inverted signals

Dear Robert and all Music-Loving Audiophiles:

I have a somewhat different take on system polarity. It seems to me that the ideal system would have a digital domain polarity switch for digital source material including internet downloads that are frequently inverted and an analog domain polarity switch for analog sources. And if you go to "The List" at http://www.ultrabitplatinum.com/?page_id=312 you see that so far of the thousands of CDs I've designated polarity for their are only two that aren't samplers with tracks from different labels that don't have all their tracks either all inverted (R) or non-inverted (N). And only about 15% of "The List's" designations are inverted but there does seem to be a higher proportion of audiophile CD labels that are inverted including those who's own vinyl record counterparts aren't inverted, e.g. Reference Recordings whose vinyl records don't seem to be inverted but its CDs seem to be inverted. I invite anyone to visit me and hear polarity for themselves on my system that's makes it very easy to hear polarity and has a CD player with a remote control digital domain polarity switch. Please email me at gslouis [at] gslouis [dot] com

George S. Louis
Perfect Polarity Pundit

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Warning: "Please observe extreme caution after reading the White Paper/the Speculation paper don't play your audio system at low volume, drive an automobile or operate heavy machinery because the papers are going to be submitted to the FDA as a non-addictive treatment for insomnia.  But you should feel free to read them when you're having difficulty falling asleep."

A Speculation Regarding Perception of Detail

I’ve observed that if an audio system sounds good, no single component of that system can be all that bad nor can the polarity of the recording be played inverted from the live performance.  Reproduced music is in absolute polarity when its compressions and rarefactions are in sync with the compressions and rarefactions of the original performance.

I have come to this conclusion because I haven’t been able to compensate for a bad component without causing some egregious sonic and musical tradeoffs.  On the other hand, if a system really sounds awful it may only be a single component or the inverted polarity of the recording that’s causing the problem.  For example, simple as it may seem, a single component could degrade the sound if its power cord is plugged into the wall outlet in less than the best sounding orientation.

A great sounding system is the result of its creator’s choice of components and musical judgment.  The only true basis for their judgment is an understanding of music and a memory of unamplified acoustic instruments and voices in a reasonable acoustic venue and heard from an aesthetically correct distance.

I believe that every choice one makes in the design of an audio system involves tradeoffs, and the only question is which tradeoffs each of us finds acceptable.  Around fifteen years ago, when I first became interested in the audibility and importance of absolute polarity, the speaker system that I’d created some ten years earlier and used for all my serious testing and musical enjoyment had second-order 12 dB Linkwitz-Riley crossovers.  Despite its many advantages it also had one major disadvantage; it wasn’t phase coherent.  Without phase coherence it was impossible for me to discern polarity or to hear music purely in or out of absolute polarity because that crossover requires some of its drivers to always play in opposite relative polarities to each other.  As a result that speaker system was inconsistent with the single absolute polarity of live music.  I listened to each separate driver connected first in one polarity and then the other.  It wasn’t all that easy in the beginning to hear the differences, especially with my sealed back electrostatic tweeters.  But since they crossed over at a relatively low 1.6 kHz I eventually decided that they, as well as all the other drivers, sounded better connected in absolute polarity.  And next, with all the drivers playing in absolute polarity, I determined that I greatly preferred hearing music in absolute polarity.  And from that day to this, I only find music played in absolute polarity to be truly emotionally satisfying and believe that the single most important sonic and musical aspect of a properly connected audio system is its ability to reproduce the polarity of live music.

Audio systems must at the very least satisfy the following three requirements to be suitable for rendering polarity judgments.  1.  The playback polarity of the source is heard in the same polarity as the original recorded source.  2.  The system is phase coherent and preferably minimum phase.  In the analog domain the only classic crossover networks that permit a speaker to preserve the phase-polarity of the input signal are 6 dB first-order Butterworth.  If you’re not sure about your speaker system, you may use single driver headphones or a single drive speaker if its frequency extremes are sufficiently extended.  3.  The system’s frequency response deviates no more than +/- 3 dB from flat between 50 and 8 kHz which is an example of an application of the rule of 400 as defined in the first edition of the Audio Cyclopedia.

