itunes polarity?

[stephen1212b]

Does anyone know of a utility that will reverse the absolute polarity of a song in itunes? Ideally Apple would eventually add this so that it could be selected and stored with each track.

[DeafMute]

You mean you want to reverse the phase?

I don't know of any plugin of that sort, but I'm wondering what it is you're trying to achieve?

Cheers

[stephen1212b]

Actually the correct term is polarity. Phase is an arbitrary shift of one frequency with respect to another. It is common to think in terms of 180 degrees of phase shift, but in relation to what?

Inverting the polarity allows for the proper relationship of electrical potential to acoustic pressure. A positive pressure leading edge to a sound-wave creates a negative electrical voltage as it deflects the membrane of a microphone away from the direction it is pointing. Ultimately a polarity reversal needs to happen so that this negative voltage will produce a positive air pressure from the speaker as it tries to reproduce the sound. The difference between pressurizing or rarifying the air can be quite striking with some speakers.

Since about half of recorded music is in one polarity and half in the other it would be nice to be able to identify and then change half of the recordings so that all recordings would be the same polarity. It is a simple matter to reverse the polarity of a digital signal so I was hoping that this feature would eventually be stored in the metadata of the music as it is a function of the recordings themselves not just the system being used for playback.

[buzzsmith]

Since about half of recorded music is in one polarity and half in the other it would be nice to be able to identify and then change half of the recordings so that all recordings would be the same polarity.

Stephen:

For some reason, this interests me, and you seem to know your subject.

Why, then, is some recorded music "reversed" as opposed to the other half?
Are there "benefits"?

=buzz=

[Phil O]

Why, then, is some recorded music "reversed" as opposed to the other half?

Forgive me for being curt, but my guess would be that half is recorded incorrectly. I doubt, however, that it's half.

Phil

[Phil O]

Does anyone know of a utility that will reverse the absolute polarity of a song in itunes? Ideally Apple would eventually add this so that it could be selected and stored with each track.

I don't know of a way to do it directly in iTunes, but if you need to go to another app, why not just do it in DP? Or were you trying to avoid that?

BTW, you are correct, sir. The proper term is polarity.

Phil

[buzzsmith]

Forgive me for being curt, but my guess would be that half is recorded incorrectly. I doubt, however, that it's half. Phil

Not curt at all, Phil!

Where, how, and why does the polarity get reversed?...someone have the invert phase buttons down?...in the mix?...the premastering?

Just piqued my curiosity!

=buzz=

[Phil O]

Where, how, and why does the polarity get reversed?...someone have the invert phase buttons down?...in the mix?...the premastering?

Just piqued my curiosity!

=buzz=

I don't have a clue. I wasn't aware that it was that big a problem. S1212b, can you shed some light on this?

Phil

[buzzsmith]

I wasn't aware that it was that big a problem.

Me, either...that's what got me curious.

=buzz=

[stephen1212b]

Some microphones are wired in reverse from standard pin 2 hot. The US used to have a different standard than the rest of the world with pin 3 hot. We have now adopted the DIN standard of pin 2 hot, but there is a lot of gear wired in reverse. Most summing amplifiers invert the polarity so that they can use the virtual ground as a summing input. This way you can add more inputs and the gain will stay the same only the noise gain is increased as more make up gain is automatically being applied. Depending on the layout of the board i.e. the number of summing stages the board might invert polarity from input to output. There is no real standard for amplifiers with some having inverting topologies and some not. Remember that the signal needs to be inverted at least once, or an odd number of times from recording to final playback because the mic faces the opposite direction from the speaker.

Fortunately more rare is the recording where some tracks, lead vocal for instance, are in the opposite polarity to other tracks such as the drums. This is a situation where you have to choose which is less offensive having softer drums or a thiner vocal. This is something we can all watch out for in our recordings. I often use many wireless mics of different brands from different sources. Here the polarity issue is obvious as two mics in close proximity to each other with opposite polarity will create a comb filter effect as different frequencies cancel themselves out.

