WORD OF THE WEEK: Distortion
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Gold Member Username: GlasswolfWisteria, Lane USA Post Number: 8219 Registered: Dec-03 | I think I'm going to try a new little feature around here. Each week I'm going to pick a new mobile audio related word, and define it. Many of you will know the words already I'm sure. Some of you may not know the words, and this will be your chance to learn them without having to ask and feel embarrassed. We'll see how it goes. Let's start with Distortion. Distortion ALL amplifiers alter input signals, generally in two ways: they make them stronger (amplify them), and they add characteristics which did not exist in the original signal. These undesirable characteristics are lumped together and called distortion. Noise can be considered a type of distortion and was discussed in the above section. Everyone is familiar with gross distortion, the sound quality that results when turning up a radio or boom box to "full blast". An excessive amount of amplifier clipping (see section below) results in hideous distortion that would be totally unsatisfactory for a sound system. However, not all distortion is blatant. In addition, there are several types, two of which will be discussed. Knowing what causes distortion will help you to prevent it from occurring. Knowing how to control distortion is important because excessive distortion can be detrimental to speaker systems (and your reputation). Harmonic distortion: One common type of distortion is harmonic distortion. Harmonics of a signal are signals which are related to the original (or fundamental) by an integer (non decimal) number. A pure tone signal has no harmonics; it consists of only one single frequency. If 100 Hz pure tone signal was applied to the input of an amplifier, we would (upon measurement with special test equipment) find that the output signal of the amplifier was no longer pure. Careful measurements would likely show that several "new" frequencies have appeared. These new frequencies are almost certain to be integer multiples of the original tone; they are the harmonics of the original signal. In the case of a 100 Hz input tone, we might expect to find tones at 200, 300, 400, 500 (etc.) Hz. We would also probably notice that the odd harmonics are much stronger than the even harmonics (we will not go into the reasons why in this article). In a good amplifier, the harmonics will be much weaker than the original tone. By much weaker, we mean on the order of a thousand times for decent amplifiers. All amplifiers are generally rated for Total Harmonic Distortion (or THD), usually at full power output over a given frequency band with a particular load. Good values are anything less than 0.5% THD. When an amplifier is measured for THD, a pure tone is applied to the input and the output is measured with special test equipment. The energy of the pure tone is measured, and the energy of the harmonics is measured. Those two values are compared, and a THD rating is calculated. A THD rating of 1% means that the total energy of all the harmonics combined is one one-hundredth of the energy in the fundamental. Harmonic distortion (although certainly undesirable) is one of the more tolerable types of distortion as long as it is kept reasonably low. Distortion levels of 10% may be very tolerable with music so long as the 10% level is only "occasional." (10% THD on a pure tone can easily be heard by the human ear... but who listens to pure tones?) The reason that a seemingly high value of THD is acceptable for music is partially because many sounds in nature are rich in harmonics. Also, most decent cassette decks (which most people agree sound pretty good) have THD (off the tape that is) of several percent. Worse, even good speakers can have THD up to 10%, especially at low frequencies! All in all, the human ear can tolerate a fair amount of THD before it becomes objectionable. Do two amplifiers with identical THD ratings sound the same, everything else being equal? Not necessarily (but differences will be subtle). The reason is that the THD specification states nothing about where the harmonics are in the frequency band. For example one amplifier could have a dominant harmonic at one frequency and a second amplifier could have a dominant harmonic at a very different frequency. Or, one amplifier could have a few "big" harmonics while a second has many weak ones. These situations could easily result in identical THD ratings. The variations could be easily measured with laboratory equipment. However do not be overly concerned. Minor variations in THD ratings will not cause major differences in sound when listening to music. With pure tones as input signals it might be fairly easy to discern which of two amplifiers was used (but again, who listens to tones?) Intermodulation distortion: Intermodulation distortion is the second "major" type of distortion that is often specified for amplifiers. Intermodulation distortion is much more objectionable to the human ear because it generates non-harmonically related "extra" signals which were not present in the original. It is analogous to someone singing way off key in a choral group Intermodulation distortion (sometimes abbreviated IM) is more complicated to test for and specify. Basically, two pure tones are simultaneously applied to the input of the amplifier. If the amplifier were perfect, the two tones (and only the two tones)would be present at the amplifier output. In the real world, the amplifier would have some harmonic distortion (as described above), but careful observation of the output signal (using laboratory equipment) would reveal that there are a number of new tones present which cannot be accounted for as a result of harmonic distortion. These "new" tones