August 17, 2022
Delay measurement
You may have noticed that SRT marketing materials always mention the low delay that is peculiar to this protocol. I would like to clarify this issue and show what you can count on when using this protocol.
For this article, we have carried out several tests in different modes:
- local network
- through the Internet (direct connection via dedicated IP)
- through the Internet in Proxy mode
- in Bonding mode (using our Bonding solution)
Basic information
We, as end users, are interested in glass-to-glass delay. As we all understand, the path from “glass-to-glass” consists of the following steps:
A) image capture
B) encoding (for example in H264 or another codec)
C) network transmission
D) decoding and display
Each stage takes a certain amount of time, which in total gives delay.
A) image capture
B) encoding (for example in H264 or another codec)
C) network transmission
D) decoding and display
Each stage takes a certain amount of time, which in total gives delay.
SRT, in turn, is only responsible for the network transmission phase, so in this equation it affects only one component. Moreover, the encoding and decoding steps often take longer than the other steps.
Once again, we want to mention: even if SRT provides ultra-low delay, this does not mean that you will have ultra-low glass-to-glass delay.
Once again, we want to mention: even if SRT provides ultra-low delay, this does not mean that you will have ultra-low glass-to-glass delay.
Some words about nature of SRT
I would like to clarify why SRT is better than RTMP. First, RTMP is based on TCP, while SRT is based on UDP.
Since 2000y, the "age" of the TCP protocol (1976) became critical due to rapid development of the Internet. The inefficiency of the TCP protocol under the new conditions has become more and more noticeable. As a result, some researchers began to work on development of special protocols, but already based on UDP.
In 2001y, Dr. Yunhong Gu started working on the UDT (UDP based) protocol. As it evolved, this protocol provided the guaranteed transfer of large data more efficiently than TCP. This work received well-deserved recognition in the IT world and its results formed the basis of SRT. We have mentioned this fact in order to emphasize that SRT has an excellent foundation.
Since 2000y, the "age" of the TCP protocol (1976) became critical due to rapid development of the Internet. The inefficiency of the TCP protocol under the new conditions has become more and more noticeable. As a result, some researchers began to work on development of special protocols, but already based on UDP.
In 2001y, Dr. Yunhong Gu started working on the UDT (UDP based) protocol. As it evolved, this protocol provided the guaranteed transfer of large data more efficiently than TCP. This work received well-deserved recognition in the IT world and its results formed the basis of SRT. We have mentioned this fact in order to emphasize that SRT has an excellent foundation.
Let's start
To carry out our tests, we addressed our colleague, who kindly provided us with his studio for testing.
We used Panasonic AG-CX350 camera, which could generate SRT stream, including 4k resolution, as an encoder.
We used Panasonic AG-CX350 camera, which could generate SRT stream, including 4k resolution, as an encoder.
We also used a BlackMagic DeckLink MiniMonitor SDI-card to output the SDI signal. Surely, we used vMix to view the signal via NDI.
Test #1: LAN
From this test, it can be seen that the delay is ～ 650ms for NDI output and ～ 900ms for SDI output.
Test #2: Through the Internet (direct connection via dedicated IP)
We conducted this test in our office using Larix Broadcaster on Android smartphone.
The results of this test show that delay is～850ms when NDI output.
Test #3: Through internet in Proxy Mode
The results of this test are shown in the photo below.
As you can see in this test, the delay is within 600ms.
Test #4: in Bonding mode (using our Bonding solution)
You can read about our bonding solution here.
The result of this test is shown on photo below. The delay was maintained within 1.5 sec.
The result of this test is shown on photo below. The delay was maintained within 1.5 sec.
Conclusion:
Our tests have shown that SRT can achieve glass-to-glass delays of less than 1 sec. Under optimized encoding parameters, we believe that the delay can be reduced to 300 - 500ms.