At the minimum you should go through all of your mics AND CABLES to determine if they are wired in the same polarity or not. If your board doesn't have input polarity switches you can reverse the wires to pins 2 and 3 on the mics that are different. Any gear that gets patched in like compressors or equalizers should be checked to make certain that it doesn't reverse the polarity from input to output. With balanced equipment it is easy to switch the input wires 2 and 3. With single ended gear you need to add a balanced converter in order to change the polarity, or simply know that the equipment inverts polarity from input to output and take appropriate measures when patching it in. Finally check your board if you use one. Sometimes the channel direct outputs are a different polarity than the mix outputs. If you use different interfaces make certain that they are all non inverting. Lastly check your monitoring set up. A quick and easy way if you have a bidirectional figure of 8 mic is to simply speak into it and then rotate it so that you are speaking into the back of the pattern. The non inverting polarity will be obvious with stronger bass and greater clarity. If this isn't the front of the mic then your overall system is non inverting and you will need to add one inversion in order to be monitoring with correct absolute polarity.

I was hoping to find a simpler solution than importing all of my CDs into DP reversing those that need it, and re-exporting to itunes for playback. By the way if you listen to a compressed MP3 style format or the radio the phase is often rotated intentionally and the absolute polarity issue becomes moot. This is just one reason why these formats don't sound as good as listening to the CD and most CDs don't sound as good as the analog original.

[John V]

Stephen, I'm going to cut to the chase here cause you've posted some good primer material for folks who might not know (or care!) about Absolute Polarity.
There are several issues that come into play and they all have been known issues in the real (not web) broadcast engineering world for a LONNNNGGG time. I'll scattershot through em as they hit me.
One is the sheer pragmatism (or lack thereof) of worrying about this on lo and mid-fi material like iTunes that's been pretty strongly data-compressed dn then played through a random field of different peoples' iPods, desktop computer systems, etc tc where every part of teh system... headphone amps, headphone wiring (which headphones are wired 'forwards'?) and small large and horrid speaker systems are resulting in which end polarity in teh air to the listener's ears?
I'd submit that there are much worse audible things done to that audio in the data-file-reduction process than what most of us would notice with a simple polarity flip... assuming in the first place that any particular multi-track production had CONSISTANT polarity throughout all tracks (thus the final mix COULD NOT have anything resembling Absolute Polarity). You;d have to know the provenance of every individual track's mic, source, analog box, console, recorder, etc... INCLUDING any loops, samples, etc that were used in addition to any live recordings used. Then there's the stuff that never went through the air on it's way into the recording: What's the AP of a direct bass track? a direct synth? Did the artist/engineer have NO THOUGHT to polarity at all? or did they indeed CHOOSE the 'sound' of a kick drum with a reversed polarity for this mix? or a kick drum mic-ed on the BEATER side... or a snare mic'd from the bottom? I'm assuming for simplicity's sake that indeed the recording's engineer folks did indeed pay attention to wrongly-reversed polarity pairs of mics that were in teh mix... though even THAT has a history in commercial top recordings of getting through. Can;t fix THAT by flopping polarity of the track overall.
It's a real issue, but not one you can do much about if you don;t know the INTENT of the producer/artist. The human voice is a VERY assymetrical
waveform, some much more than others, as I said this is an issue that broadcast engineers have dealt with for over 4 decades... how a waveform with a strong energy-loading on one polarity side or the other hits a transmitter can reduce the maximum modulation by hitting clipping on one 1/2 cycle before the other and thus a whole collection of polarity-swappers and DC-offset shifters exist to neutralise the effect on some voices and material. How that affects the end AP issue is even messier.

just a bit of the issue muddling that makes this a REAL interesting topic!

An experiment would be to use a known control waveform (a defined version of eithe a foreward or backwards sawtooth is a good one) and run it through the whole iTunes process and see how it comes back out. But you;d have to know what your own listening system is doing to tell what's 'correct in the end.

