+ Nakamichi Cassette Equalization: The Standard View

Frequency (Hz) Whenwe introduced the Nakamichi 1000 in 1973, we took the audiophile world by surprise. No one had believed that a "three-head'' cassette deck was feasible; no one had envisioned a cassette recorder capable of such extraordinary bandwidth. Shortly thereafter, a rumor began to spread that Nakamichi acheived its remarkable results by adopting a "non-standard'~qualization-that cas- settes recorded on a Nakamichi deck were "incompatible" with those recorded on competing equipment. To correct this misconception, Nakamichi issued Technical Bulletin 2 on the subject of Playback Equalization. In that paper, we demonstrated quite clearly that Nakamichi decks adhered precisely to the Philips standard (4th revision, October 1968) and that the majority of test tapes then on the market, in fact, failed to meet the requirements of the standard. (Reprints of this Technical Bulletin are available upon request.) Despite this effort to "clear the air many audiophiles persisted in the erroneous assumption that Nakamichi cassette recorders were "non-standard:' Recently, several excellent articles have appeared on the subject of "standard" cassette equali- zation, and we have reprinted two of these in this brochure. But the impetus for the brochure came from a Letter to the Editor of Modern Recording & Music from a reader concerned about the compatibility of Nakamichi recorders. We were graciously offered a chance to reply, a reprint of which also is included herein. Interestingly, we soon received a copy of a letter from a leading tape mariufacturer independently substantiating the points we had made and confirming our contention regarding early test tapes. This too is included in the brochure. We hope that a careful perusal of the articles and letters reprinted herein will establish once and for all that Nakamichi recorders are now, and always have been, in perfect compliance with international cassette-equalization standards, that tapes recorded on a Nakamichi deck are play- back compatible on any other deck adhering to international standards, and that tapes created on anv "standard" recorder will be reproduced perfectly on a Nakamichi deck.