The gist of my speculation regarding the perception of detail and polarity is as follows: When one watches film, video or computer monitors the pictures are not seen as a series of separate still images and thank goodness! It’s because the frames change or refresh fast enough, typically 24, 30 or 60 plus times per second respectively.  The actual flash rate may be up to120 frames per second, depending upon the medium, which causes our eye-brain’s persistence of vision to merge one still frame of a picture into the next.

Similarly, in audio, active noise-canceling headphones illustrate the ear-brain’s persistence of hearing with regard to high frequencies.  The way that active noise-canceling headphones work is by picking up ambient noise with built-in microphones and then generating a signal that’s exactly out of phase to the ambient noise that, at least in principle, should cancel it completely.  The specifications of active noise-canceling headphones indicate that they cancel bass frequencies much more effectively than high frequencies.  And perhaps that’s true to some extent, but much of the reduction in their apparent effectiveness at higher frequencies may be the result of our ear-brain’s persistence of hearing that merges the rapidly alternating relative phase of high frequencies into one sound that has no apparent phase/polarity.  Thus two tweeters playing high frequencies out of relative phase aren’t heard as canceling each other.  Perhaps, this phenomena could be considered a corollary of the Fletcher-Munson effect whose curves describe the reduction of our ear’s sensitivity at both frequency extremes.  Well wouldn’t it be great if noise-canceling headphones canceled the high frequencies as well as the bass frequencies? I’d surely like that and I bet you would too.  So from Shakespeare – Julius Caesar, Cassius speaking, “The fault, dear Brutus, is not in our stars [equipment], but in ourselves…”

In my experience it’s exceptionally difficult if not impossible to determine solely by ear the phase of a tweeter’s electrical connection because the phase of the high frequency signal reverses too rapidly for our ear-brain to get a fix on it.  In other words, when the highpass frequency of a tweeter is raised it eventually becomes so high that it exceeds our ear-brain’s ability to distinguish the phase of its electrical connection, and its rapid phase reversals merges both phases into a single sonic impression that’s without a discernible phase*.  For example, when two tweeters are playing a 10 kHz signal, if your head is a mere 3/10th inch (a quarter wavelength) closer to one tweeter than the other, the signal from one tweeter arrives at your ears 180 degrees out of phase with the signal from the other tweeter.  Although theoretically they should cancel each other perfectly, I believe most listeners will still hear the 10 kHz signal at full volume.

