Android Cast (POC)

Minimal LAN screen-casting proof of concept in Java. Captures screen + playback audio, encodes H.264 + AAC, streams over TCP or UDP to a receiver that decodes and plays back.

Features

• UDP broadcast discovery on the local network
• TCP or UDP stream transport (selectable; must match on both devices)
• PIN auth (SHA-256 hash, default 1234)
• Screen capture → H.264 (AVC) with quality presets and scaled capture resolution
• Playback audio capture (API 29+) → AAC-LC
• Runtime notification permission (Android 13+)

Quick start

1. Install the same APK on two devices on the same Wi‑Fi.
2. Receiver (TV): Receive → match transport (TCP/UDP) & options → tap Start listening (notification appears; service keeps running).
3. Sender (tablet): Send → same transport → pick receiver → PIN → approve screen capture.
4. Watch the notification on both devices for connection status (not only the toast).
5. Stop from the notification Stop action.

If video never appears

• Transport must match on both sides (e.g. both TCP).
• Receiver must show Listening or Ready in the notification before you cast.
• Sender notification should reach Casting via TCP · WxH — if it shows Screen capture data missing, reinstall and try again (Android 13+ intent fix).
• Keep the receiver UI open once so the video Surface attaches (or return to it after rotation).

Build

Use JDK 17 or 21 (Gradle does not run on JDK 25 yet):

export JAVA_HOME=/usr/lib/jvm/openjdk-bin-21
cd "/home/foxx/repos/My Projects/android cast"
./gradlew assembleDebug
adb install -r app/build/outputs/apk/debug/app-debug.apk

Stream options

Option	Effect
Transport TCP	Reliable delivery; default
Transport UDP	Lower latency, may drop frames (POC)
Quality Low/Med/High	Video bitrate 0.8 / 2 / 4 Mbps and FPS 20 / 24 / 30
Resolution	Scales VirtualDisplay before encode (full, 75%, 50%, or max 720p height)
Audio	AudioPlaybackCapture + AAC (Android 10+)

Protocol

Step	Description
Discovery	UDP 41234, JSON with magic, name, port, transport
Connect	TCP or UDP port 41235
Auth	HELLO → AUTH → AUTH_OK → CAST_SETTINGS
Stream	STREAM_CONFIG, VIDEO_FRAME, AUDIO_CONFIG, AUDIO_FRAME

UDP frames use ACUD header and fragmentation for payloads > ~1.2 KB.

Bluetooth / BLE

Not viable for this video stream. Practical throughput:

• BLE: ~100–300 kb/s — fine for control/metadata only
• Bluetooth Classic (SPP): often ~1–3 Mb/s theoretical — still far below 720p H.264 (~2–8 Mb/s), with high latency

A phone cannot “passthrough” a live encoded screen stream over BLE to another device for playback without a custom high-bandwidth profile (which consumer Android does not expose for arbitrary apps). Use Wi‑Fi for media; BT could be added later only for discovery/pairing if needed.

Persistent services (AIDL)

Casting runs in foreground services with tray notifications:

Service	Role
ScreenCastService	Sender — capture, encode, stream
ReceiverCastService	Receiver — listen, decode, play

Activities bind via AIDL (ICastSenderService / ICastReceiverService) for status updates and (on the receiver) attaching the video Surface. Rotating the screen or leaving the app does not stop the receiver listener; the sender keeps casting until you tap Stop in the notification.

Limitations

• No TLS encryption (LAN cleartext POC)
• UDP mode may glitch under load
• Audio captures playback routed through the system (not microphone); some apps may be silent by policy
• Single sender → single receiver