Reprinted from January 1982

"Talkback" questions are answered The Flux is the Crux the world; the one that applies to by professional engineers, many of I have heard that Nakamichi uses a dif- cassette recording is Publication 94. whose names you have probably seen ferent type of equalization for record- Publication 94 specifies the stan- listed on the credits of major pop ing than other deck manufacturers, dard recording curve in terms of the albums. Their techniques are their own and therefore tapes made on their short-circuit flux on tape as a function and might very well differ from anoth- machines are not entirely compatible of frequency. In theory, the short- er's. Thus, an answer in "Talkback" is with other machines. To the best of circuit flux can be determined by certainly not necessarily the last word. your knowledge, is this true? measuring the voltage developed We welcome all questions on the sub- -Steve Riley across the terminals of an ideal ject of recording, although the large Pocatello, Idaho playback head. Please note that it is volume of questions received precludes the recorded flux level that is our being able to answer them all. If you Thank you for this opportunity to specified-not the playback equaliza- feel that we are skirting any issues, fire clarify a common misconception regar- tion-for here is the crux of the a letter off to the editor right away. ding Nakamichi's adherence to stan- misunderstanding. "Talkback" is the Modern Recording dards. The question of what con- If the ideal playback head existed, it & Music reader's technical forum. stitutes "standard equalization" is would only be necessary to provide really quite simple-one carefully electrical equalization with a 1-7- reads and adheres to published stan- frequency break point of 3180 ps u dards. The IEC (International Elec- high-frequency break point of 120 ps trotechnical Commission) publications (for Type-I tape) and 70 ps (for Type-11, are the accepted standards throughout 111, or IV tape). Unfortunately, the ideal playback head does not exist. The magnetic gap length and the actual accordance with IEC standards, a lain differences between a "real" and tape-to-head separation. Since "work Nakamichi will have a flat bdn "ideal" playback head stem from hardening" of the magnetic material response. the finite polepiece length and the prevents the true magnetic pole from We are very sensitive to this point finite gap length of the real head, the being actually at the surface of the because some have suggested that magnetic losses in the core and the head, it is imperative that the head be Nakamichi recorders are "non- electrical ones in the windings, and the fabricated in such a way as to minimize standard" and implied that we have in less-than-perfect contact between damaging the magnetic material and some way "cheated" in order to polepiece and tape. thus losing control over where the ef- achieve the response for which we are Polepiece length affects response fective magnetic pole is located. famous. Quite the contrary; we have primarily at very low frequencies; it Play-head losses can be determined always adhered to the letter of the produces the so-called "contour effect" quite accdrately if proper care is taken standard. Actually, as play-head otherwise known as "head bumps." in the fabrication of the head. Knowing technology improves, we find several The other differences between the real the losses, one can compensate for competitive decks meet IEC standards and the ideal affect the high-frequency them in the playback electronics and so at least to as high a frequency as portion of the spectrum. The head's produce the same effect as if one had typical test tapes extend. surface finish has a major impact on started with the ideal head specified by -Ken Ohba "spacing loss" which is most severe at the standard. In fact, to be in com- Marketing Manager short wavelengths (high frequencies). pliance with the standard, one must Nakam~chlU.S.A. Corp., "Gap loss" comes into play as the compensate for the playback head 1101 Colorado Ave., recorded wavelength begins to ap- losses for the standard specifies record- Santa Monica, CA 90401 proach the effective magnetic length of ed flux as seen by aq ideal playback the gap. Similarly, the losses caused by head. It does not specify playback the head's finite electrical inductance equalization. If you think about it, this [Mr. Ohba also included a copy of are most severe at high frequencies, makes a great deal of sense. It is the Nakamichi Technical Bulletin, No. 2, and, in general, magnetic losses in the magnetic recording that is taken from which deals with the topic of Play back core also increase with frequency. For- machine to machine, and therefore it is Equalization in detail. We, of course, tunately, it is possible to either the recording that must be standardiz- were unable to reprint it here, but calculate or empirically determine ed. Playback equalizers do not hop copies of the Technical Bulletin are rany of these losses. For example, from deck to deck and it would be available free of charge from &re and winding losses are easily rather foolish to standardize them in- Nakamichi. Write for Bulletin No. 2 to determined by forcing an appropriate dependently from the playback head Nakamichi, 1101 Colorado Ave., Santa current through the windings with the with which they are used. Monica, California 90401, or call head connected to the playback Compensating for play-head losses 21 3-451-5901.I amplifier. The difference between the requires substantial additional cir- ideal response and the measured cuitry; it also requires carefully con- Reprinted by permission of response establishes the losses involv- trolled head fabrication so that the Modern Recording and Music, ed. Gap and spacing losses are readily compensation works. Thus it is not sur- copyright 1982 Cowan Publ~shing calculated if one knows the true prising that many less expensive decks avoid this complexity. It is not dif- ficult- to convince oneself that one is in compliance with standards merely by adopting a 70 or 120 microsend elec- trical playback equalization, and one can find test tapes whose high frequen- cies are boosted beyond standard level to confirm one's delusion. On such tapes, a properly equalized deck such as a Nakamichi will appear to have too hot a high end. On a tape recorded in BASF Systems Corporation

Crosby Drive Bedford, MA 01 730 Tel: (617) 271-4000 TLX: 6817069 TWX: 710-326-0456 December 16, 1981

Ms. Pam Highton, Managing Editor Modern Recording and Music 14 Vanderventer Avenue Port Washington, NY 11050 Dear Pam,

