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docs(spf): presentation timeline coordinate model
Design doc for the 3-coordinate model (startTime / startDate / startMediaTime) and the timestampOffset relocation approach the reactor + steps implement. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Claude Opus 4.8
Claude Opus 4.8
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---
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status: draft
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date: 2026-07-07
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---
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# Presentation Timeline Coordinate Model
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The coordinate model that lets an SPF engine translate between the three
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timelines a streaming presentation lives on — **media**, **presentation**, and
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**wall-clock** — and the architecture for **non-zero-PTS `timestampOffset`
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relocation** built on it.
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This is a design-in-progress for the `timestampOffset`-relocation approach to
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non-zero-PTS (spike branch `spike/spf-non-zero-pts-timestamp-offset`). It is the
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"how it's modeled and where it lives" companion to the mechanism decision in
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[../../decisions/mse-timestamp-offset.md](../../decisions/mse-timestamp-offset.md)
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and the feature framing in
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[features/non-zero-pts-support.md](./features/non-zero-pts-support.md).
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---
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## Problem
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Zero-PTS VOD let one number do everything. The encoded media, the player's
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`currentTime`, and the model's `Track.startTime` were all 0-based and identical,
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so the loader could compare `currentTime` to `segment.startTime` directly and the
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buffer's native PTS matched what the player displayed.
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Non-zero-PTS breaks that identity. A Mux instant clip (`asset_start_time=60`)
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encodes its first frame at native PTS ≈ 60s; Apple's bipbop asset starts at 10s.
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Now the buffer's native timeline (60→…) and the player's desired 0-based timeline
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diverge. Live added a **third** timeline — wall-clock, via
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`EXT-X-PROGRAM-DATE-TIME` — and, in doing so, made `Track.startTime` mean
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*different things in different engines*: the live anchor rewrites `startTime` onto
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native PTS, while VOD keeps it 0-normalized. The same field straddles two
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timelines. That ambiguity is the coordinate confusion at the root of the
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"non-zero-PTS is hard" problem.
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We want (a) one coherent model of the three timelines, and (b) an architecture
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that adds relocation to the engine **without touching the simple case** — no
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complexity and no bundle cost for the zero-PTS VOD composition that got us this
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far.
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---
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## Three timelines, one instant
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A timeline is fixed by knowing one instant's coordinate in it. Pick the
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**presentation's origin instant** and record its value in each timeline:
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| Field | Timeline | Source |
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|---|---|---|
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| `startMediaTime` | **media** (encoded/decode) | `tfdt.baseMediaDecodeTime ÷ mdhd.timescale` |
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| `startTime` | **presentation** (`currentTime`) | 0 for 0-based product semantics (existing field) |
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| `startDate` | **wall-clock** | `EXT-X-PROGRAM-DATE-TIME` (existing field, live) |
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All three advance 1:1 in seconds, so **every translation is pure subtraction** —
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no scaling (timescale is already folded into `startMediaTime`):
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```text
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timestampOffset = startTime − startMediaTime # presentation − media
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wallClock(t) = startDate + (t − startTime) # time-of-day at presentation time t
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```
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`startMediaTime` is the one genuinely new field. The relocation offset is
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**derived from the triple, not stored** — storing both would be two
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representations that drift (see [conventions/signals.md](./conventions/signals.md)).
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This model **unifies live anchoring and VOD relocation**: the live anchor is the
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`(media ↔ wall-clock)` edge; VOD relocation is the `(media ↔ presentation)` edge.
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They are two edges of one triangle, which is why the machinery converges (see
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[Open questions](#open-questions)).
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### Why `startTime` stays the presentation reference
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`startTime` remaining 0-based (not native media) is *why the relocation approach
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is simpler downstream than native-PTS*: with the buffer relocated to 0 via
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`timestampOffset`, `currentTime`, `seekable`, and the loader's
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`currentTime`-vs-`segment.startTime` window math all stay in one 0-based
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coordinate. `startMediaTime` (native) is consumed **only** to compute the offset
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at the buffer boundary — the loader never sees it. (The native-PTS branch made
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`startTime` native and paid for it with coordinate translation everywhere,
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including the initial-load stall.)
