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For the audiophile who has been using cassette recorders, there have been tremendous changes in the performance in this format in relatively few years. It wasn't that long ago (1965) Philips brought out its Cassette-corder, which was strictly for fun, family gatherings, and other low-fidelity purposes. A stereo recorder did appear the following year, but its performance was ho-hum rather than hi-fi. In this same period, of course, there was the start and growth of the software (tape) manufacturers. Ten years ago, development became more rapid, including such new products as TDK's SD (Super Dynamic) tape.
At this time, there was the need for and the practice of greater cooperation between the manufacturers of the recorders and the tape makers. The inherent nature of the magnetic inter faces made this essential: An improved tape needed improved record, play and even erase heads, and improved head designs could benefit from better tapes. It was a great year for the cassette format: In 1970 significant improvements were made in extending frequency response and lowering noise with the introduction of CrO2 tape and the Dolby B system. Some started using the expression "high fidelity," but "for a cassette" was usually added. The appearance of the Nakamichi 1000 in 1973 impressed many with its one-grand price, but many more were impressed with the sonic results. This was a time of conversion for many skeptics who had concluded that tape speed and width precluded any possibility of acceptable playback.
The next couple of years saw a great expansion in the generation of well-performing cassettes decks and new tape formulations. Some of these were not successful, including early FeCr versions which were not stable and cobalt-doped ferric which had their own sort of erratic behavior. TDK brought SA on the market in 1974, and its ion-absorption approach enabled the use of the desirable cobalt. The improvement in straight ferric tapes was continuing all during this period, and continues at this moment. Many other excellent tapes were introduced by Maxell, Scotch, BASF, Fuji, Memorex, and others. The introduction of metal-particle tapes is significant, but it is part of this relatively short history which has included a number of important changes.
Before we get into a discussion of the history of the metal-particle tapes and their magnetic properties, we should review some of the basic characteristics of tape formulations. The reader will note that there are three hysteresis loops in Fig. 1, one for a gamma ferric oxide such as Maxell UD, one for Nakamichi SX (or another similar cobalt-modified ferric), and one for Nakamichi ZX metal-particle tape.
Let's make a trip around the loop; follow any one of the three, for the basic story is the same. We also assume that the magnetic material has gone through the loop prior to our discussion, so we are not building up from zero magnetization.
As the coercivity increases in the positive direction, the retentivity also increases, very sharply at first. These steep slopes are desirable and are a goal of the designer, because this shows that the flux in the material is responding well to the magnetizing force. Alas, the material reaches a point where magnetic saturation takes place, and increasing the magnetizing force does not increase the flux in the material. With the lowering of the applied field, when coercivity has been reduced to zero, a certain flux level is retained. This is called retentivity which is the number of flux lines per cm' of the tape coating cross-section. This is flux density with the units, gauss, and the area determined by the tape width and the thickness of the coating. This is a fundamental measure of the magnetic performance of the particles, but there is a bit more to this part of the story.
Tape performance is deter mined by the actual number of lines of flux induced, not just the density. Remanence is the actual signal retention in total lines of flux (in Maxwells), contributed by the tape coating thickness and width. As we recognize that the highest frequencies would not penetrate a thick coating, we can see that the choice of coating thickness facilitates matching low- and high-frequency record sensitivities. Once again, note that retentivity (flux density) goes with the properties of the particles and the remanence (actual lines of flux) goes with the tape product.
Squareness ratio is the decimal fraction of flux at zero coercivity to the flux at saturation.
So far in our trip around the loop, we have succeeded in magnetizing the material, but what's required to de magnetize it? If a magnetizing field is applied in the reverse direction, the flux in the particles will be reduced down to zero along the lines in the second quadrant. For the other direction of induced flux, we would travel the other half of the total loop. Let's restrict our attention, however, to the important second quadrant of the figure and make some comparisons among the tapes. Note that there are two dashed lines showing the 5-kHz and 20-kHz demagnetization loss line.
These are based upon those appearing in Vogelgesang's article in Audio just a year ago.
