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README.md 

# AirPlay

A pure-Elixir **AirPlay audio sender**. Discover receivers on your network and
stream lossless audio to them using Erlang/OTP primitives: `:crypto`, `:gen_udp`
and `:gen_tcp`.

It supports classic **AirPlay 1 / RAOP**: unencrypted ALAC over RTP with the
NTP-style timing/sync receivers require. Verified streaming real audio to
shairport-sync, AirPort Express, and Apple **HomePods**.

It also has working **AirPlay 2** support under `AirPlay.V2`: transient SRP
pairing, ChaCha20-Poly1305 encrypted RTSP, binary plist SETUP/RECORD/FLUSH,
PTP timing, encrypted ALAC RTP audio, volume control, periodic `/feedback`
keepalives, and multi-room AirPlay 2 group playback.

Audio is **stream-decoded** (ffmpeg `-re` → on-demand frames, audio stream only —
embedded cover art is dropped), so playback starts after a short prebuffer instead
of decoding the whole file up front, and a multi-hour source no longer holds its
entire PCM in memory.

AirPlay 2 timing uses the receiver's own PTP clock. A HomePod's grandmaster clock
runs on its uptime (nowhere near host wall-clock time) and it does not answer
`Delay_Req`, so the offset is taken **one-way** from each `Sync`/`Follow_Up` — RTP
sync packets are then stamped in the receiver's clock frame rather than the host's,
which is what makes playback work regardless of the host OS or clock.

AirPlay 2 playback also comes in two flavours: one-shot `AirPlay.V2.Player` /
`AirPlay.V2.GroupPlayer` (pair → set up → stream one file → tear down), and
**persistent `AirPlay.V2.Session` / `AirPlay.V2.GroupSession`** that keep the
connection and PTP clock warm across tracks — so an album or audiobook changes
tracks without re-paying the ~5s pair + SETUP + PTP cold-start each time.

This is still WIP, created to scratch an itch.

## Installation

```elixir
def deps do
  [{:airplay, "~> 0.4.2"}]
end
```

## AirPlay 1 / RAOP usage

```elixir
# Discover receivers (mDNS browse of _raop._tcp.local)
AirPlay.discover()
#=> [%{name: "Office", host: "172.16.42.35", port: 7000}, ...]

# Stream a file (decoded to PCM via ffmpeg) at 40% volume
{:ok, session} = AirPlay.play("172.16.42.35", "/music/track.flac", volume: 40)
AirPlay.set_volume(session, 25)
AirPlay.stop(session)

# Or stream raw PCM you already have (44.1 kHz, signed 16-bit LE, stereo, interleaved)
{:ok, session} = AirPlay.play_pcm("172.16.42.35", pcm, volume: 40)
```

A `session` is a lightweight GenServer that streams in the background and stops
itself when the track ends; pass it to `set_volume/2` and `stop/1`.

## AirPlay 2 usage

The AirPlay 2 API is lower-level than the RAOP `AirPlay.play/3` convenience API
and currently lives under `AirPlay.V2`.

```elixir
# Play one file to an AirPlay 2 receiver. This call runs until playback finishes.
{:ok, stats} =
  AirPlay.V2.Player.play_file("172.16.42.35", "/music/track.flac",
    volume: 0.4
  )

# Play the same file to an AirPlay 2 receiver group.
{:ok, stats} =
  AirPlay.V2.GroupPlayer.play_file(
    [
      %{host: "172.16.42.35", port: 7000},
      %{host: "172.16.42.62", port: 7000}
    ],
    "/music/track.flac",
    volume: 0.4
  )
```

For long-running apps, start AirPlay 2 playback in a supervised process or task.
`AirPlay.V2.Player.set_volume/2`, `AirPlay.V2.GroupPlayer.set_volume/2`,
`AirPlay.V2.Player.stop/1`, and `AirPlay.V2.GroupPlayer.stop/1` operate on that
running playback process.

### Persistent sessions (connection reuse across tracks)

`play_file/3` pairs, sets up, runs PTP, streams one file, and tears everything
down — fine for a single track, but paying that ~5s cold-start on *every* track
of an album is wasteful. `AirPlay.V2.Session` (and `AirPlay.V2.GroupSession` for
groups) keep the connection and clock warm so the next track just flushes and
streams:

```elixir
# Connect once (pair → SETUP → RECORD → PTP). Returns a session process.
{:ok, session} = AirPlay.V2.Session.connect("172.16.42.35", owner: self())

# Stream successive files down the same connection. Each call continues the RTP
# timeline (FLUSH with the running seq/timestamp), so there is no re-pair / re-PTP.
AirPlay.V2.Session.play(session, "/music/track-1.flac")
# ... when the track ends the session sends `{AirPlay.V2.Session, :ended, gen}`
# to `owner` and goes idle, keeping the connection alive with FEEDBACK keepalives.
AirPlay.V2.Session.play(session, "/music/track-2.flac")

AirPlay.V2.Session.set_volume(session, 0.4)
AirPlay.V2.Session.stop(session)    # flush current track, stay connected
AirPlay.V2.Session.close(session)   # tear down and release the receiver
```

The session sends its `owner` (defaulting to the caller of `connect/2`):

- `{AirPlay.V2.Session, :ended, play_gen}` — the current track finished cleanly
- `{AirPlay.V2.Session, :error, play_gen, reason}` — a track failed to start; the
  session stays connected and idle

`AirPlay.V2.GroupSession` has the same API but takes a list of receivers in
`connect/2` (the first receiver is the PTP primary that drives the shared clock).

## Requirements

- Elixir ~> 1.14
- `ffmpeg` on the `PATH` — **only** for `AirPlay.play/3` (file decoding).
  `AirPlay.play_pcm/3` has no external dependency.

## How it works

`play/3` opens an RTSP control connection (`OPTIONS → ANNOUNCE → SETUP → RECORD`),
binds the UDP timing/control ports and answers the receiver's NTP timing probe
*before* `SETUP` (HomePods return `520 Origin Error` otherwise), then paces ALAC
RTP packets to the receiver against a wall-clock with periodic sync packets.

| Module | Role |
| --- | --- |
| `AirPlay.Discovery` | mDNS `_raop._tcp` browse |
| `AirPlay.Rtsp` / `AirPlay.Session` | RTSP control plane + handshake |
| `AirPlay.Player` | RTP audio streaming + timing/sync |
| `AirPlay.Rtp` / `AirPlay.Alac` / `AirPlay.Ntp` | packet builders + codecs |
| `AirPlay.Source` | file → PCM (ffmpeg) + framing |
| `AirPlay.Cast` | play/stop/volume session GenServer |
| `AirPlay.V2.Pairing` / `AirPlay.V2.Srp` / `AirPlay.V2.SecureChannel` | AirPlay 2 transient pairing + encrypted RTSP |
| `AirPlay.V2.Setup` / `AirPlay.V2.Rtsp2` / `AirPlay.V2.Plist` | AirPlay 2 control plane |
| `AirPlay.V2.PtpBmca` / `AirPlay.V2.Ptp` | AirPlay 2 PTP timing |
| `AirPlay.V2.Player` / `AirPlay.V2.GroupPlayer` | AirPlay 2 one-shot single-device and group playback |
| `AirPlay.V2.Session` / `AirPlay.V2.GroupSession` | AirPlay 2 persistent playback — connection/clock reuse across tracks |
| `AirPlay.Decoder` | streaming ffmpeg decode (bounded-memory, on-demand frames) |

## License

GNU General Public License v3.0 or later (`GPL-3.0-or-later`). See
[LICENSE](LICENSE).