Before I state the conclusion of my theory you need to know something about the use of test equipment to determine polarity.  The measurements of spectral content, frequency balance, dynamic range, distortion made on components playing music are the same regardless of the polarity of its playback.  Were it otherwise, I wouldn’t have written this think piece about how music played out of absolute polarity affects our perception of detail.  According to “The Wood Effect” many listeners can detect the polarity of asymmetrical musical signals even though test equipment and computer programs can’t.  Only sound with compressions that differ from its rarefactions (asymmetrical) has audible polarity, because changing the polarity of identical (symmetrical) compressions and rarefactions doesn’t really change anything, so there’s nothing different to hear in the same way that’s there’s nothing different to see in the mirror image of a symmetrical object.  Therefore it shouldn’t seem so contrary to common sense, scientific analysis or the least bit mysterious that measurements frequently don’t correlate well with subjective listening tests.  But on the other hand, perhaps some measurements will be more relevant to the way we hear when equipment is played in absolute polarity!  And as long as I'm speculating, I think that eventually neural computing may be able to discern the polarity of music and its software might be similar to voice print recognition software.  Now it follows, although the cutoff threshold may vary among individual listeners, as the sound’s frequency increases, above some point all listeners will perceive the music’s high frequencies as equally loud regardless of their actual polarity.  But when music is heard out of absolute polarity, the midrange, bass, and even the high frequencies below some frequency, all tend to sound somewhat recessed, rather dry, and bleached out.  Thus psycho-acoustically against a background of a sucked out and a papery dry sounding midrange, and a sucked out dry and overly taut sounding bass that's lacking in harmonic richness, the high frequencies are heard in bold relief and sound a bit harsh, which also makes the bass and mid-range seem more detailed with faster transients, although they are not.  And that can make the bass sound as if its attacks are quicker because what’s heard as the leading edge of its transients are really the sound of its harmonics which are actually reproduced by the mid-range and tweeter not the woofer.  The result gives the impression of a greatly degraded stereo image that’s rather two-dimensional with a soundstage that’s vaguely focused and somewhat confused.  That also contributes to the false impression that one is hearing more of the performance venue’s space because there seems to be more reverberation when the mid-range is less prominent.  But those effects are really only psychoacoustic artifacts of the music being played out of absolute polarity and not how acoustic instruments and voices sound live!  The most general statement I can make about how music sounds inverted is that it tends to sound concave and hollowed out instead of more convex, full bodied, and harmonically fleshed out when it's heard in polarity.  Inverted polarity will tend to reduce the music's pace, and rhythm which makes the performance rather boring and lifeless.  But if a system has a flabby lower midbass/bass/recessed high frequencies, some listeners might prefer the sound of inverted music in spite of its compromising sonic tradeoffs in an attempt to make fundamentally poor sounding system listenable.

Here are some other examples of how the psychoacoustics of audio affects our perceptions, which sometimes seems counter intuitive, but nevertheless may resonate with some listeners.  I believe when you add a subwoofer to a system it doesn’t necessarily sound as if you’ve added more bass, but more often than not, it sounds as if the highs have been reduced.  Similarly, add a super tweeter to a system and it may sound as if there’s less bass not more highs.  And if you turn off the bass/mid-bass altogether or reduce your mid-range, the sound seems more detailed when it’s not.

Music played out of phase coherence or out of absolute polarity may cause some listeners to wrongly attribute the low fidelity unpleasant sound to solid state devices or the digital process in general.  This causes some listeners to prefer what they think is the more tuneful, full bodied, and rounded sound that they associate with tube equipment or vinyl records which they believe sounds more like a live performance, when in point of fact, all they really need to hear is music played in absolute polarity.  High-fidelity equipment, tube or solid state, shouldn’t impose a sonic character of its own on the musical signal; its only tasks are to amplify the signal without distortion and control the speakers! How much tweaking and component swapping in our systems are only musically misinformed attempts to correct for music played out of absolute polarity that in Absolute Reality are bound to fail the test of high-fidelity?  Does this suggest that the conclusions of some prior listening tests should be reevaluated and repeated with music that we know for sure is played in absolute polarity?  I definitely think so, and that should include recordings as well, but each of you may answer that question for yourselves.
The Louis Objective Test of the Audibility of Relative Polarity
Someone, other than the test subject, compiles a 72-minute test CD-R or CD of 72 one-minute music tracks as follows: The first track will be a one-minute excerpt from a CD, record or tape recording of a two microphone stereo recording selected for its musical value but without regard to its actual polarity.  The second track will be the same one-minute excerpt as the first track, but it will be recorded in the same polarity as the first track or in the opposite relative polarity to the first track as determined by the toss of a coin, heads the same and tails in the opposite polarity.  The same procedure is followed for tracks 3 and 4 with different music and repeated again until 36 pairs of identical music tracks have been recorded to the CD for a total of 72-minutes.  The person making the disc memorializes the polarity of each of the even numbered tracks relative to its odd numbered counterpart, and thus he has created a test CD for The Louis Objective Test of the Audibility of Relative Polarity.  The playback system should use single driver headphones or at least a speaker system with consistent polarity, i.e.  all drivers move in the same relative phase to each other.  The actual polarity of any track or the playback system doesn’t affect the validity of the test because it’s only a test of the audibility of relative polarity not absolute polarity.  A test of a person’s ability to discern the actual polarity isn’t necessary if they can’t pass the relative polarity test.  If the polarity of the first track of each pair of tracks is randomly selected, then the test CD can also be used as a test of the ability to discern absolute polarity.  Test subjects who discern absolute polarity would by definition have also passed the relative polarity test.

