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OBS source plugin for receiving SLM (SLAY Media) streams with sub-frame latency.
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README.md

OBS SLM Source

An OBS Studio plugin that receives live video and audio from a SLAY Media (SLM) sender and injects them into OBS as low-latency input sources.

What it does

The plugin registers two OBS source types:

  • SLM Source (slay_media_source) — receives video and audio
  • SLM Audio Source (slay_media_audio_source) — audio only

Senders are discovered automatically via multicast (239.255.77.43:9878). Once a sender is found, the plugin connects and begins receiving frames. If the sender is on the same machine, it uses a shared memory ring buffer for zero-copy, sub-millisecond delivery. UDP is used otherwise.

Known senders

Sender Transport Video codec
slay-cart — SLAY Media player SHM (local) / UDP Raw NV12, H.264, H.265
slay-camera — Android camera app UDP H.265 (H.264 fallback)

Protocol

SLM is a custom binary UDP protocol (magic SLM\x01, version 3). The handshake sequence:

  1. Sender broadcasts ANNOUNCE on multicast
  2. Plugin replies with HELLO (unicast to sender's ctrl_port)
  3. Sender sends CAPS (resolution, FPS, pixel format, codec, audio params)
  4. Sender streams video chunks and audio frames

Video frames are split into 1400-byte UDP chunks and reassembled. Audio fits in a single datagram. Optional LZ4 compression is supported for raw NV12 video.

Supported video codecs:

Codec Notes
Raw NV12 Original SLM mode; optional LZ4 per-frame compression
H.264 (Annex B) Software decoded via libavcodec
H.265 / HEVC (Annex B) Software decoded via libavcodec

H.264/H.265 frames arrive as complete Annex B access units, one per logical video frame (reassembled from chunks). The decoder is initialised on CAPS and replaced if the codec changes mid-stream. IDR frames carry inline SPS/PPS so the decoder can recover after packet loss.

Supported audio format: float32 PCM, mono or stereo, 48 kHz

Transport

Transport Platforms Notes
Shared Memory Linux, Windows Zero-copy; used when sender and receiver are on the same machine
UDP All Used for remote senders; always the fallback if SHM becomes stale

The plugin tries SHM first and monitors the sender's heartbeat. If the heartbeat goes stale (no update for 5 seconds), it falls back to UDP automatically.

On Linux, SHM uses POSIX shared memory (shm_open) and named semaphores. On Windows, it uses Named File Mappings (page-file-backed CreateFileMapping) and Win32 named semaphores — the shared data layout is identical on both platforms.

Source settings

Setting Default Description
Sender name (empty) Filter by sender name. Empty accepts any sender. Dropdown auto-populates from discovery.
Port 0 (auto) UDP port to listen on. 0 lets the OS pick a free port.
Force UDP off Skip SHM and use UDP only. Enable this for remote senders on another machine.

Building

Windows

Requires Visual Studio 2022 and CMake 3.30+.

cmake --preset windows-x64
cmake --build --preset windows-x64

Install target places the plugin at %ALLUSERSPROFILE%\obs-studio\plugins\obs-slm-source\bin\64bit\obs-slm-source.dll.

Linux (Ubuntu 24.04)

Requires CMake 3.28+, ninja-build, pkg-config, and build-essential.

cmake --preset ubuntu-x86_64
cmake --build --preset ubuntu-x86_64

Platform notes

  • Windows uses Winsock2; Linux/macOS use POSIX sockets.
  • SHM works on both Linux and Windows. The Force UDP setting disables SHM on all platforms.
  • Multicast requires sender and receiver to be on the same subnet (or multicast routing to be configured for remote senders).

Planned

  • Tally signal (#21): OBS detects when a source enters or leaves the Program/Preview slots and unicasts PKT_TALLY back to the sender so remote operators know they are live.
  • Operator messaging (#21): A text field in the source properties lets the studio operator send a short message to the connected sender (e.g. "zoom in"), displayed as an overlay on the remote device.
  • Dynamic bitrate feedback (#10): Loss-rate reporting from receiver to sender to allow encoder step-down on degraded links.