audio: phase_vocoder: re-anchor synthesis phase on speed change

[singalsu]

When the user changes the speed control the component preserves the polar analysis state across the reset so that the interpolation can continue smoothly, but the synthesis phase accumulator output_phase was left drifting. Each output IFFT in stft_do_fft_ifft() advances output_phase by an interpolated one-analysis-hop delta of the form (1 - frac) * angle_delta_prev + frac * angle_delta, where frac is the current interpolation position between two consecutive analysis frames. That delta is added irrespective of the current speed, so at non-unity speed output_phase runs faster than the true polar angle. On top of that, the first post-reset IFFT does not consume a new input FFT yet still applies its phase interpolation step (with frac = 0, so the added term equals angle_delta_prev), which adds one extra angle_delta_prev per reset. After a repeated excursion (for example 1.0 -> 0.5 -> 1.0 with several intermediate steps) both effects accumulate as a random per-bin phase offset that persists after returning to unity speed. Perceptually this smears transients and dulls the sound even though the audio is being processed at speed 1.0 again.

The steady-state invariant at speed 1.0 is that after each IFFT's phase interpolation step output_phase equals polar_prev.angle. To preserve this invariant across a speed change, re-anchor output_phase to polar_prev.angle minus angle_delta_prev in reset_for_new_speed(). The first post-reset IFFT then lands output_phase exactly on polar_prev.angle, and all subsequent IFFTs advance normally with no cumulative offset. Existing NULL and channel-count guards keep the init-time call from touching buffers before they are allocated.

[Copilot]

🟢 Ready to approve

The change is small, well-guarded against uninitialized buffers, and addresses the described drift mechanism without introducing API or control-flow risk.

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Pull request overview

This PR fixes a phase drift issue in the phase vocoder when the user changes playback speed by re-anchoring the synthesis phase accumulator during speed resets, preventing cumulative per-bin phase offsets that persist after returning to unity speed.

Changes:

• Preserve analysis-state continuity across speed changes while re-initializing synthesis output_phase to maintain the steady-state invariant at speed 1.0.
• Adjust phase_vocoder_reset_for_new_speed() to set output_phase[ch][i] = unwrap(polar_prev.angle - angle_delta_prev) for all bins/channels with allocated state.

File summaries

File	Description
src/audio/phase_vocoder/phase_vocoder_common.c	Re-anchors synthesis phase during speed-change reset to prevent cumulative phase drift and post-reset offsets.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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