Listening Tests and Absolute Phase Page 2

One way, an initial compression at the microphone caused the speaker to deliver an initial compression (absolute signal phase maintained) and the other way, the initial compression at the microphone caused the speaker to deliver an initial rarefaction (absolute phase inverted). Extending this to hi-fi equipment, he found, when applying a rigorous methodology, developed with his co-worker John Vanderkooy, to two items which still sounded different when all the above-mentioned differences had been removed, that the matching of their absolute phase characteristic then removed that audible difference. Occam's razor—don't multiply entities unnecessarily—therefore must ascribe the audible difference between the two designs to the absolute phase difference and nothing else!

All the magical properties, supposedly unmeasurable by normal test methods which were supposed to exist between two different amplifiers, for example, could be boiled down to the effect on asymmetric signals of whether a design was phase inverting or non-inverting if—and this "if" is itself a significant source of real audible differences—all frequency response differences are eliminated and neither amp is being driven into even momentary overload with the speaker in use. If you're convinced that your Quad 405 doesn't sound as good as a highly touted audiophile amp, you can reverse the phase of both speaker leads and start to enjoy your record collection again—unless you use Quad Electrostatics, of course, which are themselves arbitrarily connected to be inverting (footnote 11), in which case you'll start enjoying your records less!

The problem is that there is no one standard governing signal polarity (apart from that for unbalanced mikes, which states that a positive-going pressure change should produce a positive-going voltage). After that, all amplifiers, mixing desks, tape machines and recording lathes used to produce the record are designed totally arbitrarily when it comes to preservation of signal phase polarity. The listener has no idea how many inverting or non-inverting stages the signal has gone through to get on to the record. And even if he did know, hi-fi amp design is again random. Some power amps invert; most do not. Some preamps are inverting with the tone controls switched "in" but non-inverting when switched to "cancel." Some are non-inverting except when the signal goes through a tape loop. With some the disc input is inverting but not the line level inputs. Some have an inverting headamp but a non-inverting moving-magnet input. Some tape recorders put an inverted signal onto tape but the replay amp is non-inverting. Peter Moncrieff has even reported (footnote 12) that the circuit tracking the audio signal in the Dolby-B circuitry is inverting on record but not on playback. As it incorporates a half-wave rectifier to provide the necessary DC control signal, asymmetric waveforms—such as speech and music—can give up to a 3% frequency response error due to compandor mistracking.

Now there still remains the major aspect that, despite all this undoubtedly being true, its importance does depend on the absolute phase polarity being audible on real music program. It seems that all the psycho-acoustic researchers into audibility of phase change that I have been able to find in the HFN/RR library, right the way from Helmholtz (footnote 13), who stated in 1862 that "the quality of the musical portion of a compound tone depends solely on the number and relative strength of its partial simple tones and in no respect of their differences of phase," up to Ronken (footnote 14) in 1970, Madsen/Hansen of B&O (footnote 15) in 1972 and Cabot et al (footnote 16) in 1976, have used artificial test signals—clicks, single or dual sinusoids, sinewave pulses with shifted zero points etc.—-as a (necessarily repeatable) asymmetric program source. Most of the work since Helmholtz suggests that on such signals, the human ear can detect differences, although all workers indicate that the effect is subtle.

Obviously, it is not necessarily incumbent upon the discoverers of an audible effect to show that the necessary mechanism within the ear to detect absolute phase does exist, but the necessary physiological evidence would be useful in validating such work. Mark Davis, in the Boston Audio Society Speaker (footnote 17), summarises work done on hearing mechanisms and how a change in phase on simple test signals could be detected. Although he doesn't deny the audibility of either phase reversal or the group delay of upper harmonics with respect to the fundamental on special test signals, he states that in connection with a proposed ear non-linearity phase polarity detection system, "The dynamic character of music is sufficient to render this effect inaudible. . . it seems fairly safe to conclude that distortion-dependent phase sensitivity has no effect on the reproduction of music, and can be completely discounted as a source of audible aberration." The finite resolving power of the ear's hair cell filters could also lead to a mechanism whereby phase shifting with respect to the fundamental of high order (10th) harmonics can be audible. Davis concludes, however, that "the audibility of this effect is about on a par with the phase/distortion mechanism, which is to say just barely audible under worst-case, laboratory conditions. Substitution of music . . . renders the effect inaudible by virtue of the masking ability of the music abetted again by its dynamic nature, and the fact that few musical instruments produce adequate energy at harmonics of sufficiently high order."

