Audio, Etc. (Jun. 1973)

The CD-4 Cartridge

A great truth seems to have been intruding itself; these last months, upon my observant consciousness. I haven't cultivated it-the thing just grows of its own accord. It is that the future health of the LP disc, and in particular the idea of four channels on disc, depends beyond all other factors on the phono cartridge.

Off all things! And as if we hadn't already brought that particular mechanical/electrical transducer to an absolutely extraordinary level of performance in the long quarter century during which the' modern magnetic pickup has been on the market. That performance is already far beyond anything most of us would have imagined in the ripe old days of mono, some fifteen-odd years back, at the time stereo first loomed. In fact, at that time there was much engineering grumbling as to the disastrous effect of stereo upon the elegant simplicity of the mono cartridge, then at a very advanced stage of development.

It is the engineer's privilege, of course, to grouse about the very problems which he will soon brilliantly solve. They do it every time. A kind of protective coloration, in case things don't work out. So the mono cartridge men, grumbling hard, went to work on stereo and look what They've done. Year after year, they have gone right on improving the un-improvable. And though the remaining area for advancement would seem to get smaller and smaller, somehow it doesn't; the designers just push things out a bit further in various impossible directions. Crazy and wonderful, with absolutely no reservations on my part. I can only admire.

I am not wandering in vague generalities here. These thoughts are occasioned by recently converging factors, new and revolutionary, within the parameters of the still-potent disc format. I refer, of course, to the CD-4 "discrete" quadraphonic system and its radical new cartridge requirements.

Just the thought of them has been enough to jolt the entire cartridge industry. And not without results, CD-4 or no.

It has been conventional, for instance, to measure our components according to that linguistically pleasing formula 20 to 20 KHz that being a stylized realization of more or less realistic hi-fi needs in terms of frequency range. I have before me an individual cartridge run, for example, performed on an Audio-technica cartridge now in my phono system, which reads as a virtually flat line between those two points. At each end there is a vertical drop to zero, clonck. But the specs on the cartridge suggest something a bit different: the cartridge response is listed as "5 45000" if you will forgive the old-fashioned cycles. No cloncks.

Our measuring equipment, industry-wide, has taken on the 20-20K convention as a matter of convenience, and even art, in virtually every common display including this familiar cartridge "run", pen traced, and the assorted scope trace patterns. At 20KHz the measurements just quit. They don't tail off or fade away; they stop. The pen drops to zero. The scope trace repeats itself in a bright 20K line and goes no further. But, you see, now we are up against a drastic changeover, and CD-4 has set it off, with a vengeance! The spark that sets the fire, the straw on the camel's back? Take your choice.

It's not as though these measurements couldn't go right on out, if we so desired.

It is simply that there hasn't been much of a reason to do so in the hi-fi audio measurement area. For my non-engineer mind the reasons are, well ... reasonable. We don't communicate much of importance above 10KHz and we can't hear at all when you get up to 20KHz.

True, we do need to get a ways into the supersonic, even so, since what goes on up there, mechanically and electrically, is related to effective sound propagation lower down. So-take 20KHz as a convenient round figure, safely into the supersonic but not extremely supersonic. That's about it.

Suddenly, with CD-4, we have a drastic need to go higher, much higher, and for a new reason-a specific carrier signal, two of them in every groove, set down at the extraordinary high level of 30 KHz as a mean. Not extraordinary in general sonic terms nor in electrical, but in a standard LP record groove very extraordinary. Enough to set the jitters and titillations going all over the place. How to manage it? That is-how to play such a groove? (We'll forego discussion of the problems in cutting the same, via slow-speed turntables.) Specifically, if I have picked up the proper info, we are here directly involved in the crucial matter of resonances (so vital in the bass area when it comes to loudspeakers), and in the means whereby cartridge makers have coped with that extremely delicate and difficult factor in cartridge system response. For it seems that the resonance basically determines the response extent, no matter how many good tricks you invent to cope with the same.

Non-engineer readers should understand that, generally speaking, a point of resonance is not only a point of distortion-overly easy and exaggerated response-but also a point of no extension. For it is a physical characteristic of all sorts of resonances that frequency response, whatever it is, drops off sharply beyond resonance in a tell-tale descent. It graphs like a ski slope, fast.

Elementary, Herr Watson, he said. But most extraordinarily vital right now.

Yes, cartridge resonances may be damped, flattened down, in many ingenious ways, (as in loudspeakers), often combining electrical and mechanical factors so as to cancel out and arrive as net flat, if you see what I mean. Then the response is good and the drop-off is at least non-catastrophic. But alas, it is still there. There is no hump beforehand but the drop drops, just the same.

