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This is weird. I plugged my headphones into the soundjack on the side of my Mac Air. I then go to Youtube and listen to a song, which sounds fine - I hear both the background music and the singing.

Then, I pull my headphones halfway out. The singing silences, but the background music is playing just fine. What is going on? Did I just find a cool way to hack background music for karoake?

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2 Answers 2

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You experience this effect as a mix of the hardware and the audio mix.

Hardware

Your audio connector is most likely a stereo connection. There are two wires in your cable, and they are carrying an audio signal each, which is separated in the connected/plug. Have a look at the black stripe on the plug (or white, if it's an iPod headphone!).

By connecting halfway in, you're only getting one channel of that two channel audio stream. The wiki article on connectors explains the hardware nicely.

http://en.wikipedia.org/wiki/Phone_connector_(audio)

Audio Mix

Audio is typically mixed in stereo these days. It is still important for an audio engineer to consider the various audio environments, where many radios will be mono. This is curious in 2013 to see a lot of new models of radio units released with a single mono speaker. In this case, a stereo signal is summed as a mono signal. Both streams get squished together, in a fashion.

But largely, our stereo mixes exploit the stereo format to sound interesting. A large part of mixing audio is placement both in the left and right of the perceived audio environment, and forwards and backwards by use of things like reverb. The best mixes "place" audio in this three-dimension space effectively.

A quick example; for most radio-friendly music, the vocals are largely going to be in the center. Bass is as well, this is both another foundation element, and for reasons of phase and some fun legacies of the days of cutting records. But you will notice backing vocals, guitars, and drum kits are panned across the spectrum. Or even how old Beatles records would have really dramatic panning; entire instruments hard panned.

Imagine what happens with your signal when one of these channels drops?

What You're Hearing

When you only semi-patch the connection of a stereo signal, you're only getting part of that mix. Depending on the patching of your audio chain, this might be only that channel coming through, or else summing or inverting. Normalization affects the audio path as this other Wiki article introduces.

http://en.wikipedia.org/wiki/Patch_panel

But the effect you mention is likely a product of inversion. The following article shows how you might do this with an audio editor, and gives a general idea of what's happening. In your case, the center channel is being inverted.

http://www.howtogeek.com/56335/how-to-remove-vocals-from-music-tracks-using-audacity/

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I assume your plug has 3 contacts (stereo).

The first contact is the "Tip". This is (normally) for the "Right" channel signal.

The second contact is the "Ring" immediately next to the "Tip" contact. This is (normally) for the "Left" channel signal.

The third contact is the at the "base" of the plug, immediately next to the "Ring" contact. This is the "Common" (or "Ground") for both channels.

When you pull the plug partially out, the "Tip" contact on the plug becomes connected to the place the "Ring" contact is normally connected to, and the "Ring" contact is completely disconnected. Sometimes in this situation, depending on the construction of the plug and jack, the "Ring" contact can remain connected to the "Left" channel signal while the "Tip" contact is also connected to the "Left" channel signal.

Since the "Common" ("Ground)" contact on the plug is typically fairly long, it will usually remain properly connected in this situation.

The effect of all this is that the sound on the "Right" channel is lost (disconnected), and the sound on the "Left" channel can be heard on the "Right" earpiece (or possibly both earpieces).

So, if the "Voice" was on the "Right" channel, and the background music was on the "Left" channel (or possibly both channels), then it would be normal (expected) to experience what you have described.

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