IT's not a new topic, but it IS a well-known frustrating one where you have to think whther the worry is accomplishing ANYTHING AT ALL when the field of playback systems are completely random.

[stephen1212b]

I have a little over 10,000 songs in itunes. All are in AC4 Apple loss-less format so we are talking at least CD quality. I say at least because the format is 16 bit 44.1 K sample rate, the same as is found on CD, but the asynchronous transfer from the CD to the hard disk allows multiple passes to extract an exact copy of the original data. This is not possible on even the best CD transports. I use an external Aardsync clock to pull the data off of the HD through my MOTU interfaces. I take the digital output into a modified Benchmark DAC 1 which is a wonderful A/D converter, but lacks a polarity switch. I have time aligned modified Thiel speakers so absolute polarity is readily apparent.

I find that the vast majority of my recordings have polarity consistent within themselves. That is only a few have some instruments out of polarity with others within the same song. With these as John V said there is no way to correct the situation you simply choose which instruments will be correct.

A greater number will have the polarity changing from track to track, but the vast majority are consistent throughout an entire album and often an entire label.

The term absolute polarity is misleading unless you include everything from the original source to your ear. Each playback system may be inverting or not, just as each recording may be inverting or not. The desired and audibly preferred situation is for the same polarity to reach your ears if you are hearing the sound live or through speakers. As I stated in an earlier post the recorded polarity needs to be inverted once to preserve the absolute polarity of air pressure at our ears. Any solution to absolute polarity can only be achieved locally, but if all music could be stored in a consistent polarity a one time simple adjustment to the playback system could bring proper polarity to the listening environment.

As to direct sounds these will also have a proper polarity as the leading transient is primarily a positive air pressure. As long as a positive voltage produces an increase in air pressure at the ear proper polarity is in effect. Obviously an inverted waveform can be used. You can hear the difference and would want to preserve that choice. Only a correct polarity will not alter the intent.

If anyone is interested in learning more, or has doubts about the audibility of polarity check out "the Wood Effect" discovered by Charles Wood at the Defense Research Laboratory (ISBN 0-929383-00-1). Clark Johnsen writes about this in Positive Feedback online.

[Phil O]

Stephen, after doing a little on-line research, it seems that the general consensus is that there are no standards for "correct polarity" among manufacturers. I was under the impression that makers of pro equipment were careful about maintaining input/output polarity relationships exactly for this reason (the wood effect). Have I been mistaken? It seems that most of the equipment I've dealt with has some reference to polarity somewhere in the manuals. I've assumed that this is so engineers can maintain the correct relationships throughout their studios. I've been careful when adding equipment to my studio to pay attention to polarity. Are you telling me that this is not typical among engineers? Am I the only one who pays attention to this stuff?

No sarcasm is intended in my questions. I am seriously interested in this topic and would appreciate your feedback.

thanks,
Phil

[Phil O]

It's a real issue, but not one you can do much about if you don;t know the INTENT of the producer/artist.

Yes, but shouldn't that intent be maintained. Isn't the real problem here one of standards. I'd like to know WHERE the standards are breaking down. Certainly we can't do anything about the guy who hooks up his speakers wrong, but shouldn't there be standards among manufacturers of consumer audio equipment? Are there any? Any thoughts on this?

Phil

[stephen1212b]

There is an international standard that calls for pin 2 hot, but it is not mandatory and it only addresses balanced connections. Most modern equipment maintains polarity from input to output, however it is worth checking into as I have worked on boards that have unbalanced direct outputs inverted with respect to the main outputs.

I am proposing that polarity is audible and that it would be a great feature if it could be indicated in the metadata of a song. Once all of the music was inverted as necessary so that it was consistent with each other then you would still have to make certain that your listening chain presented the music properly, but this would be a one time adjustment instead of each and every song.