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

by HERMAN BURSTEIN

Tape Speed vs. Quality

Q. Can you please tell me the difference in quality between operating at 7 1/2 and at 3 3/4 ips?

(Joseph Torneo Jr., Scranton, Pa.)

A. If you are using the finest tape machines that today's state of the art can produce, and top quality tape, there tends to be very little audible difference between results at these two speeds. Perhaps there will be a little less noise and a little more "transparency" at 7 1/2 ips.

Also, you have more "headroom" at 7 1/2 ips: there is less chance of running into distortion at high frequencies because less treble boost is applied in recording at this speed. The differences in general tend to be more obvious to measuring instruments than to the ear-provided you are using a top-notch machine and tape.

"Sour" Notes

Q. My problem is distortion in the form of "sour" notes, most noticeably in the high frequencies. I am unable to remedy the problem despite the fact that I regularly clean and demagnetize the heads. I would greatly appreciate your advice.

(D. V. Turner, Los Angeles, California )

A. The difficulty you describe appears due to wow in your tape transport. And this could be due to an out-of-round idler wheel, belt slippage, oil or other lubricant on the capstan or pressure roller, and so on. I suggest that you clean the capstan and pressure roller thoroughly with whatever fluid is recommended by the manufacturer of your tape machine. Also clean all other parts, such as guides, contacted by the tape. If this doesn't help, I am afraid the problem must be referred to an authorized service center suggested by the manufacturer.

Microphone Selection

Q. I'm shopping for a new microphone and I'm completely in the dark about the rating of sensitivity and output. I have a good tape recorder and a good microphone, but for voice and lecture recording the microphone is too sensitive. If it is touched or moved while recording, the recording sounds as though a train went by. One evaluator of this microphone said that it was great except that the output was low. The manufacturer's literature gives a sensitivity figure of about 50 dB. What gives? Specifically, what specifications do I look for to obtain a microphone which can be placed several feet, maybe as many as 15, from the person speaking and still get adequate pickup for recording?

(Rex H. Shudde, Philadelphia, Pa.)

A. Rather than risk confusing you all the more by trying to explain in limited space the various methods of rating microphone sensitivity, let me state that the figure you cited indicates that yours is a highly sensitive microphone; that is, it has relatively high output. The lower the figure given (disregarding the minus sign), the higher the sensitivity. Thus a microphone rated at-50 dB has greater sensitivity than one rated at -55 dB. Incidentally, a rating of -55 dB is about average for high-quality microphones.

As for picking up low-frequency noises, this is a problem of mounting the microphone so that it is insulated from such sounds. A boom type of mounting or other shock mounting may be necessary in your circumstances. A microphone should not be moved or touched when in use. Some microphones are available with a switch that reduces bass response, and this might help with your problem. If you plan to use a microphone with no more than about 15 feet of cable (low capacitance cable), a high-impedance microphone is ordinarily satisfactory. For longer cables, a low-impedance microphone is advisable; otherwise treble frequencies are attenuated by the cable capacitance.

Azimuth Alignment

Q. What is the proper procedure for azimuth alignment of a 4-track recorder with a full-track test tape. Should the alignment method recommended by the machine's manufacturer be followed with the two channels strapped together, or is it necessary to repeat the procedure using first one channel and then the other for playback of the azimuth test tone while adjusting the head for a maximum indication on a VTVM?

(Robert Pearson, Chicago, Illinois)

A. I think that the best procedure is one which deals with each channel separately. If a single head position does not maximize output on both tracks, then it is necessary to find a compromise position which results in about equal performance on the two tracks.

Lack of Bias

Q. My tape deck is giving me some difficulty; perhaps you can suggest some corrective measures. The most annoying and bewildering problem is the intermittent loss of the erasing and recording functions on both channels. In attempting to record during such losses, the input signal does get far enough through the record amplifier to drive the VU meters to full level, and the RECORD indicators light up when the RECORD buttons are pushed. However, on playback the tape is found to contain no signal unless the input while recording was turned up to a level which would register well above the 0 point on the VU meters, and under such circumstances the recording which does result is of course severely distorted.

Previously recorded material is not erased if a used tape is being employed in the recording process. This difficulty comes and goes unpredictably, disappearing for weeks at a time, only to reappear suddenly and remain for similar lengthy periods.

(Theodore L. Purnell; Hammonton, New Jersey)

A. It appears that your difficulty lies in the bias oscillator circuit. Loss of oscillator current would make the erase head inoperative, and it would cause the recorded signal on the tape to be very low and distorted. The trouble may be as simple as a faulty oscillator component (transistor, capacitor, or the like). It may lie in an intermittent connection. Or it may lie in a component, such as a resistor or capacitor, leading from the oscillator circuit to the heads. If you have a service manual that gives operating voltages in the record mode, you might check these voltages in the oscillator and related circuits.

(Audio magazine, Dec. 1970; Herman Burstein)

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