timecode-precision.md

Timecode Precision

Non-integer frame rates (29.97fps = 30000/1001) cause cumulative rounding errors. Follow these rules to avoid drift.

Use round(), Not int()

For float-to-frame conversion:

• int(9000 * 29.97) — wrong, truncates and loses frames
• round(9000 * 29.97) — correct, gets the right answer

Use Integer Arithmetic for Timecode Display

Convert frames → total milliseconds via:

total_ms = round(frames * fps_den * 1000 / fps_num)

Then decompose with integer division. Avoid intermediate floats that drift over long durations.

Accept ±1 Frame Tolerance

In roundtrip tests at non-integer FPS, exact equality is mathematically impossible. Accept ±1 frame tolerance.