Wednesday, February 18, 2015

Shades of Tweakdom

There's a spectrum of potential value in audio improvements and "tweaks."

1.  At the most important end of the spectrum are things that are well understand and known to academic audio engineers, such as frequency response and audible reflections, which often have easily measured effects.  In this area, there are well known things that can be done to get better sound…better speakers, better designed room--including speaker and listening position placement, room mode and reflection absorbers, etc.  Many of these effects can be easily measured…but wrt absorbing room modes, it takes far more absorption (such as filling all the corners out 3 feet) than most people would imagine or tolerate.  Meanwhile, the effect of a tiny bit of absorption might be hard to measure simply because it doesn't have much effect.  Also, contrarians may dispute whether improvements are actually being made--in which case we have kind of a fashion competition, with subjective preferences ruling.  Do you like a more or less lively sound?  In any real room, you will be getting some reflection, and reflection isn't fidelity, but some reflectiveness may make for a more enjoyable experience anyway.  Most important is the quality of the reflected sound--how much does it change the frequency content of the original sound?  And there you have tradeoffs such as trading off the frequency balance of reflection for the total amount of reflection.

2.  Then there are things that have measurable differences that are nearly within the normal audible range, but are generally believed to be below the threshold of audibility for various reasons.  For example, distortion below 0.1% is plausibly inaudible, but many seek amplifiers with PPM or less distortion.  All other things being equal, lower distortion would be better.  All other things, unfortunately, are never equal, high order harmonics may be created by attempting to cancel low orders, etc.  Frequency response to 30 or 50kHz may be in this category as well--it is just a short distance, say a doubling or tripling of frequency, from where people have known hearing ability.

3.  Then there are things which might be measured, such gigahertz frequency response, but have very implausible relationship to human listening.  Many high end cables have alleged benefits in this kind of area.

4.  Then there are things that have a plausible explanation (still plausible to a very open minded scientist, for example) but can't easily be measured.  Here I could think of the benefits of PLL over pulse count detectors.  My information loss theory regarding DS decoders may be hard to measure, certainly it can't be measured with standard technical instruments.

5.  Same as 4., but theoretically impossible to measure the effect.

6.  Things that have an explanation, but not plausible to most serious audio scientists.  For example, quantum field generators.

7.  Things that have no explanation, and are only sold on the words "listen for yourself."  These things may be pure con.  Given the high variability of listening, the placebo effect, and more, many people may perceive a worthwhile change even when nothing useful is done.

"Information" is not just my imagination

When I say that PCM systems preserve information better than Delta Sigma systems, I mean it.  I'm not only talking about my subjective experience (and I don't have a strong belief in subjective experiences, I believe we're easily fooled and fooled all the time).  I'm talking about something which I strongly believe could be measured, in principle.

One type of measurement would be to create a modulation transmission system using a digital system.  Take two 56k modems, connect one to the input of the digital sampler, and the other to the delta sigma output.  See the maximum rate of information that can be transferred.

Now a 56k modem would not be a good test as it doesn't transmit as much information as a 20kHz bandwidth system could. It would be only testing the middle frequencies available, in which delta sigma systems might be superior to some PCM systems (say, DSD vs 44.1kHz PCM).  So imagine an even better modem, a 20-20kHz modem, that perhaps transmits data at 1Mbps, I don't actually know what it could potentially do.  But such a modulation transmission system would, I believe, show PCM systems to be superior.

Meanwhile, even a 56k modem could show that low bit rate MP3 loses lots of information.  Obviously a modem stream riding on MP3 can't transmit a higher bitrate than the MP3 itself.  We are audibly fooled because of limitations in our ability to hear, but the modem isn't fooled.

With an even higher bandwidth transmission test, say 20-50kHz, high resolution PCM would be far superior to 1x DSD.  While DSD nominally has frequency response to 50kHz, it's very noisy response.

In a way, you could see the issue simply in the rising noise above 10kHz.  But static noise does not perfectly correspond to information loss because there is also dynamic noise, modulation noise, and THAT is the big problem with delta sigma systems.

Just looking at the decoder, a delta sigma or sigma delta DAC seems to have excellent amplitude characteristics.  It can be made with higher apparent linearity than a PCM DAC.  But it achieves this by smearing the time response.  You have a feedback system that takes a variable amount of time to stabilize the amplitude level, or a feedforward system based on the addition of known quantities--either way information is smeared.  True PCM systems put out the best approximation to an analog voltage level at predetermined times, times that don't vary in a way correlated with the amplitude level.  The latter preserves actual information better because any kind of correlation smears information.

I'm pretty much flying by my intuition in describing the whole problem, and I could be wrong here, but it makes sense to me.  Meanwhile, there's also a problem in that it's not clear humans can perceive the higher rate of information.  My argument remains that it may not make a difference in a single listening session, but over the course of a lifetime of listening there will be more different listening experiences because of greater information available.  Such differences would be nearly impossible, effectively impossible to produce DBT proof for.

High Resolution is not Fake Snake Oil

High resolution PCM recording is measurably better.  There is not a myth, you can look at the actual measurements of real equipment using the best test equipment in many Stereophile reviews by John Atkinson, who has published detailed measurements of more equipment than anyone, over three decades of reviewing and editing the most popular high end audio magazine.

