post last week in the comments, our man in Japan Yamamoto2002 posted a link to his page where he has an interesting test signal for all to listen to.
If we look at the test signal he used, it's an asymmetrical waveform where half of it appears to be a standard sine wave, the other half has been "flattened" off which results in numerous harmonics if we are to display it on an FFT. One of the files is simply the inverted version of the other to test whether this polarity change is audible. Notice that the waveform is bandwidth limited, and there is no "ringing" after going through a digital filter.
Here are the waveforms overlaid on each other showing the fact that indeed they are just inverted versions of each other:
Since the signals are identical other than polarity inversion, the FFT shows that the signals contain the same frequencies whether in the "initial" orientation or inverted.
Beyond the digital waveforms on the computer audio editor, I can also confirm that playback of the waveforms on both the ASUS Xonar Essence One DAC and Oppo UDP-205 using bit-perfect WASAPI/ASIO were able to reproduce the waveforms accurately... Here's confirmation using a digital oscilloscope to examine the analogue headphone output of these DACs - basically I overlaid the tracing from the inverted and "normal" polarity waveforms to show that the DACs are reproducing the waves without gross asymmetrical distortion. Don't mind the temporal shift between screen captures from the devices:
Nice. We basically see that the headphone output from both machines do not show gross distortion whether the "flat" part of the waveform is positive or negative. I wanted to do this to make sure that it wasn't the DAC itself distorting and adding to audibility.
We can see that the ASUS Essence One's output is not as accurate compared to the Oppo - the channel balance isn't as precise with the left channel louder than the right. Notice how accurate the Oppo UDP-205 is! The 2 channels essentially overlap and unless you look closely, it's hard to tell that each tracing includes overlapped 2-channel stereo audio.
Here are the 3 headphones used, lined up for some rather stress-free (!) ABX listening:
My trusty old Sony MDR-V6 with velour replacement pads, the AKG Q701 "Quincy Jones", and the Sennheiser HD800 (I still love the "speed" and clarity of these things!).
As I posted last week, clearly the difference can be easily ABX'ed in foobar. ABX log for Sony V6 + ASUS Essence One:
ABX log for Sennheiser HD800 + ASUS Essence One:foo_abx 2.0.6c reportfoobar2000 v1.4.12019-06-15 21:10:58File A: AbsolutePolarityListeningTestTone.flacSHA1: c4f4e59eedb9e838e5c393c4508e1f29113591f0File B: AbsolutePolarityListeningTestToneInverted.flacSHA1: d093c9a9723042caf0b39d1afa8d814660100808Output:DSD : WASAPI (push) : Speakers (ASUS Xonar Essence One), 24-bitCrossfading: NO21:10:58 : Test started.21:11:10 : 01/0121:11:21 : 02/0221:11:31 : 03/0321:11:40 : 04/0421:11:50 : 05/0521:12:00 : 06/0621:12:09 : 07/0721:12:17 : 08/0821:12:25 : 09/0921:12:32 : 10/1021:12:43 : 11/1121:12:53 : 12/1221:13:01 : 13/1321:13:10 : 14/1421:13:20 : 15/1521:13:28 : 16/1621:13:28 : Test finished.----------Total: 16/16p-value: 0 (0%)-- signature --9bdb7ac25c2d9e1be17598806f81114426e5d48e
foo_abx 2.0.6c reportfoobar2000 v1.4.12019-06-15 21:28:19File A: AbsolutePolarityListeningTestTone.flacSHA1: c4f4e59eedb9e838e5c393c4508e1f29113591f0File B: AbsolutePolarityListeningTestToneInverted.flacSHA1: d093c9a9723042caf0b39d1afa8d814660100808Output:DSD : WASAPI (event) : Speakers (ASUS Xonar Essence One), 24-bitCrossfading: NO21:28:19 : Test started.21:28:29 : 01/0121:28:36 : 02/0221:28:42 : 03/0321:28:47 : 04/0421:28:53 : 05/0521:28:58 : 06/0621:29:03 : 07/0721:29:09 : 08/0821:29:15 : 09/0921:29:20 : 10/1021:29:25 : 11/1121:29:31 : 12/1221:29:37 : 13/1321:29:46 : 14/1421:29:51 : 15/1521:29:56 : 16/1621:29:56 : Test finished.----------Total: 16/16p-value: 0 (0%)-- signature --994dc2ea717ee73cd8b9ad7b05005def8970cc52
And here's the ABX log for the Sennheiser HD800 + Oppo UDP-205 (best combination of headphone and DAC output):
foo_abx 2.0.6c reportI even tested the AKG Q701 with my Microsoft Surface Pro 3 laptop headphone output from the laptop and the difference was clearly audible.
File A: 01 - Yamamoto - Absolute Polarity Listening Test Tone.flac
File B: 02 - Yamamoto - Absolute Polarity Listening Test Tone Inverted.flac
ASIO : OPPO USB AUDIO 2.0 ASIO Driver
10:07:38 : Test started.