For the particular tapes shown, the retentivity increases from 1050, to 1550 to about 3500 gauss. There is no doubt about the significant increase in possible flux levels, especially at the low frequencies. Vogelgesang points out, however, that this high flux capacity is needed for good high-frequency performance, along with high coercivity.
For this property, the tapes show values of about 300, 550 and 1000 oersteds, showing increasing resistance to erasure. The demagnetization loss lines indicate the much lower likelihood of self-erasure with the metal particle tape. Consider also that with the much higher retentivity, the tape designer has a bit more flexibility in the choice of coating thickness to achieve the best combination of ex tended response and high record level.
The high coercivity is a mixed blessing in that it places much higher demands on the erase and record heads--a definite challenge to the head designer. Later on we'll take a look at some test results with both old and new machines.
Metal-Particle Tapes Arrive
The 3M Company has been working on the technology of a tape coated with pure-metal particles since 1965, which is when cassettes themselves started, though the basic work on the particle began two decades earlier. In the last couple of years, the activity in this area accelerated greatly and produced many rumors. Most of the printed discussion of problems of pure-metal particles talked about the need to prevent the actual rusting or oxidizing of the particles. There were some stories about extra fast oxidation, also known as burning, of the material.
(Now, that would be a hot tape!) 3M and others, however, were solving the processing problems, and samples began to appear more regularly in early 1978. Scotch will probably gain some points with the use of "metafine" as the name of their metal-particle tape.
TDK is using "MA," and Nakamichi has "ZX," but Fuji so far says just "metal." Maxell, BASF, Memorex and Ampex have announced intentions to manufacture metal-particle tapes, but no specific information had been received at the time of this writing.
Tests on the first-mentioned tapes, which are reported later, used a Nakamichi 582 deck. Other units compatible with these challenging tapes are being offered by Tandberg, Pioneer, JVC, Aiwa, Lux, TEAC, Sanyo, BIC/ Avnet, Technics, Eumig, and others expect, by the time this appears in print.
Fig. 5--Three-percent distortion limit for TDK SA tape in three recorders: Top, Nakamichi 582; middle, Technics RS-990005, and bottom, Harman-Kardon HK-1000. Zero reference is Dolby level.
Fig. 9--Three-percent distortion limit using ZX tape on three recorders: Top, Nakamichi 582; middle, Technics RS-990005, and bottom, Harman-Kardon HK-1000. Zero reference is Dolby level.
The various manufacturers have been gearing up the advertising departments, as well as the production lines, for output of metal-particle tape.
Some of the manufacturers have supplied technical data, but some of the "standards" used are not the same.
TDK, for example, makes comparison to TDK SA and refers to a TDK standard tape, while Scotch uses DIN references. TDK data shows the possibility of using a bias level 3.5 to 5.0 dB higher than SA bias. Increased headroom at the higher frequencies is listed as 5 to 7 dB greater. Figures 2 and 3 are photo micrographs for the particles used in TDK SA and MA (metal-particle) 51 tapes, respectively. The thinner elements in the MA tape are actually small balls strung together, somewhat like a pearl necklace. Each little ball is about 300 Angstroms in diameter. The SA particles are not in the shape of a chain, but are needlelike, about 0.5 microns in length (5,000 Angstroms).
TDK states that the coating thickness of the metal-particle tape is about 4 microns.
Bias for Scotch Metafine is shown as +6.5 dB, but this is referred to DIN ferric bias, so it is not so far removed form the bias for TDK MA as it seems at first. Scotch claims that Metafine has output twice as great as chrome tapes at low frequencies and three times as great at the high frequencies.
The manufacturer states that, overall, "this results in 5 to 10 dB greater out put over chrome." Additional data on Metafine and TDK MA is given in the tape-tests report in this same issue.
One concept that has received a fair amount of attention lately is the rating of magnetic tape performance by determining its signal capacity. With their extension of high-frequency response, metal-particle tapes have been touted by some as "greatly superior" because of the indicated increase in such signal capabilities. In the next section, consideration is given to this and other ways of assessing the improvements with metal-particle tapes.