We can manipulate the resonance point very nicely, pushing it higher and higher via more and more delicate mechanical components; but you know where that leads. Especially for people like me who are all thumbs. The devil and the deep, blue sea.

Thus the upper resonance point has always been a main concern in cartridge design, as refinements in the mechanics have pushed it gradually upwards, eventually off the scale into supersonic inaudibility. There, it would seem to have been quite safe-for you couldn't hear it. For awhile, the state-of-the-art cartridges had resonances around 15KHz which, flattened out, gave excellent sound with minimal peaking right up to that high point, already beyond plenty of ears. In recent generations of stereo cartridge that resonance has gone even higher, tending to center near 20KHz, and the nearly flat, or flattened, response has been pulled all the way out to this level before the inevitable drop-off. Adequate, to put it mildly! Adequate, that is, for all standard LP/45 discs including every variety of matrix quadraphonic, since these are contained within the standard LP frequency spread.

The new Shure V-15 Type III cartridge, announced to the press early last spring via a most interesting seminar held at Shure's home base near Chicago, is an advanced fourth generation transducer (if we count Type II and Type II Improved, with altered stylus assembly, as two generations) which beautifully sums up this whole era of steady refinement and development in the subtleties of cartridge designing. I am purposely limiting myself to one factor-there are plenty more which, in fact, go into creating the resonance and helping to flatten it down. This super refined model boasts many new "highs" in performance, most notably its extraordinary tracking ability, beyond any of its notable predecessors. But let me note merely the wider, flatter extended frequency response in the Type III. The original V-15 had a resonance at about 15KHz, with good, if not totally flat, response up to that point, then falling off. (Something is left, as you go further up, but the "slope" is fast, down to nothing.) The Type II Improved, with improved stylus, brought this figure up to 20KHz with even better flatness, out to that super-hi point. It is a lovely cartridge. The new Type III makes use of a laminated magnetic structure and new extremely light (but strong) stylus mass to extend the resonance to 23KHz, with an astonishingly flat response straight out to this point. Putting aside the Shure "trackability" which, I assure you, is of the greatest interest and importance, you can understand that in just this one respect of flat extended frequency response we have in Type HI a splendid and consistent degree of improvement over the earlier high quality cartridges in this particular line of development. What more could one ask.

Well, it's so simple. Indeed, I went out to Chicago just to enlighten myself on this very matter. You see, the CD-4 quadraphonic disc requires an even wider frequency response, out well beyond 30KHz and flat at that point, if four-channel "discrete" sound is to be reproduced with optimumly low distortion and separation.

Flat is the key word. The cartridge that plays the CD-4 disc must reproduce a frequency modulated signal, two of them, one on each face of the groove, that is based on a 30KHz carrier, mechanically inscribed in the groove and to be traced by the cartridge. Since FM signals vary up and down in frequency to each side of the central "norm" where the carrier, so to speak, is at rest when no signal is modulated, the cartridge must reproduce both sides, above as well as below, and reproduce them equally. You cannot do that on a ski-like downslope! The upper side will be weaker, lower in level, than the down side.

Now if this super-hi pair of signals were of the normal, ordinary sort, the drop-off in the higher side would be no more than that, like a bit of supersonic tone control, a roll-off in response but nothing worse. All, of course quite inaudible, but let that pass. What we have, however, is a new sort of signal (at least for commercial discs), an FM modulation of a pair of mechanically inscribed carrier waves, right in the groove. As in FM tuners, therefore, a simple but devastating phenomenon takes over in the demodulating of those signals. An unequal reproduction of the FM carrier swing, above and below the central frequency (I am using my own lay language, unofficial) results in plain, old fashioned distortion of the resulting audio signal. Ugly, unpleasant sound. And because the CD-4 system matrixes-whoops, I'd better say "electrically mixes"-its four discrete channels for recording, then un-mixes them again in the reproduction, that distortion will end up in all four quadraphonic channels. Ugh. Not good.

So your CD-4-capable cartridge, then, must (a) be able to pick up something at 30KHz (many don't) or there will be no quadraphonics at all. Just, shall I say, duo-phonics, out of four speakers.

And even more vital, (b) the cartridge response must be reasonably flat, not sloped, in that high area, from below to well above 30KHz. Otherwise, distortion.

As a matter of fact, better no response at all. You can play a CD-4 record in stereo with any cartridge, of course.

