Nature of Distortion

[mfc]

What is the fundamental nature of distortion that is so
objectionable that we seek to cover it up or reduce it to 0?

Two things come to mind:

1) Out of tune

I was listening to a live high school choir concert on Thursday.
Some of the accompaning live orchestral components were out of
tune. They stuck out like a sore thumb.

Watching a TV show on the Fox channel called American Idol. For
those who don't get this, it is a group of kids in their twenties
who sing and compete against each other to get a recording
contract. Some of the singers were out of tune and it was very
apparent.

2) Congestion

Listening to recordings of large orchestral pieces. The louder
it gets, they more it deteriorates into a blob of sound.
Apparent on recordings but not as much on listening to live sound.

Other characteristics?

Thoughts:

Is 1) the reason that higher order distortion components
are so objectionable (because they are out of tune with their
nearest corresponding notes)?

Is 2) caused by large amounts of 2nd and 3rd order distortion
(which is otherwise in tune)? A parallel thought would
be to think about what would happen if an amplifier existed
that produced 2nd and/or 3rd harmonic distortion that was
as out of tune as higher order distortion. Would it be
objectionable? I think it would be very objectionable.

This is a thread that started from some posts in another thread.
I don't know if it will go anywhere, but I thought it would be
interesting to at ask the questions.

[Graham Maynard]

Hi mfc,

Out of tune.

This might relate to the quality of an original performance, but does it really relate to reproduction ?
Francis Vaughan gave us a remarkable insight to piano tuning in another thread, but we accept that source material can be out of tune.

Of equally outstanding notability in the live performance is an individual lack of timing accuracy, and this is where I feel that power amplifier-cable-loudspeaker system interaction can impact upon reproduction performance; ie. when the system generated components become fractionally voltage/time shifted and appear to alter the harmonic character of instruments and voices. Maybe this could be heard as an un-natural alteration of tuning.


Congestion.

Or compression ?
A low power amplifier can struggle more in this regard unless it is driving efficient loudspeakers, especially with orchestral recordings.
And then there is the loudspeaker - for these virtually become non-linear the moment you can see the cone moving. Hearing many small cones in series/parallel clearly reproducing life-like sound levels whilst apparently not moving can be quite a revelation.

Just my early morning ramblings.


Cheers ............. Graham.

[anatech]

Hi Graham,
Just not a bunch of cheap small speakers (ie Bose).

Coffee time.

-Chris

[mfc]

Originally posted by Graham Maynard

...This might relate to the quality of an original performance, but does it really relate to reproduction ?
Francis Vaughan gave us a remarkable insight to piano tuning in another thread, but we accept that source material can be out of tune.

Hi,

I admit to walking on thin ice in this area and have only an
interest in these things. The reason I'll make a stronger
relationship between being out of tune in a live performance
and harmonic distortion in reproduction is that:

1) higher order harmonics (like the 7th) are out of
tune with the notes played by other instruments.
2) it is very easy to recognize a note as being out of tune or
playing in the wrong key (interpreted as the same thing by
our ear/brain?).

Therefore the reason hi order harmonics are so objectionable is
because it is so easy for human beings to recognize a pitch
as being out of tune.

I also got a lot out of Francis' posting. The out of tune aspects
of a live orchestra are a fine balancing act. Makes me think of
the reproduction chain as another instrument added to the mix
of a performance.

After staring at the attached spreadsheet you see that many
hi order harmonics aren't in pitch or key, and that this maybe
why we find hi order harmonics objectionable. OTOH lower
order harmonics are in pitch or key, and this is why it takes
much more of them before the ear/brain overloads on them as
congestion.

I think that compression, and timing distortions are also equally
valid types of distortions. Compression of course being a
distortion of all harmonics, and timing, well thats a whole
'nother topic.

Thanks,

Mike

[sam9]

2) is pretty complex and I think you need to break out some of the individual elements. The first one to think about is dynamic range and compression/limiting. This involves evet stage of the chain from microphone to the loudspeakers in your living room. It is very difficult to make an orchestral recording with out some amount of dynamic range limitation -- potentially you have to cover SPLs generated by a single flautist playing quite softly to every instumemnt going full blast. I'm not sure that the numbers are, but even with DVD-A, SACD etc it is a challenge - one that not usually met.

