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High End Fallout
Q. I have a tape deck and am not satisfied with its recording performance, specifically the high end. According to the Sony Manual the frequency response at 3-3/4 ips should be 20Hz to 17 kHz. I have tested the playback frequency response at this speed and it seems to be about 20 Hz to 15 kHz ± 3 dB, using the standard test tape. However, when I record a test record on tape, I find that the frequency response above 10 kHz is extremely poor. I have checked my phono system and receiver, and these are not at fault. If 1 return the deck to my local dealer, will he be able to change the bias to optimize the frequency response, without an increase in distortion?
-Robert Simpson, Reston, Va.
A. Your tape machine might be supplying excessive bias current to the record head. Reducing bias to extend the treble response is a relatively simple matter for a technician: However, you should be aware that as bias is decreased, there is a rise in the distortion, or to keep distortion as before will result in a decrease in the signal-to-noise ratio, i.e. you have to reduce the recording level. I feel that at 3-3/4 ips you should not aim at a response beyond something like 12 to 15 kHz, depending upon your hearing acuity. Anyway, few adults can hear much above 13 kHz.
High Speed Dubbing
Q. At one time you stated ."Little if anything is lost in the process of high speed dubbing of tapes." Assume we record a 15 kHz signal on tape and then dub it onto another tape at high speed. If the playback heads have a-3 dB at 15 kHz, the response will be a lot lower at 30 kHz-the corresponding frequency at twice the normal speed. Then the playback and record preamps would have trouble passing a 30 kHz signal.
Also, on the decks used to copy, the record heads would have to record a 30 kHz signal. In my opinion, quite a bit is lost in high speed dubbing.
What do you think?
-James Harvey, APO, San Francisco.
A. You are correct in the fact that the heads and amplifiers have a larger task to perform in high speed dubbing since they have to permit a more extended treble response. However, modern technology does provide heads and amplifiers capable of handling frequencies up to several times 15 kHz, which means that good dubbings can be made at several times the original speed. Keep in mind that treble loss of a high quality playback head is chiefly due to gap width relative to recorded wavelength; so as frequency rises and wavelength grows shorter, the loss increases. But if a 15 kHz frequency is played at twice the normal speed, the recorded wavelength on the tape stays the same, and the loss due to gap width is not increased. The loss is greater as a result of the head's winding and cable capacitance, but these can be kept minimal through use of high quality playback heads and short, low-capacitance cable.
Speed = Frequency Response
Q. When recording, why should the tape speed determine the frequency response?
- Howard Wong, Jackson Hgts., N.Y.
A. In recording on tape, losses are a function of the wavelength recorded on the tape; so the shorter the wavelength, the greater the loss owing to what is sometimes called self demagnetization or self-erasure, that is, a short recorded wavelength has its opposite poles close together, and the closer these poles are to each other, the more they tend to cancel each other out in terms of their external magnetic field.
At a high tape speed, a given signal frequency produces a relatively long wavelength on the tape because much tape passes the head during a given amount of time, hence there is little loss due to self-erasure. But at a low tape speed, the same frequency results in a short wavelength because little tape passes the head during the same time span, so now there is considerable loss due to self-erasure. In summary, a reduction of the recording speed results in decreasing treble response, and all other things remain the same.
Q. 1 would like to vary the speed of my Sony tape recorder. Would it be possible to run the motor current through a variable a.c. transformer to alter the speed? If not, can you think of another method that won't damage the motor?
-Terry Black, Springfield, III.
A. The most feasible method I can think of is to build yourself a power supply with variable frequency. These have been described in electronic literature.
Q. Is there any precise way to calibrate a tape recorder's VU meters?
-Frederick Kistler, APO, San Francisco.
A. The VU meter should be adjusted to read 0 VU when a 400 Hz signal is recorded at a level that produces 1 percent harmonic distortion on the tape, as checked in the playback.
Q. I have Sony tape on seven-inch plastic reels, and when they are placed on a flat surface they don't lie flat. On my machine when it is playing left-to-right everything is fine, but when the deck is in automatic reverse and the tape is moving right-to-left the tape can be heard scraping against the side of the reel. Alignment of the machine has been checked. What can I do about this scraping noise?
A. The first thing I suggest is that you buy new reels and transfer your tapes onto these. Of course, first check the new reels for trueness.
Then you might contact Sony, and, perhaps, they will replace the reels if they are defective. What may have happened is that your reels came from one particular lot which suffered a manufacturing defect. This can happen with any product.
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, Aug. 1977, Herman Burstein)
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