Tape Guide (Nov. 1979)

Phasing

Q. I am interested in an audio pro cess known as phasing, used effectively by many rock groups. Phasing is accomplished with three tape recorders, two used as playback units, with the same material, only slightly out of synchronization with one another. The third records the effects. As one tape recorder catches the other, phasing begins. As the two come closer together, the pitch rises to a maximum. This varying pitch is not present in either of the two original recordings. I would like to know what causes this effect.

What are the two signal waveforms doing? Is this phase modulation? Frequency modulation? In case you are not familiar with this particular technique, enclosed is a tape containing some excerpts from records which use phasing.

-Dan Keen, Avalon, N.J.

A. I am not certain about the reason for the phasing effects on the tape you sent me, but perhaps I can offer a partial explanation. When signals are in phase they augment each other and are louder; conversely for signals out of phase. Human hearing tends to pro vide an illusion of pitch change as loudness changes. Further, when a complex signal is combined out of phase with another complex signal (many frequencies), cancellation tends to be more effective at the lower frequencies. The net effect is of rising pitch. You can test this on a stereo sys tem by tuning in noise on an FM tuner and combining the two signals out of phase- assuming your amplifier pro vides a switch for combining signals out of phase. You will notice an in crease in apparent pitch of the combined signal.

Dis-Service Manual

Q. Some time ago I purchased a tape deck. After reading the manual, I appear to have bitten off a helluva lot. The dealer has been helpful in trying to interpret the manual, but he is as much dismayed by some of its con tents as I am. The manufacturer has not responded to my request for more detailed instructions. Can you suggest an approach that would enable me to obtain optimum results from the machine? What recording I have done has been hit and miss, very good and very bad.

- E.L. Boyce, San Francisco, Cal.

A. I am sorry but I am not in a position to rewrite a manufacturer's manual. I suggest that you put pressure on your dealer to obtain answers to your specific questions or refund your money. You might also ask the dealer who the manufacturer's sales representative is in your area, and put similar pressure on this individual. Let me repeat: You should prepare specific questions rather than make a broad request for more detailed instructions.

Built-In Test Tones

Q. Back in 1972 you wrote an article advocating that tape decks should have built-in test tones. To be really useful, the test tones should go down to 20 Hz and up to 25 kHz. The harmonic distortion of these tones should be low to be of value. In my opinion, adding these circuits to an already complex as well as heavy machine is not really worthwhile. There are many good oscillators available that are a valuable adjunct to a serious enthusiast's equipment.

- Ramon Valdes, New York, N.Y.

A. I tend to disagree with you about incorporation of test-tone facilities in a tape deck for the serious home recordist. Some test tones are better than no test tones. And very low distortion test tones are not needed for all purposes. I am not trying to say that built in test tones would eliminate the need -- particularly in the case of the technician--for a first-rate signal genera tor. But they would be a great convenience to the user and induce better maintenance on his part, as well as enabling him to extract maximum performance from his deck at all times.

With respect to complexity, modern technology permits signal oscillators to be built in very miniature form, occupying a minimum of space and having a minimum of weight.

Multi-Track Recordings

Q. I have a four-channel, three-head tape deck. I want to record separately on each channel in the sound-on-sound mode, and then mix down the four channels, using a stereo copy deck. The problem is that with three heads the tracks are not in sync. I have heard of "Sel-Sync," but I'm not sure how 'it works. I think it converts the record head to a. playback head on a track that has already been recorded. Please explain the Sel-Sync system and, if possible, tell me how to add it to my recorder.

-Larry Swartzwelder, Harrisburg, Penn.

A. Your basic understanding of the Sel-Sync system (also known as multi-sync, etc.) is correct. It converts the record head to a playback head on the desired channel, so that in recording on another channel the playback signal will be in time synchronization with the signal being recorded. How ever, I can't give you instructions for converting your tape deck to Sel-Sync. What you might do is obtain a service manual (costing about $2.00 to $3.00) for one of the tape decks that has Sel Sync and copy its Sel-Sync switching circuitry.

