Transmuxing — from one protocol to others, without transcoding
Updated 08 Oct 2026Transmuxing means the picture and sound pass unchanged from one protocol to another: Castreon only changes the “packaging”. An SRT source can, for example, go out at the same time as HLS, DASH, HTTP-TS, UDP multicast and RTMP — with no loss of quality and almost no CPU use.
On all plans.
How to turn it on
A channel without a transcoding profile does transmuxing: on the channel, Transcoding profile → — passthrough —. That's all.
The source can be anything: HTTP / HLS / DASH, UDP unicast or multicast, SRT, RIST, RTMP, DVB, an IP camera (RTSP) or another Castreon channel. The outputs can be any and as many as you like, all from a single read of the source:
| Outputs for viewers | Push outputs |
|---|---|
| HLS (TS or fMP4) · LL-HLS · DASH · HTTP-TS · M3U export | SRT · RIST · UDP · RTMP · push to a CDN |
The archive (catch-up) also works on a passthrough channel — see Archive, catch-up and timeshift.
What doesn't change
Transmuxing doesn't touch the picture, so:
- resolution, bitrate and codec stay those of the source;
- an interlaced source (for example 1080i) stays interlaced;
- no logo, text or multiple qualities (ABR) can be added.
If you need any of these, choose a profile — see Transcoding profiles.
When you still need a profile
- Browsers can't play the source codec: DVB sources in MPEG-2 (common on SD channels) or cameras in H.265. An H.264 profile solves it.
- RTMP: platforms that receive RTMP expect H.264 with AAC audio in practice.
- The source audio doesn't suit the output (for example MP2 or AC-3 for a player that can't play them): a profile with the picture set to copy and AAC audio copies the picture and transcodes only the audio — the load stays very low.
- WebRTC: the WebRTC output needs H.264 without B-frames, so on a passthrough channel Castreon re-encodes the picture for WebRTC only.
A profile with “copy”
Under Profiles, the copy codec can be chosen separately for picture and sound. The Passthrough recipe creates a profile that copies everything — useful when you still want a profile's settings (for example the MPEG-TS service name) without transcoding.