What strips vocals from audio when a 1/8' audio jack is partially unplugged?

http://electronics.stackexchange.com/questions/42623/what-is-it-that-strips-vocals-from-audio-when-a-1-8-audio-jack-is-partially-unp

by taylorbuley • 14 years ago

183 44 14 years ago

electronics.stackexchange.com

ArekDymalski2• 14 years ago

So called phase cancellation as often vocals are positioned in the middle of thee panorama and there's no difference between the content of left and right channel of the vocall tracks. That's thee popular way to create instrumental and acapella versions for mashups.

illicium• 14 years ago

Another way is to invert the phase of an instrumental version of the track (or the actual track) and mix them together.

MattBearman• 14 years ago

I tried doing that once, it didn't work as well as I'd hoped (ie: barely worked at all).

I'm assuming it's because the vocal track had to be removed pre-master, so the instrumental version would then mastered differently. Also the original track was a high quality cd rip, where as the instrumental was a downloaded mp3, so that would have had an adverse effect.

ydant• 14 years ago

Dan discussed the same topic recently in his usual whimsical and thorough fashion:

http://www.howtospotapsychopath.com/2012/05/16/the-music-goe...

OldSchool2• 14 years ago

I think everyone knows that there's only one correct answer and that's phase cancellation because the dry vocal track is typically dead center in the mix.

Here it happens because when the earphones' ground connection is disconnected from the source, the earpiece drivers remain connected to the L and R hot signals and their former common grounding point floats and leaves them in-series. This means that any signal present equally on L and R results in no current through the transducers. You hear only the difference signal in both ears, although to be precise, one is actually opposite in polarity.

hnriot• 14 years ago

Why repeat exactly what's in the linked page? Probably half the people reading that stackoverflow question from hn have EE degrees so the answer was obvious to most, but I don't see the point of just copying the answer over to hn.

OldSchool2• 14 years ago

I didn't think anyone had covered the small leap from "disconnected ground" to a "get a difference signal" as presented in other comments. Glad to hear there are a lot of EE's here!

nitrogen2• 14 years ago

Discovering this trick as a kid was one of the many steps on the path to my fascination with all things acoustic. The widening effect of having the two earpieces out of phase was particularly intriguing to my young mind.

clicks• 14 years ago

> ... the dry vocal track is typically dead center in the mix.

What do you mean by 'dead center in the mix'?

stdbrouw• 14 years ago

Equally loud on both the left and right tracks of a stereo recording.

mbell5• 14 years ago

Stereo audio has completely separate audio signals for the left and the right channels. One of the reasons to use separate channels is to allow creation of a "sound stage". If a track is well mastered/mixed and you have good speakers/headphones you should be able to pick out each performer's location as if they were on a stage in front of you. For example you should be able to discern where the lead guitarist is standing vs where the bass guitarist is standing. This is done by controlling the volume and phase of the sound in each channel for each performer. Generally the vocalist is placed dead center of the "sound stage" and the way to achieve that effect with stereo audio is to feed the exact same signal(in both phase and volume) into both the left and right channels. The 'mix' is just a term used for the way in which the various recordings of each performer are combined to create the final product.

There are a lot of fun things you can do acoustically with stereo sound. Some phasing effects can actually be pretty 'tripy' for lack of a better word.

ealloc• 14 years ago

As a non-EE, something that confused me briefly is that "L-R" doesn't seem symmetric but the scenario is symmetric. But I see now that you'd hear L-R in your left ear and R-L in your right ear. The R current goes the "wrong way" through the left speaker and causes the coil to move opposite the way it does in the right speaker (and vv).

matthiasl• 14 years ago

Is "phase cancellation" a general term for situations where two signals cancel out each other?

(I'm rusty at this, but when I read 'phase', I think of a time difference between two signals. In this case, there's no time difference.)

mbell5• 14 years ago

The answer from the link said 'in-phase cancellation' which isn't really a 'thing' nor accurate. He was just trying to say that when the signals are in phase you wouldn't get sound. There isn't actually any cancellation going on here. When the signal is the same in both channels there is just no voltage across the speaker so it doesn't do anything.

stewars• 14 years ago

I think in this case it's phase inversion of one of the signals as opposed to a phase shift which would imply a time difference.

anigbrowl3• 14 years ago

In audio engineering, yes. A null symbol on an analog mixer will be referred to as 'phase reversal'.

