#Sending video

Go live from OBS, ffmpeg, a hardware encoder or a browser, and the limits every publisher has to stay within.

A stream accepts video over RTMP (OBS, ffmpeg, vMix, Wirecast, most hardware encoders) or WHIP (browsers and WebRTC encoders). Both are in the response to creating a stream. Use one publisher at a time: a second connection with the same key is refused while the first is live.

#Settings that matter

#OBS Studio

Settings → Stream: Service Custom, Server = rtmp_url, Stream Key = stream_key.

Settings → Output (Output Mode: Advanced), Streaming tab:

Settings → Video: Output (Scaled) Resolution no taller than your max_resolution; 30 fps is enough for talks and classes, 60 for sport and games.

#ffmpeg

From a file, looping, without re-encoding if the file is already H.264 with 2-second keyframes:

ffmpeg -re -stream_loop -1 -i talk.mp4 -c copy -f flv "$RTMP_URL/$STREAM_KEY"

From a camera or any source, encoding with the right settings:

ffmpeg -re -i input.mp4 \
  -c:v libx264 -preset veryfast -b:v 2500k -maxrate 2500k -bufsize 5000k \
  -g 60 -keyint_min 60 -sc_threshold 0 -pix_fmt yuv420p -vf "scale=-2:720" \
  -c:a aac -b:a 128k -ar 48000 \
  -f flv "$RTMP_URL/$STREAM_KEY"

-g 60 is a keyframe every 60 frames, which is 2 seconds at 30 fps; use -g 120 at 60 fps. Quote the address: the stream key contains a ?.

#From a browser (WHIP)

whip_url accepts a standard WHIP offer from any page: your own site, a web app, a studio tool. What to send:

• One video track, H.264. Browsers prefer VP8 unless told otherwise; set H.264 as the preferred codec in the offer. A publish with more than one video layer (simulcast) is refused and the stream ends with simulcast_not_supported.
• The declared resolution from the start. Browsers start small and raise the resolution over about 15 seconds. Set degradationPreference: 'maintain-resolution' on the sender to avoid it. JustOnAir encodes a browser's top quality at your declared size either way, so viewers never see the ramp.
• Networks that block UDP. A TURN relay over TCP runs on port 3478 of the host in whip_url: turn:<host>:3478?transport=tcp, username ome, credential airen.

Two WHIP response headers aren't readable from another origin: Location (so a page can't send the WHIP DELETE; closing the peer connection ends the publish just as well) and the TURN Link header (so pass the TURN server yourself, as above).

#When the connection drops

If the publisher disconnects, the stream stays live for 60 seconds and accepts the same key again. OBS reconnects on its own. A publisher whose old connection is still half-open is refused once while the old one is cleared, then let in on the next try, usually within about 10 seconds. After 60 seconds without a publisher the stream ends with end_reason: "disconnected".

#If you lose the stream key

The key is shown once. While the stream is still pending, replace it: the old key stops working at once. Once live, the key can't change, because it is how a reconnecting publisher proves it is the same one.

#Adaptive or passthrough

profile: "abr_basic" (the default) gives viewers the quality you send plus smaller ones: 480p below a 720p source, 720p and 480p below 1080p. Viewers on weak connections switch down instead of buffering.

profile: "passthrough" delivers exactly what you send and nothing else, for a sixth of the encoding price. Choose it when your viewers have good connections, or when you already send several qualities yourself in a different way.