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.studio’s listener. Once connected, video and audio appear in the source slot.

If the port is already in use, the slot tells you which port it couldn’t bind rather than failing silently.

Connecting out instead

pMix can also dial an SRT feed you already have — paste an srt:// URL and it connects as a caller. Latency in an SRT URL is in milliseconds. It’s also available over the Control API as inputs_add_srt.

An SRT source stops claiming to have audio when a new sender isn’t sending any, so the mixer strip reflects what’s actually arriving.

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.studio 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.studio 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