Performance Improvements: Tape or Machine?
In the process of evaluating the Nakamichi 582, the reviewer was struck by the fact that its performance with FeCo tapes was superior to the majority of machines tested in the past. Further, a review of all the data revealed two interesting things. First, although the results with the metal-particle tapes were superior, there was not as much of a difference as expected. Second, the figures that have been used to indicate the expected improvement were much closer to the difference be tween the Nakamichi deck with the metal-particle tape and older recorders with chrome-type formulations.
To gain some understanding on the inter-relationships, TDK SA was used for record/playback responses at Dolby level and for maximum-record-level tests. Three recorders were used, the Nakamichi 582, the Technics RS 9900US, and the older Harman-Kardon HK1000. The bias on the first two machines was adjusted to match SA; the HK 1000 had been aligned to SA previously. The maximum record levels were determined with HDL3=3 percent from 20 Hz to 3 kHz and twin-tone IM distortion = 3 percent from 5 kHz to the upper frequency limit. Figure 4 shows that all three responses are quite good at Dolby level; the Harman-Kardon is quite impressive, considering its vintage. When the comparison is made among the machines for the distortion limit (Fig. 5), the Nakamichi has superior headroom across the entire band. The same cassette was used for the three decks, so the differences are machine related, although small shifts in bias could bring some changes.
It might be noted that the 3 percent distortion-limit curves have a slope of about-6 dB per octave and cross zero dB around 2 kHz. With the slope of much music on the order of-3 dB/ octave, however, the distortion may reach the stated limit at lower frequencies first. The ability to record a wider, undistorted bandwidth with the Nakamichi comes form greater head room at both ends of the band.
Nakamichi ZX metal-particle tape was exercised in the same way with the same three recorders, but there were some changes. First of all, no at tempt was made to adjust the Harman-Kardon deck bias to match this tape. The results would show what to expect from using such tapes in a deck actually set up for a tape similar to TDK SA. Bias was adjusted on the Nakamichi and Technics decks to match the ZX tape, using pink-noise at-20 dB. The RTA display was as expected with slight under-bias for 400 Hz with the 582 as shown in Fig. 6. Note that the highest frequencies curve upward, as they should under this condition.
With slight under-bias with the Technics, however, the rise in the highest frequencies is very mild, indicative of possible self-erasure effects from the high bias (+3.8 dB re: CrO2 zero bias).
Figure 8 shows the frequency response plots, with the expected low level and high-frequency peaking on the HK1000 with the severe under-bias condition.
For the great majority of the band (see Fig. 9.), the headroom on the 582 is superior, particularly at the frequency extremes. Note, however, that the Technics has a higher limit from 5 to a little over 10 kHz. To help put a handle on some of the comparisons that can be made, Fig. 10 shows the increase in distortion limit (or headroom) across the band for two cases. The first one examines the improvement by going from the Technics RS-9900U5 with TDK SA to the Nakamichi 582 with Nakamichi ZX tape. There is an advantage of about 5 dB for most of the band with a rapid increase above 14 kHz. In the second case, just the 582 was used, and the differences between TDK SA and Nakamichi ZX were measured. The average improvement is about 2.5 dB which is worthwhile and nice to have, but certainly less than many of the claims that have appeared. We will get back to look at these relationships from another perspective after discussing another facet of rating tape performance.
Mention was made earlier of rating tape formulations by their signal capacity. As we have just seen, the performance of a particular tape can vary a great deal from one machine to the ether. One has to be very careful, then, about firm conclusions about a tape without being certain of the effects from the machine. If we refer to Fig. 10 again, we could say that there is a great increase in signal capacity if we look at the top plot, or just a useful increase if we use the bottom plot. For analog recording, forecasts of improvements in total performance based upon the increases in signal capacity can be misleading. Some of the formulas being used treat each Hz of bandwidth as equally important. A response or distortion-limit plot on this basis would have linear frequency, such as 2 kHz for each of ten divisions.