That's just the technical limit. Then there is psychological and this is weird/ironic. Take a piano. Suppose you have one in you living room and a family member who can play it well. Measure the maximum SPL that can be produced in some of the more vigorous classiocal pieces. Now take a high quality CD (or DVD-A, whatever) and play it at the same max SPL. Even if you can do this with distressing your loudspeakers of bringing the electronics into clipping, nearly everyone including the pianist we just mentioned will find the result unpleasent.

The perception of unpleasentness will still be there even if the recording and reproduction chain is very low distortion. Appearently, we have been trained to accept much higher SPLs when they are produced by "real" instruments rather than recordings. In fact this effect setts in at SPLs much lower than the original. We just don't accept high (or even moderately) SPLs in our living room unless the source is "real" and even when the levels are not challenging equipment capabilities. I contend that even if the ultimate goal of absolutely perfect reproduction of ochestral music is achieved, we will still be subject to a psychological disconnect - our brain KNOWS there is no orchestra in our living room - it KNOWS that is an impossability and therefore cannot accept the associated SPLs.

OK so we play our CDs at lower than realistic SPLs no matter how good the recording chain. If the relative SPLs of the ochestra at max and that lone flautist are maintained, what happens? No flautists! Or at least one you can barely hear over your own breathing. Given this, I cannot fault in general the practice of compression/limiting when recording some type of performances - despite it's faults the alternative is even less listenable. It is my observation (which may be false) that among the more extremist audiophile types (some call me that) "favorite recordingd" tend to be of performances that involve by nature (small groups, vocals etc - whether classical, jazz or pop) have a restricted dynamic range where compression need rarely if ever be applied.

My point regarding comment 2). I know exactly the phenomonon you are talking about. It annoys me to no end. But I fear it won't go away even if "THD", "N", "IM", "TIM", "DIM" and all their cosins, real and imaginary, are eliminated. Only if someone finds a solution to the dynamic range problem including especially the purely psychological parts will the mush go away.

[Graham Maynard]

Hi Chris,

I've been deafened by Bose PA speakers at major expeditions, and once bought a batch of their competitively priced 'Hi-Fi' speakers for Jukebox installation extensions, but for realistic reproduction - - - time for a Brandy.


Hi Sam,

Maybe we can enjoy the unpleasantness of sound from a piano or violin etc. when we are in proximity to the live instrument and thus cannot challenge its flawed tonality, but our brains cannot always be convinced that a system is reproducing faithfully even when it has been proved to be so via blind A-B live/recorded comparison.

Maybe some of the systems that are most easy to listen to suppress some of the higher harmonics that can distress us via realistic playback. I have two requirements in audio;- high accuracy and likeable easy listening; each being so different from the other for neither to be universal.

I do agree with the comments in this thread though.


Cheers ............. Graham.

[sam9]

Graham,

Subjectively: It is much easier to forgive or perhaps not even notice a flubbed note when present at a live performance. On the other hand one must politely tolerate the fat lady wearing too much perfume to your left and the old geezer coughing his lungs out on the back of your neck.

Objectively: In addition to reducing the dynamic range, as far as I know any sort of limiting or compression introduces harmonics as dioes "soft clipping" even though it may be acomplished with more subtlety than guitar amp overdrive circuits. Some decades ago I had a 30W (that was quite a bit at the time) NAD with soft clipping switch. While I left it in off position most of the time, sometimes "ON" was better than the alternative. The point: when ever alternatives exist, it's rare that one is perfect or ideal but rather with luck one is clearly less bad.

I don't think "live vs. recorded" is really a useful comparrisson. A century after Edison it's valid to consider recorded music a separate type of performance. Bach did not have a Bosendorfer to compose for and it is silly to asert that Bach should only be performed on th instruments available in his time. The question is how well it is done. A ninteenth century symphonic composition presented in a CD format owes as much to the talent and skills of the "technical" personnel as to the individual musicians. I suspect, but don't know, that many of the best CD products are ones where everyone involved realized and accepts that end objective is a CD (or LP or . . .), not that the CD is merely an archival record of a live event.

