Short answer: The main stream is the camera’s full-quality video, used for recording and full-screen viewing. The substream is a much smaller copy of the same view, used for grids of many cameras and slow connections. Viewing sixteen cameras on their substreams can take an eighth of the bandwidth and decoding work of their main streams.
Why do cameras send two streams?
A 4 MP camera might stream 3–6 Mbps. In a grid of sixteen, each tile is a few hundred pixels wide, yet decoding sixteen full streams means handling up to 100 Mbps and sixteen full-resolution decodes. The substream gives the grid a picture sized for the tile:
| Main stream | Substream | |
|---|---|---|
| Typical resolution | Full sensor, 2–8 MP | About 640×360 or 704×480 |
| Typical frame rate | 15–25 fps | 10–15 fps |
| Typical bitrate | 2–12 Mbps | 0.3–1 Mbps |
| Used for | Recording, full screen, export | Grids, phones, slow links |
How should I set up the substream?
- Keep the same aspect ratio as the main stream. Otherwise the picture stretches or jumps when a viewer switches between them.
- Use a short keyframe interval (one or two seconds) on both streams, so switching and starting playback is quick.
- Don’t starve it. Around 0.5 Mbps is enough for a clear tile; very low bitrates turn movement to mush.
- Leave the main stream alone for recording. That’s the evidence.
Some cameras offer a third stream, useful for mobile apps or a second recorder.
Where do the streams show up?
Over ONVIF, each stream appears as a separate media profile; over RTSP, each has its own URL. NVRs usually expose a main and a substream per channel too.
How Supervision uses them
When Supervision brings in a camera or an NVR, it detects the main stream and substream for each channel, and grids use the lighter substream. Together with WebRTC, that’s how live video starts in under a second.
Common questions
Should I record the substream?
Only as a second, lighter copy. Record the main stream; recording the substream alone gives you footage too small to identify anyone.
My camera has no substream. What now?
Some older and budget cameras offer only one stream. Grids will still work, but each tile costs a full stream, so watch the viewing machine’s load and your network.
Does the substream use more of the camera’s bandwidth?
A little: the camera now sends both. The substream is small, and the saving on every viewer is far larger.
Supervision is self-hosted video management software for any camera brand. If this is the kind of problem you're solving, tell us about your cameras and we'll talk it through, or email hello@bipolarfactory.com.