Quarter-Track Vs. Half-Track Decks

Q. My problem is whether to get a quarter-track or half-track tape deck. I'm really not sure what the advantages of the half-track configuration are.

-Stephen Irwin, Columbia, Ohio.

A. For home use I see no net advantage in half-track, other than an improvement of about 3 db in signal-to noise ratio. While the extra 3 db is not to be sneezed at, a top quality home machine has sufficiently good S/N-particularly with Dolby- so that the deterioration in performance is hardly, if at all, noticeable. Meanwhile, quarter-track offers twice as much time per reel of tape, and has the advantage that errors in azimuth alignment do not adversely affect treble response as much as in the case of half-track. On the other hand, half-track is superior with respect to dropouts, because the wider track permits dropouts to aver age out better; still, top-quality tape today is so good that dropouts are rarely a problem. All in all, my vote is for quarter-track for the home, assuming you are not going to be editing tapes. If you are, which involves cut ting and splicing, then you might get half-track.

Adding a Dolby Unit

Q. My tape deck has a non-Dolby single-pass noise suppressor. I have decided to add a Dolby unit. Can this unit and the built-in noise suppressor be used together?

- Gary Leeds, South Orange, N.J.

A. Probably you should not use the two together. The built-in suppressor most likely works on the principle of cutting high frequencies, whereas the virtue of the Dolby unit is that it enables you to maintain treble response while at the same time reducing noise.

You may want to use the built-in suppressor for tapes you recorded before acquiring the Dolby unit.

Bias Adjustment for Thicker Tape

Q. I have been told that using a thicker recording rape produces less print-through. However, the thicker tape is harder to magnetize. Would it sound any better using the thicker tape if the bias could be adjusted for maximum undistorted output? How about equalization?

-Susan Mackey, Kew Garden Hills, N.Y.

A. To my knowledge, thicker tape is not harder to magnetize. Any tape should be optimized in the sense of adjusting the tape deck's record equalization (treble boost), audio drive cur rent, bias current, and record-level indication in accordance with the suggestions of the tape deck manufacturer. Usually this requires the services of a competent technician. Most tape decks come adjusted for a particular tape, such as Manufacturer X's low noise/high-output tape No.-.

Reel Tape Differences

Q. Is there any difference between high quality 1800-foot and 1200-foot, seven-in. open-reel tapes?

-Fred Waites, Charlotte, N.C.

A. The advantage of 1 1/2-mil tape (1200 feet per seven-in. reel) over 1-mil tape (1800 feet) would lie in higher signal output and lower print-through.

However, from a practical point of view, these advantages are sufficiently slight so that the home recordist should not be inhibited from using 1-mil tape. In ordinary use, I doubt that you hear any difference between 1-mil and 1 1/2-mil tape, everything else (oxide formulation, etc.) being the same.

Improving High Frequency Response

Q. To improve the high frequency response of my tape system, I want to purchase a graphic equalizer which would be inserted between my Dolby noise reduction unit and the deck. What is the likelihood of success?

- David Ohde, Weaverville, Cal.

A. If inserted between the Dolby unit and the deck, the equalizer would tend to upset proper operation of the Dolby N/R. Keep in mind that the Dolby unit provides treble boost only for low-level signals, and for such signals it provides corresponding treble cut in playback. The presence of the equalizer would alter the information needed for proper Dolby decoding. Therefore, the equalizer should be introduced prior to the Dolby unit.

Specifically, the signal would be fed from your amplifier to the equalizer and thence to the tape deck. But be alert to the fact that treble boost sup plied by the equalizer increases the chance of tape saturation at high frequencies. Accordingly, it may be advisable to reduce recording level several dB.

(Adapted from: Audio magazine, Nov. 1979; Herman Burstein)

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