[WebRTC_Demo] Duplex audio can be silent/choppy due to int16 resample path in backend

[chatsci]

Summary

In demo/web_demo/WebRTC_Demo, duplex mode can show normal subtitles/state transitions but produce no audible output, or very choppy output (sentence split into discontinuous fragments).

Environment

• macOS (Apple Silicon), M1 Max, 64GB RAM
• WebRTC_Demo started via bash oneclick.sh start
• model: openbmb/MiniCPM-o-4_5-gguf
• LiveKit + backend + cpp server all healthy

Symptoms

• Frontend receives <state><audio_start>, <state><generate_end>, subtitles are correct
• Browser side shows remote audio track attached, but sound is missing or highly discontinuous
• C++ logs show generation succeeded and WAV chunks sent

Example (from logs):

• total generation time ~4.7s
• total audio duration ~2.0s
• RTF ~2.35x
• only a few chunks sent

Root Cause (confirmed locally)

In backend audio path, int16 PCM is directly fed into scipy.signal.resample_poly(...) in voice_chat/omni_stream.py. In this path, resampled output can become near-zero/all-zero for some inputs, so LiveKit receives effectively silent frames.

Additionally, coarse chunking + queue underflow + aggressive generate_end/play_end transition can make output choppy.

Minimal Fix

Before resampling:

• Convert source audio to normalized float32 in [-1, 1]
• Run resample_poly on float32
• Convert back to int16 after resample

This fixed the silent-audio issue immediately in local verification.

Suggested Code Locations

• WebRTC_Demo/WebRTC_Demo/omini_backend_code/code/voice_chat/omni_stream.py
  • model wav -> WebRTC path
  • local TTS wav -> WebRTC path
  • any helper resampling functions

Optional Improvements for Choppy Playback

• reduce output chunk size (finer granularity)
• avoid queue starvation in output_audio
• delay play_end decision to avoid premature turn-end during short gaps

Repro Steps

1. cd demo/web_demo/WebRTC_Demo
2. bash oneclick.sh start
3. open frontend URL, start duplex voice conversation
4. observe subtitle/state normal but audio silent/choppy