Additional Information
Frequently Asked Questions
SRT Deployment Guide, v1.1, Issue 01 45
You could think of a skipped packet as a packet that the decoder drops, except that it
doesn’t tell the encoder. The decoder sends a positive ACK, even though the packet is
“lost” from the point of view of the decoder. It might drop an entire frame because it
is corrupted, but the encoder doesn't know it. This is because when things are going
badly, the last thing you want to do is to increase the handshake traffic. A packet has a
certain time-to-live, and if it doesn't play after that time then it is skipped.
You might see the number of skipped packets increasing on the decoder, without a cor-
responding effect on the number of dropped packets seen on the encoder. If the number
of skipped packets on the decoder increases slowly, you should increase the SRT La-
tency. If the number of skipped packets on the decoder increments in large jumps, the
best thing to do is lower your video bitrate, or increase your Bandwidth Overhead % if
you have available capacity.
Do I control the latency value at the source or destination?
You can control the latency, the bit rate, and the overhead percentage on the source de-
vice. But you can configure the latency on the destination device as well. The two de-
vices will settle on the maximum value.
How do I decide whether to boost the latency or lower the bit rate?
You have to decide what is most important for you: quality or latency. If low latency
is more important to you, then you may see that you are not using your bandwidth as
effectively, and image quality may not be optimal. If you want more quality, you need
to use the maximum available bandwidth, with minimal overhead, and therefore more
latency. An SRT transmission needs either bandwidth or time. So if you are using your
maximum bandwidth at a higher bit rate because you want higher quality, you’ll have
to allow more time to recover from faults.
What happens if I set my latency value too low?
If the delay/latency setting is too low, you will see blue lines (Send Buffer) climbing
up past the orange line (Latency) in the graph on the encoder. This will be reflected at
the destination as visible artifacts, corresponding to skipped packets on the decoder.
If I see my Send Buffer repeatedly spiking by one or two seconds, by how
much should I increase my latency?
In early SRT versions, there was little tolerance. The Send Buffer values had to remain
below the Latency to obtain good results. With the most recent SRT version the Send
Buffer can occasionally go over one second without being dropped. However, this can
cause a problem at the other end, because while the packets are shown as “delivered”
on the encoder, they may not arrive at the other end.
Does SRT support Pro-MPEG Forward Error Correction:
No, SRT does not support PRO-MPEG error recovery at this time. Its error recovery
mechanism is based on retransmission requests, in the form of NAK (Negative Ac-
knowledgment) packets. The destination device sends NAKs back to the source device