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Disc Mapping Design

This document describes a robust, deterministic design for reconstructing a frame-accurate disc map from a noisy field capture in ld-decode. The design explicitly prioritizes VBI-derived picture numbering (CAV / CLV), uses field phase only as a rejection and tie-breaking mechanism, and cleanly separates field pairing from frame numbering and gap detection.

1. Problem Summary

Input is a sequential capture of fields that may be: - Missing, duplicated, reordered, or corrupted - Carrying inconsistent or missing VBI data - Affected by noise, dropouts, or capture glitches

Goal: - Reconstruct an ordered sequence of frames - Assign each frame a picture number (if possible) - Detect and pad missing frames - Choose the best instance among duplicates - Preserve correct absolute positioning on the disc timeline

2. Definitions

Field

A captured video field with: - FieldID (capture order; not guaranteed contiguous) - Parity (first / second field; may be unreliable) - Phase hint (NTSC 1–4, PAL 1–8) - Per-field VBI data (CAV PN, CLV timecode, chapter/program) - Quality metrics

Frame

A logical pairing of two related fields representing one picture on disc.

Picture Number (PN)

A monotonically increasing integer index: - CAV: directly from VBI picture number - CLV: derived from timecode using format FPS - PN == 0 is invalid (lead-in/out)

3. Source-of-Truth Priority

  1. CAV picture number
  2. CLV timecode → picture number
  3. Chapter / program number (weak hint only)
  4. FieldID order, cadence, phase (supporting evidence only)

Phase and cadence must never override valid VBI numbering.

4. High-Level Pipeline

Fields ↓ Per-field VBI normalization ↓ Candidate field pairing ↓ Candidate frame construction ↓ VBI-driven frame selection & deduplication ↓ Gap detection & PAD insertion ↓ Final disc frame map

5. Per-Field Metadata Normalization

For each field:

  1. Attempt to decode:
  2. CAV picture number
  3. CLV timecode → picture number
  4. Validate:
  5. PN ≠ 0
  6. CRC / structural sanity
  7. Assign confidence score
  8. Mark lead-in/out or invalid data

All picture numbers are normalized to a single integer PN space.

6. Field Pairing (Candidate Frame Generation)

6.1 Pairing Strategy

  • Fields are paired based on capture order (FieldID)
  • Only pair within a small sliding window
  • Prefer first → second parity but allow uncertainty

6.2 Frame Construction Rules

For each candidate pair:

  • Select PN using priority:
  • First field CAV PN
  • Second field CAV PN
  • First field CLV PN
  • Second field CLV PN
  • If both fields provide PN of same type:
  • Values must agree
  • Otherwise keep first-field value unless invalid
  • Assign PN confidence
  • Combine quality metrics
  • Check phase consistency

6.3 Phase Handling

  • Phase is used only to veto obviously bad frames
  • Phase must not override trusted VBI PN
  • Phase disagreement is recorded for later tie-breaking

Result: a set of candidate frames, some with PN, some without.

7. Frame Validation and Filtering

  • Drop frames with PN == 0
  • Retain frames with no PN for continuity
  • Retain phase-invalid frames only if PN confidence is high
  • Mark PN-disagreement and phase issues for later scoring

8. Deduplication and Frame Selection

8.1 Group by Picture Number

Frames with PN are grouped by PN value.

8.2 Duplicate Resolution

For each PN group, select the best frame using:

  1. Highest PN confidence
  2. Phase-valid preferred
  3. Highest quality
  4. Capture-order coherence with neighbors

All other frames for that PN are discarded.

9. Disc Timeline Construction

9.1 Ordered PN Sequence

  • Sort selected frames by PN
  • Enforce monotonic PN order
  • Non-monotonic PN implies duplication or error

9.2 Gap Detection

For consecutive frames:

  • If PN increments by 1 → OK
  • If PN jumps forward:
  • Missing frames = ΔPN − 1
  • Insert PAD frames for missing PN values

Frames without PN: - Do not block gap insertion - Do not define numbering

10. PAD Frame Semantics

PAD frames represent missing frames on disc: - Carry a PN - Contain no image data - Preserve absolute disc positioning - Enable correct downstream indexing

11. Handling Regions with No VBI

When no trusted PN exists for a region:

  • Do not invent PN
  • Optionally estimate missing frames using:
  • FieldID gaps
  • Expected cadence (2 fields/frame)
  • Phase continuity
  • Resume PN-driven mapping when VBI reappears

12. Consistency Checks

CAV

  • PN must strictly increment by 1
  • Any repeat is a duplicate
  • Any jump implies missing frames

CLV

  • PN must be monotonic
  • Large absolute PN values are expected
  • Conversion must be consistent with disc format

13. Algorithmic Components

Stage Technique
Field pairing Sliding-window matching / min-cost
Frame deduplication Deterministic scoring
Gap detection PN monotone scan
Optional robustness Viterbi / HMM over PN

14. Design Principles

  • VBI numbering defines ground truth
  • Phase and cadence are supporting evidence only
  • Field pairing is local; numbering is global
  • Missing data is explicit (PAD), not implicit
  • Prefer determinism over heuristics where possible

15. Outcome

This design produces: - A stable, frame-accurate disc map - Correct absolute picture numbering - Explicit representation of missing content - Robust behavior under severe capture corruption

16. Implementation

Each stage shall produce a clear description of the decision making process/pipeline - with each stage description becoming part of a report to be given to the user alongside that actual outcome (which will be a field map stage parameter (the field map list)).