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Variable Pitch Q. Is there any way to install a vari able-speed (pitch) control in a tape deck that doesn't have one? -Mark Mayhew; Platteville, Wis. A. There is no easy way that I know. One can build a variable-frequency power supply-assuming that the speed of the capstan motor varies with frequency--if one has sufficient knowledge and skill. Articles on building such a power supply have appeared from time to time in various electronic periodicals which your local library might have. (One such article, if you can still find it, is "Programmable Control" by Gary McLellan, in Popular Electronics, April 1981.) Time for a New Deck? Q. I have an eight-year-old tape deck whose frequency response and signal-to-noise ratio are not as good as another, three-year-old deck which I also own. The older deck is in need of repair. Since I need a second deck for dubbing, I'd like to know if I should repair the older deck or buy a new one. -John DeRosa, Mattapan, Mass. A. In a time when very good performance can be obtained from cassette decks costing less than $300, it is questionable whether it pays to try to bring an eight-year-old deck back to its original level of performance. Such a deck may have any number of problems, including pronounced head wear, excessive wow and flutter, improper frequency response, etc. You should get an estimate as to what the appropriate repairs would cost. If the amount approaches $100, it would probably be best to apply this sum toward a new deck. Emphasizing Treble Response Q. To accentuate treble response, I understand that some people record on high-bias tapes with the proper equalization but play back with normal equalization and with Dolby NR. Can I get the same or better results if, using a deck with fine bias adjustment, I slightly lower the bias while recording but in playback use the proper equalization? What adverse effects would this have? -Gabriel Vazquez Padua, Bayamon. P.R. A. First let me state that using Dolby noise reduction only in playback and not in recording will reduce treble response. On the other hand, using normal (120-µS) instead of high-bias (70-uS) playback equalization will accentuate treble response. The net effect would appear to be a moderate cut in treble. Recording with underbias in order to emphasize the high frequencies tends to increase distortion. Whether the effects of underbiasing are audible depends on the extent to which you underbias, on your recording level, and on the sensitivity of your hearing. The better course, if you want to moderately emphasize treble, is to use 120- instead of 70-µS playback equalization. Or you could judiciously use your audio system's treble control. I suggest that you experiment with various combinations of bias, playback equalization, use of Dolby NR only in the record mode, and use of your system's treble control to learn what best satisfies your taste. Dolby Level Q. The term "Dolby level" crops up in the literature frequently. What does it mean? Why is it indicated on some decks and not on others? Why is it referenced in some deck specs and not in others? How can the average schnook use this information? -Russell Stepanchak, Columbia, Pa. A. The Dolby B and Dolby C noise-reduction systems work by boosting weak treble signals in recording and cutting them by an equal amount in playback, thereby restoring flat response and cutting high-frequency noise. The amount of treble boost varies with the high-frequency signal level; for the treble cut in playback to be complementary to this boost, the play back circuits must be able to deter mine what the original levels were. To make this possible, both record and playback circuits require a reference level of magnetism on the tape. This Dolby reference level is 200 nanoWebers per meter for a 400-Hz signal. When properly adjusted for a given tape, a 400-Hz signal which just reaches the Dolby-level mark on the meters in recording will be recorded at 200 nWb/m on the tape, and a signal of that level will register at the meters' Dolby mark in playback. Some decks come pre-calibrated for correct Dolby-level matching on specific tapes; since tapes differ in their sensitivity, other decks can be adjusted by the user for correct matching on any tape. Improper matching of recording and playback levels ("mistracking") when using Dolby NR will adversely affect frequency response, usually in the form of treble loss or rise. Mistracking of up to 2 dB is usually not considered serious. Inaccurate Speed Q. I recently purchased a home cassette deck which I use mostly to record tapes for playback on my car deck. But when I play these tapes they seem to run slower than on my home deck. A friend who owns the same home deck says he too notices slow playback on his car deck. What could be the problem? -Mike Egan, Lindenhurst, N.Y. A. Either your two car decks are both running slow or your two home decks run fast. It's more likely that the home decks are at fault, since identical decks are more likely than dissimilar ones to develop identical problems. Buy some prerecorded cassettes which list the timings of selections on the liner notes or labels. Listen to these cassettes on both home and both car decks, and check their timings on all four decks. If you notice audible speed differences, they are probably on the order of 6%, enough to alter the music's pitch by a semitone. Most people do not notice pitch deviation until it's an appreciable part of a semitone, though very few can detect a pitch deviation as small as 1%. Once you have found out which decks are at fault, have them adjusted by a qualified service shop. Azimuth Alignment Q. Please explain how I can check the alignment of my cassette deck's head through the entire frequency range, not just at one frequency. -Edward Kalinowski, Philadelphia, Pa. A. Presumably you refer to azimuth alignment of the record-playback head or playback head. Inasmuch as accurate azimuth grows more critical as frequency increases, it is tested at a high frequency, such as 12 kHz. If the check is performed at much lower frequencies, such as 1 kHz, little is learned. That is, an error in azimuth which produces insignificant loss at 1 kHz may produce disastrous loss above 10 kHz or so. On the other hand, an azimuth check isn't ordinarily made at a very high frequency, say 18 kHz, because many decks have very little response this high in the audio range. Further, the playback level may tend to fluctuate considerably, making it rather difficult to ascertain the peak reading when adjusting the head's azimuth. Third, at very high frequencies one tends to encounter false peaks in head response, so that an incorrect azimuth alignment may result. (To avoid aligning to a false peak, the procedure sometimes followed is to preliminarily align the head on the bias of a frequency in the low treble range, such as 5 kHz, and then touch up the alignment on the basis of a high frequency such as 12 kHz.) The customary way of checking and adjusting azimuth is to play a test tape containing a high-frequency tone, say 12 kHz, and adjust the tilt of the head until you obtain maximum output as indicated by an audio voltmeter or an oscilloscope. If the cassette deck's meter indicates playback level, this can be used, too. In the case of decks with separate record and playback heads, the playback head is aligned first. Then the record head is aligned by simultaneously recording and playing a high frequency tone and adjusting the record head for maximum output in playback. Dropouts on Splices Q. I am having difficulty when splicing cassette tape. I have tried both perpendicular and diagonal cuts. The trouble is the dropout of 1S or so that I hear when the spliced area (which is 1/2 inch long) passes over the heads. The splicing tape seems to be considerably stiffer than the cassette tape, and my guess is that, in playback, the portion of the tape with the splice loses contact with the heads. -Leo Unger; Los Angeles, Cal. A. The pressure pad should be effective in maintaining tape-to-head contact. Are you by any chance putting the splicing tape on the wrong side of the audio tape? It must go on the side away from the heads-the shiny side of most tapes (though some newer types are shiny on both sides). The two ends of the tape should butt up against each other perfectly, with no gap through which the splicing tape can be seen or through which its adhesive could ooze onto the heads or the next tape layer. Diagonal splices offer a smoother audible and mechanical transition between the tape sections than do perpendicular splices. (Audio magazine, Mar. 1986, HERMAN BURSTEIN) = = = = |
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