graham@convex.UUCP (05/15/84)
#N:convex:32900004:000:299
convex!graham May 15 14:34:00 1984
I have been passively reading net.audio for some time now. I have a "dumb"
question which I'm sure most of you can answer: what does "distortion" sound
like? That is, how do I tell when it is occurring in my system?
Marv Graham; ConVex Computer Corp. {allegra,ihnp4,uiucdcs,ctvax}!convex!grahampmr@drutx.UUCP (05/17/84)
Distortion: If it doesn't sound like the real thing (i.e., the actual performance), it's distorted. Yours for higher fidelity, Phil Rastocny AT&T-ISL ..!drutx!pmr
fish@ihu1g.UUCP (Bob Fishell) (05/17/84)
(oo)
"Distortion" is a catch-all term which is used to describe any deviation
in the shape of time-varying signal from input to output, except for
amplitude. There are several subtypes, the most commonly referenced are
harmonic and intermodulation distortion. Harmonic distortion is a
measure of how accurately a component can reproduce a pure sinusoid;
it gets its name from the fact that any waveform other than a pure
sinusoid produces harmonics, and can be expressed by a Fourier infinite
series that includes the fundamental frequency and all its harmonics.
IM distortion, on the other hand, is a measure of how well a system
can reproduce a complex waveform consisting of many frequencies mixed
together.
Figures for THD (Total harmonic distortion) and IM distortion are
usually given with components, but they only represent part of the
picture; amplifiers with identical THD and IMD figures can sound
different. However, most people can't tell the difference between
0.1% THD and 0.001% THD. When you can actually hear distortion,
there's usually something wrong, like overdriving and amplifier,
which produces clipping. This is a strident, garbled noise that
is an indication of the signal amplitude trying to exceed the
saturation voltage of the output devices. This type of distortion
destroys tweeters. Worn phono styli also produce audible distortion,
as do dirty or worn tape heads and failing internal devices. These
are only a few of the most common sources of distortion; it can occur
from many other sources.
--
Bob Fishell
ihnp4!ihu1g!fishmwg@mouton.UUCP (05/18/84)
++ Did you ever see (experience) those guys walking down the streets of New York with their 'Getto Blasters' playing disco or whatever at max volume? Well, distortion sounds like a mixture of that and music. -Marko
rcd@opus.UUCP (Dick Dunn) (05/19/84)
>Distortion: If it doesn't sound like the real thing (i.e., the actual >performance), it's distorted. I can't tell you all just how helpful I find some of the postings in this group! (If I told you, I'd have to use net.flame...) So if I've never heard the real thing, does that mean that distortion is nonexistent? Or is it infinite? How about albums like "Arc of a Diver", which were created by extensive mixing? If a note falls in the mixdown, does anyone hear it? (Oops, move that to net.philosophy...) OK, suppose I have some idea that it doesn't sound like the real thing...so what's going on, and why? If you are an audiophile, and nothing else, you are entitled to say that it doesn't matter...but then you're going to have to find someone else to FIX the distortion for you - and THEY'RE going to have to understand distortion - I mean what kinds there are, how to measure them, which you can hear and which you can't, and so on. I'm going on to read what fish wrote - I peeked ahead; it looks like he at least tried to answer the question. I don't see the point in Phil's non- answer. -- ...A friend of the devil is a friend of mine. Dick Dunn {hao,ucbvax,allegra}!nbires!rcd (303) 444-5710 x3086
kar@ritcv.UUCP (Kenneth A. Reek) (05/19/84)
<>
Come on, people, the question was, "what does distortion SOUND like",
not, "how is distortion mathematically described" or something like that!
Being an admitted audio novice, I'm interested in this answer myself.
Bob Fishell claims that if you can hear any distortion, something must
be wrong, such as dirty or worn stylus, dirty or worn tape head, etc. This
sounds to me like a direct frontal assault on the purists who only buy audio
equipment that needs a bigger mortgage than they have for their houses. By
this they imply that they can hear differences in distortion that are many
orders of magnitude less than that produced, for example, by a big gob of dust
and hair glued to the stylus with Discwasher sludge.
This group ought to be interesting for the next few weeks....
Ken Reek, Rochester Institute of Technology
{allegra,seismo}!rochester!ritcv!karsimard@loral.UUCP (Ray Simard) (05/20/84)
One thing that seems to be overlooked here is that there is more than one kind of distortion. There's frequency distortion, in which some frequencies are reproduced at a different level from others relative to the original source material, phase distortion in which some frequencies are shifted in phase relative to others, and harmonic distortion, in which frequencies are generated that were never present in the original material. Of these, the first is sometimes desirable (that's why equalizers, not to mention tone controls, exist). I cannot at the moment imagine a useful purpose for the second, though there may be one. The third is just plain trash (unless used for effect, like a rock guitarist's fuzzbox). It's my guess that harmonic distortion is the prime subject of the 'what does it sound like' question. If so, the answer is somewhat complicated, and subject to the listener's interpretation. To me, a harmonically distorted program sounds strained and muddy, as if something in the audio chain is being asked to do something of which it is not capable. Sometimes very short peaks of high-frequency material are clipped which results in a strange perception of irritation. Although the harmonics generated are above human hearing, there is a perceptible effect anyway. In most modern equipment of any quality, THD (total harmonic distortion) levels are so low that one usually cannot hear the distortion directly. However, I find that when listening to a superlative system there is a clarity and distinctness to the sound that is lacking in middle-of-the-road gear. Also, stereo imaging is more definite, adding to the illusion of an actual performance. A friend of mine has a Hafler amp and a pair of Dahlquist speakers, and a high-end turntable (I don't recall the name). He has also furnished his room to avoid resonances and other damaging effects. To hear this system is to enter a very special world. Ray Simard
wm@tekchips.UUCP (05/20/84)
A simple definition of audio distortion is anything that happens to the
music that you didn't want to happen. If this sounds stupid, let me
give a few examples, and then I will try and answer what I really think
you were asking.
