Audio, Etc. "Transient InterModulation Distortion"

Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting

Distortion! A fascinating subject both for the audio engineer and the avid listener. It is the very obverse of fi.

At last fall's AES Convention in New York, for instance, there was a memorable give-and-take at one of the morning technical sessions on a new aspect of this old subject, TIM. "TIM" for Timothy, otherwise known as Transient InterModulation. I'm not about to enter the Timothy fray nor any other of the sort, but I do approve. I also love to argue about distortions and I have my own ideas, as derived from the listener's hot seat. TIM has merely reached that point where before we can cope with it, even measure it, we have to codify and standardize and fence it in. It happens all the time and it needs to.

Hence the big arguments.

Of course audio distortions tend to occur simultaneously, all at once, in the natural audio state. But we don't work that way. We have to isolate before we can measure, and we have to discover before we can isolate. So each of the great major types of distortion; the obverse of fi, has had its big moment in the engineering limelight over these years, almost like a series of high-level fads. If they're arguing TIM right now, wasn't it Ph.D. last year? That's phase distortion. And some time before that, quite awhile, we went into a prolonged hassle over plain IM, without the T, measured by nice continuous pairs of clean sine waves. Then there was T by itself, minus IM. Big thing, back then, even if TIM would seem to be a close younger relative. Transients, mixed, all kinds, like mixed nuts.

Back at our beginnings came the granddaddy of all distortions, the old original, now shortened to HD. It was the only kind I ever heard about when I got into this biz, and my earliest power amps were proud of their low, low four or five percent harmonic distortion. Honestly, truly. And don't think it wasn't audible. Most people enjoyed the sound, not having heard anything different other than live, which didn't really count. Good, solid harmonic distortion gave you a bright, metallic sound that could cut through the soggy layers of early audio out of low-pass pickups, cactus needles, and tubby speakers minus tweet. It was good. And it sounded loud, too.

Sometimes I really miss it.

I take pleasure in the audible audio distortions because when they go away I can always hear the difference.

Also, and maybe more important, because I find to my pleasure that each and every type of electronic distortion has its fair analog in live musical sound, not as distortion at all but as part of the signal. That's interesting.

Take IM, for instance. As I see it, IM corresponds to what musicians call beats. Sure, there might be a few li'l technical differences, but have you ever played a recorder (finger holes and no cassette) right next to someone else who is also playing a recorder? What you hear, in addition to the musical notes, is a shattering series of loud buzzes and beepings, seemingly inside your head, bzzz, BZZZZZ, bheeee, Brrrr, wildly different in pitch and totally unrelated to the music.

These are sum and difference tones, which is to say, IM, but they are not distortion; they are "natural." You often hear them, too, in brass ensembles.

(Oddly, a similar effect can be electronically generated pure distortion in faulty audio playback of the same music.)

Call Me Reproducible

Transients have any number of names in music, which is overpoweringly full of them that's what we try so hard to reproduce. The now prized "chiff" of the old or Baroque organ is one transient that the 19th-century organ builders (misguidedly, according to present thought) were able to eliminate via "nicking" of the pipes, bleeding away the transients in favor of steady state. Then there is piano. I will not forget my first piano edits. Turn a piano tape backwards and you have a sort of wheeze box no transients at the beginning and a sort of subdued cough (transients in reverse) at the end. But turn it frontwards, cut off the transient "head" and you still have a piano sound. I was dumbfounded, the first time I heard this. The original transient burst is merely replaced by an electronic burst of distortion caused as the sharp edge of the steady tone goes by your tape head. But is it distortion? A question. Plenty of extant piano notes on a million different records have at least a few of these synthetic tone bursts, masquerading as perfectly good signal.

In singing and speaking there are glottal sounds, a breathy transient from down in the throat. And consonants what else is a consonant but a bundle of transients? In almost every musical instrument there are deliberately induced percussive transients, from the bounce of a violin bow on the string to the takataka tonguing of wind instruments. As for nonpitched percussion, notably the drum, the sound is a mass of transients and not much else. So we musicians are quite familiar with transients, though not as distortion.