However, Davis here, in straying onto the audible effect of group delay has left the absolute phase argument and so this last piece of evidence, though interesting, is perhaps irrelevant (except to designers of linear phase loudspeakers). Certainly other workers have reported more positively on the effect of absolute phase reversal on music program. Ralph West (footnote 18) recalls an experiment using recorded bass drum beats (shown to have an asymmetric waveform on an oscilloscope) which had slightly positive results, while Lipshitz and Moncrieff both quote more easily audible effects using music as a source. However, whereas Lipshitz is very careful in extrapolating to the possible effects on music program, Moncrieff has no doubts at all, stating that (footnote 19) "experiments...have demonstrated that absolute phase polarity is far more audible than most people have suspected," and that "The characteristic sonic quality caused by phase inversion is so striking that it can be perceived through many other changed and changing sonic properties."

As indicated, only asymmetric waveforms should produce an audible difference upon having their polarity reversed, speech being the most obvious of examples. However, as all continuous waveforms can be shown by Fourier analysis to be equivalent to a combination of symmetrical—by definition—sinewaves, one might reasonably suspect that audible differences would most possibly be detected using program material consisting mainly of transients, which can be markedly asymmetric—Ralph West's bass drum, for instance, or handclaps. Similarly, plucked and struck instruments should all produce waveforms possessing such asymmetry, and yet the work carried out by Moncrieff gave inconclusive results when using such program material. Perhaps the transient in these cases is over too quickly for the ear/brain to be aware of whether it was positive-going or negative-going.

However, Stan Lipshitz did find that the handclaps on a DG Archiv disc, Golden Dance Hits of 1600 (2533 184), were particularly revealing. Richard Cabot, then with Tektronix (footnote 20) noted that music signals off-record that he was using for slew rate analysis had a distinct waveform asymmetry with, in general, the greater degree of asymmetry being associated with music played on a smaller number of instruments. This asymmetry polarity varied from disc to disc, presumably reflecting the arbitrary final phase condition of the record. I would have thought that this asymmetry was connected with the polarity of transients rather than of continuous signals, and this is what would be audible—if at all—but Moncrieff notes (footnote 21) that "the most dramatic difference occurs instead during sustained notes by a massed ensemble of human voices, strings, or wind instruments. . . The sonic effect of inverting the absolute phase is that of both a tonal balance depression and suck-out through the midrange and upper midrange. The altered tonal balance in turn provides psycho-acoustic illusions of an apparent loss of presence, solidity, and midrange clarity. There may also seem to be less midrange definition, less depth within the ensemble, and more depth between the listener and ensemble location."

Listening Tests
We have never been that positive ourselves about the audible effects of polarity reversal, but since then, in true pragmatic spirit, HFN/RR has been careful, whenever possible, to preserve absolute phase polarity in equipment reviews, apart from a lapse in November 1978 when we inadvertently carried out a listening test between the Quad 33 (inverting) and Hafler DH101 (non-inverting) preamps without maintaining the same phase polarity. A sharp reminder from Lipshitz (footnote 22) forced us to redo the tests, thankfully without having to modify our original conclusions. And after all, only on truly phase-coherent recordings will it matter, anyway. On a normal multimike recording, the absolute phase of even different sections of the orchestra might be opposite, so changing the absolute phase won't then introduce any significant overall improvement at all. However, compulsive hi-fi neurosis led to nagging thoughts that it might be important on enough records to do some further investigation. My colleague Ivor Humphreys designed a suitable line-level black box and we did some serious listening.

The results were interesting, to say the least. Following precedent, we decided first to see if any difference in phase polarity could be heard using an artificial, "worst case" signal, in this case a regular series (roughly 5Hz) of clicks derived from a simple CMOS astable circuit. Initially thinking it best to use a "linear-phase" loudspeaker, we decided on the trusty Quad Electrostatics, as a "time-smeared" design would perhaps introduce competing sonic effects. With the pulse width set to about 50ms, blind A/B-ing over a long period between the 5Hz pulse-streams producing compressions and a rarefactions showed that a difference could be reliably detected, but that the incidence of correct detection varied widely from listener to listener, being from 50% (no detection) to 100% (complete detection).

Ralph West had pondered whether the ability to detect differences in absolute phase was similar to that of "perfect pitch" and it was instructive that one listener detecting the change in absolute phase 100% of the time did have perfect pitch. He reported the subjective difference between the two phase states as being that of approximately a semitone change of pitch, and this would tie in with the B&O study, which also indicated that the change in absolute phase was interpreted by the listener as a pitch change.