Example:
When you turn the volume control on your stereo up, you probably want
the music to get louder. If instead, turning up the volume control
overdrives the amplifier so that it clips, or overdrives the speakers
so that their voice coils try to "reach out and touch someone", then
that's distortion.
If you purposely turned up the volume control so that the amplifier
would clip (ignoring what that will do to the rest of the system, or to
your ears), because you like to listen to your favorite Spinal Tap
album that way, then that is NOT distortion (to you).
'nother example:
If the heads on your tape deck get dirty, so that you can't hear the
high frequencies as well, then that is distortion. If you turn down
the treble control on your stereo so you can't hear the high
frequencies as well, that is not distortion (to you).
To a "golden ears" listening to classical music, then, a useful
definition of distortion is anything that makes the music sound
different from "the real thing" (whatever that really is).
What I really think you are asking: commonly heard distortions in a
stereo system, and what you can do about them.
Clipping.
When an audio signal is larger than the circuit trying to handle it was
designed for. Called clipping because the tops and/or bottoms of the
waveform get clipped off, like with a machete. Sounds just like that,
like someone took a machete to the music. Note that it is VERY common
to plug electric guitars into what is called a "Fuzz Box" which does
exactly that, it clips the sound. When done on purpose it is not
distortion, as noted above. When not done on purpose, it can destroy
amplifiers, speakers, ears. To hear what it sounds like, turn the
volume control on your stereo up until your cat leaves the room. If
you have no cat, fake it. Clipping mostly causes Harmonic Distortion.
To fix, get a more powerful amplifier and speakers that can handle the
extra power. And new neighbors. Or use headphones, and go deaf alone.
Actually, most any normal amplifier is clipping quite often. To
reproduce the peaks of the waveform from a piano, at normal room
volume, with reasonable speakers, would take at least 5000 watts. Make
that 15,000 just to give you a couple of dB headroom. Not very many
people without football stadiums have amps like that. So the point is
not to eliminate clipping, but both to minimize it, and to clip
"softly", i.e., round the corners off smoothly, instead of cutting them
off sharply. Sharp corners have lots of high frequencies, and get on
your nerves.
Another interesting effect. Music sounds "too loud" because of
clipping, more than because of the actual sound level. Try this
experiment: Play a piano. Observe mother come in and praise you for
such pretty music. Play similar piano music on standard (less than
15000 watts) stereo at same volume. Observe mother come in and tell
you to turn the stereo down before you go deaf. Fuzz boxes are one
reason that rock music sounds "loud and obnoxious" even when played at
low levels.
Harmonic distortion sounds like one frequency turning into a higher,
related frequency. Play middle C on your piano, then play the C above
middle C. The higher note is the second harmonic of the lower C. If a
concert pianist plays middle C, and it come out of your stereo like 15
different notes, most of which are higher in frequency than the real
note, that is harmonic distortion.
Intermodulation Distortion.
Caused by anything that changes the shape of a waveform in a way that
is not desired, other than clipping. Tone controls cause
intermodulation, but as noted above it is not distortion if you want
it. Purists get stereos that can switch the tone controls out of the
circuit. All the same applies to graphic equilizers.
Intermodulation is caused by two different frequencies going through a
non-linear circuit and modulating each other. If you have two notes,
of frequencies A and B, and pass them through a non-linear circuit,
then you get A, you get B, but you also get A+B, A-B, B-A. If you also
have some harmonic distortion, you also get things like 2*A-B, and any
other conceivable combination.
Intermodulation distortion doesn't sound as bad as harmonic distortion
(especially clipping), but it can make things sound muddy, or
indistinct, with all those other frequencies jumping around that have
nothing to do with the original music.
Hum and Noise.
Hum and noise collectively make up the "noise" part of signal to noise
specs.
The power line is 60Hz, which most everyone can hear. Even when not
directly audible, this can cause problems, because the hum signal can
intermodulate with the music and make everything mushy.
Noise typically comes from high gain, low level stages, like the phono
or tape preamp. To hear what noise sounds like, turn your FM tuner
between stations, with muting off. To actually hear the noise (and
hum) in your system, without playing a record switch your stereo to
phono, and crank the volume up. The stuff that sounds like the ocean,
and isn't hum, is noise. It is caused by electrons bouncing around
when they get excited and not doing what your stereo tells them to do
because you didn't pay enough money for it and electrons don't take
orders from cheap stereos.
