Tape Guide (Q and A) (Dec. 1976)

Dolby Update

Q. With the advent of Dolby equipment and 4-channel stereo, I am wondering what will happen with the prerecorded open-reel tapes? Will the record companies be issuing a lot of their former tapes now Dolbyized and later in a 4-channel format? If they might, would it be better to wait and not buy the tapes at the present moment?

-W. D. Robertson, Vancouver, B.C., Canada

A. I think that the present 2-channel stereo format, using non-Dolby tapes, will be around for a while. There is, of course, a trend to the 4-channel format and to Dolbyizing everything in sight, but it takes time to work out problems and to secure industry agreement on standards. Moreover, it is not clear that the Dolby noise reduction system has permanently sewed up the audio market. There are some competitive systems, like dbx or Burwen, which promise even greater noise reduction than Dolby. Or, conceivably, the Dolby system might be modified to further reduce noise, but all this takes time. In the meantime, it seems a pity to consign yourself to an indefinite waiting period and deny yourself the pleasure of listening to tapes you would enjoy. Furthermore, when changes do come, there is usually a strong attempt to maintain compatibility with previous procedures, so that there is a good chance that your previously acquired audio equipment, tapes, etc. will not be made obsolete.

Dolby vs. Burwen

Q. I have read articles on the Dolby and Burwen noise reduction systems, but I'm confused as to how the Burwen system works. What enables it to give such a greater noise reduction than the Dolby system and at the same time allow much lower recording levels?

-Wallace Bacon, FPO San Francisco, Calif.

A. The Dolby system operates on the principle of boosting the treble frequencies at low signal levels in recording, and correspondingly deemphasizing the treble frequencies at low signal levels in playback, thereby restoring flat response and at the same time reducing noise that occurs in recording and playback. The Burwen system similarly employs treble boost in recording and treble cut in playback. In addition, it compresses the recording signal and correspondingly expands the playback signal.

Such compression permits a greater degree of treble pre-emphasis in recording, without overloading the tape, than does the Dolby system. Hence the Burwen system can apparently achieve greater noise reduction.

In the above, I have been referring to the Dolby-B system widely used for home application. For professional use, the full-scale Dolby system divides the audio range into four bands and applies its noise reduction technique to each. Since noise is most apparent in the treble range, the Dolby B system is quite effective.

Whistle Filter

Q. Could you please refer me to some source which would give details on making a filter to cope with my tape recorder whistle? I have realigned my tuner, a Dynakit FM-3, according to the instruction manual, but I get a high-frequency whistle on tape when recording a stereo broadcast. There is no whistling on mono broadcasts. I tried feeding the tape recorder from the "main output" jacks of my preamp, with the scratch filter on. This eliminated the whistle, but also eliminated the high frequencies of the audio signal. I'd appreciate your help.

-Sheldon Isaac, Phila, Pa.

A. When you have a specific problem with a specific component, it is a good idea to write first to the manufacturer of that component. In the meantime, I suggest that you place a trap tuned to 19 kHz across each output of your tuner. This would consist of an inductance and capacitance in series, placed between the hot and ground terminals. If you use an inductance of about 10 mH, then you would need a capacitance of about 0.007 µF. Given the value of inductance, the required capacitance is calculated from C = 25,000,000/L2F, where C is capacitance in pF, L is inductance in mH, and F is frequency in kHz.

Preamp Problem

Q. I have been having a problem with the record preamps of my tape deck, which uses tubes. They have been producing a quite audible distortion. The distortion is present be fore the signal enters the record heads; it is audible in the "source" position of the monitor switch. The distortion increases as the record gain control is turned up and is equally bad on both channels. My first inclination would be that there is a bad tube in the first stage(s) of the preamps. However, no one tube (except the oscillator) is shared by the two channels.

-Foster Action, Nashville, Tenn.

A. Your problem may be due to a defect in the power supply. Have you checked the rectifier tube and other power supply components? If voltages have seriously changed owing to a defect in either the power supply or coupling circuits, this may cause one or more tubes to distort, and distortion would increase as gain was increased.

Tape Developer

Q. Please describe the "developing" process by which recorded signals are made visible on the tape.

-Reg Fulton, Phila., Pa.

A. The information I have on Magna-See, made by Reeves Soundcraft Corp., Great Pasture Rd., Danbury, Conn., states: "Magna-See is a nontoxic and non-inflammable fluid into which you simply dip a strip of recorded tape, let it dry, and you can actually see the track recorded on the tape. By doing so, you can check azimuth, head alignment, and track uniformity." For more information, I suggest you write to them.

Track Comparison

Q. Is half-track that much better than quarter-track? Can you get quality performance from an automatic reversing deck?

-Sheldon Neider, Houston, Tex.

A. Half-track operation permits a higher signal-to-noise ratio-about 3 or 4 dB better than quarter-track.

The top quality reversing tape machines provide about equally good performance in either direction on automatic reverse. When such machines first came out, there were problems in maintaining equally good performance in each direction, but these problems appear to have been largely surmounted.

If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.

( Audio magazine, Herman Burstein)

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