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The cost: because all tracks relocate by the shared *min*, a skewed non-primary
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track's 0-based `startTime` sits ~skew below its true buffer presentation time
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(Apple: ~44ms for video). Sub-frame, only affects coarse load-window planning —
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vastly better than native-PTS's full-origin (60s) mismatch.
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---
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## One offset, applied polymorphically
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Whatever `startMediaTime` a track is given (own in Tier 1, shared `min` in
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Tier 2), the relocation applies the same way — `presentation = native +
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timestampOffset`, `timestampOffset = startTime − startMediaTime` — only the
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*application* differs:
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```text
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buffer sample: presentation = sampleNativePTS + timestampOffset # MSE sets sb.timestampOffset
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text cue: cueFinal = cueNative + timestampOffset # we compute it
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cueNative = LOCAL + MPEGTS/90000 (X-TIMESTAMP-MAP) | absoluteCueTime (no map)
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```
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Text has no `SourceBuffer`, so it *effectively* has a `timestampOffset` applied
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as cue arithmetic. This is the live single-anchor rule
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([../../decisions/live-presentation-anchor.md](../../decisions/live-presentation-anchor.md))
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in VOD form: **established from the A/V tracks, applied to all tracks including
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text.**
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**Shared-`min` across A/V (Tier 2).** When A/V is skewed, the shared
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`startMediaTime` is `min` across the audio and video tracks' native origins
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(Tier 1 gives each track its own). `min` (not video-primary, not per-track-own):
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- keeps every track's earliest **DTS ≥ 0** (relocating by anything larger drives
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the lower track negative → Chromium append failure);
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- **preserves real A/V skew** (Apple's 44ms audio-lead is retained; relocating
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each track to its own 0 would flatten it).
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---
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## Capability axes (not a tier ladder)
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The "tiers" are really **three orthogonal opt-in axes** — a composition enables
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what its platform needs:
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| Axis | Off / simple | On / complex | Driven by |
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|---|---|---|---|
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| **Relocation** | no `startMediaTime`, offset 0 | read + apply | is the source non-zero-PTS? |
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| **Parser** | presumptive (first box) | track-id-matched | container packaging (muxed `clcp`/extra track?) |
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| **Coordination** | per-type own (opt-out) | `min`-reduce across A/V (default) | is A/V actually skewed? |
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They cross freely — muxed-captions-but-aligned (track-id + single), or
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separate-file-A/V-but-skewed (presumptive + `min`). The parser axis is already
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built as two tree-shakeable exports (`media/mp4/timestamp-origin.ts`,
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committed `cf8aaca45`): `readFirstMediaTimescale`/`readFirstBaseMediaDecodeTime`
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(presumptive) vs `findMediaTrack`/`readBaseMediaDecodeTime` (track-id).
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---
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## Architecture: discover → derive → apply, established by a reactor
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A per-source reactor, **`establishStartMediaTime`**, owns the coordinate
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establishment; the byte-level work rides **steps** in a plain config
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`messagePipelines` array (`relocationMessagePipelines`) woven into the segment
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loader's pipelines. The loader ships a Tier-0 `fetch → dispatch` pipeline and stays
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oblivious to relocation; the steps read composition `state` from their call-time
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`deps` (no closures, no context), so the reactor and the pipeline steps are
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decoupled — **the reactor never touches pipelines**. Enabling relocation = composing
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the reactor + supplying the steps as config (+ optionally a `deriveStartMediaTime`
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seam); a Tier-0 composition does neither.
