[LeeO86]

What

MXL 218ddaa docs/Architecture.md (video/smpte291): a grain stores the RFC 8331 payload from the Length field on. Length (16), ANC_Count (8), F (2) and reserved (22) are 6 bytes; the first ANC packet follows at byte 6 (in the RTP packet that is byte 20, 32-bit aligned; MXL drops the first 14 bytes).

encodeRfc8331 padded the header to 8 bytes (alignTo32 after reserved), so every packet sat 2 bytes late. mxl-st2110-gateway's ANC egress, MXL's mxl-data-probe and gst-mxl-rs read packets at byte 6 and would take the 2 padding bytes as the start of packet 1.

Now:

• no header padding; Length counts the bytes after the 6-byte header;
• each packet is padded to a multiple of 32 bits from its own start (RFC 8331 word_align);
• F bits unchanged;
• decodeRfc8331 (tests) reads the same layout.

The output side does not read ANC, so nothing else changes.

C bit (second commit)

RFC 8331 C = 1 means the colour-difference (chroma) data stream. The writer set C = 1 for every packet with a known line. Now C follows the SDK's GetDataStreamIndex() (third_party/decklink/DeckLinkAPI.h:996: "Usually zero. Can only be 1 if non-SD and the first data stream is completely full"): 0 = luma (and SD) gives C = 0, 1 = chroma gives C = 1, anything else counts as luma.

The same index was also written to S and StreamNum. RFC 8331 keeps those for the link number of a multi-stream interface (e.g. ST 425-3), which the SDK does not give, so S and StreamNum are now 0. The decoder (tests) maps C back to the index.

Tests

• New unit test: two packets (the gateway's AFD golden packet with its original C = 0, and a chroma-stream packet with C = 1; field 1) encode to exactly the reference vector made by mxl-st2110-gateway's codec::serialiseGrain, and decode back.
• New unit test: the C bit per SDK data stream index (0, 1, unknown).
• Checked once against the gateway's parser (codec::parseGrain): no parity or checksum errors; an empty grain is byte-identical too.
• Existing tests updated for the 6-byte header.

Version 1.0.7 (CMake, src/version.hpp, Kubernetes), CHANGELOG, SPECIFICATION §2.3, IMPLEMENTATION_PLAN.

Companion PRs: LeeO86/mxl-test-player#15, LeeO86/mxl-replay#28.

🤖 Generated with Claude Code
https://claude.ai/code/session_01St7QwmonpoG9EEEqaLFXLi