs
5. Create stage config YAML defining both stages, memory allocation, and model paths
6. Create stage input processor for prompt building
7. Write end2end.py test script

Critical Parameters to Get Right

Parameter	Impact if Wrong
Hop length	Audio duration wrong, streaming noise
Token ID mapping	Garbage codes -> noise output
Codebook count/size	Shape mismatch crashes
Stop token	Generation never stops or stops too early
dtype / autocast	Numerical issues, silent quality degradation
Repetition penalty	Must match reference (often 1.0 for TTS)

Debugging Priority (from experience)

When audio output is wrong, check in this order:

1. RoPE / attention: Are position encodings correct? Is the attention mask right?
2. Normalization: RMSNorm epsilon, layer norm placement (pre vs post)
3. Hop length: Product of all upsample rates in the codec decoder
4. Token mapping: Are codec IDs correctly offset from the vocabulary base?
5. Sampling parameters: Temperature, top_k, top_p, repetition_penalty
6. Tensor layout: Codebook-major vs frame-major ordering
7. dtype: Float32 for codec decoders (autocast can corrupt audio)

Deliverables

• Model files in vllm_omni/model_executor/models/<model_name>/
• Stage config YAML
• Working end2end.py with correct audio output
• README.md in the example directory

Phase 3: Online Serving

Goal: Expose the model via /v1/audio/speech API endpoint.

Steps

1. Register in serving_speech.py:
   • Add model stage name to _TTS_MODEL_STAGES set
   • Add model detection flag (e.g., _is_fish_speech)
   • Implement prompt builder method (e.g., _build_fish_speech_prompt())
2. Handle model-specific parameters:
   • Voice cloning: ref_audio encoding and prompt injection
   • max_new_tokens override in sampling params
   • Model-specific default values
3. Use tokens_input() for prompt construction: All TTS prompt builders must use vllm.inputs.tokens_input(prompt_token_ids=...) instead of raw dicts. This avoids deprecated preprocess() fallback warnings. The returned dict has "type": "token" automatically. Add model-specific keys (e.g., additional_information) to the returned dict.
4. Create client scripts: speech_client.py, run_server.sh
5. Test all response formats: wav, mp3, flac, pcm
6. Add Gradio demo: Interactive web UI with streaming support

Voice Cloning Pattern

import base64
from pathlib import Path
from vllm.inputs import tokens_input

def build_voice_clone_prompt(ref_audio_path: str, text: str, codec, tokenizer) -> dict:
    """Build prompt with reference audio for voice cloning in serving_speech.py."""
    audio_bytes = Path(ref_audio_path).read_bytes()
    codes = codec.encode(audio_bytes)  # Encode on CPU using model's codec (e.g., DAC)
    token_ids = [code + codec.vocab_offset for code in codes.flatten().tolist()]

    # Build prompt with voice tokens and tokenize
    voice_tokens = "".join(chr(t) for t in token_ids)
    prompt = f"<|voice|>{voice_tokens}{text}"
    prompt_token_ids = tokenizer.encode(prompt)

    # Use tokens_input() to avoid deprecated preprocess() fallback
    result = tokens_input(prompt_token_ids=prompt_token_ids)
    return result

Deliverables

• Updated serving_speech.py with model-specific prompt builder
• Client scripts and server launcher
• Gradio demo with streaming and voice cloning UI
• Documentation (offline + online serving docs)

Phase 4: Async Chunk (Streaming)

Goal: Enable inter-stage streaming so audio chunks are produced while AR generation continues.

Steps

1. Update stage config YAML:

   async_chunk: true
   codec_chunk_frames: 25      # inter-stage streaming window
   codec_left_context_frames: 25  # overlap for smooth boundaries
   initial_codec_chunk_frames: 10 # first chunk only (optional, lowers TTFA)
   

2. Implement chunk handling in Stage 1:
   • Accept partial input (chunk of codec codes)
   • Handle left context for smooth audio boundaries
   • Return partial audio in OmniOutput
3. Separate connector chunking from decoder chunking:
   • codec_chunk_frames / codec_left_context_frames control inter-stage streaming windows
   • decode_chunk_frames / decode_left_context_frames control the decoder's internal processing (independent)
   • Use initial_codec_chunk_frames for a smaller first chunk to lower TTFA, then return to normal codec_chunk_frames for subsequent chunks
4. Test streaming:
   • Verify audio quality matches non-streaming output
   • Check for artifacts at chunk boundaries
   • Measure TTFA (time to first audio)
5. Update online serving to support stream=true with PCM output

Streaming Architecture

Stage 0 (AR)                    Stage 1 (Decoder)
  |                                |
  |-- chunk 0 (25 frames) ------> decode -> audio chunk 0 -> client
  |-- chunk 1 (25 frames) ------> decode -> audio chunk 1 -> client
  |-- chunk 2 (25 frames) ------> decode -> audio chunk 2 -> client
  ...