I read the letter in the December "Talk Back" column written by Mr. Ohba of Nakamichi Research, and I thought that I could shed some light on the playback equalization controversy. I spoke to Mr. Ohba about this matter, and we both agreed that information from a calibration tape manufac- turer might clarify the issue. Mr. Ohba's thorough description of the equalization process is accurate & in every detail. An interesting point that might not be clear, however, is that the only way to measure magnetic flux on a tape is to measure the voltage induced across a head. When the German DIN standards established the 120-ps calibration tape standard, BASF and Philips used the best ferrite heads available at the time (mid-60's) as reference heads. It is always an uncomfortable fact that the time for initial standards is also the time when little information is available and equipment is relatively crude. When the cassette came of age, vastly improved heads, especially the Nakamichi crystalloy head with its incredibly small gap, showed how accurate the original reference head was. The calibration standard had too much high frequency compensation added. The new heads could better resolve the short wavelength flux and produced a rising high frequency response. In 1974 DIN decided to reduce the level of short wavelength flux on the ca1 ibration tapes but remain close to the original but technically "wrong" standard in order to maintain compatibility. DIN also made several other minor changes over the years, but Japan was never fully informed about what was happening in Europe. A great deal of confusion and misunderstanding arose from the lack of technical communication. What everyone needed was communication and cooperation on an international basis. The IEC (International Electrotechnical Commission) was established to provide a forum and to set "the accepted standards throughout the world," as Mr. Ohba points out. BASF ~~~TEAcworked together on the question of calibration accuracy and compatibility, and both companies manufacture the IEC calibration tapes used to align the heads and playback amplifiers of cassette recorders for flat frequency response at both 120-ps and 70-JJS equal ization. Playback EQ can be a complicated matter because mechanical azimuth misalignment can easily disguise the electrical accuracy of the tape and the amp1 ifier. Nakamichi 's abi 1 i ty to resolve incredibly short wave- lengths for extended high frequency response is due to the design and finish of heads with extremely small playback gaps and not to "tricks" with equal ization. The IEC calibration standard manufactured by BASF will &d show flat frequency response on all Nakamichi recorders produced for the last few years. This compatibility assures complete compatibility with all other recorders adhering to international standards.

Terence D. 0' Kel ly, Manager Technical Marketing Services TDOK: sen1 cc K. Ohba, Nakamichi Corporation Opinion and comment on the changing audio scene by Robert Long

