Tape Guide (Nov. 1981)

Metal Treble

Q. I use metal tapes with my cassette deck. The manual says that the equalization should be set to 70 µS for metal tape playback. But the sound is much better if I use the 120-uS setting. Why?

-Thomas Muhlethaler, Kloten, Switzerland

A. The 70-uS playback equalization denotes bass boost commencing at 2274 Hz; 120 µS, at 1326 Hz. In relative terms, say relative to 1000 Hz, this signifies more treble cut at 70 µS than at 120 µS. Accordingly, there will be more pronounced treble response when using 120-uS playback equalization. For metal tape, 70 µS is appropriate.

If a cassette deck is properly adjusted in terms of bias, record equalization, and playback equalization, it should produce substantially flat response to at least 14,000 or 15,000 Hz, particularly with metal-particle tape. The fact that your deck sounds better with 120 µS than with 70 µS equalization when using metal tape indicates a fault in your deck. It might be something as simple as a failure to clean the heads. It may be excessive bias, improper record equalization, improper playback equalization, or some other factor that causes the treble loss. It is probably advisable to have your deck checked by an authorized service station.

Which ips?

Q. Does having an open-reel tape deck with three speeds give more versatility? If so, in what way?

-Mark Belloto, Struthers, Ohio

A. Three speeds (presumably 7 1/2, 3 3/4 and 1 7/8 ips) permit you to make a trade-off between performance (wide and flat frequency response, low distortion, low noise, and low wow and flutter) and playing time. At the two higher speeds you can get generally excellent performance. With a high-quality deck, chances are that you may not be audibly able to detect a significant difference between the two top speeds, although instruments can measure such a difference easily. At 1 7/8 ips, you will probably detect a difference--more noise and less highs--but not so great as to prevent enjoyable use of this speed. For material such as background music, speech, etc., the 1 7/8-ips speed can give you tremendously long recording time (3 hours and 12 minutes in each direction with a seven-inch reel containing 1800 feet of long-playing tape) along with satisfactory performance.

Former Fan

Q. I found that the left side of my open-reel deck gets hot after about 30 minutes of operation, so I opened it up and installed a small fan. When the fan is on, I detect a slight hum in my speakers. The fan is plugged directly into the wall socket.

-Mark Jackson, Hinesville, Ga.

A. The signal produced by a tape playback head is very weak and very deficient in bass. Therefore, this signal must not only be amplified a great deal but also subjected to much bass boost.

At the same time, any hum due to a magnetic field-such as is , apparently being picked up from the fan's motor is also greatly amplified. Relocating or turning the fan or interposing a magnetic shield between the fan and the circuitry might help. Probably the best course is to remove the fan. A number of tape decks do get quite hot but are designed to withstand this heat without damage. It would be advisable to get the opinion of an authorized technician on this.

EQ and Bias Basics

Q. I'm confused about tape types, bias, and equalization. Please give me the basic definitions or a basic explanation.

--David Barr, Wyandotte, Mich.

A. Because recording on magnetic tape is not a linear process, a high-frequency bias signal is added to the audio signal to get the recording process into a more linear area and reduce distortion.

All of the Type I tapes, which are made with ferric oxide, require roughly the same amount of bias current for recording in the linear area, while the Type II tapes, which use a chromium-dioxide or a cobalt-modified ferric particle, require more bias. Type III or ferrichrome tapes need still more bias, and Type IV or metal-particle tapes require yet more.

Even with the bias properly set to achieve the lowest distortion with a given tape, the recording process is not perfectly linear since there are magnetic and other losses which occur. What's done in this case is equalization, essentially a substantial treble boost. Record equalization may also consist of a small bass boost to compensate for the leveling off at 50 Hz, but the amounts of treble and bass boost will vary according to the maker's idea of what is required with a particular tape and his conception of flat response. The treble boost also varies with the amount of bias current, which, in turn, depends on the manufacturer's intent on distortion, noise, and frequency response. Altogether, record equalization on a single tape deck will probably vary with the tape type settings, and it may well vary from one make of deck to another.

Playback equalization, however, has been standardized by the industry. It consists mainly of a very large bass boost to compensate for magnetic characteristics of the tape head, that is, declining signal output with declining frequency. Playback equalization may also include a relatively small amount of treble boost to compensate for treble losses of the playback head. The industry standards call for 120-µS equalization for Type I tapes, which means that bass boost begins at 1326 Hz. For the remaining standardized tapes, Types II, III, and IV, 70-uS equalization is called for by the standard, meaning that bass boost begins at 2274 Hz. Thus, 70-µS provides more bass boost than 120 µS. In all cases, bass boost begins to level off at 50 Hz.

Lethal Levels

Q. How can you tell when amplified music is loud enough to be harmful? What specific type of ear protection is recommended?

-Con Schieder, Delta, B.C., Canada

A. Sound levels above 90 dB must be regarded with caution, particularly if sustained for an appreciable time. Levels above approximately 100 dB tend to be dangerous, even for short periods. Rock and disco levels may reach 120 dB or more, and they can produce a good deal o1 hearing impairment, sometimes permanent. A person whose hearing is already impaired may not realize how loud the sound is, and therefore expose himself to further injury. A sound level meter can tell you what you need to know; such units come at varying prices, some of them quite affordable. Industrial-type ear plugs or ear muffs can help prevent injury.

Editor's Note: The January 1981 issue of Sound and Vibration contains a buyer's guide to hearing conservation equipment and lists a dozen makers of either ear muffs or plugs for hearing protection. Of these, I can recommend three from personal experience: Muffs from David Clark Co., 360 Franklin St., Worcester, Mass. 01604, and plugs from E-A-R Corporation, 7911 Zionsville Rd., Indianapolis, Ind. 46268 and from Flents Products, 14 Orchard St., Norwalk. Conn. 06850. Such items are invaluable to Editors-in-Chief when Editorial Assistants are asking for raises, not to mention when receiving flack from irate advertisers.

-E.P.

(Adapted from: Audio magazine, Nov. 1981; Herman Burstein )

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