The standard way to scientifically compare component A against component B is by double blind ABX testing.  In order to make ABX testing a bit easier I’ve added non-X that allows the test subject to hear non-X but only knowing that it’s non-X.  When the purpose of the test is to find whether component A or B is preferred, I have a more direct protocol.  Component A and component B are played alternately double blind first one and then the other as many times as needed to state a preference.  Then the sequence is repeated with the order of A and B chosen at random for the next set of alternating comparisons.  The protocol is repeated until the results are statistically significant.  This single test will determine directly both the test subject’s ability to distinguish A from B as well as preference.

Given the relatively high proportion of audiophile CD labels that are polarity inverted (please see "The List" at http://www.ultrabitplatinum.com/?page_id=312) including those labels with non inverted vinyl record counterparts, one might speculate that a major reason for the preference of many music-loving audiophiles and musical artists for the presentation and musicality of vinyl records is that they're comparing them to inverted CDs.  And it also seems to me that some hi-res internet downloads are inverted.

* After I wrote the think piece I came across an article in the October 1999 Sound stage at http://www.soundstage.com titled Audio Signal Polarity - A Look Behind the Curtain by Doug Blackburn that's chock full of good polarity information and references a book that Mike VansEvers found titled An Introduction to the Physiology of Hearing (James O.  Pickles, Academic Press, sixth printing 1999).  Mr.  Pickles was in the Department of Physiology, University of Birmingham, Birmingham, England.  Mr.  Pickles states that below 5kHz we hear only one polarity at a time but above 5kHz we hear both polarities and that would seem to corroborate my empirical observations that I discuss in this think piece.
History and Afterword

I’ll never know if I would have realized the importance of correct polarity if it hadn’t been for Clark Johnsen and his seminal book The Wood Effect. Reading it changed everything for me because I never gave a thought to polarity before, although I always made sure the left and right channels of my speaker system were in phase with each other.  Until I heard the effects of polarity on my own speaker system I was totally unaware of the importance of absolute polarity to the fidelity, musicality, and the degree of emotional connection I could make with music by hearing sound in absolute polarity as it exists in nature.  I’ll always be grateful to Clark Johnsen for bringing the problem of polarity to my attention.  After listening to the effects of polarity on each driver of my 3-way electronically crossed over speaker system with its second order Linkwist-Riley crossover*, I decided that each driver sounded better when connected in polarity.  Since then I always connect all the drivers in my speaker systems in polarity.

In The Wood Effect Clark states he first learned about the audibility of the polarity of asymmetrical sound from discoveries made by Charles Wood at the Defense Research Laboratory in 1957.  The timbre of all non-electronic sound is asymmetrical.  Only electrical sound sources have the possibility of generating sound waves that are sufficiently symmetrical as to not have an audible polarity.