raverbashing• 14 years ago

If you have a CD player (last century tech) that had a karaoke button that's what happens when you press that button, the two channels are merged.

daeken2• 14 years ago

Not merged, but subtracted; in effect, taking one side out of phase and adding that. If you just merge them, common components (sounds in the middle) double, rather than canceling out.

corwinstephen• 14 years ago

It's not just the vocals you're losing, you're losing everything in the center of the mix. Typically that means kick drums, deep basses and anything else lower than about 120Hz (these pieces are always mixed in mono), plus vocals, and a lot of times snare drums.

hammock• 14 years ago

Interesting note that mp3 compression does not keep the L/R channels perfectly independent. Since this means on playback the L/R fidelity is not 100%, you will have better results creating a difference signal with a lossless/uncompressed file than an mp3.

mikeash3• 14 years ago

Doesn't that depend on what mode you use? Joint Stereo behaves as you describe, and is the common mode, but regular stereo mode encodes the two channels independently.

g_lined• 14 years ago

The difference in the signal in one channel will make the encoder throw away different parts of the audio compared to the other channel. This produces data loss in both the 'mono' portion of the signal and the 'stereo' part of the signal. So the 'mono' part of the signal (the vocal) ends up being different between the tracks since data loss is applied across the whole frequency range (or large chunks). Note that Joint Stereo has the same problem because the 'joint stereo-ness' only applies to the top part of the frequency spectrum.

gojomo• 14 years ago

Could you use this to put an obfuscated message in a stereo recording... only audible when in this half-plugged-in difference-mode?

waxjar• 14 years ago

I don't think so, it would still be audible when the signal L and R have in common is transmitted. Maybe if put at a low enough volume?

I'm just guessing here, though. Definitely not an expert.

daeken2• 14 years ago

I'm not positive about this, but what if you take white noise (or pink noise to keep the frequency distortion to a minimum) and then add your hidden message to the left and right channels? Split it so that every other frequency band is on the other channel (L, R, L, R, etc) to keep it roughly even, and make sure the noise is on both channels equally (centered). The noise would drown out the message generally, and the message would only "come out" when the center was subtracted.

anigbrowl3• 14 years ago

Yes, but with several caveats. You could to put it in a frequency band unused by the surround audio, but masked by the center - but this would show up very distinctly on spectrograms or phase analysis. Likewise, one could modulate the panning of a particular frequency band across the left and right, but that would also be quite noticeable on visual analysis.

You could do wideband modulation that would be much harder to detect, but only if your music is repetitive...such that you subtract, say, the instrumental content of the first verse from that of the second to reveal your hidden message. That's not so difficult, given the ubiquity of loop-based and electronic music nowadays.

One other issue with all these schemes is that the more well-hidden your steganographic message is, the more likely it is to be corrupted or destroyed by conversion to lossy data formats such as mp3. If you absolutely had to hide a message in musical form, I think it would be better to do in the manner of the purloined letter, ie haiding it in plain site. Encode it into the rhythm of the cymbals, or the sequence of pitch differentials in the primary melody or some other foregrounded musical element. You can use binary strings or a sequence of rational integers as musical elements very easily, and said sequences can be pre-encrypted. Sure, the signal is even more visible in that context, but who's to say that it's covert or merely artistic?

zura• 14 years ago

Then I guess this won't affect The Beatles's "Eleanor Rigby" (stereo variant).

jermy• 14 years ago

You can get a similar effect, but in a different manner, with attempting to plug a CD or MP3 player into certain mixing desks. The obvious approach, when you have a 3.5mm (1/8") stereo jack from the device, to attach it into one of the 1/4" line input sockets with a suitable stereo adaptor.

In most cases with older desks, this will be fine (you'll only get the left channel from the source). Some newer/smaller desks have a balanced jack input instead or as well as a balanced XLR, and plugging in an audio source will get exactly the same effect as described in the thread, since the left and right channels will be treated as a balanced pair and subtracted.

Cheapest approach to connect such a device to a mixer is a 3.5mm to 2x phono plugs and a pair of phono to 1/4" jack adaptors, connect to two channels on the mixer, and fully pan left and right.

(Never really thought about it before, but here in the UK it seems perfectly normal to refer to 1/4" and 3.5mm plugs at the same time)

What strips vocals from audio when a 1/8' audio jack is partially unplugged?

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