It ignores the carrier signals. They don't register (though they may perhaps invoke complications in your stylus motion-that is a moot point). You can play your cheap standard cartridge through a CD-4 demodulator and get quite a pleasant sound, providing the cartridge is totally insensitive at 30KHz.

The regular pair of groove signals comes through and is nicely distributed in, as I say, four-way duo-phonic stereo. No 30KHz response, no distortion! But if your cartridge has some response on the lower side of 30KHz, and less response on the upper side, then you will have yourself a problem.

A dose of muddy sound, together with the discrete four-way separation. So it goes, as well as I can gather, a rank non-engineer.

We may now bounce back to the new cartridges and their present and future characteristics. First, you are asking me whether all this really matters. I.e., will there be lots and lots of interesting CD-4 discs around to get played by our cartridges? I know no more than the next "expert" but I can put it safely this way.

There is not the slightest chance that the CD-4 disc, from RCA and other companies, will not be available in the near future. (Nice double negative.) Nobody has to play them, but many will want to, and in full four-way "discrete" sound. Whether the long-absent decoding equipment will also be around in force, soon, is another story but let that pass, too. We're talking about cartridges. Any way you look at it, CD-4 is now a factor and will be, conservatively speaking, at least in the nearby future. (Some say it is dead already; others say it is on the verge of triumph. I say merely that it is here.) And so we will need new cartridges, in all ranges of excellence and cost, developed or modified to meet the new requirements. Even the junkiest record player that claims to play discrete discs in quadraphony will have to cope somehow with that elusive little pair of super-signals in the new groove walls.

Now you are asking me what Shure had in mind in respect to its V-15 Type III, which I am "shure" is going to prove to be one of the best top-of the-line American made cartridges. I will have you know that I am not on the Shure staff, and cannot speak for Shure.

I will also say, on the side, that Shure really has me baffled. The Type III, by their own specs and charts, is 'way down at 30KHz and sloping off like a ski jump, too. I do not see how it can be expected to play a CD-4 disc quadraphonically, though it should do an absolutely superb job on any other type of disc, whatever. So that is that, for all to see and hear. Why? Strictly on my own, I would suggest several speculative possibilities (and I have not consulted Shure, aside from the company's own amply provided seminar and publicity material).

1. Perhaps Shure has deliberately put the CD-4 disc aside as unlikely to be an important factor in the market for their Type III cartridge. Possible. Reasonable from some points of view. CBS and Sony would agree. This viewpoint may be right. Or it may be wrong. My own feeling, cautiously, is to disagree.

2. On the other hand, perhaps Shure has ready some modifications of its V-15 Type III which could quickly extend its range to accommodate the 30KHz carriers with top quality (flat) response? Could be! But they have not said so. Somehow, with all those problems of resonance and the known exhibits of Shure's V 15s to date, moving up from 15KHz to a high of 23KHz through such beautifully calculated refinements in all parameters-and especially in trackability-I wonder whether a sudden big jump, say flat out to 40 or 45KHz before rolling off, would so quickly be possible? But this is really beyond my competence even for guessing. And if possible, why wait? Why bother to make two Type III models when one would do? Dunt esk, as Archie the cockroach used to say.

3. This third possibility seems most unlikely. Perhaps Shure, like many big companies, is so immersed in its own dynamic work that it doesn't accurately evaluate what is going on outside. After all, research on the Type III must have begun years back, before the CD-4 disc was announced. Maybe, just maybe, that methodical development has simply gone forward without fail, all the way to its triumphant conclusion, without any influence at all via this new need for a 45KHz response? Could be. But I don't believe it.

So I really am baffled. What would you do, if in mid-development of a splendid cartridge like this Type III a sudden new emergency declared itself? Give up, in the face of CD-4, and start all over again? Modify your design, perhaps with considerable anguish and compromise, to accommodate the new possibility? Develop alternative models, one with CD-4 capability and another (very likely a better cartridge under the circumstances) without? Believe me, these are tough questions involving very difficult technical decisions, and all the cartridge makers are having to go through the process. I do not think any of us, at least outside the cartridge engineer's own domain (and maybe not even there), can know just what has been in Shure's mind. My best guess, is sheer honesty and integrity.

I should suppose that Shure was unwilling to compromise a superb design, so carefully matured over the years, by any sort of sudden, drastic alteration. I would have to agree. To find out for yourself how it is, just go out and look and listen to this Type III cartridge, and compare its performance with that of its own predecessors in the line. Not even CD-4 should be allowed to get in the way of this beauty. Maybe it's as simple as that.

