SRT Sources
SRT (Secure Reliable Transport) is a broadcast-grade protocol for sending video over IP networks. It provides reliable delivery with error correction, making it suitable for unstable or long-distance connections where NDI might not perform well.
Adding an SRT source
- Tap + on an empty source slot
- Select SRT
- The iPad starts listening on the configured port
- On the sending device, connect to the iPad’s IP address and port
The SRT source connects as a caller to pMix’s listener. Once connected, video and audio appear in the source slot.
How SRT works
SRT uses UDP with an ARQ (Automatic Repeat Request) retransmission mechanism:
- Lost packets are retransmitted — Unlike raw UDP, SRT detects and recovers from packet loss
- Late packets are dropped — If a packet arrives too late to be useful, it’s discarded to maintain real-time playback
- Configurable latency — A latency buffer absorbs network jitter; higher latency = more resilience, lower latency = more responsive
Video codec support
SRT sources in pMix support:
| Codec | Description |
|---|---|
| H.264 | Broadest compatibility, standard quality |
| H.265 (HEVC) | ~40% better compression — better quality at the same bitrate, or same quality at lower bitrate |
The codec is determined by the sending device. pMix automatically detects and decodes both.
Typical latency
SRT sources typically have 180–300 ms of latency due to:
- The SRT latency buffer (minimum 180 ms)
- MPEG-TS muxing/demuxing overhead
- Decode and display time
This is higher than iPhone cameras (~100–200 ms) but acceptable for most production workflows.
Common SRT sources
- OBS Studio — Output → Advanced → SRT
- vMix — SRT output support
- Hardware encoders — Kiloview, Haivision, Teradek
- FFmpeg —
ffmpeg -f lavfi -i testsrc -c:v libx264 -f mpegts srt://ipad-ip:port
SRT vs NDI
| SRT | NDI | |
|---|---|---|
| Bandwidth | Low (4–8 Mbps typical) | High (100–150 Mbps for full NDI) |
| Latency | 180–300 ms | 30–60 ms on local network |
| Reliability | Built-in error correction | Best-effort (no retransmission) |
| Best for | Remote/WAN connections, unreliable networks | Local network, low-latency switching |
Tips
- Use SRT for remote sources — When the source is on a different network or over the internet
- Use NDI for local sources — When everything is on the same LAN and you need low latency
- H.265 over SRT gives excellent quality at modest bitrates — ideal for bandwidth-constrained links
- Firewall — Ensure the SRT port is open on your network if receiving from an external source