Of course, it might be argued that it is not absolute pitch, per se, that is being detected, but a secondary effect; maybe the room itself reacts differently to positive-going pulses than to negative-going ones, or that the loudspeaker cabinet has different resonances excited in the two cases. However, repeating the pulse train experiment in another room with moving-coil speakers (B&W 801s) produced much the same results, as did repeating it in the open air and with headphones. And in any case, whether a primary effect of absolute phase, or a secondary effect, the fact remains that an audible difference is produced on the pulse train by reversing its absolute phase polarity.

An important point to note is that we didn't have any idea which way round was which, so the "lower-pitched" pulse-train could have been either inverted or non-inverted: this knowledge isn't relevant when trying to detect differences on such an artificial signal. On music program, though, one way round is "correct," as then the phase polarity will be that existing at the original microphone. However, if a difference can be heard when reversing the absolute phase of a record, then there is still no easy way of knowing which position would be correct, apart from making a cynical assumption that it must be that which makes the record more enjoyable. In the absence of evidence to the contrary, "reality" is the condition leading to increased happiness!

Following the click experiments, we carried out a number of blind A/B tests using both homebrewed speech recordings and commercial crossed-pair music recordings, as well as live applause conveniently provided by the BBC for the Prom broadcasts. This time, the results were not nearly so conclusive within the limited time permitted by a rapidly approaching copy date; Martin Colloms indicating that any audible difference was of the order of that between nominally identical drive-units, for instance. I wouldn't go as far as to say that the effect of maintaining absolute phase polarity is not that important, but more that it is up to the individual listener to determine the importance with his record collection, on his equipment, for himself, using something like the the HFN/RR phase box.

If you can then reliably detect a difference, then all that is necessary to do is to mark the relevant sleeve either "+" or "–," depending on which way round you prefer, remembering to press the appropriate button when you put the record on. To quote Lipshitz in his original BAS Speaker article: "How many of your records would sound better when reproduced with reversed polarity?" Only you can decide what the answer is.



Footnote 11: Letter to the Editor, HFN/RR, July 1978 p.83

Footnote 12: "Phase Inversion," J. Peter Moncrieff IAR 4, 1979 p.13.

Footnote 13: "On the sensations of tone," H. L. F. Helmholtz, Dover 1954.

Footnote 14: "Monaural detection of a phase difference between clicks," D. A. Ronken, J. Acoust Soc. Am Vol.47 p.109.

Footnote 15: "On aural phase detection," V. Hansen & E.R. Madsen, JAES Vol.22 No.1.

Footnote 16: "Detection of phase shifts in harmonically related tones", Cabot et al, JAES Vol.24 No.1.

Footnote 17: Comment on Lipshitz article, Mark Davis, BAS Speaker March 1978.

Footnote 18: Letter to the Editor, HFN/RR March 1978 p.65.

Footnote 19: Moncrieff IAR 4.

Footnote 20: "Measurement of audio signal slew rate," Richard Cabot, preprint 1414 61st AES Convention 1978.

Footnote 21: Moncrieff IAR 4

Footnote 22: Letter to the Editor, HFN/RR April 1979 p.95.

ARTICLE CONTENTS

COMMENTS
Anton's picture

I am so old that I actually shopped for a preamp with the ability to switch absolute phase.

I grabbed a Krell KRC HR back in the day. Still a great piece of gear.

I can't recall the last time I read a review or saw a demo that still offered this feature or discussed this 'blast from the past!'

Maybe I haven't been looking.

Is it still a thing?

Herb Reichert's picture

...has a button to reverse phase

supamark's picture

makes good schiit (I've got a headphone amp and DAC from them, very happy with both).

Glotz's picture

This is exactly the type of material we need more online here. A wonderful dovetail to Jim Austin's Pass Labs coverage. Excellent piece, and love HiFi News.

PS Audio GCPH phono preamp has a phase switch, and it is handy from time to time... 10-year old product now, though.

(Dipolar) speakers (and their out-of-phase back wave) are also a 'variable' as JA wrote about in understanding phase and the effects of other components' system interactions. They are almost an infinite amount variables that affect a system, depending on complexity. Getting great sound is easy today, vs. decades ago, given greater component linearity in general.

"Carrying out any test non-blind ie, with the identity of the device under test known to be the listener, brings in all the above-mentioned additional stimuli, totally invalidating any conclusions drawn."
Read more at https://www.stereophile.com/content/listening-tests-and-absolute-phase#h5I3gBUkUqAY4wKZ.99

Bingo! The answer to the age-old blind listening argument! The holy.. never mind. Yet more variables enter the picture, and begs the question- what is one truly listening for?

supamark's picture
Quote:

Yet more variables enter the picture, and begs the question- what is one truly listening for?