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**The `establishStartMediaTime` reactor** has three states, driven by a `monitor`:
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`inactive` (no resolved presentation — clears the transient `mediaContainerData`
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slot on entry, so each source starts fresh) → `monitoring` (runs the **derive
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effect**) → `established` (the selected A/V tracks carry `startMediaTime`; disables
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the derive — establish-once, sticky per source, like the live anchor). Selection
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signals are **optional/defensive**, so the one reactor composes across video-only /
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audio-only / both. Per-source freshness is structural (the `inactive` transition
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clears the slot), not a hand-rolled reset.
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- **Discover** (per-type steps). Head-peek steps read the init (`mdhd` timescale)
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and the first media segment (`tfdt` baseMediaDecodeTime, plus that segment's
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0-based `startTime`) and write them into `mediaContainerData`, a
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`Signal<Record<TrackType, { timescale?; baseMediaDecodeTime?; segmentStartTime? }>>`
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**keyed by track type** (`'video'`/`'audio'`) — one init+media pair per type
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suffices, and ABR rungs of a type share the entry. It's a **two-source** read
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across two appends, but the slot *is* the shared state (init writes `timescale`,
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the first media segment writes `baseMediaDecodeTime`+`segmentStartTime`), so the
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steps are independent — no shared closure, no self-discrimination. Writes are
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**synchronous** RMW of disjoint keys (the sync-merge invariant keeps this clear
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of the #1746 hazard). Runs on the *fetched* stream, so transport stays pure
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`fetchBytes`. `segmentStartTime` is recorded because the origin is
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`baseMediaDecodeTime/timescale − segmentStartTime` — the first *loaded* segment
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isn't necessarily the 0th (non-zero initial `currentTime`, live/DVR).
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- **Derive** (reactor effect). Watches `mediaContainerData` (+ selection) and, via
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the injected **`deriveStartMediaTime`** seam, writes the settled per-type
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`startMediaTime` onto the `Track`s (each type's value stamped on every track of that
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type). The seam is **pure** —
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`(mediaContainerData, ctx) => Record<TrackType, number | undefined>` — `undefined`
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means "not ready yet"; the effect is the sole writer of the field. It's the
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**one coordination knob**, and the default `deriveSharedMinStartMediaTime` collapses
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what were "Tier 1" and "Tier 2" into a single reduce: relocate every track by the
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`min` across the *selected* A/V origins. This subsumes per-type — aligned A/V →
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`min` equals each origin, skewed A/V → `min` keeps every DTS ≥ 0 *and* preserves the
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real skew (per-type would flatten it), single-type → its own origin. It returns
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`undefined` until every selected type is discovered (the shared-`min` barrier). The
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same value goes to the model here and to the buffer via the tier-agnostic apply. No
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Presentation-level field is needed — the min is denormalized across the per-type
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entries. `derivePerTypeStartMediaTime` remains as the barrier-free opt-out for a
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composition that knows its A/V is aligned.
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- **Apply** (per-track step). A stamp step relocates via `timestampOffset =
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−startMediaTime`, which the SourceBufferActor applies to
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`SourceBuffer.timestampOffset`. The apply is **tier-agnostic**: it runs the *same*
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`deriveStartMediaTime` seam the reactor uses, over the shared `mediaContainerData`
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slot — so the buffer offset always equals the model's stamped `startMediaTime`, and
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it's robust to `established` + late tracks (the slot persists and discover gates
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per-type; the model value may not be re-stamped once the reactor goes sticky). It's
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**async with an `awaitDefined` holdback**: per-type resolves at once (own origin
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discovered earlier in the same pipeline); shared-`min` waits until every selected
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A/V origin is in (the barrier), filled by the other type's discover step — no
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deadlock, since discover precedes the awaiting stamp. A **liveness guard** short-
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circuits first: if this type's own origin was never discovered (TS / containerless /
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a mock/0-PTS-no-`tfdt` source) it leaves the append native and does not wait. A
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derived `0` also leaves it native (setting `timestampOffset` at all can ripple).
