text-to-speech

Practical mastering steps for TTS audio: cleanup, loudness normalization, alignment, and delivery specs.

SKILL: TTS Audio Mastering

This skill focuses on producing clean, consistent, and delivery-ready TTS audio for video tasks. It covers speech cleanup, loudness normalization, segment boundaries, and export specs.

1. TTS Engine & Output Basics

Choose a TTS engine based on deployment constraints and quality needs:

• Neural offline (e.g., Kokoro): stable, high quality, no network dependency.
• Cloud TTS (e.g., Edge-TTS / OpenAI TTS): convenient, higher naturalness but network-dependent.
• Formant TTS (e.g., espeak-ng): for prototyping only; often less natural.

Key rule: Always confirm the native sample rate of the generated audio before resampling for video delivery.

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2. Speech Cleanup (Per Segment)

Apply lightweight processing to avoid common artifacts:

• Rumble/DC removal: high-pass filter around 20 Hz
• Harshness control: optional low-pass around 16 kHz (helps remove digital fizz)
• Click/pop prevention: short fades at boundaries (e.g., 50 ms fade-in and fade-out)

Recommended FFmpeg pattern (example):

• Add filters in a single chain, and keep them consistent across segments.

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3. Loudness Normalization

Target loudness depends on the benchmark/task spec. A common target is ITU-R BS.1770 loudness measurement:

• Integrated loudness: -23 LUFS
• True peak: around -1.5 dBTP
• LRA: around 11 (optional)

Recommended workflow:

1. Measure loudness using FFmpeg ebur128 (or equivalent meter).
2. Apply normalization (e.g., loudnorm) as the final step after cleanup and timing edits.
3. If you adjust tempo/duration after normalization, re-normalize again.

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4. Timing & Segment Boundary Handling

When stitching segment-level TTS into a full track:

• Match each segment to its target window as closely as possible.
• If a segment is shorter than its window, pad with silence.
• If a segment is longer, use gentle duration control (small speed change) or truncate carefully.
• Always apply boundary fades after padding/trimming to avoid clicks.

Sync guideline: keep end-to-end drift small (e.g., <= 0.2s) unless the task states otherwise.