SRT FAQ

SRT is better RTMP ?

Yes, because:

  • allows use many codecs (h264, HEVC, mpeg2 etc). RTMP - only h264.
  • allows multi-channel audio. RTMP - only 2channels.
  • uses modern methods for quick transfer data by UDP through unstable connection. RTMP uses TCP.

What is Drops, Losts and Latency ?

Internally SRT splits data to small packets before sending. Each packet has length 1316 bytes.

During transmission packet in theory can be lost (because UDP allows it). In this case SRT marks the packet as "lost". But SRT try re-send this packet again. Again, again and again...

So we have :

Question: how many times SRT try re-send this packet (10 ms or 1mins etc) ?

Answer: this time is set by "latency" parameter. By default it equals 120ms.

By default SRT try re-send lost packet 120ms. If you have unstable connection then you can increase "latency" value (maximum is 5000ms)

Next:

Question: what happens if SRT can't successfully delivery lost packet in time defined in "latency"?

Answer: in this case this packet marked as "dropped" and SRT forgot about it.

In other words:

  • "Lost" means sicked packets
  • "Drops" means died packets
  • "Latency" is treatment time for each sicked packet. High value means more chance to repair sicked packet.

Next:

Question: ok, I will use 5000ms for latency always, what is problem ?

Answer: no problem, but be aware - your Studio gets data with delay equals "latency". Even if you have excellent Internet connection.

In other word "latency" affected to all packets not only for sicked. SRT does this to equalize of the data flow.

So "latency" value is compromise between "speed of delivery" and "picture quality".

many Drops means artefacts for picture

How to setup "Latency" ?

Question: What is value for latency I should use for my connection ?

Answer: use special SRT-speed measurement tool as described here

You can setup "latency" on both side of connection - encoder and server. SRT will use maximum value for connection.

For example: SRTStreamer/OBS/vMix uses "latency" value as 100 ms and SRTMiniServer uses 300ms. So SRT will uses 300ms for this connection.

setup latency for SRTMiniServer. Will be applied as minimum for all connections

how to setup latency for OBS

About Passphrase and Stream ID

Stream ID - it very similar to "stream key" for RTMP and used for access control. It works like password (not passphrase). This feature was introduced in SRT 1.3.3 and allows use one port for many connections. If your encoder is older and does not have field "stream id" then please read this post

Passphrase - it's secret phrase for encrypt your stream. It means you need ~20% overhead for bandwidth.

NOTES: we does not support passphrase in our products yet because 20% bandwidth overhead not applied for our clients. For real encryption purpose we recommended use VPN.

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