A phone as an SRT camera, over 4G, into vMix or OBS
A modern phone is a decent camera with a built-in H.264 encoder and its own internet connection. What it lacks is a way to be reached: mobile networks put you behind carrier-grade NAT, so nothing can connect to your phone. The phone has to dial out — which is exactly what SRT caller mode does.
What you need
- Larix Broadcaster (free, iOS and Android) — the usual choice because it exposes real SRT settings rather than hiding them
- An SRT destination the phone can reach: a relay input address, or a listener on a public IP
- A production machine running vMix or OBS to receive
1. Get an input address
In @inoutsrt_bot create a stream. You get an input like srt://de.inoutsrt.com:4012 for the phone and an output like srt://de.inoutsrt.com:6023 for vMix or OBS. Free sessions run 30 minutes, three a day — plenty to walk the venue and check coverage before the event.
Pick the node nearest the phone, not the nearest the studio: the mobile leg is the fragile one.
2. Larix Broadcaster settings
Settings → Connections → New connection:
| Field | Value |
|---|---|
| URL | srt://de.inoutsrt.com:4012 |
| Mode | Caller |
| Latency | 800 ms (mobile; see below) |
| Passphrase | only if your stream uses one |
Settings → Video:
| Field | Value | Why |
|---|---|---|
| Resolution | 1280×720 | 720p that never breaks beats 1080p that does |
| Frame rate | 25 or 30 | match your production |
| Bitrate | 2500–3500 kbit/s | mobile uplinks are advertised optimistically |
| Codec | H.264 | HEVC will not decode everywhere downstream |
| Keyframe interval | 1 s | fast recovery and fast joins |
| Adaptive bitrate | on | it drops the bitrate instead of the connection |
Settings → Audio: AAC, 48 kHz, 128 kbit/s. If you use a wired or Bluetooth mic, check which input the phone actually selected — Larix shows it, and Bluetooth adds its own delay.
Adaptive bitrate is the single most valuable setting on a phone. A fixed 6 Mbit/s looks glorious on the walk-through and collapses when the venue fills with people and their phones. Let it back off to 1.5 and climb again.
3. Latency for mobile networks
Mobile loss is bursty: fine for minutes, then a clump of packets vanishes as you cross cells. Bursts need more buffer than the loss percentage alone suggests.
| Situation | Latency |
|---|---|
| Strong 5G, stationary | 400–600 ms |
| Ordinary 4G, stationary | 600–800 ms |
| Moving, or a crowded venue | 1000–1500 ms |
Put your measured RTT and loss into the calculator for the exact figure, then round up for bursts. Nobody watching the programme can tell whether the camera is 0.6 s or 1.2 s behind — but everyone sees a freeze.
4. Receive it in vMix or OBS
vMix: Add Input → Stream/SRT → SRT Caller, hostname de.inoutsrt.com, port 6023, latency matching the phone.
OBS: Media Source, uncheck Local File, Input srt://de.inoutsrt.com:6023?latency=800, Input Format mpegts, Network Buffering 0 MB.
5. Walk the venue before you trust it
The point of testing is not "does it work on my desk" but "does it work where the camera will stand".
- Start the stream and walk the full route the operator will take, watching the receiving end.
- Stand where the crowd will be — bodies absorb signal, and a hall at capacity behaves nothing like an empty one.
- Note where Larix's bitrate indicator dives. Those are the spots where you either accept lower quality or change position.
- If you need a second opinion on the receiving side, point vMix at our test signal — if the bars are clean but the phone is not, the phone's network is the problem.
Troubleshooting
| Symptom | Cause | Fix |
|---|---|---|
| Connects, drops in seconds, repeatedly | Latency too low for bursty loss | 1200 ms and retest |
| Video fine, audio crackles | Bluetooth mic or sample-rate mismatch | Wired mic, 48 kHz |
| Works on Wi-Fi, fails on mobile | Carrier blocks the port, or APN restrictions | Try another port from the bot; test on a different SIM |
| Picture soft after every movement | Bitrate too low for the motion | Raise bitrate or drop to 25 fps |
| Phone overheats and quality drops | Thermal throttling, common outdoors | Shade, remove the case, lower to 720p25, keep it charging |
Several phones at once
Each phone gets its own stream and its own output address; vMix or OBS pulls them as separate inputs and you cut between them. Two or three phones on the same cell will fight for the same uplink capacity, so budget the bitrate across all of them, not each one alone.
Need a tunnel that carries your own video?
IN/OUT-SRT is a hosted SRT relay in Germany and the USA. Your encoder connects in, your receiver connects out — no static IP, no port forwarding, nothing installed. Three free 30-minute sessions every day; a full production day is $5.