Many astute music-loving audiophiles such as the late Lars Fredell and Doug Blackburn, to name a couple, also wrote about the importance of correct polarity.  Lars Fredell published a list of his polarity designations of 90 CD labels along with his impressions of the sound of inverted versus non-inverted polarity.  Doug Blackburn, in a paper that I reference above, discussed the technical difficulties of setting up an audio system so that the method of switching its polarity doesn’t itself contribute to any changes that might be mistaken for a difference in the sound of inverted and non inverted polarity.  Contrary to Doug Blackburn’s assertions, based on my thousands of hours of empirical experience to discerning polarity, I’m convinced that once a listener has learned to discriminate polarity any sonic differences, which Doug Blackburn attributes to non electrically neutral polarity inversion switching, are at most tertiary and would never confuse an experienced discriminator of polarity.
More About the Audible Effects of Polarity
Many who’ve read my think piece have asked me to elaborate about some of the sonic attributes and impacts of the two polarities on musicality beyond just the perception of increased detail.  But first it’s important to point out that the difficulty of discerning polarity varies from one recording to the next because of the variations of recording techniques and the acoustics of the recording venue that may vary from multimiked studio mixes (note microphone placement and reflected sound only results in phase differences at or between microphones but not their polarity) to live outdoor coincident single stereo microphone recordings in real-time and every possible venue in between.  Consequently, some of the attributes of polarity described below may not apply equally well to all recordings.  I believe that practice makes perfect when it comes to polarity discrimination… more practice more perfection.  I’ve found the better the playback system the more clearly it reproduces polarity differences.  Thus changes to your system that make polarity distinctions easier to hear probably mean you’ve made real improvements to your system’s fidelity and musicality.  And while I'm speculating, perhaps some day neural computing might be used to objectively discern the polarity of music with software that's similar to voice print recognition software.

The most important aspect of polarity is its effect on instrumental and vocal timbre, because when one knows the sound of unamplified instruments and voices they can use that as an infallible way to discern polarity even for those who are deaf in one ear.  That’s also true for monophonic recordings with no real depth but which still give the impression of depth for listeners with normal hearing as well as those with only one good ear.  If one doesn’t have the imprint of unamplified music in their head and heart, I don’t feel they really understand what better high fidelity means.  Listeners who know acoustic timbre well can almost always determine polarity in 5 to 15 seconds without comparing both polarities even on systems of first impression such as those at the Consumer Electronic Show.

A good system gives strong spatial cues with live recordings and a listener can learn to reliably judge polarity from just the sound of applause.  Good audio systems usually have a much greater soundstage depth, better delineation of instruments and voices with a more holographic presentation when live recordings are played in polarity rather than inverted polarity.  After both polarities of a live recording are heard on a great system the polarity that recovers the most spatial information is an almost infallible indicator of which polarity is correct.  But trying to use the soundstage depth as a reliable cue on first listening to a great audio system may be difficult because better audio systems give a greater impression of the depth and holographic imaging effects even when they’re playing in inverted polarity.  Higher fidelity audio systems will also give pace and rhythm cues that may be almost as reliable for polarity determination as its spatial cues.  Of course, the increased dynamics, better inter-transient silence, higher emotional intensity, generally smoother, and more natural flow of the music of in polarity versus inverted polarity music is also more apparent on better systems when they are playing in polarity versus playing in inverted polarity.  Some listeners say that playing music inverted diminishes its emotional impact like a "phoned in" performance, and that inverted playback is to in polarity playback, as a dress rehearsal is to the actual performance.

In sum, music heard in polarity will be less fatiguing and allow music-lovers to make a closer emotional connection with music then when its polarity is inverted.  For what this all means, read 30 Years of Digital at http://www.UltraBitPlatinum.com/?page_id=725.

*Second order Linkwitz-Riley crossover (LR2, LR-2) Second order Linkwitz-Riley crossovers (LR2) have a 12 dB/octave (40 dB/decade) slope.  They can be realized by cascading two one-pole filters, or using a Sallen Key filter topology with a Q0 value of 0.5.  There's a 180° phase difference between the lowpass and highpass output of the filter, which can be corrected by inverting one signal.  In loudspeakers this is usually done by reversing the polarity of one driver if the crossover is passive.  For active crossovers inversion is usually done using a unity gain inverting op-amp (http://en.wikipedia.org/wiki/Linkwitz-Riley_filter).
George S.  Louis, Perfect Polarity Pundit™ Chief Polarity Buster of the Polarity Police™
Copyright 2006 All rights reserved by George S.  Louis, Digital Systems & Solutions 1573 Kimberly Woods Drive, El Cajon, CA 92020-7261, Phone: 888-588-9542 toll free

www.ultrabitplatinum.com

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