All of which I think makes clear what we mean by distortion. We are trying to be faithful to an original, a "given," and that includes the faithful reproduction of all these sound effects. We can only define distortion, then, in terms of a prior model, and the definition of that model can be a tough problem. Oversimplification is our greatest trap, the easy way out.

The trouble with IM is that it doesn't include TIM; we get false readings when it comes to actual listening sound. On the other hand, what do you do about reproducing an "original" that is distributed over 24 separate tape tracks? Or how about the hi-fi reproduction of those electronic fuzz noises in pop music, deliberate electronic distortions (are they, though?) which must be distortionlessly reproduced! It's a slippery world, and no wonder there are arguments. I think, though, that it is reassuring to 20 know that real-life natural sound so neatly parallels the unwanted, falsely generated electronic signals which in fact do constitute our carefully studied types of distortion.

As a listener, I tend to make my own categories of audible distortions, just as I hear them out at the very end of the long audio chain. I fancy that these categories do indeed have some relationship to those of the engineers in our business that is, I am hearing what they are talking about. But as a musician I can never be sure; I only know what I like, as the music hater says.

Categorically Speaking

I have assembled for this month no less than three Canby Categories of audible distortion, and you may make what you will of them. The first, shall I say my 'first-order" category, is the kind which, if I am right, is caused by harmonics generated in the system, here, there, everywhere. It is very uncommon now, and usually unmeasurable, but it wasn't so in the past. We used to hear positively enormous quantities of this distortion back in the 78-rpm days, early through late. Vast harmonic content, variably transient, via the old shellac disc and its crude "needle" and cartridge and via many a brave new early loudspeaker. I can hear all those sounds in my head and

they weren't always so bad. Various terms come to mind, pro and con we talked about buzz, blast, grind, needle chatter, and about worn grooves and worn needles, sometimes minus their points, or the tips bent into a hook. There were cracked crystals, strained speaker cones, tired tubes, and I don't know what else the resulting sounds had a clear family relationship, from the gentlest to the most raucous. Harmonic? Mainly.

If so, that is important. Because every such distortion is directly related, for the ear, to the "fundamental" sound, the original signal. Harmonics make not only harmony but, more importantly, the whole range of audible tone color. So we were able to "read" this distortion to a remarkable extent as a kind of added coloration, often grotesque and hideous, but often relatively pleasant. Some brand-new speakers in early hi-fi were actually advertised as "golden" in sound note the coloration though this was perhaps to avoid the thought of a less flattering term, "tinny."

Yes, there must have been plenty of the other known distortions in these earlier reproduced sounds, even did not then recognize them. But, really, the biggest factor was harmonic distortion in enormously high percentages. So I would think.

I cannot say exactly how my "second-order" category of audible distortion will look in the figures, but it is certainly a very different kind of unpleasantness, a new sound, cropping up mostly via late-model advanced circuitry thou h I first heard it myself on early FM. I till do. Strong adjacent FM station tries to butt in on your music.

Sput, sputter, splat! Unmistakable and most unattractive. There is no harmonic message n this ugly sound and no coloration. It is just loud, jagged, a shattering, tearing noise that is completely destructive. I hate it. But I hear it more and ore often.

Definitely this is the ugliest of all the gross distortions, far more unpleasant than any ordinary buzzing or blasting.

If there is music in its origin, an occasional bit of musical pitch often gets through with the splat, but it is no more than a hideous gargle, not music.

Purely indigestible, this stuff, and you can give me Caruso on an acoustic portable with a broken needle any day.

A particularly jarring example of this sputtery distortion was to be heard a good while back via those brave earlier four-channel "discrete" record systems. It came at the dramatic peaks of volume and particularly via the innermost record grooves. Few of us would care to listen to any music with that sort of loud sputter coming through every so often! The CD-4 people made heroic efforts to improve, and in their later demodulators the splats were gone. But, alas, it was too late. No more. Curious that here we were also dealing with FM circuitry, via the CD-4 supersonic modulation of the groove walls. Curious, too, that none of the "matrix" decode equipment I ever tried produced this splatty sort of distortion, not even those with logic.

