Tape Guide (Mar. 1979)

Abrasive Characteristics

Q. What are the abrasive characteristics of chrome dioxide tape vs. iron oxide? How much wear can one expect over a long period of heavy use? What happens to the performance of a deck as the heads wear?

- Alexander Davenport, Phila., Pa

A. So far as I know, the abrasiveness of chrome dioxide and iron oxide tapes is substantially the same. Permalloy heads are reputed to have a life expectancy of between 2000 and 4000 hours of use, perhaps more in the case of hard Permalloy. Tape speed is, of course, a factor ... the slower the tape speed the more hours of use. Ferrite and other types of long life heads are reputed to have a life expectancy of something like 10 or more times that of Permalloy heads, which comes out to about 20,000 to 40,000 hours.

Head wear is of particular concern with respect to playback. Head wear causes the playback head's gap to widen, resulting in a loss of the high frequencies. The slower the tape speed, the higher the loss. A worn record head may result in distortion.

Recorder Redundance

Q. Why do some tape decks (those with separate record and playback heads) have a monitor switch? Isn't this redundant since there's probably a monitor switch on either the amp or the receiver?

- Larry Joe, Toronto, Ont., Canada

A. A monitor switch at the tape deck is usually more convenient than having to use the monitor switch of an amplifier or receiver some distance away. This is particularly true in a case where one is checking or adjusting the performance of the deck.

Head Impedance

Q. I read about high-impedance and low-impedance tape heads. Kindly let me know the meaning of these two types of heads.

- P. U. Sukhadia, Bombay, India

A. High-impedance heads provide high voltage and low current, while low-impedance heads provide low voltage and high current. A high-impedance head is desirable for playback in order to obtain a playback signal with high voltage, thus maximizing the signal-to-noise ratio. A low-impedance head is desirable for recording in order to permit passage of a large amount of signal and bias current and, correspondingly, produce a large magnetic field.

Dubbing Loss

Q. When dubbing a 7 1/2 ips tape to a 7 1/2 ips copy, will performing the operation at 15 ips cause one to lose half the frequencies? And if it does, then how do they commercially duplicate tapes at speeds as high as 240 ips?

-Alan Korwin, Bronx, N.Y.

A. Chances are that you won't lose "half the frequencies" if you dub at 15 ips. The nature of the loss will depend upon the upper frequency limit of the tape electronics. If the electronics are good, say, to 30 kHz, this will maintain a response to 15 kHz for a 7 1/2-ips tape.

However, if you dub a 7 1/2-ips tape at 15 ips you will run into another problem ... equalization. The record and playback equalization will not be such as to conform to NAB equalization for a 7 1/2-ips tape.

High-speed dubbing requires that the record and playback electronics be able to handle the high frequencies entailed. If, for example, dubbing is at 10 times the normal speed, the electronics must be able to handle frequencies up to, at least, 150 kHz. Also, the bias frequency has to be greatly increased to avoid beats with the audio frequencies, and the equalization has to be appropriately changed.

Tape Loudness

Q. I have a problem recording tapes from records and maintaining the same volume throughout the full length of the tape. This is true for all three open-reel decks I own. I try to copy the records so they all produce the same readings on the VU meters.

Some seem to come out stronger on the tape and I can't understand why.

-Steve Jones, Houston, Texas.

A. The apparent loudness of pro gram material depends, in a good part, upon the average level of the material, and the nature of the program material determines the degree to which the average level is below the peak signal indicated by the VU meters. For some program material, the average level may be only 6 to 8 dB below the level indicated by the VU meters, while on other program material it can be as much 20 dB below the peak levels.

Therefore, the loudness levels of re corded selections can be found to vary substantially.

Another possible factor is the frequency content of the material. Sensitivity in the human ear is most pronounced around the vicinity of 3000 Hz. Selections with a good deal of frequency content around 3000 Hz will sound louder than selections with less.

AM Taping

Q. Should you ever use a tape speed higher than 3 ¾-ips when recording AM broadcasts?

- Alan Kuhn, Rochester, N.Y.

A. Generally one doesn't need to, but not necessarily in all cases. A more specific answer depends upon the quality of the tape machine and the quality of the source. If the tape deck performs in mediocre fashion at 3 ¾-ips, it may be worthwhile to go to 7 ½-ips even for AM broadcasts. Keep in mind that some AM stations do go out to 10,000 Hz or more, and a few (too few) AM tuners can do justice to such extended treble response.

Running Times

Q. Please give me the running times for 5-, 7-. and 10 ½-inch reels at the various tape speeds.

-Name withheld by request.

A. Let us start with the standard tape speed of 7 1/2 ips on the commonly used 7-in. reel with 1 1/2-mil tape. The reel holds 1200 feet and runs for 32 minutes in one direction. As speed is reduced, running time is proportionately increased ... thus it runs 64 minutes at 3 ¾-ips, but would only run 16 minutes at 15 ips. If the reel is run in both directions, then the total running time is doubled.

If a 5-inch reel is used, then the running time is cut in half, while if a 10 1/2-inch reel is used then the running time is doubled, compared to the 7-inch reel.

If 1-mil tape is used, allowing 1800 feet on a 7-inch reel, then the running time is increased by 50 percent. In the case of 1/2-mil tape, the running times are doubled compared with the 1 1/2-mil tapes.

(Source: Audio magazine, Mar. 1979; Herman Burstein )

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