Stereo to Mono Using The Side Channel

If you haven't heard my podcast, hear it. It's what I do in some of my other spare time. When editing it, I always create a mono (or one channel) file. Since it is only two people talking back and forth, it kind of helps save on file size, and ups efficiency.

I also put some music in there, but that causes a slight problem. If you haven't realized, most music is CD quality: two channels, 44,100 samples per second per channel, 16 bits per sample. That's great, but my podcast has only one channel to it. I've noticed that when you convert two channels to one, you lose something. When you take two samples and average them, you should be fine, right?

No, not really, and I just lived with it. Previous to one of my shows, I investigated how FM radio works. I took note on how the signal carried stereo sound. It turns out that it doesn't have distinct left and right signals. It uses mid and side signals. The mid signal carries the left plus right sound, and the side signal carries the left minus right sound. To reconstruct the sound, the left channel is mid plus side, and the right channel is mid minus side.

I twiddled around with this knowledge in Audacity. It turns out that when I listen to stereo sound, my brain is somehow hearing three channels: the sound in my left ear, the sound in my right ear, and the difference between the two. Having all three in one mono track makes it not sound flat. Here is what I do in Audacity:

  1. Start with the stereo sound file.
  2. Copy it.
  3. On the copy track, set the gain slider to -4 db.
  4. Click the name of the second track, and Split Stereo to Mono.
  5. Select the second split track. In the Effect menu, Invert.
  6. Select both mono tracks. In the Tracks menu, Mix and Render. This creates the side channel.
  7. Select the other stereo track. In the Tracks menu, Stereo Track to Mono. This creates the mid channel.
  8. On that new mono track, set the gain slider to -4 db.
  9. Select both tracks. In the Tracks menu, Mix and Render.

The -4 db part makes it not sound louder when it is mixed together; adjust to your liking. It varies greatly depending on the specific piece you process, but this can make a great difference on music. When you do this, listen to the side channel before you put it back together. It often brings out different parts of the music. I would love to make a script for this, but the built in filter chain setup is inadequate.

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