When you listen to music as a job, where you're mixing a lot of separate tracks down to 2 like a puzzle, making them sonically fit together with EQ, compression, and panning (a good arrangement makes your job so much easier), it's hard to turn that off when you're listening to music for pleasure. It took 10 to 15 years after I stopped working as an engineer to mostly turn off that analytical stuff.

Unfortunately, I'm still very good at hearing deep into a mix now that I've got my system set up properly again and a lot of crap still gets on my nerves. Example:

I like Queensryche's Operation: Mindcrime musically, but its lack of dynamics always kinda bothered me. I listened to Revolution Calling (track 3) for the first time in many years, with a modern DAC, and instantly figured out what had been bothering me - the mix engineer ran a compressor across the mix buss and screwed it up so when the snare hits the whole damn mix pumps down for a moment (incl. the snare) destroying the dynamics. I assume it was the mix engineer because Bob Ludwig mastered it and he knows how to properly use a compressor. The only other explaination I can think of is they used the same master to send to LP and CD plants in 1988 (it's an hour long, which doesn't really fit well on a single LP) but still, Bub Ludwig knows how to properly use a compressor and he mastered it (you can't really "undo" compression). Hearing it clearly made my ears hurt lol. Empire is a bit better in this regard, but sounds like they ran it through a BBE "sonic maximizer" unit.

Yes' Yessongs is the worst recording that I still love - the performances are so much better than studio albums but the sound is just... awful but I still wore out 2 LP's of it before I got it on CD. Little errors, like the bad edit in Pink Floyd's Another Brick in the Wall pt. 2 (it's that stutter in the drums at about 1:05 on the CD), amuse more than bother me thankfully.

All a long winded way of saying there's no telling what a person's preferences are. I mean, compare Art and Jason's (AD vs JVS) systems - it's obvious they value (very) different things in their playback systems but both bring their respective owners a lot of satisfaction.

Anton's picture

That was a fun post to read, Supamark!

supamark's picture

Some speakers wire one or more drivers out of phase.

Even in the 1990's, some pro audio mfgs wired their equipment with pin 3 hot instead of the AES standard pin 2 hot on their balanced connectors. In pop/rock, one or more tracks may have the phase reversed for any number of reasons (i.e., bottom snare mic to put it in phase with the mic on top).

Anton's picture

Sometimes this hobby feels like we are trying to turn hamburger back into steak.

As a side note: I always thought the speaker wiring schemes were to get 'in phase' arrival at the listening position based on how the drivers are set up. I may be way off, so I have no fact based assertion on that.

supamark's picture

it's mostly about x-over slopes (1st/2nd/etc order slopes) and how the drivers interact at the x-over point(s). Some slopes invert phase (2nd/4th), others don't (1st/3rd) from my understanding. Others can give you better answers.

DougM's picture

You realize you've now condemned us all to be furiously switching our speaker polarity back and forth with every record we play, trying to determine which sounds better. Time to buy more screw on banana plugs!

myrantz's picture

Interesting to see this article pop up (from 1980 no less). Have phase OCD ever since I mixed 2-Hot with 3-Hot gear. Before this did not even realise phase is even a thing.

Many audiophiles I visited have somehow managed to wire up their system incorrectly. They fall into these three categories:
1. Left/Right swap
2. One speaker out of phase
3. both speakers on inverted phase

3 was the most difficult to prove until a friend told me there is already a test: Speaker Pop Test.

Play that track, and run Studio Six's iPhone Speaker Polarity test. With this you can tell straight away if the system is + (IMO correct) or - (IMO incorrect). Or look at the cones, they should pop out (not get sucked in).

The problem is in hifi gear. Mixing EU and US gear is one cause. Another reason is some singled ended gear appears to be on - phase (e.g. I think one Conrad Johnson pre-amp owned by an audiophile I visited a few years back may be on -).

Music polarity is a lot harder to determine, as it's pretty much subjective and fatigue can set in quickly. To determine if the phase is inverted, I use cues like audio localisation, imaging and timbre. Last but not least, I use this totally subjective cue called 'emotional engagement'.

Some tracks which IMHO are on - phase:
1. Dark Side Of The Moon (30th SACD), e.g. Time and Money
2. Musik Wie Von Einem Anderen Stern Manger Test CD, e.g. Walking On The Moon and Jazz Variants

The tracks above sound weird IMO when played on a + system.

