FA-78543 / Broadcast timecode arithmetic / Member archive

2:3 pulldown film frame placement: cadence group size · case 03

Film frames after the first cadence group map to video frames far too early.

Member previewVariant 3 · 3 implementations · 9 checks per implementation

Case contract

Film frames follow the A2 B3 C2 D3 field cadence (10 fields per 4 film frames). phase is the cadence position (0=A..3=D) of film frame 0, which starts at video field 0. Negative n returns None. For film frame n return [video frame of its first field, "upper" for an even field index else "lower", number of fields].

Why this case matters

Timecode arithmetic errors misplace edits, commercial breaks and captions against the broadcast clock.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: cadence group size	`[5, "upper", 2]`	`[10, "upper", 2]`	Failed

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