What’s this?

This was a fun one.

I like to repair stuff. Fixing a thing is good for the planet. Not only do you continue to take advantage of all of the energy that went into obtaining the materials and crafting the thing, but you also save it from being buried in a hole in the ground. It can save you a bit of money, too.

Yesterday, I decided to repair my son’s headphones. He had been propping them up so that they pressed into the headphone jack of his computer at angle, because if they sat at the wrong angle, they didn’t work. I figured the connector was bad, I surely had an 1/8″ stereo headphone jack laying around, and it should be an easy repair. In order to hear the problem for myself, I plugged his headphones into a device and started to watch Nightmare Before Christmas. Sure enough, I couldn’t hear Jack Skellington singing “What’s This?” at all.

But wait. I could hear the orchestra, and I could hear them in both ears. I turned it up. I listened carefully. There was no trace of Skeleton Jack, but the instrumentals were clear, if muted. Pressing on the connector at the proper angle brought Jack’s vocals in perfectly. If the headphones didn’t work, I would expect there to be no sound at all, either in one ear or both ears. But what defect would cause just the vocals to disappear?

The answer turns out to be that the stereo audio of the movie I was testing with was encoded for Dolby Pro Logic surround. Before there were 7.1 channels of pure digital audio piped along our HDMI cables, there was Dolby Pro Logic. A stereo audio signal encoded for DPL has some material out of phase between the two channels. When you add the two signals together, the out of phase signal cancels out, and you get just what should be in the surround channel. When you subtract the signals, you get just what should be in the center channel. 4 channels of surround sound out of 2 channels of audio. Pretty neat.

What was happening in the headphones is that the two audio channels were shorting together, cancelling out what should be in the center channel and leaving me with just what should be in the surround channel. I clipped off the end of the cable, spent a few minutes learning which color meant which conductor of the headphone plug, and soldered on a new connector. All better!


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