Constant Bitrate (CBR)

Constant Bitrate (CBR) is a method of filling bitrate gaps with null packets to achieve a constant rate, which is required for DVB modulators.

The diagram below shows a stream without CBR, where bitrate varies and has gaps:

When CBR is enabled, the stream is filled with null packets to maintain a constant bitrate, as shown in the diagram below:

How to configure CBR

To configure CBR in Astra, you need to add the cbr parameter to your UDP output address. The cbr parameter is only available for UDP outputs. For example:

udp://239.255.1.1#cbr=4000

This will enable CBR with a target bitrate of 4000 Kbps. You can adjust the bitrate value according to your requirements.

Note

CBR doesn’t reduce bitrate. If your stream’s bitrate exceeds the CBR limit, all excess packets will be dropped.

How to reduce bitrate

The cbr option in Astra does not reduce the stream’s bitrate. It only fills bitrate gaps with null packets to achieve a constant rate. To actually reduce the bitrate, you need to transcode the stream. This can be done using a tool like Senta or FFmpeg.