AirPlay Audio Receiver for Galaxy Ace 4

Turns a Samsung SM-G357FZ (Galaxy Ace 4 / Ace Style LTE, Android 4.4.4 / API 19) into an AirPlay speaker over its 3.5mm headphone jack. See docs/decisions.md for the toolchain research and architecture decisions behind this setup.

Status: volume sync (one direction) + Now Playing metadata/artwork + status-dial UI

Verified end-to-end against a real iPhone on 2026-08-12: the notification shows the current track's title/artist/album and cover art, and the iPhone's own AirPlay volume slider/buttons control the Ace4's actual output volume. The Now Playing view is dark-themed and minimal (no title bar), built around one large circular status dial that's color-coded per state (teal = advertising, amber = reconnecting, red = error) instead of a plain system button, and the app/notification icons are a small generated glyph reused throughout, including as the idle artwork placeholder. Reopening the app while the service is still running now shows the correct state immediately, the keyboard no longer opens automatically on launch, and the speaker-name field locks (with an explanatory label) while advertising. The notification itself is now styled like a music player's - tinted with a color pulled straight from the current track's artwork, with a bigger, wider artwork banner in its expanded view.

Verified working over an iPhone Personal Hotspot connection on 2026-08-14, not just shared home WiFi. Fixed a real cold-start bug that caused a stutter at the start of every track (AudioTrack was told to start playing before any audio had been buffered) plus a double-attenuation bug that made output quieter than intended below max iPhone volume, and added a conservative (+3dB) digital volume boost with clipping protection. Also hardened against mobile-connection jitter: a WiFi high-performance lock during playback, higher audio-thread priority, and a bounded retry for lost resend requests. One known gap remains under investigation: no jitter/ reorder buffer yet, so a resent packet still plays out of order if it arrives late - see decisions doc.

Not working, and not expected to start working without a change on Apple's side: pushing the Ace4's volume-button presses back to the iPhone (DACP requests are consistently rejected with 400 Bad Request - looks like a real iOS-side restriction, not a bug here), and transport/volume controls in the notification or lock screen (tried two different Android APIs for this; neither rendered on this Samsung TouchWiz build). Both attempts were reverted rather than left half-working - see decisions doc for the full investigation.

Also done: WiFi drop/reconnect handling (mDNS re-advertises automatically once WiFi returns, dead connections get cleaned up on a timeout) and RTP resend/retransmit for lost packets (gaps in the audio sequence trigger a recovery request to the iPhone). Both verified on real hardware. NTP-style clock sync was deliberately skipped in favor of resend/retransmit - see decisions doc for why.

Build

Requires:

• JDK 17+ (Android Studio's bundled JBR works: Android Studio/jbr)
• Android SDK with platform 34 + build-tools 34.0.0 installed

./gradlew assembleDebug

Output: app/build/outputs/apk/debug/app-debug.apk

Install & run

1. Enable "Unknown sources" on the Galaxy Ace 4 (Settings → Security).
2. Copy app-debug.apk to the device and install it, or: adb install app/build/outputs/apk/debug/app-debug.apk
3. Connect the phone to the same WiFi network as your iPhone, plug headphones or a speaker into its 3.5mm jack.
4. Open the app, optionally rename the speaker, tap Start.
5. On the iPhone, open Control Center → AirPlay, select the speaker, and start playing audio - it should come out of the jack. To watch what's happening: adb logcat -s RaopRtspServer shows the handshake (ANNOUNCE: encrypted session established) followed by a stream of Audio RTP packet #N lines as audio decodes and plays.

Project layout

/app                            # Android app module (Kotlin)
  /src/main/java/com/ace4/airplayreceiver
    *.kt                        # service (incl. WiFi reconnect handling), mDNS advertisement, UI
    /raop                       # RTSP server, RSA/AES crypto, ALAC decoder wrapper
  /src/main/java/com/beatofthedrum/alacdecoder
    *.java                      # vendored pure-Java ALAC decoder (third-party, BSD-style license)
/docs                            # protocol notes, decisions, test logs
  /images/social-preview.png     # repo social-preview / link-thumbnail image (Settings -> Social preview)

Toolchain pins

• AGP 8.7.3, Gradle 8.13, Kotlin 2.0.21
• compileSdk 34, minSdk 19, targetSdk 19 (targetSdk intentionally pinned to match the real device — see decisions doc for why)
• No NDK. Both the RTSP/crypto handshake (milestone 2) and ALAC decode (milestone 3) turned out not to need it — javax.crypto handles RSA/AES, and a vendored pure-Java ALAC decoder (com.beatofthedrum.alacdecoder, ported from soiaf/Java-Apple-Lossless-decoder) handles decode, fast enough in real-time on the actual Snapdragon 410 hardware. See decisions doc for the full reasoning and the one bug hit while wiring it up.