With noticeable increases in head room between 10 and 20kHz, there's a great increase in signal capacity. Be fore you get bowled over by numbers derived in this way, remind yourself of a couple facts. First of all, there is no way that the 10-kHz band from 10 to 20 kHz will ever be as important as the 10-kHz band from zero to 10 kHz for analog recording. Second, the levels of the harmonics keep dropping with frequency, except in rare cases. The gains with this type of recording should be assessed with frequency on a log basis.
If we consider digital recording, how ever, the gains in signal capacity with high-end improvements can be directly helpful, they could be essential for a digital system using the cassette for mat.
There are some other machine-tape relationships which merit discussion.
Scotch states that an erasing field of 3000 oersteds is required, and all manufacturers have commented on the problems of adequate erasure with decks not designed for metal-particle tapes, even if they have the bias capability for record purposes. A few tests confirmed that the problem is real.
The Nakamichi 582 was the only one of the three used for the previous tests that was able to erase greater than 60 dB across the audio band. In most places, erasure was greater than 70 dB.
On the other hand, the Technics deck had erasure of only 40 dB at lower frequencies with metal-particle tapes.
Nakamichi had stated that its deck was able to do a better job than many bulk erasers. I had discounted this claim until I found that I had to use the 582 deck to do what my bulk eraser could not.
The severe challenge to using metal-particle tape in a present deck thus includes many factors. Even if the unit can generate enough bias drive to the record head for the mid-frequencies, limitations in head design could cause a drastic self-demagnetization of the higher frequencies. The user may also have to face the problem of being unable to erase what was put on the tape. Further, he is likely to find out that his bulk eraser can't hack it either.
Mine looks impressive, and it says "professional" on it, but it didn't do the job.
Is Metal-Particle Tape Worth It?
The new metal-particle tapes do provide worthwhile improvements in total sonic performance when used with a well-designed deck. It should be clear from the previous discussion that using such tape is not a simple case of throwing another cassette into your present machine. It is quite probable that the new tapes will stick to the 70-uS EQ, which would allow playing pre-recorded tapes with such formulations on existing machines.
There are certain to be some hobbyists who will make modifications to their present machines, but the challenges are many, and this approach cannot be given a general recommendation.
There will be an increasing number of new decks that will have the basic capability to utilize metal-particle tapes. As the text above showed, the requirements for record and erase heads are very severe and a great challenge to the designer. Some will obviously be more successful than others.
Specific points to check when contemplating purchase include the following. (1) erasure, particularly at low frequencies, (2) headroom across the en tire audio band, and, (3) the means of setting and checking bias for best response. The combination of a deck, well designed in these and other respects, and metal-particle tape could very well be a most worthwhile change for many owners of present decks. It is also possible that other new decks will offer improved performance will all formulations, as evidenced with the Nakamichi 582.
It is to be expected that the metal-particle tapes will continue to improve; the results reported elsewhere in this issue used early-run samples. Perhaps the prices will be quite close to those for FeCo and CrO2 tapes. And, we should expect to see improvement in performance because of up-dating in the deck designs, particularly the heads. In other words, we will see greater headroom and wider response in the future, with contributions from both types of manufacturers. Areas worthy of particular attention by the engineers are improved consistency, lower modulation noise and lower distortion at the frequency extremes. Perhaps there will be standards established for bias to minimize the possible spread in tape bias requirements without such a guideline.
Refinements should be expected to continue with ferric, FeCo and CrO2 formulations, upgrading their performance. Du Pont states that CrO2 still has considerable undeveloped potential for audio cassettes. The FeCr tapes and their future is problematical: There are only three formulations, so deck makers may not want to keep such a switch position with the metal-particle tapes on the scene--and they are here to stay.
Metal Cassette Tests
by Howard A. Roberson
Sample cassettes of the new metal-particle tape formulations have be come available just in time for this issue. There has been great anticipation of their appearance, and the results provide evidence of truly significant improvement in most performance parameters. With the limited number of samples avail able, it was not possible to obtain our usual three samples of each length, which number facilitates checking consistency and determining what is typical. Cassettes were received from Fuji, Nakamichi, Scotch and TDK.