of some of their specifics. (In fact, a tape- essarlly pass the heads in a perfectly The High test standard is largely complete.) The straight line between tape guides, and its IEC's standard heads, In particular, have "bending" can introduce azimuth skew. become obsolete, and so have the tapes Nor IS the magnetic azimuth of a tape head Price of based on them. If you measure a current necessarily dead straight along the center- Nakam~chideck wlth a current test tape, line of the gap So achieving perpendicular- Progress you'll probably find a perfectly flat hlgh ity is rather like trying to draw a square box end. That's because today's narrow-gap with no aids except a rubber T-square and a EVERYTIME I HEAR the ~tatl~ticlan'~term playback heads (~ncludingNakamlchl's) French curve. "standard devlat~on" it strikes me afresh display l~ttleif any hlgh-frequency loss HIGHFIDELITY used to measure play- that there's something vaguely absurd wlthin the audio band, so nelther the deck's back response-and, indirectly, azimuth- about ~t.If deviation 1s a departure from electronics nor the test tape need compen- with Philips test tapes. We found them (like some sort of standard course or condition, sate substantially for such losses; both used all brands, to qome extent) a little inconsis- how can a deviation be standard? A soph- to compensate for the some loss and thus tent from sample to sample. But when we istry, perhaps, but disturbingly simllar to turned it lnto a gain-which explalns the changed to the TDK test tape to get its the situation in which the cassette medium greater reach into the high-frequency range finds itself, where some standards are hon- Improvement and and ~tsmodem bass equalization (the Phil- ored more in the deviatlon than In the adher- ips tapes follow an older standard and hence ence and others suffer from a multiplicity of standardization of represent yet another example of "standard references, each deviating from the other. deviatlon"), we found still larger inconsis- We have, for example, received a few the ca tencies between results wlth the two brands letters citing what the wrlters called the medium are than we had with our various samples of the "nonstandardness" of Nakamichi decks Philips tapes. And other tests suggested that and pointlng to the International Electro- necessary but ne~therbrand would match the results with technical Comm~ssionstandards and to a Teac test tape. Of course, we might have playback-response test tapes based on those inhibit each other. come to different conclusions with different standards to "prove" their point. And lf rismg high end in the measured response of samples of these same test-tape brands. but you measure an old Nakamichi deck with early Nakamichi decks. the polnt remains that there is no unanimity an old test tape, you may well find that the One more example of how standards of azimuth among quality brands and hence response curve turns upward at the extreme can trip us up, and then I'll get down to my no standard-de jure or ds facto-for azi- high end. Why? Because Nakamichi fol- real polnt. If you look at a lot of our cas- muth adjustment! lowed the IEC standard, that's why sette-deck playback-response curves, All of which may sound like an elab- How can this be, you ask. Well, the you've surely noticed many that turn down orate way of introducing a plea for compre- IEC wrote its primary specification in terms instead of up at the high-frequency end. hensive, comprehensible, cast-iron stan- of flux density on the tape and playback- The reason usually is a "disagreement" dards for the cassette medium. Well, yes equalization time-constants. So far so over cassette head azimuth. In fact, if the and no. In the scant decade that has passed good. But because there was no way of response begins to sag at frequencies below since the cassette became a serious contend- measuring flux dens~tiesdirectly (and still 8 or 10 kHz in any deck that is above the er for grace in the high fidelity firmament, isn't for practical purposes), the IEC spec- budget-price level, you can be fairly certain there have been many calls for more (or lfied elsewhere what heads would be con- that its azimuth does not match that of the more useful) standards. There have also sidered standard for playing back (and thus test tape; if the recordiplay response curves been complaints that the restrictions Phllips measuring indirectly) recorded flux densi- all stand up well to hlgher frequencies, the incorporated into its licensing agreement ties. And in order to be "standard," test point can be considered proven. were barriers to progress in the cassette for- tapes were devised to give flat results with Azimuth is the effective perpendicu- mat. Would the immense changes that have the existing heads. Meanwhile, however, larlty of the head gap to the tape path. If the taken place in the last ten years have been Nakamichi had calculated head behavior playback head is rotated out of perpendicu- possible with more comprehensive stan- for the standard flux densities and time- larity, one edge of the gap will read the dards? I tend to think not-not entirely, at constants and was producing decks whose recorded signal a little ahead of the other; as least. Yet obviously standards are needed performance was flat on that basis. Because recorded frequency rises-and recorded and, in some areas, even overdue. the calculations took head-gap losses into wavelength consequently shrmks-some This dichotomy between the radical account and the progression by which the frequency eventually will be reached where and the conservative is a fascinating para- test tapes were arrived at didn't, there was a the trough of the waveform is being read at dox. Extremism in either direction exacts a discrepancy between the end results. Yet one edge of the track while the peak is bemg heavy toll; improvement and standardiza- each was "standard!" read at the other, canceiling each other and tion inhibit each other, yet each is a neces- There's actually a lot more to rt than reducing output. It sounds as though the sity to a healthy and viable cassette medl- this resume suggests. Addenda to the orig- cure 1s simple: Just make sure that all head um. The industry must continue to try to inal IEC documents tend to confound any gaps are perfectly perpendicular to the tape write "perfect" standards, but while one attempt to derive a clear picture of what the path. But it's not that easy. Depending on hand is codifying the past the other must standards say, and time has made nonsense the drive mechanics, the tape doesn't nec- always be reaching for the future. HF

