FFmpeg Tee Muxer: Record and Transcode Live Streams

This article explains how to use the FFmpeg tee muxer to simultaneously transcode an incoming live stream and write the output to multiple destinations, such as a local backup file and a remote streaming server. By using the tee muxer, you can perform the resource-intensive transcoding process only once and safely duplicate the resulting stream, saving significant CPU and memory resources.

Understanding the Tee Muxer

The tee muxer behaves similarly to the standard Unix tee command. It takes a single encoded audio/video stream and splits it into multiple identical outputs. This is highly efficient for live streaming workflows where you need to archive a broadcast locally while broadcasting it live to platforms like YouTube, Twitch, or a private RTMP server.

The Command Structure

To transcode an incoming stream and output it to both a local MP4 file and a remote RTMP destination, use the following basic command structure:

ffmpeg -i rtmp://source-server/live/input \
  -c:v libx264 -preset veryfast -b:v 4000k -g 60 \
  -c:a aac -b:a 128k \
  -f tee -map 0:v -map 0:a \
  "local_record.mp4|[f=flv]rtmp://dest-server/live/output"

Parameter Breakdown

• -i rtmp://source-server/live/input: Defines the incoming live stream source.
• -c:v libx264 -preset veryfast -b:v 4000k -g 60: Transcodes the incoming video to H.264 at a 4000k bitrate. The -g 60 option sets the keyframe interval (GOP size), which is crucial for stable live streaming.
• -c:a aac -b:a 128k: Transcodes the audio to AAC at 128 kbps.
• -f tee: Tells FFmpeg to use the tee muxer for the output.
• -map 0:v -map 0:a: Explicitly maps the transcoded video and audio streams to the tee muxer.
• "local_record.mp4|[f=flv]rtmp://dest-server/live/output": Specifies the outputs, separated by a pipe (|) character.
  • local_record.mp4 is the path for the local recording. FFmpeg automatically infers the container format from the .mp4 file extension.
  • [f=flv]rtmp://... specifies the RTMP destination. Because RTMP destinations do not have a standard file extension, you must explicitly define the format using the [f=flv] prefix option.

Handling Network Failures with onfail

During a live stream, remote network connections can drop. By default, if one of the tee muxer’s outputs fails, the entire FFmpeg process will stop. To prevent a remote network glitch from interrupting your local recording, you can use the onfail=ignore option.

ffmpeg -i rtmp://source-server/live/input \
  -c:v libx264 -preset veryfast -b:v 4000k \
  -c:a aac -b:a 128k \
  -f tee -map 0:v -map 0:a \
  "local_record.mp4|[f=flv:onfail=ignore]rtmp://dest-server/live/output"

In this configuration, if the connection to rtmp://dest-server/live/output drops, FFmpeg will print a warning to the console but will continue transcoding and saving the local file local_record.mp4 without interruption.