Technical data sheets were provided by some of the manufacturers, but there was quite a range in the amount of information delivered. The entries shown in Table I do not cover all the details, but much essential data is shown. Take especial note of the values for coercivity and retentivity for the various formulations. Note there is about a 2:1 in crease in coercivity going from high-bias tapes to the metal-particle tapes, approximately 600 to 1000 or more oersteds. We would expect to find improving high-frequency performance with these changes. As for retentivity, there is a jump of about 2:1 in going to the metal-particle formulations, so we should find measurable improvements in maximum record levels. There are a number of inter-related factors which are discussed more fully in the separate article on metal tape characteristics.
Fig. 2--Frequency responses and three-percent distortion limit (dashed line) for Nakamichi ZX tape.
* The record sensitivities shown are those relative to the sensitivity of Nakamichi ZX.
** The figures shown for bias for the metal-particle tapes are not actual bias values. Listed for each of the tapes is the change in its response at 15 kHz when shifting bias to that for Nakamichi ZX tape. See text.
As in the previous tests on cassettes, bias of the test recorder was adjusted to show each formulation at its best.
There was no so-called standard tape, which would make others look poorer in comparison. The metal-particle cassettes were evaluated on the Nakamichi 582, which has the requisite capabilities for recording and erasing these high-coercivity tapes. Because there are no industry standards as of this date, record sensitivity was referenced to Nakamichi ZX. As there was no bias-current monitor, an indication of the bias needs for each of these tapes was gained, in a relative sense, by changing bias from optimum to that for Nakamichi ZX and noting the change in response at 15 kHz. The reference level used for all tests was a fluxivity of 200 nWb/m at 400 Hz, which is Dolby level for cassette tapes.
The reference record level, then, was that which obtained this flux level.
Record/playback responses were run at Dolby level and 20 dB below that.
For each tape, adjustments to bias and the record-head azimuth were made just before sweeping. The Crown RTA 2 was used for the pink-noise source and the 1/3-octave real-time display, which greatly facilitated these adjustments.
The source for the swept plots was an Exact 128 function generator with portions of a UREI 200 plotting system feeding an MFE X-Y recorder. A Sound Technology 1701A was used for a signal source, level monitoring and to measure distortion at 100 Hz, 400 Hz, 1 kHz and 2 kHz. A Ferrograph RTS-1 was used as the second signal source for twin-tone IM tests, with the tones 1 kHz apart. Data was taken for HDL3 =3 percent for the single tones and also 3 percent for the (2f1-f2) distortion product for the twin-tone tests. Two checks were made, with the lower frequency, f1, equal to 5 kHz and 10 kHz.
A Hewlett-Packard 3580A spectrum analyzer showed the levels of both fundamentals and upper and lower sidebands. It should be understood that this twin-tone IM distortion limit is more restrictive than a simple saturation test. The signal-to-noise ratios were referred to the 3 percent distortion limit for 400 Hz and used IEC "A" weighting. A Nakamichi T-100 secured the dBA figures, and was also used as the bias level monitor for the Technics RS-990005, as well as for some brief flutter checks. Past experience had shown that the great majority of cassettes are quite similar as far as flutter performance is concerned. Most of them give a variation in readings just from stopping and starting, reinserting, etc. A few are consistently better than most, and there's the occasional dog that is much worse.
Modulation noise was measured by recording a 1-kHz tone at reference level, rewinding and playing it back. A UREI 560 feedback suppressor notched out the tone, and band limiting was introduced at 500 Hz and 1500 Hz with a Gen. Rad. 1952 filter set. The checks on amplitude stability and drop-outs were made with the H-P 3580A in zero scan mode tuned to the 3 kHz tone that was recorded. A sweep rate of 2S per division showed the slower variations, and the sweep rate of 50 mS/div. showed slower detail on drop-outs.
The consistency from cassette to cassette among the samples supplied was verified primarily with the Crown RTA-2. Adjustments, such as bias and head azimuth, were made with one cassette. Then, all other samples were tried with these same settings. Bias or skew differences immediately appeared as a roll-off at the highest frequencies. Sensitivity variations were revealed with checks with the test oscillators built into each test recorder.
