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

~~~
matthiasl
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.)

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mbell
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.

~~~
kortuk
Their signals are cancelling out because they are in phase, this is a thing.
It describes what is happening and why. You are arguing semantics here more
than a technical point here, from my perspective. I agree with the physical
reason, just because it is simple does not mean you cannot have a term for it
in the case.

~~~
mbell
I am in a way arguing semantics because I believe its confusing to non EE
people reading this to use a term like 'phase cancellation' in a way that is
very different from its usage in almost every other context. In general it
refers to the sum of two waves equaling zero (or some reduced value), i.e.
destructive interference. In this case the opposite is happening and the
vocals are removed not due to a sum of the waves but rather a difference
equating to zero.

Side note - welcome to hacker news! Assuming your the same kortuk from the EE
stack exchange I believe we've interacted a few times. My account there is:
<http://electronics.stackexchange.com/users/1438/mark>

~~~
kortuk
Yes, we have interacted there a few times. I would probably use the term
"common-mode cancellation." But it is something that is a very base concept in
EE and the other terminology makes sense to me, although for non-EE I could
see the issue.

You should make hacker news your website on EE? Your account looks some bare
there, add some information!

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

[http://www.howtospotapsychopath.com/2012/05/16/the-music-
goe...](http://www.howtospotapsychopath.com/2012/05/16/the-music-goes-round-
and-round-and-comes-out-backwards/)

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corwinstephen
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.

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

~~~
MattBearman
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.

~~~
JagMicker
It depends on a few things...

-As mentioned earlier, how the mp3 was prepared, whether split or joint stereo encoding.

-The source of the instrumental version. If the source is vinyl, all bets are off. If the source is a proper "Instrumental Version", it would have been mastered from an identical mix but without vocals and vocal effects (ex. reverb), and with the exact same settings as the original master. That is how major labels typically do it --- if it's apparent that the album will be popular, they'll usually have an instrumental mix (and master) created at the same time. This is done because the label wants to have instrumental versions for later potential use in films, TV shows, award ceremonies, commercials, etc. It's much more expensive to go back and "recall" settings in order to "mix-down" instrumental versions. This usually requires hiring the original mix engineer or one of their assistants and renting the studio at which it was mixed. Many pieces of "analog" mixing gear are not "stepped" with repeatable settings. Analog gear also has differences between individual pieces of equipment, even if the same model. Equipment breaks, is sold or removed, and this makes the task of a recall that much more difficult. Recalling mastering settings isn't nearly as daunting, as mastering studios typically retain all of their equipment and keep it in working order. This means that good documentation is required, similar to the software development process. For many of your all-tim favorite albums, there exists binders full of charts, diagrams, and notes detailing microphone types and positioning in the room and relative to the instrument/speaker, channel assignments, mixing equipment settings, the type/speed/bias/etc. of tape, lyrics and notation, even the types and tuning of instruments. It's a very complex process to document and recreate.

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raverbashing
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.

~~~
daeken
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.

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

~~~
daeken
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.

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jermy
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)

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hammock
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.

~~~
mikeash
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
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.

~~~
mikeash
Interesting point. Just to make sure I understand you correctly, you're saying
that even in independent stereo mode, the fact that the two channels have a
limited maximum total bitrate means that the encoder can and will trade off
bits between the two channels in order to account for one channel or the other
needing more data at any given time?

Seems like you could easily create an encoder that didn't do that, and just
e.g. encoded each channel at 64kbps when a total of 128kbps is requested, but
maybe none of the real-world encoders actually do this.

~~~
apendleton
No, I think it's that even if each channel gets the same bitrate, if there's
more "going on" in one channel than the other, there may be fewer bits to go
around and that channel may have more qualitative loss. So if they both have
the vocals equally, but one also has bass drum, the one with the base drum may
lose more fidelity on the vocals than the channel with vocals alone.

~~~
mikeash
Excellent point! I hadn't thought of that. It strikes me, then, that Joint
Stereo would actually be better in this case, as it effectively encodes the
mono signal and the L/R difference, which is what you're after anyway for this
purpose.

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