Those are the easy ones. Unfortunately a lot of music are often mixed with different phases, and sometimes Left and Right are swapped. Girl From Ipanema by Stan Getz is the classic example for me - I have 2 different CDs and 1 SACD. And still don't know which is the correct one - none seem correct.

Glotz's picture

Great info from everyone! Great to see insights from the pro recording world. Supamark's and Myrantz's comments are excellent examples.

myrantz's picture

Do you know why CJ gear are designed like so? That pre-amp has bugged me for ages and I never got to verify it.

John Atkinson's picture
myrantz wrote:
Do you know why CJ gear are designed like so?

Presumably to keep the number of amplification stages to the minimum. Unless used as a cathode follower, which doesn't have any voltage gain, a single tube stage inverts polarity.

John Atkinson
Editor, Stereophile

myrantz's picture

That's sticking to a 'keep it simple' policy and gotta say I love it. Hopefully in the future will have a chance to listen to a Conrad Johnson in + phase system and see how it really sounds.

Knowing this is like finally scratch out that itch. lol. This has bugged me for a while. Thanks guys.

spacehound's picture

That unlike us, who are enthusiasts with OCD about this sort of stuff, The record 'manufacturers' are sane people who don't care about phase (or anything much else as long as it sells) at all.

What's more, being professionals rather than bankers, brain surgeons or garbage collectors who falsely believe they know relevant 'HiFi' stuff, like we are, they KNOW absolute phase can't possibly make any difference to the MUSIC or voice, 'pop' test or not.

So their records have a 50-50 chance of the phase being 'wrong' or 'right' anyway.

supamark's picture

recording engineers are sooo much worse. I've spent literally hours listening to the same 30 to 60 seconds of a song on loop adjusting stuff and I'm hardly unique.

bpw's picture

Kudos for mentioning Clark Johnsen's book.

A quip by Keith Herron of Herron Audio some years ago at CES:
"We found out we were out of phase, so we ordered more."

Brian Walsh

ok's picture

Various phase shifts mostly at the high frequency range due to small inch-movements of the listener’s head – let alone room reflections – render absolute phase evaluation impossible in any meaningful sense through loudspeakers. I've switched "invert" countless times and never got nowhere. Anyway discerning absolute phase in blind A/B stereo tests seems as plausible as telling back/front page of any given A4 white paper. Listening to mono recordings through headphones while separately inverting each channel’s phase could arguably be a more sensible way for the brain to locate relative L-R phase shifts without interfering external cancelations (not as practical though, since hardware dissection possibly required..)

Timbo in Oz's picture

Because I don't like the imaging, and don't like the feel of things on my ears. And, my ears stick out, not flat to my head.

Hearing polarity on speakers is not impossible for me and many I know. Some can't hear it, ever.

Timbo in Oz's picture

A long time ago, in the early 1960s when I was still a 'tween, and a cathedral chorister under the Royal School of Church Music's scheme, our choirmaster/organist presented me with my leader's badge on its purple ribbon.

Privately he told me he believed I had perfect pitch, paused and said 'doesn't help much, does it?'. Equal temperament has been a good thing for most musicians, but I still wince.

Shrill sopranos which are VERY common, are a bete-noire of mine. Everyone else claps like mad!!!

I have been listening to phase and time coherent speakers for a very long time, and IME&O polarity does matter, but usually only on good (simply miked) recordings, some of which I have made myself.

(I've taken time out from that volunteer role, but hope to get back to it.)

We all do hear differently and are also differently affected by music, by the standards of playing and engineering. The assumption - built in to much of the argie-bargie about audio and testing - that we all hear and are affected by music, in the same way and to the same extent is false.

Bottom summed the reality up, in 'A Midsummer Night's Dream'.

For me, once polarity was shown to me, it became a chore to swap leads, and thus I had built two (L&R) spkr level switch boxes with two* DPDT relays. At the same time we took the HP-only xovers out of the 'spheres and put them in the boxes.

* I wanted to bi-amp the spheres. With two matching rethought LEAK St20s.

My wife hears polarity, too. Which helped with the budget.

I am slowly building a new system which will have similar relays to all the speakers, incluidng the four subs arrayed ala "swarm' ideas under & to the side of QUAD 63s in an arc.

A good way to test if you are right with polarity is to move as far to one side as possible. When the distant spkr is most clear that's the right setting.

For me the issue might well relate to timbre and expression, both of which depend most heavily on attacks and decays.

A good example here is the very different sound of tracker-action organs and electro-pneumatic organs. I was fortunate to hear this difference for several years, during those early 1960s, while our organ was being rebuilt.

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