Dirt.
Dirt on your tape heads. Dust on your phono needle. To see what it
sounds like, clean it off and compare to what it sounded like before.
Mistracking.
When the needle on your phono is literally trying to jump out of the
record grooves. Most often caused by warped records, but even unwarped
records can mistrack due to the wrong stylus pressure (typically too
little). A stylus with too little pressure that is mistracking can
cause more damage to records than using more pressure. To see what it
sounds like, take a record you don't want any more, and decrease the
stylus pressure until it mistracks. Sounds somewhat similar to
clipping, but different enough so that you should be able to recognize
it.
TIM.
Sort of like the electronics of your stereo mistracking because they
can't move as fast as the sound. Mainly caused by using op amps with
too much negative feedback. Can only be heard by golden ears using
test records.
Other stuff.
Distortion caused by not using gold plated everything, and by sparing
any expense for your stereo. Very difficult to get rid of this kind of
distortion, because they will always find something else for you to
spend your money on. And just when you think you have gotten rid of
all that nasty distortion, you start to get a new kind of distortion,
that caused by creditors ringing your doorbell while you are trying to
listen to music.
Digital.
I won't try and go into this in detail. There is distortion caused by
quantization errors (the fact that volume has to change in discrete
steps). There is aliasing, and sampling errors. Most of these types
of distortion are fairly subtle. Don't worry about them until your
stereo costs more than $3000.
WHAT DISTORTION SOUNDS LIKE.
What is more important is what it doesn't sound like. Stereos are of
good enough quality, even for fairly cheap systems, that you will
probably not directly hear distortion most of the time. But distortion
makes listening to your stereo less enjoyable. Do you find your
friends asking you to turn the music up or down? Do you suddenly get
the urge to switch stations, when you are listening to a tape? Do you
usually come home from work and feel like listening to music? Stereo
systems are to enjoy. If you are not enjoying yours, it might just be
because of distortion.
Wm Leler 503/627-5151
wm.Tektronix@csnet-relay
{ucbvax|allegra|decvax|ihnp4}!tektronix!wmcsc@watmath.UUCP (Computer Sci Club) (05/22/84)
This question actually merits more than the superficial treatment it
has been getting so far. Description of sounds is rather difficult
as the terms, "muddy", "crisp", "metalic" etc. often seen on this net
are somewhat ambiguous and can be quite useless to the neophyte.
The best way to describe sounds is to give examples.
Noise: The most straightforward type of distortion, noise is additions
to the signal you are amplfying that are not related to the signal.
Noise can be heard quite easily, listen to a system with no input
and turn up the volume, or "white noise is the noise you hear when
you turn on the TV and there isn't any station". To hear noise
together with music play a tape and turn of the Dolby, (if the
deck doesn't have Dolby play a tape :-) ). Clicks and pops due
to scratches on records are another common type of noise.
Noise is generaly fairly easy to identify, you hear it as
something seperate from the music, "tape hiss" for instance.
Frequency Distortion. This occurs when some frequencies are amplified
more than others. To hear it fool around with the tone controls
on your amplifier. If you have access to an equalizer fool around
with that to hear more subtle frequency distortion.
Frequency distortion is much harder to pin down. You cannot
seperate it from the music. If you turn the bass way down or
the treble way up it is fairly clear what is happening, but
if you just tweak the controls a bit the music will sound
different but it is not clear why. It has been shown that
humans can hear quite small amounts of frequency distortion, but
that they do not always identify it as such. Frequency distortion
has been cited as the reason some people perceive a difference
between CD's and Lp's
Pitch change (mathemaically I think this is harmonic distortion but
who cares :-) ) If the pitch of the music is shifted uniformly up
or down, we have another type of distortion. A good example of
this in the extreme form is a 33 1/3 record played at 45. Small
amounts of pitch change are not annoying (or even likely to be
detected unless you have perfect pitch) if they are constant.
If the pitch change varies with time (e.g. the record speeds up
and slows down) this can be very annoying. The best way to
detect this is to play some long steady note and listen for
variation (test records and tapes have such notes for this reason).
Harmonic distortion, This is distortion that introduces frequency components
not in the original signal. An extreme example is the distortion
of an electric guitar used for effect by many groups.
In its extreme form harmonic distortion is quite easy to detect.
(The usual description is that the music sounds "fuzzy") In milder
forms it is much more difficult to hear. There is a large body
of opinion that maintains that the amount of harmonic distortion
produced by any reasonably good amplifier is quite inaudible. There
are no unanimous opinions in the audio field.
Small amounts of distortion are very difficult to hear. In the end you can
only compare to some standard, "actual" sound, very difficult to do if you
have poor audio memory. I am of the heretical if it sounds good to you
go for it school of audiophiles, who listen quite happily to our $800.00
systems with our tin ears. How low a level of distortion is acceptable
and whether some types of distortion actually sound better, can be
determined by listening to a wide variety of music on a wide variety of
systems.
William Hughes
P.S. Anyone have any other tricks for detecting small amounts of distortion
(like listening to steady tones to detect variation in pitch)