Behind The Scenes (Oct. 1975)

by Bert Whyte

IF YOU belong to that select breed known as "ardent audiophile," sooner or later you'll acquire an open-reel tape recorder. As a quick look in Audio's annual directory issue will confirm, there is a mind-boggling selection of tape recorders available in a wide range of price categories. No matter what tape recorder you eventually choose, two things are certain. One is that few pieces of audio gear can give you as much pleasure as a good tape machine; the other is that you will have acquired what is probably the most technically sophisticated unit in your hi-fi system, an interfacing of mechanical and electronic parts of bewildering complexity.

It is perfectly true that even a "Danny Dullskull" can lay hands upon this technological marvel and by "following the simple directions from A to Z," come up with a high-quality tape recording. Manufacturers like to build in this sort of capability in their products. However, this ease of use is generally confined to the most simple functions and the most elemental recording tasks. There is a certain "mystique" to tape recorders, and even our "ardent audiophile" is a bit intimidated in his first confrontation with the formidable jargon of magnetic recording ... coercivity, remanence, bias, equalization, azimuth, zenith, headroom, tape saturation, flux densities, modulation noise, tape skew, scrape flutter, and on and on. Of course, you don't have to know about these things to make a good tape recording, but the very complexity of the subject is part of its attraction and a challenge for the doughty audiophile.

Having said this, I must point out that much of this information is hard to come by, so good intentions aside, most audiophiles' technical abilities in magnetic recording are rather limited.

There is another factor operating too, which is responsible for many audiophiles being very apprehensive about making even some of the most basic adjustments to their recorders. This can be summed up as the manufacturers exhortations in their instruction manuals to "keep your cotton pickin' hands off of things you don't understand!" This followed inevitably by ... "refer to authorized service station for adjustments to thus and so, etc., etc." Okay, we'll agree that these warnings have their point. You can bet many a tape recorder has been rendered hors de combat by the inexpert diddlings of their technically incompetent owners. On the other hand, those warnings have spooked even the more knowledgeable audiophiles to the extent that recorders which are in almost daily use go for months... even years ... with little or no checkout and maintenance other than routine cleaning and demagnetization.

The owner of one of these recorders may hear some sonic aberration ... subtle perhaps ... but persistent and annoying. He may suspect that his machine's high frequency response is attenuated. In monitor checking between source and copy, there is a perceptible diminution of quality in the copy. Frustrated audiophiles ask if there are some ways to check on certain basic performance parameters of their tape recorders without having extensive (and expensive) test instrumentation and a knowledge of how to conduct these tests.

The answer is that a "sonic stethoscope" has been available for many years in the form of precision test tapes. You will note I emphasize precision and I don't mean the cheap gimcrack test tapes found in the tape accessory department of many retailers. Until fairly recently, Ampex was the principal source of the high quality test tapes used throughout the professional recording industry. Now we also have professional test tapes from Taber Manufacturing and Engineering Co., 2081 Edison Ave., San Leandro, California 94577; and the test tapes I happen to prefer, those of the Magnetic Reference Laboratory, 999 Commercial Street, Palo Alto, California 94303. The MRL tapes get the nod from me for a number of reasons. For one thing they have a very comprehensive variety of test tapes available in virtually every tape-head configuration. Plus they have several unique test tapes containing material that is not available elsewhere. Lastly, MRL is under the direction of John (Jay) McKnight, formerly of Ampex and now dubbed (that's a pun!) the "Wizard of Watergate" since his impeccable technical credentials as one of the foremost experts on magnetic recording in this country earned him a place on the technical committee analyzing those infamous tapes.

Our "Sonic Stethoscope" is an MRL alignment tape. For the usual audiophile-type quarter-or half-track open-reel recorder using quarter inch tape, you obtain a full-track test tape which has been recorded at the highest speed of your machine, 7 1/2 or 15 ips, for example. Why full track? For one thing it simplifies manufacture and keeps costs down, plus you can use the tape on a variety of track configurations ... instead of having to buy the specific tape for the specific track format. From a technical standpoint, any height error of the reproducing head does not introduce gain setting or frequency response errors. Now without disturbing a single adjustment in your tape head assembly, you can playback this tape and, with some simple rules of interpretation, it will give you an accurate indication of the total frequency response of your recorder.

With a simple manipulation in the tape head assembly, this same test tape will enable you to check the azimuth of your reproduce head.

(Azimuth in this context is the degree of perpendicularity of the tape head gap to the direction of tape travel.

Ideally, the gap is perfectly vertical.

Deviations from this condition will result in attenuation of the high frequency response.) Let's take a closer look at the makeup of a typical alignment tape. A reference fluxivity tone at 1 kHz is recorded at the beginning and end of every test tape. This reference fluxivity is a value for the magnitude of the magnetic signal and is expressed as nanoWebers per meter of track width.