10:07:47 : 01/01
10:08:01 : 02/02
10:08:10 : 03/03
10:08:17 : 04/04
10:08:25 : 05/05
10:08:34 : 06/06
10:08:43 : 07/07
10:08:53 : 08/08
10:09:02 : 09/09
10:09:11 : 10/10
10:09:19 : 11/11
10:09:28 : 12/12
10:09:37 : 13/13
10:09:49 : 14/14
10:09:58 : 15/15
10:10:07 : 16/16
10:10:07 : Test finished.
p-value: 0 (0%)
-- signature --
Initially I did wonder whether the polarity difference was more audible with the ASUS Essence One versus the Oppo UDP-205. Hard to say once I started running the ABX test. Either way, like Yamamoto-san, I was able to achieve 16/16 correct. I got 15/16 on the AKG Q701 + Surface Pro 3 combination so maybe that's indication that a lower fidelity device might make it less easy to hear.
With each DAC and headphone the subjective sound was indeed consistent and similar to Yamamoto's description on his page. While I did not perceive any right/left change in balance (other than that R-L imbalance shown above with the ASUS Essence One), I agree that the non-inverted version sounded "thicker", more "direct". In comparison the inverted sample seemed to be a little "thinner", tonally slightly higher pitched to my ears, perhaps a little more "laid back".
While it was easier with headphones, it was also not a problem with my AudioEngine A2 desktop speakers - 15/16 (p<0.0003) correct match using ABX Comparator.
For completeness, since my family was asking what the heck I was doing with this annoying sound, I gave the ABX test to my wife and kids through the Sennheiser HD800 and ASUS DAC :-). My wife and I are both in our mid-40's, my son is 14 and daughter 12 years old. No problem with ABX score of at least 14/16 correct for everyone (easily p<0.05).
Therefore, my expectation is that Yamamoto's test files should be generally audible. I would definitely expect "golden ears" to have no problem with hearing the polarity effect with this test file.
Conclusion...Well guys and gals, indeed absolute polarity/phase is an audible phenomenon. This is an interesting property of the physiology of human hearing. I'm a believer! And was definitely wrong last week to assume that "absolute polarity" was about whether the first wave was up/down going, and the J.G. Holt Stereophile article from 1980 could use a little updating to include the physiology of the human inner ear.
I looked into what we understand about the mammalian hearing mechanism, and indeed, it looks like the mechanicoelectrical sound transduction with release of neurotransmitters from the cochlear hair cells is directional in nature (much of the basic research appears to have appeared decades ago, here's a 2017 review paper from Comprehensive Physiology addressing some of the known details these days). "Positive deviation" of the hair cells will open primarily potassium and calcium channels whereas negative deviation closes these channels.
This results in a "half-wave rectification" effect in our hearing mechanism... Polarity of the basilar membrane deviation in the cochlea will move the hair cells, trigger action potentials that induces neurotransmission (glutamate release and stimulation of post-synaptic AMPA receptors) in one direction. Check out this detailed PowerPoint presentation which looks like the slide set from University College London, especially slide 42 regarding the rectification process.
This past week, I also had the pleasure of conversing with Matthias Carstens of RME about this. He posted some test files using sawtooth waves to demonstrate the audibility of absolute phase as well in 2017. Here's the waveform from the 200Hz test file at the point of the polarity flip:
Have a listen. Again, the difference in sound is very evident. Matthias notes that the effect is reduced with higher frequencies and almost gone by 1kHz.
In the comments section last week, Yamamoto and GillesP discussed how trombone music tends to show waveform asymmetry. Brass instruments like trumpets and trombones tend to show waveform asymmetry because the air is blown forward and there is impedance of vibrations back towards the mouthpiece. I imagine that this would be especially pronounced with close miking.
Here is a file package you can download - have a listen to this. It contains both the trombone sample track from GillesP last week (correct labeling for the inverted track), plus a 26-second segment from Trombone Shorty's "Laveau Dirge No. 1" from his album Parking Lot Symphony (2017), a more complex piece, with backing instruments. As usual, if I post audio files containing copyrighted content, the sample is provided on the basis of Fair Use for the purpose of education and commentary. Please delete when done and purchase the music if you like it.
This is what the waveforms look like with the asymmetry evident; "normal" polarity versions shown:
|The short segment from Gilles.|
Can you hear a difference between the "normal" polarity and the inverted polarity version of each of the tracks? Does one version sound "fuller", more "real"? Are there differences in the perceived soundstage between normal polarity and when inverted?
Personally, while I can easily pass the ABX test with the synthetic samples from Yamamoto and the difference is obvious with RME's sawtooth files, I do not hear enough of a difference with Gilles' example or this Trombone Shorty segment to be able to pass a blind test. For me, it seems that although the effect can be demonstrated with test tones, the effect disappears with more complex audio, it appears to be unnoticeable (to me at least). Let me know if you can ABX a difference!
To be clear, remember that what I said last week could still be true about asymmetrical distortions from transducers. Do not assume that just hearing a difference automatically means that one's speakers, amps, or DAC is "good" - double check that you're not hearing a system defect first!
