Docs/Guide
From SRT to NDI.
A camera far away, over the internet, and your production on the local network. SRT carries the signal to your PC; NDI shares it across your network. Here is how the two fit together.
- SRT · RTMP
- NDI® 6
- vMix · OBS · TriCaster
Concepts
SRT, RTMP and NDI, plainly.
SRT (Secure Reliable Transport) is an open protocol for sending video over the internet with low latency: it recovers the packets lost along the way, can be encrypted and runs over UDP. One end listens (listener) and the other calls (caller); the latency you choose is the margin it has to recover what was lost.
RTMP is the classic protocol of streaming platforms: many cameras, phones and encoders can only send RTMP. It runs over TCP, so it does not lose data, but it usually has more delay than SRT. RTMPS is encrypted RTMP.
NDI® carries production-quality video over the local network with very little delay, and any compatible program (vMix, OBS with its plugin, TriCaster and many more) sees the sources of the whole network. It is not designed to cross the internet: that is why it is paired with SRT.
Options
Three ways to bring it into production.
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01
The switcher's SRT input
Some programs, such as vMix, receive SRT directly as one more input. It is the simplest option when only that program on that PC needs the signal.
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02
A hardware decoder
A box that receives SRT and outputs SDI or HDMI into a capture card. It is robust, but each camera needs its own decoder and capture input.
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03
SRT Server: SRT and RTMP to NDI
A Windows PC receives several SRT (caller or listener) and RTMP/RTMPS feeds and delivers them as NDI 6 sources to the whole network: vMix, OBS, TriCaster or any NDI program, on that PC or others.
When SRT Server makes sense
What it adds to the signal.
| If you need… | With SRT Server |
|---|---|
| Several remote cameras at once | One channel per camera (1, 4 or 8 depending on the license), each independent, plus an RTMP channel. |
| Cameras in sync with each other | Adjustable delay per channel: by hand, frame by frame, or with phone-based sync. |
| Cameras or phones that only send RTMP | RTMP and RTMPS input as well as SRT. |
| The signal in more than one program or PC | NDI 6 sources for the whole local network. |
| Receiving even if you cannot open ports | Tunnel through your NAS, Amazon Lightsail or the SAMedia service (hotels, CGNAT, company networks). |
| Talking to operators and giving them tally | Intercom and Tally Web in each operator's phone browser. |
NDI® is a registered trademark of Vizrt NDI AB. vMix, OBS and TriCaster belong to their owners.
Questions
SRT to NDI, quick questions.
What SRT latency should I use?
SRT latency is the margin to recover lost packets. On a good connection a small value is enough; on 4G/5G or unstable networks, raise it. In SRT Server it is set per channel, and the encoder's should be similar.
Listener or caller?
Usually SRT Server listens (listener) and the encoder calls (caller) your PC's address. If the encoder is the one listening, SRT Server can call it (caller).
Can I send from a phone?
Yes, with any app that streams over SRT or RTMP. The phone sends to the channel's address and port, over the local network or the internet.
Does it work with OBS instead of vMix?
Yes. SRT Server delivers NDI 6 sources; any NDI-compatible program sees them, including OBS with its NDI plugin and TriCaster.
Do I have to open router ports?
To receive from the internet directly, yes (the App opens them via UPnP if the router allows it). If you cannot, use the tunnel: your PC connects out to a server and the feeds come in through it. The «Access from the internet» guide explains it.