Different values of reference fluxivity apply to various recording tapes.

Typically, 200 nWb/m is the reference fluxivity for general-purpose tape used in home recording, whereas 250 nWb/m is the value for such tapes as 3M 206 or Ampex 406. In use, the 1-kHz reference fluxivity signal is used to set a standard zero VU on your VU meter. Once set, the playback gain control on your recorder is not touched during the playback of the test signals on the tape. Ideally, a recorder with a perfectly flat playback frequency response will not vary in gain at the reference fluxivity zero VU point. The reference fluxivity signal is also known as "operating level." On a 15 ips test tape, all the signals are recorded at this operating level. On a 7 1/2-ips tape, for technical reasons only the reference fluxivity signal is recorded at operating level, the other signals being recorded at minus 10VU. After the reference fluxivity signal, there is an 8-kHz signal for a course setting of azimuth, followed by a 16-kHz signal for fine setting of azimuth.

How do you set the azimuth of your machine's reproduce head? Well, friends, here is where some tape recorder manufacturers may get a bit miffed with me. Almost without exception, on a typical audiophile recorder once the manufacturer has set the azimuth, the azimuth adjustment screw is sealed with some variety of glue. Now this is all right to a degree, but head wear and other factors can conspire to put the gap out of alignment. Of course, enough head wear to cause a "grooving" or "lipping" of the head, and no alignment tape is going to help then because you need a new head. In any case, it is nice to know the actual setting of the azimuth, and any audiophile worth his salt will want to adjust the setting for optimum high-frequency response. Thus, after making absolutely certain which is the azimuth adjustment screw in your tape head assembly and using an appropriate tool (some units need an Allen wrench), boldly break the glop seal, and turn the screw to give you the maximum peak reading on your VU meter. If you should have a recorder that does not have a VU meter that reads playback signals, you will have to use an external VOM or VTVM. (Obviously, you would need a meter to read the other signals on the tape as well.) Following the 16-kHz azimuth adjustment signal, the alignment tape has a series of discrete test frequencies at 31.5, 63, 125, 500, 1k, 2k, 4k, 8k, 10k, 12.5k, 16k, and 20k Hertz. The tape ends with a repeat of the 1-kHz reference fluxivity signal. Thus, the alignment tape can give you a quick checkout of your recorder's playback performance. I should mention that at the low frequencies of 31.5, 63, 125, and on up to 1kHz, there are elevated readings above the zero VU point because of what are known as "fringing effects when a full-track tape is used on a multi-track head. Along with a calibration graph of the accuracy of the particular test tape you purchased, MRL furnishes a table of correction factors to subtract from the readings at the low frequencies.

What can you do if the alignment tape reveals that your recorder does not have a flat playback frequency response? This is a problem of adjusting the equalization and unfortunately in the usual audiophile recorder, this circuitry is usually in the innards of the unit and generally beyond the ministrations of the average audiophile. So, in this case, friends, it is back to the manufacturer. Ironically, in the professional tape recorders, like my Ampex 440, all the equalization circuitry is on plug-in cards accessible from the front panel and subsequently very easy to adjust.

The important advantages of using an alignment tape to check your playback response are that the method is quick, easy, and repeatable. It thus provides the knowledge required to put the recorder in perfect playback alignment so that any necessary recording adjustments can be undertaken.

MRL also makes a very clever "Difference Method" azimuth adjustment test tape. With this tape, one can check the azimuth of the reproduce head without actually changing any azimuth screw settings. On the tape, a medium frequency tone is alternately recorded at two azimuth angles symmetrically displaced from true azimuth. On playback the sequence is Tone A (at one angle) for 2 seconds, then a 0.1-second pause, the Tone B (at the other angle) for 2 seconds, then a 0.6-second pause. This is repeated throughout the length of the tape. When the reproduce azimuth is correctly set, the level output from tones A and B will be equal.

In practice, if you obtain a reading within one dB of each other, the azimuth is close enough for practical purposes. Of course, if the two tones are further apart in level, then you must use the azimuth adjustment screw to make them equal.

For the advanced audiophile who has an oscilloscope with a triggered sweep, MRL has available a rapid frequency sweep test tape that sweeps all frequencies from 500 to 20,000 Hz and gives the appearance of a continuous display of all frequencies at once. An MRL film positive graticule is necessary for use with this tape. Placed over the face of the scope, it is available in 3-, 4-, and 5-in. sizes.

MRL has an interesting catalog, listing all their test tapes. For the tape recorder enthusiast, even the most basic alignment tape is invaluable.

They're not cheap; $30.00 for the 7 1/2 or 15-ips quarter-inch tape, for example, but they are good investments, and given good care, as per the MRL instructions, these tapes will remain serviceable for along time.

(Audio magazine, Oct. 1975; Bert Whyte)

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