Tape Guide (Q and A) (Mar. 1978)

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Impedance Mismatch

Q. My two tape decks have input impedances of 100 kilohms and 50 kilohms. I wish to record radio broadcasts. May I record into the tape deck using the earphone jack of my transistor radio? I am concerned about an impedance mismatch.

-David Martin, APO San Francisco.

A. While I don't know the exact impedance of the earphone on the radio you are using, I am quite sure that it is low compared to the input impedances of your tape decks.

Therefore, you may safely make a connection from the earphone jack to the tape decks. I have done this a number of times with satisfactory results.

Re-recording Noise

Q. How many times can sound -on sound recordings be made for one song before there is a great amount of distortion?

- Preston Schaffer, Brooklyn, N.Y.

A. Distortion does not tend to rise appreciably with successive sound-on-sound recordings unless these recordings are made at excessively high levels. What does tend to go up is noise--about 3 dB with each recording. Also, there tends to be a progressive loss of treble response from the earlier recordings.

Tape Calibration

Q. My tape deck has a switch for changing between conventional and low -noise tape. As I understand it, the switch just changes equalization and doesn't affect the bias. I have been contemplating lowering the sensitivity of the VU meter so that a VU reading of 0 dB would really be +3 dB. I have been told that I don't really have to drive the tape with a higher input signal if I adjust the machine for the proper bias .. . but this is not my understanding. Please clear this matter up.

-Thomas Arntson, APO New York

A. Tape switches generally adjust the bias in order to give the user a choice between conventional and low noise tape. Sometimes the switch also adjusts treble boost in recording. For low -noise tape, more bias and less treble boost is required. Furthermore, for low -noise tape the sensitivity of the VU meter should be reduced about 2 to 3 dB so that it takes a stronger signal to drive the meter to 0 VU. The proper calibration of the meter is such that, using the tape you plan to record on, a 400 -Hz signal causes the meter to read 0 VU when the signal level is such to produce about 1 percent distortion on the tape.

Static Electricity

Q. A peculiar problem has occurred to myself and two friends in recording on high -output tape at a high volume ... while rewinding the tape every few seconds a spark jumps from the reel to the tape deck. I realize that somehow there is a static buildup on the reel that increases until it is large enough to arc over the machine's ground. What causes this buildup and do you think this will cause the metallic surface of the machine to become pit marked? What can be done to remedy this situation?

-Michael Smith, Middle Village, N.Y.

A. I cannot give you a specific explanation for the phenomenon you describe except to associate it with the tape material and conditions of low humidity in the tape environment. I doubt that this arcing will cause pit marks, but it depends on the nature of your machine's metallic surface. To alleviate this problem you might try a ground between the chassis of your tape deck and earth, or you might try to conduct the electric charge on the tape to chassis by mounting a strip of aluminum or some other metallic foil so that it contacts both the chassis and the tape. Also, try increasing the tape's humidity by storing it for a few days in a box with moist blotting paper.

VU Level Settings

Q. What is the proper VU meter setting when recording on a low -noise, high -output tape and on a standard tape? If 0 VU is the correct overload level for standard tape, would +3 to +6 dB be the proper overload level for the low -noise, high -output tape?

-Larry Scherer, St. Joe, Mich.

A. If the record level indicator is properly calibrated for standard tape, the maximum indication when recording should be about 0 VU for standard tapes and about 2 to 3 dB higher for the low -noise tapes.

Peak Recording Levels

Q. Can you please give me an idea as to the relationships between the European CCIR and the American NAB peak recording levels and the readings given by the VU meter?

-R. Newbury, Ottawa, Ont., Canada

A. In the U.S., the standard NAB recording level is approximately that at which a 400 to 700 Hz signal results in 1 percent harmonic distortion on tape. However, the maximum permissible distortion is widely considered to be about 3 percent and one would think that the VU meters would be set to read 0 VU at a signal level producing 3 percent distortion, since this level is about 6 to 8 dB above the signal producing 1 percent distortion.

But the VU meter is an average -reading device rather than a peak-reading one, and on brief, strong signals it may read substantially below the true signal level. Therefore, a margin of safety of about 6 to 8 dB is provided in calibrating the VU meter, though it is generally set to read 0 VU for a signal producing about 1 percent distortion.

European practice is to use meters that, as I understand, are closer to peak -reading devices than average reading ones. Thus, they could be set to read 0 VU for a signal that produces about 3 percent distortion on the tape-in other words, for a signal about 6 to 8 dB higher. Apparently, the European standard's level (CCIR) hasn't gone all the way in this respect. They have, as I gather, elected a standard recording level about 4 dB higher than the U.S. one, hence the two standard levels reconcile within 2 dB-not a profound difference.

If you have a problem or question on tape recording, write to Mr. Herman Burstein at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered.

Please enclose a stamped, self-addressed envelope.

(Source: Audio magazine, Mar. 1978, Herman Burstein)

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