Fig. 4--Frequency responses and three-percent distortion limit (dashed line) for TDK MA-R tape.
There were a total of four metal-particle cassettes tested, including ones from Fuji, Nakamichi, Scotch, and TDK. They have, of course, generated a good deal of interest, particularly among the technical community.
Now, we get to the highlight of this report: How well did the metal-particle tapes do? It has become somewhat common practice to refer to these formulations as "metal" tape, but we hope our readers know that the base of the 0.150-in. wide tape is still Mylar.
The particles on the base may be pure, non-oxidized metal, but we do not have a solid metal ribbon. When com paring the results in Table II with the text below, remember that further in sight into a number of the inter-related factors is provided in the accompanying article on metal-particle tape.
Fuji Metal: Reference To Table I will show that this formulation offers the highest headroom at low and mid frequencies, up to +10.6 dB (!), and the highest signal/noise ratio in this category. The responses are extended at both 0 dB and at-20 dB, particularly in comparison with the other types of tape in the table. C-60s were consistent in all respects, as were the C-46 samples received. There was very little amplitude variation with time, perhaps 0.1 dB. There were rare, unimportant drop-outs.
Nakamichi ZX: This tape had head room of +9.0 dB at 1 kHz and wide responses similar to the Fuji tape. The sensitivity and bias settings for this tape were used as reference points for the other tapes. Insufficient samples were on hand to check for consistency. There were some random amplitude variations, usually less than 0.25 dB. There were occasional drop-outs of a minor nature.
Scotch Metafine: If nothing else, Scotch ought to get a few points for naming their formulation "Metafine." In the actual performance, this tape had the most extended better-than-average headroom across the band and better-than-most signal/ noise ratio. The headroom at 5 and 10 kHz was the best of the group. The modulation noise, however, was the highest of the category. The C-46 samples were generally consistent for sensitivity, skew and bias needs, but there was a skew change with turning cassettes over. Some earlier C-60 samples were actually very similar in all respects. There was a general variation of 0.3 dB in amplitude that was quite continuous. There were a few minor drop-outs shown on the analyzer, which were not detectable in listening.
TDK MA-R: This entry into the metal-particle sweepstakes had responses very close to Scotch Metafine and the lowest modulation noise. The head room was a bit less than the others at the lower frequencies, but quite close over the rest of the range. The C-60 samples were exactly consistent in sensitivity, bias and skew, including turning the cassettes over. This could very well have been aided by the re fined cassette "mechanism" used by TDK. The amplitude was generally very steady and smooth, with just an occasional variation of were rare, medium drop-outs which were not detectable in listening.
There is no doubt that this new category of tape has more to offer than any of the others. Improvements are evident in frequency response, head room all across the band, and signal to-noise ratio. The modulation noise did not show the same improvement, however, nor was consistency better than a number of older tapes. This is the beginning of production though, and it is to be expected that there are more advances to be made with the new materials. There are a number of things to be standardized for the met al-particle tapes, and there must be standard test tapes for everyone to refer to. Along with that there may be a standard bias established. In the results reported here, note that the Scotch Metafine would be down 5 dB at 15 kHz with Nakamichi ZX bias, or the Nakamichi ZX would be up about 5 dB with Metafine bias. Competition is intense in the tape area, and we can expect to see continuing improvement from all of the manufacturers.
Can I Use Metal-Particle Tape?
The results given here were obtained with a Nakamichi 582, which was designed to use metal-particle tape. The performance figures given go with those tapes tested and the particular machine. With further changes, it is possible (read probable) that the data will be even closer to open-reel results. Using a metal-particle tape on a machine not designed for it could actually be quite disappointing, with both recording and erasing problems. Such use is not recommend ed. Reference should be made to the accompanying article which includes coverage of such facets. It could aid in any evaluation of a deck offering metal-particle-tape compatibility.
(Source: Audio magazine, Sept. 1979)
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