1 I Reprinted by permission of HlGH FIDELITY, February 1982, A Publication of ABC Leisure Magazines, Inc. AuGho~,d~~ b

A Dempa Publication December 1981 Cassette equalization: ~echnicalside the standard view With audio as a world wide business, the interchangeability of cassettes is ever more vital By Ed Foster Considering the number of years cas- promulgate standards - the IEEE and the main playback losses are caused by settes have been around, it might be EIA (into which the old IHF has been the finite length of the play head gap, surprising to hear that cassette equali- merged) in the U.S., and the EIAJ in the unavoidable separation between zation is not as straightforward a prop- Japan. Last, but certainly not least, the tape and the play head poletips, the osition as meets the eye. It would be there is the IEC - the International magnetic losses in the play head core. easy to explain the confusion on an Electrotechnical Committee - which and whatever electrical losses might East-vs.-West basis, in that the cas- has representatives from both Western occur in the interface between the in- sette was originally a European devel- and Comnlunist countries and sccks to ductive head winding and the pream- opment (invented by Philips of the establish inrrrrzarional norms. Consid- plifier. There arc two other play head Netherlands). But that would be a ering the world-wide popularity of the peculiarities: the so-called "contour rather simplistic view; Japanese manu- cassette recorder, it would appear that effect" produced by the finite pole- %U facturers do not all agree with each this is the organization to turn to at piece length used in the play head (and other, much less with the Europeans. least insofar as interchangeability of resulting in "head bumps" or irregu- One might also point out that the cas- software is concerned. larities in low-frequency response): sette was originally envisioned as a The establishment of an equalization and the 6dBioctave rising response dictation means, and for that purpose. standard is simply a means of insuring (for constant flux level on tape) that is standardization of equalization is less interchangeability of cassettes. As characteristic of all rate-of-change-of- important than in its present use as a long as a cassette is recorded and re- flux-sensitive devices such as the nor- high-fidelity music medium. But, produced on the same machine, it re- mal play back head. again, that explanation is weak, for ally makes no difference which equali- On the recording side of the ledger, seldom has any company ruled its li- zation is used. For that matter. track there are core losses and separation censees more thoroughly than has width, track placement, and azimuth losses too. The size and shape of the Philips in such things as tape speed, need only be standardized so that a gap also plays a role in establishing track width and location, and the phys- tape recorded on one machine can be how deeply into the magnetic coating ical characteristics of the cassette it- played properly on another. But in or- the tape is recorded and how sharply self. In the final analysis, it is not in1- der to insure interchangeability of soft- defined the "critical recording zone" portant why there are differences but ware, the reproducer must have heads is. In conjunction with the tape formu- only that the differences exist. whose track width. placement. and az- lation itself and the choice of bias imuth angle match those of the deck on level, these determine the relative Alphabet standards which the tape was recorded. Sin~i- strength of short-wavelength (high- There are many organizations whose larly, to reproduce the tape with the frequency) information compared with purpose it is to standardize either correct frequency response, the play- long-wavelength (low-frequency) in- methods of measurement or perfor- back equalization must complement formation. mance (or both). or to establish means the recording equalization so that, to- by which compatibility among equip- gether. they compensate for recording Record or playback equalization? ment is ensured. Most major countries and playback losses. Insofar as the playback losses are, in r have national standards organizations. What are those losses'? Ignoring the the main, caused by imperfections in L such as ANSI in this country, DIN in loss due to azimuth misalignment the playback head and its interface Germany, and the CCIR in France. (which, at least theoretically, with the playback electronics, it would Then there arc manufacturers' organi- shouldn't occur if both record and sccm logical that compensation for zations and engineering institutes that playback heads are properly aligned). these losses should take place in the playback electronics and the losses tance (read, no core losses) and is in and, for internal use, created a fre- should not be "pre-compensated" for intimate contact with the surface of the quency response test tape. Early Am- in the recording equalization. Simi- tape (read, no spacing loss) over an in- pex recorders used a playback head larly, it would seem sensible to correct finite length (read, no contour ef- with a 250 microinch gap, and its test for recording losses in the recording fect)." It then goes on to specify the tape was designed to compensate for equalization so that, despite imperfec- recorded tape flux characteristic in that playback gap. Ampex then was in- tions in the recording head, each re- terms of the short-circuit tape flux duced to sell these tapes. corder produces as theoretically per- versus frequency as the result of the Everything went swimmingIy until fect a recording as is possible. In this combination of two curves, i.e. the Ampex adopted narrower playback way, a designer could choose whatever 3180 and 120 (or 70) microsecond gaps (100 microinches) in order to ex- bias field he deemed most appropriate curves that we have always used. tend the bandwidth of the deck. Tested and use whatever recording preem- Clearly, this seems to indicate that the with the old tape, the new decks had a phasis is required to create a "proper" pIayback system is assumed to be rising high-end. Little by little, the test response with a theoretically perfect "ideal," and that playback losses tapes were deemphasized to show flat playback system. Only in this way is should be compensated for in the play- response on the newer and better true tape interchangeability assured; back electronics so that the "real" sys- players. every recorder would assume that the tem is as close to ideal as is feasible. The NAB standard, which was in tape would be reproduced on an ideal general acceptance at that time, was playback system, and every playback Pre-equalized test-tapes written in terms of a playback equali- system would assume it was reproduc- I may seem to be belaboring the ob- zation rather than a recorded flux char- ing a theoretically ideal recording. vious, but the point seems to have es- acteristic, and that, in a sense, blessed Of course, there has to be some gen- caped most designers. More often than a rather questionable practice. For that eralized equalizat~onimplied. Here is not, cassette decks are engineered matter, there may be some cassette where the curves come in: the so- without correction for playback losses standards extant today that follow the

