# Pcm Telemetry Decommutation > Use when you must decommutate a serial PCM telemetry stream: acquire frame sync by locating the sync word in the captured word stream, walk the locked minor frames and count sync misses, and demultiplex the recovered telemetry channels into time series, with fixed channels read from their word slot every frame, supercommutated channels read from their multiple word slots per frame, and subcommutated channels keyed by the subframe id into the per-subframe value lists. Produce the locked frame count, the sync miss report, the subframe id trace, and the recovered per-channel time series that feed data reduction. Trigger: pcm telemetry decommutation, pcm frame sync, supercommutated demux, subcommutated demux, minor frame decode. - Skill: `ashfordeou/pcm-telemetry-decommutation` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/pcm-telemetry-decommutation` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/pcm-telemetry-decommutation/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Docs & Writing - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/pcm-telemetry-decommutation --- # PCM Telemetry Decommutation (flight-test-operations/planning/pcm-telemetry-decommutation) Use when the task is decoding a serial PCM minor-frame telemetry stream at the ground station decode side: finding the sync word in the captured word stream, locking the minor frames, counting sync misses, and splitting each locked frame back into the fixed, supercommutated and subcommutated channel time series that feed data reduction. This leaf implements the standard decommutation walk in pure Python, stdlib only. It pairs with flight-test-operations/planning/telemetry-data-acquisition (the design leaf, which sizes the frame and assigns the channels and stops at the transmitter) and with flight-test-operations/planning/flight-test-data-reduction (the post decomm processing leaf that calibrates, aligns and smooths these recovered series). ## Domain quick reference - Wire format: a minor frame on the wire is the sync word followed by the data words, then the idle words before the next sync word. The frame period in words is 1 + data_words_per_frame + idle_words, and frame_period_words(data_words_per_frame, idle_words) returns that period. Example: 8 data words and 1 idle word give a 10 word period. - Frame sync acquisition: the sync word is a fixed bit pattern that repeats at every frame boundary. sync_search finds its first occurrence at or after a start index; acquire_lock reports first_sync_index and scan_length (words examined up to and including the sync word) for the first lock in the stream, or raises ValueError when no sync word exists in the scanned region. - Locked-frame walk: with the frame lock at first_sync_index, the next sync word is expected one frame period later. At each expected sync position, a word equal to the sync word locks the frame and the following data_words_per_frame words are recorded; any other word is a sync miss, and the walk resyncs by scanning forward from the missed position, bounded to the next two frame periods. - Fixed channel: one word slot sampled every frame, so it yields one value per locked frame. - Supercommutation: a channel occupies multiple word slots per frame (a list of slot indices) and yields one sample per slot, so it yields len(slots) values per locked frame, in slot order. - Subcommutation: a channel occupies one word slot that carries M sub-values, cycled across frames. A designated subframe id word, masked to its low bits, selects which sub-value each frame carries; the sample joins the per-subframe value list for that id. - Subframe id: read from the frame data word at sid_word_index and masked with sid_mask, so the recovered trace is the id itself for every locked frame. - All channel values are recovered as transmitted: no calibration, alignment or smoothing happens here (those belong to flight-test-data-reduction). ## Workflow 1. State the format: data_words_per_frame and idle_words, and confirm the period with frame_period_words(data_words_per_frame, idle_words). 2. Acquire the frame lock: acquire_lock(stream, sync_word, data_words_per_frame, idle_words, max_scan) returns first_sync_index and scan_length; a ValueError means the stream holds no sync word in the scanned region and cannot be decoded. 3. Walk the frames: decode_frames(stream, sync_word, data_words_per_frame, idle_words) returns frames_locked, sync_misses and frame_data, the per-frame data word lists of every locked frame. Corrupted sync words drop the affected frame and add one to sync_misses. 4. Demultiplex with demultiplex(frame_data, sid_word_index, sid_mask, supercommutated, subcommutated): pass the supercommutated map {channel: [word slots]} and the subcommutated map {channel: {"word_index": i, "subframes": M}}. Recover super[channel] (values in frame order then slot order), sub[channel][subframe_id] (values per subframe id) and subframe_ids. 5. For a one-call decode, run decommutation_summary(stream, sync_word, data_words_per_frame, idle_words, format) with the format dict {"sid_word_index", "sid_mask", "supercommutated", "subcommutated"}; it returns frames_locked, sync_misses, super, sub and subframe_ids together. 6. Hand the recovered per-channel series to flight-test-data-reduction for its post-decomm processing (its correction, alignment and smoothing steps). 