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Text-cue relocation reuses the model value the reactor stamped (see the text step).
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### Consume: `startMediaTime` lives on the model
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The settled `startMediaTime` lands per-track on the **CMAF-HAM `Track`**, a peer
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of `startTime` (presentation) and `startDate` (wall-clock). It's a coordinate base
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value that *defines the timeline relationships*, so it belongs on the model, not a
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parallel slot; `timestampOffset` stays derived (`startTime − startMediaTime`),
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never stored.
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The **churn** — partial per-track reads across appends — stays in the transient
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`mediaContainerData` slot and never touches `presentation`. Only the *settled*
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value reaches the model, written by the derive effect as **sole writer** of the
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field. That write does share `presentation`'s existing multi-writer situation
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(#1746, addressed at the presentation-ownership level); we accept that rather than
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distort the model to dodge it. This is also where relocation and the live anchor
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**converge** — both are "an establishment unit writing coordinate base values onto
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tracks" (the anchor writes `startTime`/`startDate`, relocation writes
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`startMediaTime`) — the flagged eventual dedup.
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### Branch-free always-present actors
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The pieces present in *every* composition carry **no relocation vocabulary at
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all** — not even a no-op'd seam. Relocation lives entirely in the reactor and its
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injected steps; the Tier-0 pipeline is literally `[fetch, dispatch]`:
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- **SegmentLoaderActor**: owns only the invariant skeleton (`fetchStep`,
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`dispatchStep`, in-flight bookkeeping, between-step abort checks) and a
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`messagePipelines` factory that defaults to `fetch → dispatch`. It does not
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fetch-whole, parse, wait, or know what a `timestampOffset` is.
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- **SourceBufferActor**: sets `sb.timestampOffset` only when the append meta
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carries one, idempotent-guarded (`meta.timestampOffset != null &&
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sb.timestampOffset !== meta.timestampOffset`) so re-stamping the constant offset
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on later appends is a no-op. Absent = untouched. Apply-only.
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All tier variation collapses to **the reactor and its injected steps + derive
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seam**; the loader and buffer actors are shared, and Tier 0 imports no relocation
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code.
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---
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## Key decisions
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Documented because they were debated.
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### `startTime` stays the presentation reference; add `startMediaTime` (media)
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**Decision:** `startTime` remains 0-based (presentation); add `startMediaTime`
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(native media origin) as the new field. `startDate` stays wall-clock.
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**Alternatives:**
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- *Make `startTime` honest (native media).* Clean three-field correspondence, but
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ripples through the parser (stop 0-normalizing), the loader (translate every
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`currentTime`-vs-`startTime` comparison), and live's `startDate` math — i.e. it
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*is* the #1746 model cleanup.
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**Rationale:** Relocation keeps everything downstream 0-based, so presentation is
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the *correct* timeline for `startTime` here; the native origin is needed only to
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derive the offset. Keeps the loader coordinate-consistent and the simple case
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untouched. The honest-`startTime`-everywhere convergence (which would also
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reconcile live's native `startTime`) is a separate, named future effort.
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### Relocation is a per-source reactor; `startMediaTime` on the model via a pure derive seam
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**Decision:** relocation is the `establishStartMediaTime` reactor (per-source
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lifecycle), not a config bundle of loose signals. Discovery and apply are loader
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pipeline steps; **derive** is the reactor's effect, driven by a **pure injected
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`deriveStartMediaTime` seam** that maps `mediaContainerData` → per-track
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`startMediaTime`. The churn lives in the transient `mediaContainerData` slot; the
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settled value lands on `Track` (the effect is its sole writer).
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**Alternatives:** *a behavior with a hand-rolled reset effect* (the reactor's
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entry/exit gives per-source reset + teardown for free, structurally); *the reduce
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as a `computed`* (can't write the model — and putting `startMediaTime` on the
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model is the point, per the consume decision); *keeping the transient state
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module-local rather than on `state`* (it's coordinate state the model consumes, so
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it belongs on `state`, defined through the reactor).
