Tape Guide (Q and A) (Sept. 1970)

Engine noise, fading etc.

Q. Would you be so kind as to answer the following three questions concerning tape recording:

1. I now have installed in my automobile a 4-track auto tape playback cartridge machine. I installed the speakers for the machine in the rear seat area, using standard zip cord to connect them with the machine, which is mounted under the dashboard. I am experiencing noise mixed with the tape program while the engine is running; apparently the source is the alternator or other part of the ignition system. I realize that shielded speaker leads and/or ignition noise-suppression devices installed in the engine compartment are probably the answer to my problem, but do not know how the shielded leads should be connected, or what type of suppression devices are required. Please advise.

2. In an electronics publication I noticed a simple gadget that was suggested as an improvement for tape recorders. It is a simple pot connected so that the erase head of the recorder could be put into operation gradually.

The suggested use was at the end of a recording, so that a gradual fadeout could be accomplished. Would you recommend installation of this device in my tape recorder, which is a semiprofessional machine costing $600? Or would the problems that I might run into outweigh any advantage that might be gained?

3. I am very interested in learning more about tape recording, leading possibly to a career in this field at a later date. However, I know of no sources for information concerning the practical aspects of professional recording. Do you know of any textbooks or sources that are offered on the subject? I have learned of some broadcast engineering courses offered by correspondence institutes. Would these be worthwhile?

-Richard L. Wayner, North Palm Beach, Florida.

A. (1) I doubt that your ignition noise is picked up by the speaker leads. More likely it is picked up by the tape electronics, and you would go about eliminating this pickup in much the same way as for a car radio. If I am wrong and the speaker leads are the culprit, you could use microphone cable as the speaker leads. Three -wire cable (two inner conductors) might work better; the outer (ground) wire would be connected to ground of your tape machine.

2. I am dubious about making any serious modification of an expensive tape machine, such as you mention in connection with a fadeout device.

You might try installation of the device, note whether performance deteriorates in any way, and remove the device in case performance does deteriorate. I suspect that changing the amount of oscillator current reaching the erase head will affect the amount of bias current reaching the record head, and in turn performance will be affected in terms of treble response, distortion, and signal-to-noise ratio.

3. I cannot advise you on correspondence courses having to do with recording. As for texts, I suggest you consult your library. There is an index of textbooks, by subject, which you might find in your library. One book of professional caliber on magnetic recording is that of W. Earl Stewart, Magnetic Recording Techniques, McGraw-Hill Book Company, Inc., New York, N.Y.

Tape Prices

Q. I can get 1800 feet of 1-mil polyester tape for $1.25 a reel, or similar tape for $2.25 a reel. Which is the best value, or are both tapes not worth buying?

(George Katzmarek, Jr., Warren, Michigan)

A. You tend to get what you pay for. Chances are that the more expensive tape will be better lubricated, more precisely slit (and thereby avoid skewing Tape Guide (cont.) or sticking), have better magnetic characteristics ( frequency response, distortion for a given recording level, etc.), have more uniform characteristics throughout the reel and from one reel to the next, and so forth. True, one does sometimes come across a good buy in "unbranded" tape, but the tape which may be a good buy one day may not be so at another point of time.

Recording Bias

Q. I have read that "... a specified value for bias voltage across the record head more nearly assures a constant bias field than a specified current." Is this true?

(C. J. Woodcock, Chicago, Illinois)

A. This statement is new to me, both in terms of what appears elsewhere in the literature and my own experience.

The usual professional way of measuring bias current through the record head is by measuring the voltage across a small resistor in series with the ground leg of the head. The resistor is much smaller in value than the impedance of the head at the bias frequency, and therefore has negligible effect on bias current. Current through the head is calculated by Ohm's Law, based on measured voltage and resistor value: I=E/R.

D. C. Bias

Q. With reference to the use of d.c. bias in recording, just how linear is it?

(C. J. Woodcock, Chicago, Illinois)

A. I don't know that d.c. bias produces more distortion than a.c. bias. The principal objection to d.c. bias, so far as I know, is that it results in more noise than a.c. bias and tends to magnetize the tape heads. The amount of distortion depends upon the amount of bias. current and upon the level of the audio signal.

Computer Tape

Q. I have access to high-quality digital and analog computer recording tape. I would appreciate comments on the subject.

(Dr. John R. Sturgul, Tucson, Arizona)

A. Tape made for computers may have substantially different frequency -response characteristics than tape made for audio, requiring changes in bias current and equalization.

(Audio magazine, Sept. 1970; Herman Burstein)

= = = =