Ultimately, as a "perfectionist" audiophile, intellectually, I think it would be elegant to maintain absolute polarity from the studio all the way to home playback to maintain the highest fidelity. However, I'm not sure if this would improve the sound, nor provide greater enjoyment. Furthermore, other than the most direct 2-channel recordings of acoustic performances, I don't see how multitrack studio productions can ensure that every microphone input, every (pre)amp, every sample, of every track be in "absolute" polarity. The cost in time and effort just doesn't seem beneficial; perhaps even impossible in some situations.
Maybe this is a good example of one of those times where we as audiophiles might say "everything matters" (including the physiology of hearing absolute polarity/phase with isolated test samples), but compared to a multitude of other factors that make sound/music "good", it's probably reasonable to think of this as being much lower on the list of priorities.
After I wrote that conclusion above, I read this comment from reader mp which I think sums things up very nicely:
Folks interested in the audibility of 'absolute phase' are referred to Audio Critic issues 18 and 22 letters section (on-line with a search). Another commentor cites it, but I'd like to expand. In the early '90s a man named Clark Johnsen was citing something called the "Wood Effect", arguing that absolute phase was not only audible, but a strong determinant in reproduced sound quality. He wrote a book about it, I think. Of course he did not 'prove' his claim using DBT, but made an assertion, and came up with a 'new' test protocol that, in his opinion, could be used to demonstrate phase audibility to anyone's satisfaction.
Jeff Corey (professor of experimental psychology) critiqued Johnsen's "triple blind test protocol" as nonsense, and R.A. Greiner (professor emeritus EE) wrote in to discuss his recent JAES paper on polarity. Dr. Greiner's experiments showed:
1) acoustic polarity is audible with steady state (monotone) signals.
2) polarity with simple musical passages was 'extremely difficult' to ascertain in a 'highly idealized simplified laboratory environment'.
3) on complex musical passages in a normal living room no subject was able to reliably determine phase differences.
Dr Greiner stated that he discussed the issue with both Drs (Richard C) Heyser and (Stanley) Lipshitz at AES meetings. Their joint conclusion was that while it would be ideal if the recording industry adhered to a standard, in the scheme of recording problems absolute phase is pretty low, to not a concern at all, in the list of special effects that need addressing.What a perfect summary with the 3 points, thanks mp! I see that not long ago, there's this Positive Feedback article from Johnsen, so it looks like the discussion continued. Notice in that article the allegations of "certain agencies" being aware of this polarity issue and the alleged "subterfuge" - I don't believe this is healthy thinking.
One last thought before signing off - isn't modern audiophilia great!? Obviously, "absolute phase" is a topic much discussed over the decades (Greiner referred to his presentations and papers starting in 1991 and his letter in Audio Critic issue 22 from 1995 is great). However, there is a huge difference between reading that Issue 22 letter, imagining what Greiner did in his "Electroacoustics Laboratory" if we were in the 90's versus me downloading these test files, examining the contents in an audio editor, putting the signal through an inexpensive oscilloscope and ultimately listening to them myself in 2019 to develop a sense of the experience of the perception. That link between subjective experience and objective verification is something that all audiophiles can partake in if we wanted to.
Isn't it wonderful that these days we have the technologies easily available for transfer of ideas, images, and of course music files to experience first hand?! Isn't it wonderful that highly accurate consumer gear is so easily within reach and testing equipment is essentially ubiquitous to anyone who wants to explore and understand for ourselves without needing to go through intermediaries (especially the "high priest" audio reviewers telling us what to believe!)? Of course, this does not mean we're all "experts", but with experience and discussion with others, we can all learn and up our game in accruing knowledge and wisdom beyond what we're simply told...
While there are numerous concerns we can raise about the Internet like all the "fake news" out there, pseudointellectual nonsense, antisocial fringe groups, concerns around privacy, active monitoring by organizations, etc. let's not forget the benefits of this freedom and the innovations these technologies catalyze. This combination of freedom of access to knowledge, sharing of ideas, and the means to seek truth through empirical techniques IMO represents what I believe is a much necessary rise of a rational and "more objective" audiophile hobby. One where the younger generations of audiophiles need not conform to the "received wisdom" of nonsense like $$$$ cables having major effects on sound, ridiculous jitter FUD, or bowing down to unnecessary "new formats" designed for corporations and promoted by a sponsored press.
While I have been critical of many aspects of audiophilia, as an audiophile, I am personally very excited about the future of this hobby! Perhaps some of what we discuss here and what I see around the Internet with the sharing on forums is representative of what the future holds. A democratization of knowledge, encouragement for experimentation, and openness to mindfully experience the truth around music reproduction technology. While we can never anticipate the irrationality of humans, I believe that the rise of more reasonable and knowledgeable voices discussing audio hardware will result in rewarding the true commercial innovators while the fate of snake-oil artists by right eventually results in obscurity and ridicule. Such is necessary for the integrity and health of any industry (and IMO there are many industries these days that must face this beyond audio).
Have a great week everyone! Happy summer and I hope you're enjoy the music...