Probably just didn't happen to have the right elements in their circuits to … Whatever 6 my matrix y ... mild exam sat-like, came aQ dbx noiseless … before I … `mod up as to levels F e y overload, proba S' ,did emit just two istakable, before it , subsequently perfect i criticism but, en[…]nerely indicated that here, too, was a modern circuit which if forced to malfunction would give forth not a harmonic blast but a second-order splat. There must be a lot more of this around, at least in the potential. Can any reader pin it down?


I won't dwell long on my "third-order" audible distortion because, well, it gets into aesthetics. But I have to bring it up because it just happens to be our greatest concern these days the hi-fi reproduction of unwanted noise, the sounds we never asked for but get anyway. Our equipment can't tell the diff. between noise and signal, and that is the rub. We are frantically going this way and that in our search for the means to get rid of noise without getting rid of signal too. We have gone far. Autocorrelators (excellent), new metal tapes, plus hopefully better disc surfaces, Dolby, dbx, direct-to-disc, digital, maybe even some more uses of the letter "D" for all I know. Yet at home I still hear unwanted noises, just as you do. They are terrifyingly diverse we have to scatter our fire. FM stereo may give out splats but it is also still pretty hissy. Even the natural ambient sound of a concert hall or church can get to be a big problem but there we get into the aesthetics because that ambience is part of the signal, even a part of the musical effect, and therefore we must treat it with deference.

The nearer we come to true silence in our media background, the more careful we must be of all the tiny noises that are still strictly signal.

Where we now have that new silence, in all-digital recording, via dbx encoded discs, even in a few direct-to-disc releases (by no means all of them!), we suddenly find exasperating new problems thanks to the ear's uncanny ability to hear ultra-low-level residual noises once they are unmasked.

Don't, misunderstand me-I have been fascinated by my first batch of dbx noiseless discs and am certain that this is the most important new system we have seen in recent years that uses the still-viable LP record as its base. It has caused me absolutely no trouble since I got the levels of in and out adjusted properly-never a splat again.

But there are a few mighty interesting further items to be noted in connection with dbx.

For example, on a number of dbx discs from various labels, I became aware that some tape editor had repeatedly cut off the die-away reverbs at the end of the musical segments before they had quite disappeared. Now that is an old fault, seldom heard today thanks .to more aware and careful editing. So why here? I suspect that the answer is as you might guess-the noiseless disc has uncovered a new area of audible die-away where formerly the sound merged into general surface noise. Now we have to edit further down, virtually to zero. It's astonishing the way the ear can pick up these tiny faults, down in what used to be (still normally is) the proverbial mud. Caveat editor! I also noticed on the dbx discs that the problem of built-in hall ambience is not always adequately solved. It is quite normal today for a record producer to fade hall noise down to zero between movements, where there is normally a strip of leader tape inserted to fill up the time. OK-we mostly don't notice this because once again the ambient noise blends into the surface sound. Now, alas, we must rethink this problem. On a number of dbx discs I became unpleasantly aware of these between-movement breaks, because not only is the ambient sound nakedly clear-distant buses, cars, faint clunks and bumps, even maybe the ghost of a voice or two-but the sudden fade-out for the leader strip leaves you thinking your system has just died. Total, disconcerting silence.

And a clear disruption of the musical continuity.

We'll have to devise better ways, once it is established for the ear, to keep this ambient signal noise going (or fade it with care when it does have to go). In that way it is subliminal, which is the way it must be.

Well, the direct-to-disc boys and girls should be happy with this. No editing. Just one continuous play, straight through in real time, and ambience all the way. Just don't be tempted to close down your pots between numbers, you people. I'd much rather hear a few discreet coughs and a creaking chair than have to go through more of that unnerving sudden total silence in midstream. Aren't coughs and chair scrapes a part of musical performance? Is not silence, in this case, a clear distortion? You'd better start thinking so.

by Edward Tatnall Canby (adapted from Audio magazine, Feb 1980)

= = = =

Prev. | Next

Top of Page    Home

Updated: Saturday, 2017-07-22 9:12 PST