---
status: draft
date: 2026-05-20
definition: technical
---

# 5.1 surround selection

Capability-aware audio rendition selection on the channel-count axis:
filter `presentation.audioTracks` to channel-count / codec
combinations the browser supports (with implicit stereo fallback),
plus customer-driven channel-count preference (config), plus engine-
inferred preference based on the downstream playback environment
(runtime), plus optional mid-stream codec switching via
`SourceBuffer.changeType()` between channel/codec configurations.

The audio sibling of [hevc-variant-selection](./hevc-variant-selection.md):
both are **codec-change variant selection consumers** of
[capability-probing](./capability-probing.md). HEVC operates on the
video codec axis (AVC ↔ HEVC); this feature operates on the audio
channel-count axis (stereo ↔ 5.1 surround, with codecs typically
differing between the two — AAC stereo vs AC-3 / EAC-3 5.1). The
parallel sibling structure is intentional; the channel-count axis has
one additional Tier 2 phase (downstream-environment-aware preference)
that doesn't have a HEVC analog.

A **Media-src feature** in the framing from
[clusters.md § Feature classification axes](./clusters.md#feature-classification-axes)
at Tier 1; a **Player feature** at Tier 2 (customer-driven overrides
+ engine-inferred preferences layered on top).

## Status

- **Composition:** not implemented in `createSimpleHlsEngine`. Today
  `selectAudioTrack` runs the default picker (`pickFirstTrackId`) over
  all audio renditions without channel-count filtering. The parser
  surfaces `channels` on audio renditions (`parse-multivariant.ts`)
  but the value isn't consumed for selection or filtering — see
  [audio-playback.md](./audio-playback.md)'s "Channels exposure vs
  use" open question.
- **Definition depth:** technical — scope and constraints articulated;
  no implementation. Source material: [SPF Epics Working Doc — epic
  #23 5.1 Surround Sound](https://www.notion.so/35f97a7f89d08123a13fecab1ca1cac4)
  (cluster C+D, Media-src, eng M, validation M, "Audio rendition
  capability gating").
- **Hard prerequisite:** [capability-probing](./capability-probing.md).
  Tier 1 filter applies that feature's "Multivariant CODECS-attribute
  filtering" mechanism to the audio channel/codec combination; Tier 2
  mid-stream `changeType()` consumes that feature's "Cross-codec
  transition (`changeType()`) probing" phase (which explicitly cites
  AAC stereo ↔ AC-3 5.1 as a transition pair).

## Phases of complexity

[Tier 1 / Tier 2 framing](./clusters.md#tier-1-spec-compliant-baseline-vs-tier-2-custom-behavior)
per the Notion epics doc (5.1 row; sibling `hevc-variant-selection`
and `multi-language-audio` precedents).

