Using SRT Protocol in FAB Subtitler LIVE

The SRT protocol is often used to transfer video over the internet because it allows for recovery of lost packets.

FAB Subtitler LIVE can display live video received over SRT protocol on the PC screen and also reproduce the audio over the PC speakers or headphones. Because live video streams over SRT protocol cannot be received directly by FAB Subtitler it is necessary to convert SRT to UDP/RTP multicast. By converting SRT to UDP/RTP multicast it is then possible to display live video simultaneously on multiple PCs with FAB Subtitler LIVE provided that the Ethernet switch supports UDP multicast.

Converting SRT to UDP/RTP Multicast

To convert SRT to UDP/RTP multicast the easiest way is to use the srt-live-transmit program under Linux. Use the following parameters to start the program so that SRT traffic from 192.168.216.34:6600 will be converted to UDP/RTP multicast on 239.192.1.251:1234

srt-live-transmit srt://192.168.216.34:6600?latency=400 udp://239.192.1.251:1234

Always use udp:// prefix, independently of whether the video stream is RTP or UDP packed.

The parameter latency might have to be adjusted but it shall never be less than 80 milliseconds. To see the status of the connection add the following parameters: -v -s:2000

Details about how to configure the SRT protocol correctly can be found here:

https://www.haivision.com/blog/all/how-to-configure-srt-settings-video-encoder-optimal-performance/

A raspberry pi or a Linux virtual machine are very convenient for srt-live-transmit because it only takes a few minutes to set them up for srt-live-transmit.

Displaying LIVE video in FAB Subtitler

To display live video please use the Display tab and the following functions:

  • Editor over video: Displays the video in the active editor window
  • Video in a window: Displays the video in a separate window which can also be displayed in full-screen mode on a secondary PC monitor
  • Teletext decoder: Displays a window with a teletext decoder
  • Video source: Allows changing the video source if the configured video in a window source supports multiple sources (i.e. the channel number for analogue antenna reception or the URL for video over IP)

To configure FAB Subtitler to received Video over UDP/RTP please set the following settings in Options:

By clicking on the button “Channels” you can define the URLs which are used for every channel that shall be received over IP and displayed. The URL shall be specified as:

udp://239.192.1.251:1234

Converting UDP/RTP to SRT

To convert an UDP or RTP video stream to SRT the easiest way is to use the srt-live-transmit program. Use the following parameters to start the program so that UDP/RTP on 239.192.1.251:1234 will be converted to SRT an made available for client SRT connection on UDP port 6600:

srt-live-transmit udp://239.192.1.251:1234 srt://:6600?latency=400 

Always use udp:// prefix, independently of whether the video stream is RTP or UDP packed.

The parameter latency might have to be adjusted but it shall never be less than 80 milliseconds. To see the status of the connection add the following parameters: -v -s:2000

FAB strongly recommends to use RTP packed transport streams as the input to srt-live-transmit whenever possible. UDP stream without RTP encapsulation is much more susceptible to errors and might therefore result in occasional errors in the displayed video.

Detecting problems in SRT/UDP/RTP transfers

The freely available program TSDuck can be downloaded from https://tsduck.io and can be used to analyze problems in SRT/UDP/RTP connections.

The following command line can be used to use TSDuck to analyze a SRT stream:

"c:\Program Files\TSDuck\bin\tsp.exe" --verbose -I srt 192.168.211.114:6600 --latency 400 --messageapi -P until --seconds 60 -P analyze --error-analysis --wide-display -O drop

The following command line can be used to use TSDuck to analyze a UDP stream:

"c:\Program Files\TSDuck\bin\tsp.exe" --verbose -I ip 192.168.211.114:6600 --latency 400 -P until --seconds 60 -P analyze --error-analysis --wide-display -O drop

The following command line can be used to use TSDuck to analyze a RTP stream:

"c:\Program Files\TSDuck\bin\tsp.exe" --verbose -I ip -r 192.168.211.114:6600 --latency 400 -P until --seconds 60 -P analyze --error-analysis --wide-display -O drop