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**Rationale:** the reactor matches the per-source lifecycle relocation actually
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has (and that `setupBufferActors` already models), makes stale-write races
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structural rather than guarded, and isolates the one tier difference into a pure,
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testable seam.
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### Shared-`min` origin (the Tier-2 `deriveStartMediaTime`)
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**Decision:** the coordination axis is entirely the `deriveStartMediaTime` seam.
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Tier 1 writes each track its own origin; Tier 2 writes the `min` across the
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selected A/V origins onto **every** track. `startMediaTime` stays per-track on
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`Track` in both tiers — Tier 2 denormalizes the min across the per-track slots, so
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no Presentation-level field is required and **apply is unchanged** across tiers.
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**Alternatives:** *per-track-own for skewed A/V* (flattens real A/V skew, lossy);
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*video-primary* (the field norm — VHS/hls.js — but they sidestep negative-DTS via
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transmux/offset math we don't have); *a Presentation-level shared value* (not
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needed — the seam denormalizing onto tracks keeps the read path uniform).
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**Rationale:** `min` is the only choice that keeps every DTS ≥ 0 *and* preserves
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real inter-track skew; expressing it as a swap of the derive seam means Tier 2 is
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one function change with no movement in discover, apply, or wiring.
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### Offset derived, not stored; two parsers split for tree-shaking
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`timestampOffset` is a `computed` from the model triple, never a stored field.
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The presumptive vs track-id parsers are separate exports (not one
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optional-selector function) so a caption-free platform tree-shakes the
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track-selection machinery (~37% smaller, verified). Application rides optional
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per-op append metadata so the always-present actors stay branch-free.
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### Offset applied via append meta, not a dedicated message (revisitable)
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**Decision:** the offset is carried on each media segment's append meta
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(`meta.timestampOffset`) and applied by the SourceBufferActor's
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`appendSegmentTask` (idempotent-guarded). A relocating composition's `stampOffset`
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pipeline step writes it onto the meta before `dispatch` (absent in Tier 0, whose
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pipeline has no such step; the step may be async — see
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["One offset, applied polymorphically"](#one-offset-applied-polymorphically)).
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Setting the offset is **not** its own SourceBuffer message today.
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**Alternative:** a dedicated `set-timestamp-offset` SourceBuffer message the
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loader schedules as its own task — cleaner separation of buffer configuration
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from append payload, and the natural shape once each actor's *messages* are
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decoupled from the concrete *Tasks* they translate into (segment-loader
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`planTasks`/`makeLoadTask`/`scheduleAll`; SourceBufferActor `messageTaskFactories`).
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**Rationale:** the offset is constant per source, so meta-carried + idempotent
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apply costs nothing and needs no new protocol. And meta-per-segment *generalizes*:
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HLS discontinuities and DASH multi-period re-base `timestampOffset` per period, and
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carrying it on each append gets correct in-order application for free via the
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`SerialRunner` — a standalone message would have to be interleaved into the append
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stream to match. "Set once" is the special case we happen to be in.
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**Left open:** promoting offset-setting to its own message remains a valid future
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move; the mid-stream offset-*change* case (discontinuities / multi-period) is the
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likely forcing function, though even then meta-per-segment may still win. Nothing
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in the current seam precludes the switch — this is a deliberate "not yet," not a
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closed door.
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---
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## Open questions
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- ~~**Generalize the live establishment behavior.**~~ *Resolved (mechanism).* VOD
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relocation is a **sibling** of `anchor-presentation-timeline`, not literally it:
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a per-source establishment unit (`establishStartMediaTime` reactor) writing a
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coordinate base value onto tracks, with the tier logic in a pure derive seam
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rather than a `computed`. The two share the "establish-once, apply-to-all"
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*shape* (and both write per-track base values → the #1746 multi-writer surface).