| Phase | Tier | What | Notes |
|---|---|---|---|
| Capability-aware initial selection | Tier 1 | Filter `presentation.audioTracks` to channel-count / codec combinations the browser supports per capability-probing's verdict. 5.1 variants (typically AC-3 / EAC-3 / AAC 5.1) retained if the codec+channel combination is supported; excluded otherwise. Stereo fallback emerges from the filter: 5.1 variants removed → stereo variants remain in the candidate set → `selectAudioTrack` picks among the remainder | Mechanism lives in [capability-probing.md](./capability-probing.md)'s "Multivariant CODECS-attribute filtering" phase. This feature is the applied use case for the channel-count axis. Same codec/channel combination for the session — no `changeType()`, no setup re-entry. The `channels` field already surfaced by the parser is the input axis |
| Customer channel-count preference (config) | Tier 2 | Config-driven biases on top of the Tier 1 filter: `audioChannelPreference: 'stereo'` excludes 5.1 even when supported (the force-stereo override called out in the Notion epic); `audioChannelPreference: 'surround'` biases toward 5.1 when available | **Policy** mechanism per [clusters.md § Feature classification axes](./clusters.md#composition-vs-policy-vs-middle-pattern). Constraint slot read by the selection filter step; same shape as HEVC's customer override. Customer-policy-driven; same codec/channel combination for the session |
| Downstream-environment-aware channel preference (runtime) | Tier 2 | When both 5.1 and stereo codec/channel combinations are supported, infer a channel preference from the playback environment — e.g., `AudioContext.destination.maxChannelCount === 2` suggests the effective playback is stereo (5.1 would be downsampled by the browser anyway, wasting bandwidth). Engine-inferred, not stated by config | **Middle pattern** per [clusters.md § Feature classification axes](./clusters.md#composition-vs-policy-vs-middle-pattern) — a new state-producing behavior monitors the audio context and writes a preference slot; the selection filter step reads it. Parallel shape to viewport-driven cap in [rendition-selection-caps.md](./rendition-selection-caps.md). No HEVC analog (video codec selection has no equivalent downstream-environment signal). Composes with the Tier 2 config-driven override above: explicit config wins over engine-inferred preference if both set |
| Mid-stream codec change via `changeType()` | Tier 2 | Allow channel-count / codec switching between AAC-stereo and AC-3 / EAC-3 5.1 (or other channel-count-distinct codec pairs) mid-stream when the customer/environment preference changes, or when audio ABR (if introduced) crosses a channel-count boundary. Buffer-side `SourceBuffer.changeType(newMimeCodec)` call before appending segments of the new codec | Depends on [capability-probing.md](./capability-probing.md)'s "Cross-codec transition probing" phase landing — capability-probing answers "can the browser changeType from AAC to AC-3?"; this feature owns the actual call site for the audio buffer specifically. Mirror of HEVC's mid-stream phase; significantly more complex than the prior phases (codec re-init, buffer-state coordination, MSE quirks). Browser support for audio cross-codec changeType is fragile and pair-specific |

## What's in scope vs out of scope

**In scope:**
- All four phases above for channel-count-distinct audio variant
  selection (typically stereo ↔ 5.1, but also stereo ↔ 7.1 or similar
  higher-channel-count combinations)
- Audio-side channel-count preference state slot + filter step on
  `presentation.audioTracks`
- `AudioContext.destination.maxChannelCount` (or equivalent) detection
  hook for the runtime-inferred preference phase
- `changeType()` call site for cross-codec audio transitions in the
  buffer-setup neighborhood
- Customer-facing config surface (`audioChannelPreference`)
- Stereo fallback semantics (inherent in the capability filter — 5.1
  excluded ⇒ stereo remains)

**Out of scope (separate Media-src candidate features):**
- **[hevc-variant-selection](./hevc-variant-selection.md)** — parallel
  sibling on the video codec axis. Same structural shape; the
  downstream-environment phase is 5.1-specific.
- **[audio-abr](./audio-abr.md)** — audio bandwidth-driven quality
  switching. May eventually cross channel-count boundaries (a low-
  bandwidth 5.1 variant vs a higher-bandwidth stereo variant), which
  would consume this feature's `changeType()` phase.
- **`[multi-language-audio]`** — Tier 2 mid-stream language switching.
  Different axis (language) but consumes the same `changeType()`
  primitive for codec-distinct language variants.

**Out of scope (different architectural layer):**
- Adapter-layer customer-facing API surfaces (e.g., a hypothetical Mux
  Video element `force-stereo` or `prefer-channels` attribute). The
  SPF feature owns the state slot + filter + detection hook;
  consumer-facing config attribute names live above-engine.
- Above-engine "is 5.1 playing" UI affordances. Consume the resolved
  audio track's `channels` value via existing audio-track surfaces;
  not SPF concerns.
- `AudioContext` lifecycle management outside the engine. The runtime-
  detection phase needs an `AudioContext` or equivalent capability
  query; whether the engine creates its own short-lived context for
  probing or consumes one provided by the adapter is open (see Open
  questions).