The question is, can you hear the difference.  Some say yes, including many industry professionals who are engineering graduates, or the top recording and mastering engineers who have won grammy awards, many reviewers, many high end audio equipment buyers.

But none of those people have the certified published and replicated DBT results that show that there is an audible difference.  At least this is the belief of mainstream academic audio engineering, also the likes of Hydrogen Audio and the Boston Audio Society, and it is verified in a critical scan of published articles in the Journal of the Audio Engineering Society.

Now, in many cases the Actual Mastering done on a high resolution recording is different, including the possible use of less limiting and compression, lower average level, and so on.  It would seem to me it would be worth the bother of high resolution PCM to get that.  Maybe others think it is a scam which they'll boycott.

But anyway, this is not tiny rocks, or crystalline wire compounds, or cryogenically treated AC outlets--such things as have no measurement even remotely relevant to audio measurements.

The same would largely be true of special cables, at least the ones that don't actually distort the sound in some way.  Measurements are essentially invisible to the kind of standard tests John Atkinson performs.   Sometimes cable differences are there if you perform the right measurements, say at gigahertz frequencies and so on that wouldn't be measured by John Atkinson.  That would be true of the principled cable solutions from Cardas…based on litz wire same as used in very high frequency instrumentation.  There ARE measurable differences, but not on standard tests, on frequencies way outside normal ranges.  So cable differences of THIS type are in sort of a middle ground.  Most cable differences are pure hype based on construction or imagination rather than measurable differences.*  Dialectric absorption of different dielectrics can be measured, but then with a low frequency exception to their being measured in conventional tests.  But most cable differences are hype, or at least people could get nearly all the benefit from a professional grade cable with polyethylene rather than vinyl dielectric.

(*Same is true of many other audio tweaks, the crystals, quantum field generators, cryogenic treatment, and so on.  Though separated wires is possibly measurably better if not actually audibly better.)

So Just Thought I'd Point This Out, to the serious people decrying high resolution recorded audio.  It is measurably better.  All evidence suggests that if it is ever proven to be audibly different, it would be audibly different and better.  At the same time, it has no support form serious evidence (at least that hasn't subsequently proven unreplicable) that it is audibly better.

That's now and in general.  If suddenly my hearing gets much better, I could hear it easily.

But most important may be the argument I've made elsewhere, which is that even if differences can't be reliably discerned, there may be a wider space of possible experience which would become evident over a large number of listenings.  The wider space of possible experiences is made possible by there being more information.  Supposedly inaudible frequencies nevertheless apply variance in this way to the experience space.

So it's not in the same category as many audiophool things.  It's provably better, just hasn't been proven for being audibly better.  (Many people insist it is.)

I think it's a good idea at 24/96.  But I might draw the line at some point.  Maybe.

Saturday, February 14, 2015

Acoustat vs Sub Impulse Response
Above is the graph made by Tact when I was time aligning an Acoustat panel with an SVS sub.

Notably they both initially seem to respond out-of-polarity initially, and the sub for a longer period.  But taken as a whole, the impulse has correct polarity in both cases.  I have in other ways verified that the Acoustat polarity is correct, such as with an old "SoftPolarityTest" signal, which I can view on Android oscilloscope.  The sum total of an audio signal is correct, but a tiny leading transient (exaggerated in the Tact HF measurement shown above) is out-of-polarity.

Today I added one more verification, using the Android PolarityChecker app.  I copied the mp3 files from the phone storage (where the App puts them) onto my Mac, and then updated my Sonos libraries. Then I was playing the polarity test in living room and bedroom.  Both systems show correct polarity, and the Acoustats individually.  This is clearest full range, where I get the green signal more than 4 times out of 5.  With the 4kHz test, about 1/3 of the time it shows incorrect polarity.  I believe this is because, as the plot above suggests, there is significant phase shift, sufficient to cause an effective out-of-polarity initial transient at the higher frequencies, or at least look that way to a test comparator a significant fraction of the time.  The acoustic transient response shown above suggests serious phase shift above 10kHz, perhaps as low as 3kHz.  But this is also where audibility of phase shift is lowest.

Likewise the sub has phase shift in it's initial response due to roll off above 300 Hz in the speaker itself, and 80 Hz in the very steep crossover.  That could explain it's long initial out-of-polarity component.

My guess for now is that the Tact image is roughly correct.  But it has also occurred to me that the picture shown might be of the Tact transient itself, which might have a significant leading out-of-polarity component.  That was why I wanted to resurrect my old LAUD 3 measurement tool, which computes a very clean transient response (from a MLS signal).  But so far that effort has been hampered by the old computer with ISA bus that it requires for the vintage DSP card from Turtle Beach.  Some time ago the Fiji card was removed, perhaps it had issues.  I tried installing a spare Pinnacle card, but the floppy drive wasn't reading the drivers.  I later found I had disabled the floppy drive because it was stuck and preventing the ATX power supply from ever turning off.  So it needs a new floppy, or I could burn the drivers to a CDROM, which I may do soon.