7. Confirm the deterministic checks with the contract test scripts/test_pcm_telemetry_decommutation.py. ## Worked example 16-bit words, sync word 0xEB90, 8 data words per frame, 1 idle word, so the frame period is 10 words. Frame k (k = 0..39) carries: word0 = k & 3 (subframe id), word1 = 1000 + k (channel A, fixed), word2 = 2000 + (k & 3) * 100 + (k // 4) (channel S, subcommutated, 4 subframes), word3 = 3000 + k (channel A2, supercommutated slot 1), word4 = 4000 + k, word5 = 5000 + k (channel A2, supercommutated slot 2), word6 = 6000 + k, word7 = 7000 + k. Idle word 0x0000. The wire stream is the 40 frames concatenated (400 words). Real module outputs on this fixture: - frame_period_words(8, 1) = 10. - acquire_lock: first_sync_index = 0, scan_length = 1 (clean, aligned stream). - decode_frames: frames_locked = 40, sync_misses = 0, and every frame_data[k] equals the transmitted data words of frame k elementwise. - demultiplex with sid_word_index 0, sid_mask 0x0003, supercommutated {"A": [1], "A2": [3, 5]}, subcommutated {"S": {"word_index": 2, "subframes": 4}}: - super A = [1000, 1001, ..., 1039] (40 values). - super A2 = [3000, 5000, 3001, 5001, ..., 3039, 5039] (80 values, frame order then slot order). - sub S: subframe 0 = [2000..2009], subframe 1 = [2100..2109], subframe 2 = [2200..2209], subframe 3 = [2300..2309] (10 values each). - subframe_ids = [0, 1, 2, 3, 0, 1, ...] (40 ids, cycling). Corruption check on the same 40-frame stream with frame 20 sync word replaced by 0xFFFF: frames_locked = 39, sync_misses = 1; channel A recovers 39 values (the 1020 sample is dropped with frame 20); channel A2 recovers 78 values (3020 and 5020 dropped); subframe 0 of channel S recovers 9 values (the frame 20 sample, value 2005, is dropped). ## Pitfalls - Demultiplexing without the frame layout: supercommutated channels interleave in frame order then slot order (A2 reads [3000, 5000, 3001, 5001, ...] for slots [3, 5]), while subcommutated channels key on the subframe id carried in the sid word. - Reading the subframe id with the wrong mask or word index: the ids cycle through 0..3 under sid_mask 0x0003 at sid_word_index 0, and an out-of-range sid_word_index raises ValueError. - Ignoring sync corruption in the recovery: a corrupted sync word drops the whole frame from every channel (39 locked with 1 miss), so channel A recovers 39 values and subframe 0 of channel S only 9 - downstream analysis must expect the missing samples. - Locking onto an unaligned stream without the scan bound: three junk words prepended shift first_sync_index to 3 with the same recovered frames, and max_scan bounds the sync search. - Feeding malformed format or subcommutation dicts: empty frame lists, ragged frame lengths, supercommutated slots outside the frame, subframes below 1, and subcommutated entries missing their word_index or subframes keys all raise ValueError. - Recomputing the frame period from the data words alone: the period is data plus idle words (8 + 1 = 10 here), and the layout must match the transmitter-side design owned by telemetry-data-acquisition. ## Verification - Confirm frame_period_words(8, 1) = 10, (8, 0) = 9, (0, 0) = 1, and that negative data_words_per_frame or idle_words raises ValueError. - Confirm sync_search finds the first sync word at or after start and returns None when the sync word is absent; empty streams and negative sync words raise ValueError. - Confirm acquire_lock on the clean stream reports first_sync_index 0; three junk words prepended shift it to 3 with the same recovered frames. - Confirm on a clean stream every transmitted frame is locked (sync_misses == 0) and the recovered channel values equal the transmitted ramp values elementwise (deterministic identity). - Confirm a frame whose sync word is corrupted is not locked and increments sync_misses by one, with its samples absent from every recovered channel. - Confirm demultiplex raises ValueError on an empty frame list, an out-of-range sid_word_index, a ragged frame length, a supercommutated slot outside the frame, subframes below 1, and subcommutated entries missing their word_index or subframes keys. - Confirm decommutation_summary returns exactly the keys frames_locked, sync_misses, super, sub, subframe_ids and matches the decode then demultiplex result. - Run the contract test offline: python3 scripts/test_pcm_telemetry_decommutation.py (27 tests, deterministic). ## Related leaves - flight-test-operations/planning/telemetry-data-acquisition: the design side, sizes the PCM frame, assigns the supercommutated and subcommutated channels and stops at the transmitter. - flight-test-operations/planning/flight-test-data-reduction: the post decomm processing side, calibrates, aligns and smooths the series this leaf recovers. - flight-test-operations/planning/flight-test-instrumentation: the sensor and recorder chain upstream of the telemetry stream. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_pcm_telemetry_decommutation.py The test covers the frame period arithmetic, sync word search, frame lock acquisition on clean and unaligned streams with max_scan bounds, the locked-frame walk on clean and corrupted streams (40 frames locked with zero misses versus 39 locked with one miss), fixed and supercommutated channel recovery including the interleaved multi slot channel, subcommutated recovery keyed by the subframe id, the subframe id trace, the elementwise ramp identity, the corruption dropped-sample checks, and ValueError rejection of empty streams, no sync word, negative frame words, out-of-range slots and malformed format and subcommutation dicts. ## Compliance - Standards referenced, not reproduced: FAR-25 sets the flight test and certification context; PCM decommutation practice (frame sync acquisition, minor frame locking, supercommutation and subcommutation demultiplexing) is common telemetry methodology, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.