Live jazz probably has additional nuances . . . .

[bear]

Multiple issues, multiple problems...

The "congestion" issue is simply a combination of amplifier deficits and speaker deficits combining... it's not all that difficult to put together both speakers and amps that *don't do that*. But, it isn't inexpensive nor does it correlate exactly to "price point" either. To get this right requires some significant insight into the system and space you're involved with.

The short and simple version for amps:
- power supplies that don't duck on peaks
- output current and voltage capability that is large compared to the load (generally speaking)
- circuitry that is not riddled with sources of non-linearity, or sources of higher order (albiet low level) distortions (especially those that rise significantly near clipping)

For speakers:
- low distortion
- ample sensitivity & power handling to remain within linearity for the space

For speakers, my experience is that they often don't meet even the most basic requirements in these two regards...

As far as I am concerned, the issue of "being in tune" is not related to the reproduction of sound via a hi-fi system. As far as American Idol is concerned, to my ears the band itself is not tuned properly to a standard (so-called) "440" in some cases - OR the tune has been transposed to another key so that the contestants can sing it, and it doesn't "sound right" in the other key ( a common effect ).

The higher order harmonics don't make a given bit of reproduced music or voice sound "flat" or "sharp."

Two things to remember here:

- that humans are "programmed" to expect to hear sound in a rather primordial or primitive way. This is why we can hear things in "stereo" and make out a three dimensional sound field! We're reassembling the sound according to a pre-programmed understanding and expectation of how sound works "naturally." This takes *brain cycles and effort* to pull off.

The *less* sound that does not correspond to the brain's expectations is presented the easier it is to "hear". The more that does not correspond to the natural "expectation" (in an already artifical situation, btw) the more difficult it is to reassemble the information into meaningful sound.

- The second thing to remember is that these higher order harmonics may or may not conform to or interfere with a "natural" sounding presentation... when they do interfere you'd probably hear things like "grain" "strain" or "harshness" to some degree...

Many musical instruments embody higher order harmonics - violins and trumpets - so merely *having* higher order harmonics present in and of itself doesn't tell you all that much on the surface... although the difference between a really great violin and a not so great violin doesn't seem to be all that much in terms of differences in the harmonics, yet it's these differences that let us human ears define which are great and which are not.

_-_-bear

[mfc]

Hi,

Originally posted by bear
...Many musical instruments embody higher order harmonics - violins and trumpets - so merely *having* higher order harmonics present in and of itself doesn't tell you all that much on the surface... although the difference between a really great violin and a not so great violin doesn't seem to be all that much in terms of differences in the harmonics, yet it's these differences that let us human ears define which are great and which are not.

Yes, otherwise we'd be listening purely to electronic tones.
This has been found to be dull and boring and its the original
harmonics that make it interesting.

The point is that hi order harmonics are more out of tune then lo
order harmonics when compared with their corresponding notes
in the equal tempered scale. It is easy for the ear/brain to hear
things that are out of tune. So hi order harmonics should be used
carefully...both by instrument makers as well as avoided, masked,
or greatly reduced in reproduction equipment.

Maybe getting distortion in reproduction equipment down
below .002% across the audio band is all that is neccessary to
preserve the original harmonic recipe? This seems to be the
solution that most designers have chosen.

[bear]

Maybe getting distortion in reproduction equipment down
below .002% across the audio band is all that is neccessary to
preserve the original harmonic recipe? This seems to be the
solution that most designers have chosen.

Probably not.

Unfortunately a figure like 0.002% does not correspond to much of anything in terms of perception... there have been some recent studies that indicate that vanishingly low percentages of higher order harmonics in the *right situation* can be objectionable.

The same studies seem to show that the harmonic spectrum spewed out by much higher distortion circuits (for example triodes) do not exhibit the same objectionable perception.

I'm painting a very broad brush above, so just focus on the idea that merely having apparently low distortion figures does not correlate well to perception.

Oh, and other than speakers, achieving the quoted figure (on the surface) is rather trivial as almost any Japanese branded receiver will do that trick, as will almost any CD player...

_-_-bear