Tape Guide (Q and A) (Nov. 1974)

Pinning the Needles

Q. I own a Sony professional stereo tape deck, Model TC-355. The only thing which I regret is that I do not have a limiter built into the recorder. I make a great number of piano recordings, and since most of the time I am the pianist, it is hard to be an engineer and run the recorder at the same time. I find it difficult to set the re cording volume level, because the soft passages turn out fine during playback, but when I play crescendo, the sound pins the needles of the VU meters. Al though the level does exceed the distortion range on the meters, there is no audible distortion during playback due to the quality of the recorder. But I am sure you understand that it does absolutely no good to the meter to have the needle pinned at the peak levels. Therefore I am seeking a solution to the problem, namely how to install a limiter into the tape machine's circuitry, so that the limiter can be turned on or off as needed for certain recordings.

-Zoltan Zeisky, Trenton, New Jersey

A. If you want to associate a limiter with your tape recorder, I suggest that you address your question to the manufacturers of such equipment. It seems that a simpler solution might be to re-calibrate your VU meters. The fact that you do not get noticeable distortion when the VU meter hits hard all the way right suggests possible miscalibration. Also, let me point out that VU meters (genuine ones, not cheap meters with VU scales) are constructed to take a good deal of overload, so that you really may be doing no harm at all to the meters by pinning them. In a well designed tape machine, signal amplitude is restricted to a range which will not harm a genuine VU meter. Finally, keep in mind that use of a limiter in recording may deleteriously alter the quality of the recorded sound.

With or Without Dolby

Q. I am planning to purchase a tape deck. Will the use of a Dolby unit, such as the Advent Model 101, produce tapes at 3-3/4 ips that are audibly better than tapes made without the Advent unit?

-Dennis M. De Santis, Philadelphia, Pennsylvania

A. All other things being equal, a system that includes the Dolby will produce a better signal-to-noise ratio than the same system without the Dolby. If the tape recorder is of very high quality, the improvement added by the Dolby may tend to be unnoticeable. If the tape recorder is of rather low grade, the Dolby will probably pro duce considerable improvement. How ever, an inferior tape recorder with the Dolby will not necessarily come out with as good S/N as a superior ma chine without the Dolby. In other words, you can't make a silk purse out of a sow's ear.

Dolby Operation

Q. Can you answer the following questions for me? (1) Is there a discernible background noise, or hiss, in a good non-Dolby tape player? Does this noise exceed that of a record player? If so, does the Dolby system reduce this noise noticeably? (2) What is the principle of operation of the Dolby system? Does it require special tape? Added tracks? (3) How would you compare cassettes, cartridges, and open-reel tape for home music systems?

-George Jeromson, Sherman Oaks, California

A. A top-quality home tape recorder, without Dolby, can record and play back with virtually no discernible noise at 7 1/2 ips. A few can do so at 3 3/4 ips as well. On the other hand, if volume is turned up to near-thunderous levels, or if one recorded at much too low a volume, noise does become discernible. I believe that Dolby would perceptibly reduce such noise. On the whole, noise of a really fine tape system compares well with that of a good disc system.

Dolby does not require special tape or added tracks. It requires the input signal to the tape machine to be fed through a "black box" and the output signal from the tape machine to be fed through a complementary "black box." These boxes, containing electronic circuitry, serve to boost the signal in the noise area (treble range) in the case of input; and to correspondingly decrease the signal in the noise area, and thereby the noise as well, in the case of output. The boxes only act on signals of low magnitude, which is when noise is more discernible; and by confining their action to signals of low magnitude, the boxes avoid overloading the tape system.

I think that in the present state of the art, open-reel tape machines still have some superiority over cassette and cartridge machines. However, the cassette machines have been improving fast and narrowing the margin of performance between them and open-reel units. Cartridge machines do not appear to be improving as fast.

Bias Frequency

Q. Would you please explain the term "bias frequency."

-Rick Bacon, Gainesville, Florida

A. Bias frequency is a high frequency signal, usually in the range of 70 kHz to 100 kHz (occasionally to as high as 150 kHz) in high quality tape recorders, that is applied to the tape simultaneously with the audio signal. The magnitude of the bias current is something like 10 times that of the audio current. Usually the bias signal and the audio signal are both fed to the record head. Sometimes, in the crossfield method, the bias is fed to a separate head located exactly opposite the record head, so that bias is applied through the back of the tape. The purpose of bias is two fold: (1) to reduce recorded distortion on the tape; (2) to maximize the amount of audio signal recorded on the tape, thereby resulting in increased signal-to-noise ratio.

(Audio magazine, Nov. 1974; Herman Burstein)

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