Key Considerations

• Left context overlap: Prevents audible artifacts at chunk boundaries
• Hop length matters: context_audio_samples = context_frames * hop_length
• First chunk latency: Can use larger initial chunk for better quality, then smaller chunks

Deliverables

• Updated stage config with async_chunk enabled
• Smooth streaming audio without boundary artifacts
• TTFA metrics

Phase 5: CUDA Graph Acceleration

Goal: Capture the AR loop as a CUDA graph for significant speedup.

Steps

1. Identify the hot loop: The AR decoding loop that runs N steps per token
2. Create static buffers:
   • KV caches with fixed max sequence length
   • Pre-built causal masks and position tensors per step
   • Static input/output tensors
3. Implement graph capture:
   • Warm up with real data
   • Capture the forward pass
   • Replay with updated inputs
4. Handle constraints:
   • Use torch.argmax instead of torch.multinomial (graph-safe)
   • Fixed batch size (fall back to eager for other sizes)
   • No dynamic control flow inside the graph

Example: Code Predictor CUDA Graph (Qwen3-TTS)

import torch

class CodePredictorGraph:
    """Captures the 16-step code predictor AR loop as a single CUDA graph."""

    def setup_graph(self, device: torch.device, kv_heads: int = 4, head_dim: int = 64):
        self.num_steps = 16
        self.kv_cache = torch.zeros(1, kv_heads, self.num_steps, head_dim, device=device)
        self.positions = torch.arange(self.num_steps, device=device)
        self.causal_mask = torch.tril(torch.ones(self.num_steps, self.num_steps, device=device))
        self.input_buf = torch.zeros(1, 1, kv_heads * head_dim, device=device)
        self.output_buf = torch.zeros(1, self.num_steps, device=device, dtype=torch.long)
        # Warm up, then: self.graph = torch.cuda.CUDAGraph(); self.graph.capture(...)

    def run_graph(self, initial_input: torch.Tensor) -> torch.Tensor:
        self.input_buf.copy_(initial_input)
        self.graph.replay()
        return self.output_buf.clone()

Performance Expectations

Based on Qwen3-TTS code predictor experience:

• 3-5x speedup for the graphed component
• Only effective for fixed batch sizes (typically batch_size=1)
• Falls back to eager mode for unsupported configurations

Deliverables

• CUDA graph implementation for the AR hot loop
• Benchmark script comparing eager vs graph performance
• Documentation of constraints and fallback behavior

Integration Checklist

Use this checklist when integrating a new TTS model:

Phase 1: HF Reference

• Reference model runs and produces correct audio
• Architecture documented (stages, codebooks, tokens, sample rate)
• Reference audio samples saved for comparison

Phase 2: Stage Separation

• Model registered in registry.py
• Config classes created with model_type registration
• Stage 0 (AR) implemented and generates correct tokens
• Stage 1 (Decoder) produces correct audio from tokens
• Stage config YAML created
• end2end.py produces audio matching reference quality
• README.md written

Phase 3: Online Serving

• Model added to serving_speech.py
• Prompt builder uses tokens_input() from vllm.inputs (not raw dicts)
• Prompt builder handles text input correctly
• Voice cloning works (if supported)
• All response formats work (wav, mp3, flac, pcm)
• Client scripts and server launcher created
• Gradio demo working
• Documentation added (offline + online docs, nav, supported models)

Phase 4: Async Chunk

• Stage config updated with async_chunk: true
• Stage 1 handles partial chunks correctly
• No audio artifacts at chunk boundaries
• Streaming via API (stream=true) works
• TTFA measured and acceptable

Phase 5: CUDA Graph

• Hot loop identified and profiled
• Static buffers allocated
• Graph captured and replays correctly
• Benchmark shows meaningful speedup
• Fallback to eager works for unsupported configs

References

• TTS audio skill -- supported models and usage
• Fish Speech integration -- complete example of Phases 1-3
• Qwen3-TTS reference -- complete example of all 5 phases
• Adding a TTS model (developer guide)