called " 120-microsecond"(for Type-I in the playback equalizer, which simply same philosophy. My only point is that - ferric) and "70-microsecond" (for contains the composite of the two IEC Publication 94 does not seem to Types 11, 111, and IV - chrome, curves (3180 and 70 or 120 microsec- be one of them, and that, both from the fenichrome, and metal respectively). onds). All additional losses are cor- point of view of its international rec- The purpose of these curves (and the rected in the recording equalization. ognition and from the point of view of 3 180-microsecond low-frequency Nakamichi, the one compLny that, common sense, it seems to be the one break now standard for all tapes) is to practically from time immemorial, has to follow. Recent indications are that d make a "first cut" at correcting for the compensated for pkyback losses in the gpe deck manufacturers other than 6dBloctave rising response of the typ- playback equalizer, has frequently Nakarnichi also seem to be adopLng ical playback head, the energy distri- been accused of "non-standard equx- this posture, and are compensatin~for bution of music, and the basic high- zation" when, in fact, its app~h pLayback losses in the playback equal- frequency losses common to all seems to be in precise adherence to izer. Common sense and good engi- recording systems. IEC standards. neering practice may yet win out. AVI The nut of the question is whether This peculiar state of affairs is attrib- these "standard playback equaliza- utable, in large measure, to the pro- pensity of test tape manufacturers to tions" apply to the playback elec- Reprinted by permission of tronics or to the playback s-ystern. That pre-equalize their frequency response Audio Video International, December 1981. is, should one bimply dial these fixed tapes for a presumed playback gap A Publ~cationof Dempa Publications, Inc. equalization parameters into the play- length and spacing loss. Insofar as the back electronics and then juggle the re- losses in a playback head match the cording equalization to compensate for presumption on which the test tape other playback losses, or should one was created, it will appear to yield flat instead use the "standard playback response withour additional correction. equalizations" as a starting point and A deck that has correction for play- modify them for the playback losses back loss built into the playback equal- peculiar to that deck? It would seem izer - and thus, as a s-ystem, appears sensible to adopt the latter approach, ideal - will exhibit a rising high-end Nakamichi and, indeed, a careful reading of the when tested with a tape whose high- IEC standard (Publication 94 is the end has been boosted to "correct" for Nakamichi Corporation ~okyoOff~ce one that applies) would imply that this presumed playback losses. Sh~njukuDallchl Selrnel Bulldlng This idea of generating a test tape 7-7-1 N~sh~sh~n~~~ku,Shlnjuku ku, Tokyo is the "correct" technique. Phone (63) 345-4461 Telex 2324721 (NAKAM J) The first amendment to IEC Publi- based upon the presumption of certain Nakamichi U S A. Corporation cation 94 defines the "short-circuit playback losses is not a new one; it 1101 Colorado Avenue Santa Mon~caCahforn~a 90401 flux" of a magnetic tape as "The flux goes back to the early days of open Phone (213) 451 5901 New York Offlce 220 Westbury Avenue which flows through the core of a re- reel. At that time, Ampex was proba- Carle Place New York 11514 producing head which has a zero reluc- bly the leading tape deck manufacturer Phone (516) 333-5440