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Whether they should be **one unit** — the anchor writing `startDate`/`startTime`
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and relocation writing `startMediaTime` folded into a single establisher — is
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the remaining, larger dedup, still future. See
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[../../decisions/live-presentation-anchor.md](../../decisions/live-presentation-anchor.md).
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- **Barrier liveness.** For Tier 2, `deriveStartMediaTime` returns `undefined`
|
||||
until the selected A/V origins are all present, and the apply-side
|
||||
`awaitDefined` holds each first append until then — so audio erroring, disabled,
|
||||
or absent must not block forever. The holdback needs a bound (timeout /
|
||||
audio-disabled short-circuit inside the seam's "when"); both VHS and hls.js
|
||||
special-case this.
|
||||
- **Holding the first segment across the wait.** The first media segment is held
|
||||
(buffered) while the offset resolves, then appended; steady-state streaming is
|
||||
untouched. The first-chunk peek is validated to contain the `moof` with a
|
||||
~60–600× margin (first chunk ≥128KB, `moof` ≈0.2–2KB), so no streaming
|
||||
box-peeker is needed — but the hold's interaction with preempt/replan needs
|
||||
care.
|
||||
- ~~**`startMediaTime` storage granularity.**~~ *Resolved.* Per-track on `Track`
|
||||
(the self-describing triple), in both tiers — Tier 2 denormalizes the shared
|
||||
`min` across the per-track slots rather than introducing a presentation-level
|
||||
field. Promotion to a `Presentation`-level base value is a possible future move
|
||||
if a real de-dup need appears, but Tier 2 does not force it.
|
||||
- **Text-only sources.** No A/V `tfdt` to establish from — but the
|
||||
`X-TIMESTAMP-MAP` `MPEGTS` *is* a media-timeline reference, so text could
|
||||
self-establish. Deferrable special path.
|
||||
- ~~**Discovery doesn't belong in the fetch abstraction.**~~ *Resolved.* Discover
|
||||
is now a head-peek discover step run on the *fetched* byte stream inside the
|
||||
loader, not a fetch decorator — transport stays pure `fetchBytes`, and parsing
|
||||
mp4 boxes / writing the raw values into `mediaContainerData` sits where it
|
||||
belongs (a content concern). It keeps the per-track, byte-first, no-double-fetch
|
||||
properties.
|
||||
- **Convergence to honest `startMediaTime` everywhere** (the "A" option above),
|
||||
which would let live and VOD share one `startTime` semantic. Its own effort.
|
||||
|
||||
---
|
||||
|
||||
## See also
|
||||
|
||||
- [presentation-modeling.md](./presentation-modeling.md),
|
||||
[live-presentation-modeling.md](./live-presentation-modeling.md) — the data
|
||||
model this extends; #1746 (the concurrently-RMW'd `presentation` hazard) is why
|
||||
the churn stays in the transient `mediaContainerData` slot.
|
||||
- [../../decisions/mse-timestamp-offset.md](../../decisions/mse-timestamp-offset.md)
|
||||
— the mechanism decision (native-PTS default; relocation for the 0-based cases).
|
||||
- [../../decisions/live-presentation-anchor.md](../../decisions/live-presentation-anchor.md),
|
||||
[../../decisions/live-timeline-anchoring.md](../../decisions/live-timeline-anchoring.md)
|
||||
— the live anchor this generalizes.
|
||||
- [features/non-zero-pts-support.md](./features/non-zero-pts-support.md) — the
|
||||
feature framing.
|
||||
- `packages/spf/src/media/mp4/` — the committed box parser (presumptive +
|
||||
track-selected).
|
||||
- `.claude/plans/spf-non-zero-pts-timestamp-offset-spike.md` — the spike's
|
||||
running notes; OSS prior-art survey (buffer-whole is universal;
|
||||
establish-then-gate cross-track) captured in agent memory.
|
||||
Reference in New Issue
Block a user