## Likely cross-cutting impact

Things this feature probably forces decisions on, not just additions:

- **Filter-then-select shape.** Same open question as
  [hevc-variant-selection.md](./hevc-variant-selection.md) and
  [rendition-selection-caps.md](./rendition-selection-caps.md): does
  the selection picker take the pre-filtered candidate set (option a
  — keeps the picker pure, matches the precedent), or extend its
  signature to take preference slots and filter internally (option b)?
  Cross-cuts with the parallel features; whichever lands first sets
  the audio-side shape.
- **Channel-preference slot granularity.** Single slot like
  `audioChannelPreference: 'stereo' | 'surround' | undefined`, or
  numeric (`audioMaxChannels: 2 | 6 | 8`) for finer control across
  5.1 / 7.1 / Atmos / etc.? Numeric is more flexible; symbolic is
  simpler. Open question shared with HEVC's analog slot-shape
  question.
- **Override-vs-runtime-detection precedence.** When the Tier 2
  config-driven slot AND the Tier 2 runtime-detected slot both express
  a preference, which wins? Default: config wins (explicit customer
  intent overrides engine inference). Open whether to expose a
  "merge-priority" config field for finer control.
- **Pipeline ordering with capability-filter and codec-preference.**
  When all relevant features land, the audio-side filter pipeline
  becomes: capability filter (physics) → channel-count preference
  (config + runtime) → audio-side selection. Mirror of the video-side
  pipeline established in `hevc-variant-selection.md` /
  `rendition-selection-caps.md`.
- **MSE codec-change boundary.** Tier 1 and Tier 2 customer-override
  and runtime-detection-only do not require `changeType()` — the
  codec/channel combination is fixed for the session. Tier 2 mid-
  stream `changeType()` is the exception; it crosses into
  [mse-mms-pipeline.md](./mse-mms-pipeline.md)'s buffer-setup
  neighborhood. The MSE codec-change check fires only for the mid-
  stream phase.
- **`AudioContext` lifecycle.** The runtime-detection phase needs to
  query the audio output environment. Options: (a) the engine creates
  a short-lived `AudioContext` for the channel-count query and
  immediately closes it (low cost on most browsers, but not free);
  (b) the engine reads from an `AudioContext` provided by the adapter
  / consumer; (c) the engine uses a different probing API entirely.
  Decision affects whether this feature has a DOM dependency for
  detection or stays consumer-driven.
- **Environment changes mid-stream.** The downstream playback
  environment can change (user plugs in headphones, switches output
  device). Detecting and reacting to this change crosses the runtime-
  detection phase into the mid-stream-changeType phase. The two are
  composable but the interaction shape — does the runtime-detection
  signal trigger an automatic mid-stream switch? — is open.

## Open questions

- **Channel-preference slot granularity — symbolic vs numeric.** Per
  the cross-cutting note: `audioChannelPreference: 'stereo' |
  'surround'` vs `audioMaxChannels: number`. Customer use cases drive
  the call.
- **`AudioContext` lifecycle for runtime detection.** Short-lived
  engine-managed context vs adapter-provided vs alternative probing
  API. Affects feature shape (one behavior owns DOM detection vs one
  slot the adapter writes) and SSR / non-browser contexts.
- **Override-vs-detection precedence merge policy.** Config-driven
  override vs runtime-detected preference: default "config wins,"
  but expose a merge-priority config field? Open until use cases
  surface.
- **Environment-change mid-stream reaction.** When the runtime-
  detected preference changes mid-stream (user plugs in headphones),
  does the engine automatically trigger a `changeType()` switch, or
  does the preference change only affect future selection? The
  bandwidth-savings argument suggests automatic switch; UX continuity
  may prefer stay-current.
- **Mid-stream `changeType()` trigger policy.** Only when capability
  requires (e.g., manifest reload introduces new codec), or also
  preference-driven (config or detection change), or also bandwidth-
  driven if audio-ABR crosses channel-count boundaries? Three trigger
  sources; policy may differ per source.
- **Buffer-state coordination during audio `changeType()`.** Flush vs
  continue-append vs other coordination, dependent on browser pair
  behavior (AAC ↔ AC-3 specifically). Likely lives in this feature
  but interacts with buffer-management's planner.
- **Audio-side codec-priority comparator (if audio-ABR lands).**
  Equal-bandwidth-different-codec/channel tie-breaking at audio-ABR
  upgrade time. May be `[multi-signal-abr]` follow-on work rather
  than this feature.
- **Fallback-when-empty chain.** If capability-filter narrows to zero
  AND customer-override further excludes the residue, what happens?
  Same chain as HEVC; refer to capability-probing.md's broader
  fallback-when-empty open question.
- **Multivariant `CHANNELS` parsing.** `parse-multivariant.ts`
  hard-codes `channels: 2` as default (lines 225, 255); actual
  channel info would need to flow from the media playlist or from
  `EXT-X-MEDIA:CHANNELS=...` parsing if the multivariant carries it.
  Whether the multivariant or only the media playlist carries
  channel info varies by encoder; worth verifying with test fixtures.

