Tape Guide (Oct. 1981)

Hightailing High End

Q. My cassette tapes lose much of their high end within six months after recording. The heads are cleaned regularly but unfortunately demagnetized very infrequently. Could this be the reason for the treble loss?

-Glen Carnrick, Sandy Hook, Conn.

A. In general, tapes tend to lose a bit of their high-end response with repeated playing, although most of the effect occurs during the first few plays; in the case of high-quality tapes, the loss is quite slight. The losses you mention seem more severe than the ones I have just described, and they could very well be due to magnetized tape heads. Some heads apparently become more easily magnetized than others. For example, according to Nakamichi, ferrite heads are more susceptible than other types to becoming magnetized. This seems to be a function not only of use but also of time (i.e. stresses in the head material tend to produce magnetization even when the head isn't used). Hence, in the future, I would urge more frequent demagnetization of the heads.

Homing in on Dolby

Q. Which is appropriate for home use, Dolby A or Dolby B noise reduction?

-Donnie Andrews, Jr., Brooklyn, N.Y.

A. Dolby A noise reduction is intended for professional use. It divides the audio spectrum into three parts and deals with each part separately. Dolby B noise reduction is intended for home use, is far less expensive than Dolby A circuitry, and deals only with the treble range, where noise is most audible to the human ear.

Suggestion Vox

Q. I record old 78-rpm records and am trying to make good tapes from these recordings. May I have your suggestions?

-Emmett Smith, Baltimore, Md.

A. Taping at slow speed no more than 3 3/4 ips--may provide a useful cutoff of high frequencies, which contain more noise than desired audio signal; this works best with the older tape machines that go out only to about 8,000 or 10,000 Hz at 3 3/4 ips. You can achieve a sharp cutoff of high frequencies by increasing bias. One of the single-ended noise-reduction devices would serve to filter out noise; these devices reduce high-frequency response only at low signal levels, thus eliminating noise when it is most apparent and affecting highs when they are least audible. Sometimes you will find that an elliptical stylus intended for microgroove records will do a good job on 78s as well.

Treblesome Swish

Q. When recording strong plucked bass notes at 3 3/4 fps, on playback I can hear a swishing noise that accompanies each bass note. If I switch to 7 1/2 ips, the swishing disappears. Why?

-Guy Camenisch, Bischwiller, France.

A. On a strong plucked bass note, there is an accompanying transient which may be considerably stronger than indicated by your record level meter. This transient is of high frequency and subject to a good deal of boost by the record equalization circuit of your tape deck. Such treble boost is substantially greater at 3 3/4 ips than at 7 1/2 ips in order to compensate for the more pronounced treble losses which occur in recording at the lower speed. Hence, at 3 3/4 ips there is more chance of overloading the record electronics, the tape head, the tape, or any combination of these. Have you tried recording at reduced volume? This may eliminate the swishing.

Compromising Position

Q. I noticed in Audio's Annual Equipment Directory that the most expensive open-reel tape deck also has virtually the poorest high-end frequency response, namely to only 16 kHz even at 15 ips.

Why?

-Daniel Temianka, Palos Verdes Estates, Cal.

A. Electronically, the criteria of good performance are extended and flat frequency response, low noise, and low distortion. In designing a tape deck, particularly at the lower speeds (3 3/4 and 7 1/2 ips), the design engineer has to achieve a compromise among these three aspects of good performance. In other words, an improvement in one aspect often has to be traded off for a deterioration in another. For example, treble response can be extended at the cost of higher noise or higher distortion or both.

The designers of the deck in question may have decided to extend response "only" to 16 kHz in order to minimize noise and distortion.

Further, there is the question of delivering what one has promised. To make sure that every unit produced will permit no more than, say, a 1.5-dB loss at 16 kHz, a manufacturer must allow a margin of safety: This means that most of its units permit the 1.5-dB loss no earlier than 18 or 20 kHz. Another consideration is to ensure that treble response won't plummet suddenly. If a deck is down 1.5 dB at 16 kHz, it may still be no more than 3 dB down at something like 20 kHz or higher. A responsible manufacturer also allows for factors such as wear of a deck, tapes that don't always deliver their promise, slight gradual misalignment, etc. The company has to keep in mind that you will require response out to at least 16 kHz not only today but also tomorrow. Finally, it is possible that the manufacturer has deliberately decided to cut off response above 16 kHz on the premise that signals above this frequency can do more harm than good. Such harm may be in the form of oscillation, beating with the oscillator frequency, etc. He may have possibly decided that since there is little or no music at these frequencies, there would be no real point in providing response there, except as a demonstration of technical expertise.

Mismatched Couples?

Q. I am contemplating the purchase of a receiver which specifies a tape input impedance of 33 kilohms and a cassette deck which specifies a line input (load) impedance of 50 kilohms. Do these specs rule out coupling these units?

-Gustav Mattersdorf, Lakehurst, N.J.

A. I doubt that there is anything significant to worry about. The general rule is that the source impedance (at the output of the unit supplying the audio signal) should be one-tenth or less of the load impedance (at the input to the unit to which the signal is fed) in order to avoid distortion or perceptible alteration of frequency. response. The source impedance of most decks is on the order of several hundred ohms, so that in your case the ratio of source/load impedance will be well under one-tenth. Today one rarely finds a serious mismatch.

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

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