Route Left Channel to Both Channels in Ecasound

Ecasound is fully capable of routing the left channel of a stereo file to both output channels. By using Ecasound's built-in channel-copying operator, you can duplicate audio from one channel to another in real time or during file processing. This article details how to execute this routing using straightforward command-line options.

The Solution: Using the -erc Operator

To route the left channel to both the left and right outputs, use the -erc (ecasound route channels / copy channel) operator. In Ecasound, audio channels are indexed starting at 1, meaning channel 1 represents the left channel and channel 2 represents the right channel.

The basic syntax for the operator is:

-erc:from_channel,to_channel

To copy the left channel (1) to the right channel (2), the parameter is -erc:1,2. This duplicates the signal from channel 1 onto channel 2, while leaving channel 1 intact. Both output channels will then play the contents of the original left channel.

Command Examples

Processing a File to a New Output File

To process an existing stereo audio file and save the result with the left channel duplicated to both outputs:

ecasound -i input.wav -erc:1,2 -o output.wav

Real-Time Playback via ALSA

To apply the channel route during live playback through your default ALSA soundcard:

ecasound -i input.wav -erc:1,2 -o alsa

Live Jack Audio Server Routing

If you are working within a JACK environment, route the file to JACK outputs directly:

ecasound -i input.wav -erc:1,2 -o jack

How It Works

  1. Input Stage: Ecasound opens input.wav as a 2-channel (stereo) stream.
  2. Chain Operator: The -erc:1,2 operator copies the audio buffer from channel 1 directly into the buffer for channel 2, overwriting whatever data channel 2 previously held.
  3. Output Stage: The stream is sent to the output target with channel 1 and channel 2 carrying identical left-channel audio data.