SRT

Secure Reliable Transport (SRT) is an open-source transport protocol designed for delivering high-quality, low-latency video over unreliable networks such as the Internet. SRT uses end-to-end encryption, data integrity verification, and congestion control to deliver video streams securely and reliably. It is designed to overcome the challenges of streaming video over unpredictable networks with high packet loss and variable bandwidth.

Aximmetry can stream AVC or HEVC video with AAC audio, encapsulated in MPEG-TS, using the SRT protocol. All three SRT connection modes (Caller, Listener, and Rendezvous) are supported.

Setting up SRT Connections

You can set up SRT connections in the Device Manager of the Aximmetry Composer Startup Configuration

Or when Aximmetry Composer is already running, under Edit / Manage Devices...

Video Input

On the left, in the device list, under the Video category, select SRT.

Click Add...

The SRT device setup window will appear:

NOTE: For SRT video inputs the most common choice is the use of Listener mode. Aximmetry also uses Listener as the default Connection mode for SRT video inputs.

Adjustable Input Parameters

Name

Adjust the Name of the video input device displayed in Aximmetry.

Connection Mode

You can select one of the three connection modes here: Caller, Listener, and Rendezvous.
NOTE: Based on the Connection mode of your choice, some adjustable parameters may not appear to you.

Host

The Host can be specified by name (e.g., localhost, media.myserver.com) or by IP address (e.g., 127.0.0.1).
NOTE: Starting from Aximmetry 2023.2.2 the Host parameter only appears in the Caller and Rendezvous connection modes.

Adapter

By selecting an Adapter you can limit incoming connections to the specified network adapter only.

Port Numbers

No special ports are officially assigned to the SRT protocol so any port number can be used except 0. (Port numbers between 1024 and 49151 are recommended.)
Valid values for Port and Local port are between 1 and 65535.
NOTE: Based on the Connection mode of your choice, some adjustable parameters may not appear to you.
NOTE: In Caller mode, both Port and Local port can be set. In this mode, Local port is where answer packets can come back to. When left empty, the operation system chooses a port automatically. Defining it is mostly useful for diagnosing network problems.

Passphrase

If the stream is encrypted you have to fill in the Passphrase as well.

Latency (ms)

This can be between 20 and 10000 ms, the default is 120.
You only have to change this value if the Round Trip Time (RTT) is too long and the transmission is breaking up. With higher latency, there will be more time to recover lost packets.

Video Output

In the Manage Devices... menu, on the left, in the device list, under the Video Out category, select SRT.

Click Add...

The SRT device setup window will appear:

NOTE: For SRT video outputs the most common choice is the use of Caller mode. Aximmetry also uses Caller as the default Connection mode for SRT video outputs.

Adjustable Output Parameters

Name

Name your SRT connection here.

Connection Mode

You can select one of the three connection modes here: Caller, Listener, and Rendezvous.

Host

The Host can be specified by name (e.g., localhost, media.myserver.com) or by IP address (e.g., 127.0.0.1).

No special ports are officially assigned to the SRT protocol so any port number can be used except 0. (Port numbers between 1024 and 49151 are recommended.)
Valid values for Port and Local port are between 1 and 65535.
NOTE: Based on the Connection mode of your choice, some adjustable parameters may not appear to you.
NOTE: In Caller mode, both Port and Local port can be set. In this mode, Local port is where answer packets can come back to. When left empty, the operation system chooses a port automatically. Defining it is mostly useful for diagnosing network problems.

Passphrase

In order to use encryption, you have to specify a passphrase that is used to encrypt/decrypt the stream.
For the SRT protocol, the Passphrase must be between 10 and 79 characters.

Note: Aximmetry uses AES-256 encryption, which cannot be changed.

If you leave this field empty, the stream will not be encrypted.

Latency

This can be between 20 and 10000 ms, the default is 120.
You only have to change this value if the Round Trip Time (RTT) is too long and the transmission is breaking up. With higher latency, there will be more time to recover lost packets.

Bandwidth Overhead

This is an extra bandwidth on top of the video and audio bandwidths. It will be used to recover lost packets.

Video Encoding

As mentioned in the introduction, this can be AVC (H.264) or HEVC (H.265).

Video Mbps

The video bitrate is in megabits per second. Valid values are between 0.01 (10 Kbps) and 128.

Audio Channel

Adjusts the number of audio channels in the encoded stream.

Audio Kbps

Adjusts the audio bitrate.

Connections Modes

With SRT you can choose any Connection mode both for input and output. Each Connection mode has parameters that must be specified, as described below.

Listener

In Listener mode, you only have to specify the Local port used to receive the stream. The Listener mode is the default value for SRT input.

IMPORTANT:  The Local port must be specified.
NOTE: If the stream is encrypted you have to fill in the Passphrase as well.

Caller

In Caller mode, you have to tell Aximmetry where to connect to, and using which port, so Host and Port are required. The Caller mode is the default value for SRT output.

IMPORTANT: The Host and Port must be specified.
NOTE: The specification of the Adapter and Local port are optional.
NOTE: In order to use encryption, you have to specify a passphrase that is used to encrypt/decrypt the stream.

The specification of the Adapter and Local port are optional.
You can select an Adapter if you want to send the stream through this adapter only.
If the Local port is 0, Windows will assign a port between 49152 and 65535 (ephemeral range).

Rendezvous

Rendezvous is a special mode that can be used from behind firewalls without the need to set up port forwarding.

IMPORTANT: The Host and Port must be specified.

The Adapter can be selected just like in the two other cases. For Rendezvous mode, the Local port cannot be specified, as it is always the same as the remote port.