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AUDIO QUESTIONS and ANSWERS: Advice on readers' technical problems, by LARRY KLEIN, Technical Editor Double-Dolby Dubbing Q. Consider the fallowing situation: I want to dub a Dolbyized reel-to-reel tape onto a cassette. Should I decode the signal from my open-reel machine and then re-en code it with the Dolby circuits in my cassette deck, or should I feed the Dolbyized open-reel recording into the cassette machine with all Dolby circuits shut off? I've tried it both ways, and the best results occur when I activate the Dolby circuits in both units. In other words, I decode the output of the reel-to-reel unit and then encode the signal as it is being fed into the cassette. However, this seems like unnecessary processing. Is it? JOHN KADOR; Durham, N.C. A. Your observation is correct; best results are achieved in duplicating tapes when you decode the Dolbyized signal and then re-encode when making the copy. Although this may seem like a redundant process, it is necessary because accurate decoding of a Dolbyized signal can only be achieved if the decoder "tracks" the encoder both in signal level and frequency. When you copy a Dol byized signal, the odds are that it is recorded on the new tape at a higher or lower level than on the original tape. This can confuse the Dolby decoding circuits, because the Dolby-level reference point has shifted. In addition, any frequency aberration that occurs in the signal after the encoding (because of the use of a hotter tape for the copy, for example) would be emphasized by the decoding pro cess. In short, if an encoded signal is not fed to the decoder circuits at exactly the same reference level at all frequencies that it had when encoded, then the Dolby circuits simply cannot react properly. The result is a diminution of the noise-reduction potential of the Dolby circuits and some high-frequency boost (or loss) in playback of low-level (soft) audio signals. Distorted Views Q. Much of the equipment evaluation in your magazine is based on the degree of deviation from the accurate reproduction of a sine wave. The various modes of distortion are given in percentages of such deviations. Yet, isn't it true that any musical instrument manifests at least some distortion of the sinusoidal waveform and isn't such deviation to be expected? Although such deviations from an ideal sine wave may be measurable in a laboratory, they have absolutely no meaning in musical listening enjoyment. The ear is the only final judge. G. M. KOSOLAPOFF; Auburn, Alabama A. Mr. Kosolapoff's letter shows a basic confusion between sound production and reproduction. It is true that one rarely finds a pure sine wave (meaning a tone without any harmonic content) among natural sounds or even those produced by musical instruments. However, this fact has nothing to do with what we demand of our audio equipment. Ideally, a component should be able to do a perfect job of amplification-or transduction--of any audio signal, whether it be a sine wave from a signal generator, a trill from a Stradivarius violin, or the mating call of an impassioned yak. My definition of "perfect" is this context is: good enough that trained critical ears cannot detect any difference in quality between the component's input and output signals. Note that there is no attempt to define in numbers how good is good enough. We just don't know enough about the sensitivities of the ear to various kinds of waveform distortions to state with assurance, for example, that x percent of any given type of distortion is audible or inaudible and under what circumstances. Of course, it's easy enough to determine that gross distortions of one type or another are readily audible, but when you get below 5 percent or so, then the nature of the distortion, the complexity of the program material (or test signal), the acuity of the listener, and the ability of the other components to pass on whatever fine distinctions exist, can play a determining role in whether or not a measurable distortion is also audible. To return to Mr. Kosolapoff, he almost touched on a matter of real concern to many audio engineers: the validity of the commonly used test signals as indicators of component quality. All of the distortion test procedures work the same way. A signal (or signals) of known characteristics is fed into the component. The signal(s) at the output of the component is analyzed to determine the degree to which it has been undesirably modified by the action of the component. Although the percentage numbers provided by the analyzing instruments do correlate somewhat with the perceptions of trained listeners, the correlation is far from absolute. Many investigators think the problem arises from lack of a test signal that properly simulates the characteristics of music. Others feel that the available test signals are adequate, but that the effect of the component on them is not analyzed in sufficient detail to disclose the audibly significant factors. Both may be right. Given all of the above, we've come to an area where Mr. Kosolapoff and I agree. For the present, the ear is the final judge. But, unfortunately, the acuity, the consistency, and the "taste" of the ears making the judgment must also be care fully evaluated, and that is a whole 'nother ball game. Equalization for 78's Q. I am taping a number of 78-rpm discs and am trying to locate a listing of the old equalization curves used by the various recording companies so that I can compensate for them with my graphic equalizer. Do you know where I could find such a list? JOSHUA P. HILL; New York, N.Y. A. many listeners, the search for the "correct" record-playback equalization seems to have achieved the status of a holy quest. But consider: every time a recording engineer or producer twitches a frequency-equalization knob during the recording or mixing process he is, in effect, injecting his own deviation from the recommended RIAA (Recording Industry Association of America) equalization curves. And any time you second-guess the engineer (it is frequently necessary to do so) by adjusting your tone controls, you are in a sense departing from his notion of the proper RI AA equalization. During the 78-rpm days and the early LP days, the recording-equalization situation was quite chaotic. There were at least seven eight different equalizations in common use, and many preamplifiers had two separate controls-one to set the low-frequency turnover and the other the high-frequency rolloff--to be manipulated according to the recording company's notion of how it should be done. Frequently, a change in chief engineers would result in a change in a company's record equalization; the same record label might therefore require any of several different equalizations, depending on the date of a particular pressing. In the light of all this, it seems pointless to be concerned about duplicating the original specified equalization for each specific disc. But there are other considerations also. All these discs were engineered to play through what would today be thought of as very low-fidelity phonographs. The equalizations employed were intended to get the best possible performance despite the technical recording and playback limitations of the day. It doesn't necessarily make sense therefore to duplicate the old equalization curve when the disc is going to be played through modern equipment. Why not simply adjust your equalizer until you get the best possible sound from each disc as heard through your system and make your tapes using those settings? If that idea strikes you as cheating or diminishing the "authenticity" of the original, then I would suggest that your only recourse would be to find a phono graph of the same vintage as your discs and do a "live" recording of its acoustic output. Incidentally, I'm sure that you are aware that many of the old 78's were not recorded at exactly 78 rpm. Therefore, if the music seems a little sharp or flat, try adjusting the playing speed slightly up or down before applying the equalization.
Also see: EQUIPMENT TEST REPORTS: Hirsch-Houck Laboratory test results on the: JVC JR-S600 AM/FM stereo receiver, Marantz Model 1250 integrated stereo amplifier, KLH Model 354 speaker system, and Micro Seiki DDX-1000 turntable and MA-505 tonearm NEW PRODUCTS: A roundup of the latest in high-fidelity equipment AUDIO BASICS: Glossary of Technical Terms, RALPH HODGES Source: Stereo Review (USA magazine) |
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