It remains, then, to look around briefly elsewhere-not everywhere, since this is not a cartridge survey. A year and more ago, for instance, I witnessed a Stanton Magnetics demo at which the slogan, if I remember, was something like " Stanton is ready". There were two phono set-ups on hand. One used a regular Stanton cartridge. The other was what looked like an identical model. But it was designed to play CD-4 discs. Not in production, that one. But you can see how the thinking went chez Stanton, back then.

And now I have me an Audio-technica cartridge (a Japanese line which briefly bore the Electro-Voice name awhile back, and now is launched on its own). Mine is the next-to-top in a complete line in which four out of eight models are advertised as having a frequency response out to 45KHz, specifically to play the CD-4 discs.

(The very top model is derived from the next-to-top by special quality control and selection.) Moreover, the top three cartridges have a nude-mounted Shibata stylus, some form of which looks to be increasingly necessary to track the tiny CD-4 modulations without wiping them off or rubbing them down. (The Shibata bears on a much larger area of the groove than the elliptical stylus and thus the pressure at any one point, for a given stylus "weight", is considerably reduced.) Here is clearly an all-out design effort to meet the new requirements head-on. It will not be the last, needless to say.

I removed my long-time Shure V-15 Type II and in a few moments had the Audio-technica AT15S playing, right in the same old arm. It is a pleasure to be able to switch cartridges so easily today, in comparison with the bad old times when non-standard connections, dribbly solder and melted plastic, hopelessly differing weights, screws, mounting sizes and shapes, made life miserable for the cartridge user. It is also a pleasure to report that in no gross respect did I notice any dramatic difference in sound. We are now close enough to total fidelity so that cartridge differences are micro and minute, if important (notably, again, tracking ability), and the over-all sound of all good cartridges is gratifyingly the sound of our records.

No-I wasn't even demodulating CD-4 discs! Just playing them through an SQ decoder into pseudo-quadraphonics.

Also playing SQ discs, per se, plus regular stereo, mono and everything else at hand, all via Shibata and wide range Audio-technica, flat to maybe 45KHz, right there in my system. No quirks, no freakishness, no problems.

Play everything. (No problems yet, anyhow.) Hopefully, this is a good taste of what we could have, once the changeover to a wider-range cartridge and modified stylus tip makes for complete playing compatibility for all systems, as far as the cartridge goes. I like the idea.

How was this cartridge designed? Gotta be careful-I'm not a cartridge engineer. But it gets its performance basically via a dual magnet assembly, one for each side of the groove, and less mass, hence (I'm assuming) a higher resonance and extended response.

In any case, it has the response, and the sound is good. No tests. Not me.

Except that I'll be trying out Audio-technica with some glee on Shure's horrendous new set of testing discs, designed to wring the withers out of anything less sturdy than the Shure Type III-including the Type II, as I heard and saw for myself at Chicago.

That'll be fun, you may be Shure.

After all, frequency response isn't the first parameter in a fine cartridge. It lust happens to be very important at the moment, courtesy of CD-4.

* * * * * * * * * As usual in my writing, this has been an attempt to put an evaluation on a new factor in the hi-fi scene, without necessarily taking sides, which I haven't.

There are other ways, combinations of ways, to achieve a CD-4 type frequency response, even in lower-priced cartridges.

Why not? We have an extraordinary cumulative cartridge know-how at this date. And here is the sort of problem where, given the impetus, we are supremely able to make acute, quick improvements, towards a specific end.

I see no reason why we should not soon have a complete range of CD-4 capable cartridges, all the way up and down. Starting of course in the top brackets, which is merely normal. (Remember the Fairchild stereo SM-I and the SM-2? The first stereo-capable cartridges on the market.) But from thence, rapidly descending, with all the normal grades of performance, down to the very bottom, short of sheer nothing. It's surely about to happen, and in awhile the new models should wholly supplant the old, just as the stereo cartridge took over from the mono to play everything available. All this, surely, unless the CD-4 dies tomorrow morning.

Suppose it did? Or next year. Do you think that the world of cartridge design would remain unaffected? My feeling is that the mere existence of the possibility of commercial discs with a 30KHz carrier has long since had an immense stirring-up influence on cartridge design.

This is the sort of challenge that gets the designers' best blood up. So things will not be the same again. Whether CD-4 is alive or dead. From the cartridge viewpoint it really doesn't matter.

(Audio magazine, Jun. 1973; Edward Tatnall Canby)

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