## Related features

- **[capability-probing](./capability-probing.md)** *(hard
  prerequisite)* — provides the multivariant-CODECS filter (Tier 1)
  + the `changeType()` probe (Tier 2 mid-stream, explicitly citing
  AAC stereo ↔ AC-3 5.1 as a transition pair). This feature is
  scoped explicitly as a consumer in capability-probing.md's Out of
  scope ("Same shape as HEVC") and Related features.
- **[hevc-variant-selection](./hevc-variant-selection.md)** —
  parallel sibling on the video codec axis. The codec-preference slot
  pattern, customer-override phase, and mid-stream `changeType()`
  phase mirror across the two. The downstream-environment-aware
  preference phase is 5.1-specific (no video analog).
- **[audio-playback](./audio-playback.md)** — baseline this feature
  extends. Today's `selectAudioTrack` is channel-count-unaware; this
  feature wires `channels` into the selection logic.
- **[audio-abr](./audio-abr.md)** — audio quality switching. May
  eventually cross channel-count boundaries, consuming this feature's
  `changeType()` phase. Algorithm change for audio-ABR is its own
  feature; this feature provides the cross-codec primitive when
  audio-ABR needs it.
- **`[multi-language-audio]`** — Tier 2 mid-stream language switching.
  Different selection axis (language) but consumes the same
  `changeType()` primitive for codec-distinct language variants.
  Worth harmonizing the audio-side `changeType()` call site shape.
- **[mse-mms-pipeline](./mse-mms-pipeline.md)** — `changeType()` call
  site lives in the audio buffer-setup neighborhood (Tier 2 mid-
  stream phase only). Same-codec phases don't touch MSE setup.
- **[rendition-selection-caps](./rendition-selection-caps.md)** —
  sibling pattern: the middle-pattern mechanism (state-producing
  behavior monitors signal, selection consumer respects it) is the
  same shape used by the viewport-driven cap. The audio context
  detection is the analog of the resize observer.
- **[errors](./errors.md)** — when the filter pipeline narrows the
  audio candidate set to zero, the error surfaces through that
  feature's phase 2 (SVTA 2012, no supported audio track).

## See also

- [hevc-variant-selection.md](./hevc-variant-selection.md) — parallel
  sibling; the doc this one mirrors on the channel-count axis
- [capability-probing.md](./capability-probing.md) — hard
  prerequisite; provides the filter mechanism and the changeType
  probe (citing AAC ↔ AC-3 5.1 as a transition pair)
- [audio-playback.md](./audio-playback.md) — baseline feature this
  extends; the "Channels exposure vs use" open question is resolved
  by this feature
- [clusters.md § Track & variant registry](./clusters.md#track--variant-registry)
  — cluster C description
- [clusters.md § Capability probing](./clusters.md#capability-probing)
  — cluster D description; this feature is a consumer
- [clusters.md § Feature classification axes](./clusters.md#feature-classification-axes)
  — Tier 1 / Tier 2 framing the phases map to; Composition vs Policy
  vs middle pattern (this feature's runtime-detection phase is the
  middle pattern)
- [SPF Epics Working Doc](https://www.notion.so/35f97a7f89d08123a13fecab1ca1cac4)
  — source material; epic #23 (5.1 Surround Sound), Tier 1 / Tier 2
  table for the 5.1 row
- [Mux Video Permutations Matrix](https://www.notion.so/32c97a7f89d08191b84dd30f06685490)
  — Stream Type / Audio section