are called "beat products" or "sum and difference" frequencies, and are a result of the interaction of the two pure tones within the amplifier. No amplifier is perfect, all have some non linear characteristics. Whenever two signals are applied to a nonlinear system, new signals (in addition to the original two) are generated. For a good amplifier, the new signals are very small in relation to the two original tones. This is fortunate, since the ear can detect much lower levels of intermodulation distortion as compared to harmonic distortion. It should be noted that distortion measurements on amplifiers are made with test tones. These tones are usually sine waves (pure tones), which represent the simplest possible test signal to measure and quantify. A music signal is an extremely complicated waveform consisting of many constantly changing sine waves. Since music has so many harmonics and frequencies present, quantifying how two different amplifiers will sound by using simple THD and IM specifications is extremely difficult. In other words, just because two amplifiers have the same published specs for THD and IM does not mean that they are equivalent. Fully and completely quantifying the technical performance of an amplifier would be extremely complicated and costly (and would probably have little benefit in the end). Most amplifiers available today (from reputable manufacturers) have THD and IM levels low enough to yield excellent performance (so long as they are not overdriven). |
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Silver Member Username: Big_osoPost Number: 168 Registered: Jun-05 | Another great post glass very detailed now i hope people might use the search before asking questions when it comes to this subject |
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Gold Member Username: James1115Wilton, Ct Post Number: 3597 Registered: Dec-04 | I am a big fan of this glass  good idea, and very very informative. |
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Gold Member Username: CarguyPost Number: 3587 Registered: Nov-04 | Good info. I hope you didn't had to type all that. |
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Gold Member Username: InsearchofbassPost Number: 3338 Registered: Jun-04 | yes a very good idea |
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Gold Member Username: James1115Wilton, Ct Post Number: 3602 Registered: Dec-04 | I am pretty sure that was already up on his web site. atleast for his sake I hope it was. |
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New member Username: CstmsamiCrystal lake , Il United states Post Number: 8 Registered: Jun-05 | cant wait till next weeks |
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Silver Member Username: Touche6784USA Post Number: 491 Registered: Nov-04 | i thought distortion applied to any amp? |
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Silver Member Username: HellbenderLos Angeles, California USA Post Number: 124 Registered: Apr-05 | Great Idea, thanks glasswolf! |
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Silver Member Username: Dominirica012Charlotte, North Carolina US Post Number: 391 Registered: May-05 | good stuff...... |
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Silver Member Username: LowdoseSt. Louis, Mo Post Number: 111 Registered: Apr-05 | Very Good Idea Glass Props bro. |
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Silver Member Username: Mikechec9Chicago/atlanta Post Number: 401 Registered: May-05 | something to look forward to each week. like peewee's playhouse, lol (probably showin my age w/that one). anyway, good idea! Glass, what is a decent way to simulate the 2nd and 3rd harmonic distortion that used to be a result of analog tapes (or is that gone for good)? I ask because most people (including myself) found these rather pleasing. you could actually feel them, not necessarily accurately perceive them with your ear. in my opinion, it is like a tube preamp, vs regular digital playback. they both kind of warmed things up. just wondering because i'm almost done w/my install. |
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Bronze Member Username: WinnSanta Ana, California US Post Number: 78 Registered: Jun-05 | a great idea & help for this forum.. Thanks Glass |
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WC Unregistered guest | Glass you should take over this forum lol. whoever is running this forum isnt doing a good job. or everybody move to glasswolf website and leave this one bhind to all the dumb@sses who wanna post under stupid names and talk sh!t. All in favor for glasswolf taking over say AYE!!! |
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Gold Member Username: GlasswolfWisteria, Lane USA Post Number: 8233 Registered: Dec-03 | I have wav files with distortion demonstrated on them, but aside from that, to get a clear idea of isolated distortion, you'd need to generate it. otherwise usually you'e talking about coloration that's a byproduct of an enclosure design and hard to isolate for identifying. for the record, distortion applies to anything raelly.. speakers, amplifiers, and even components for signal processing. speakers produce a lot of distortion on their own, but generally when we discuss distortion, it's in relation to amplifiers, which if it gets out of hand, is most noticeable, and can cause the most damage.. mainly due to the fact that any distortion is getting amplified hundreds of times so it's going to be more audible. yeah that one came from my site.. I was in a hurry. some may not be that in depth. anyay, |
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bigyellowtaxi Unregistered guest | AYE!!! Glasswolf take over this Bit*h.lol |
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