TAPE GUIDE (Aug. 1987)

Riding Gain

Q. What is "riding gain"?

-Anthony Mauldin, Lewisville, Tex.

A. Riding gain means varying the record level while making a recording, to keep the signal level high enough to be clearly heard above the tape noise while ensuring that high -amplitude signals do not overload the tape. Such overload would cause distortion and possible treble loss as well.

Before the recording art developed to the point where cassette decks with signal-to-noise ratios well over 60 dB became common, it was very important to record at as high a level as possible, in order to maximize S/N. It was frequent practice to record at levels that would cause tape saturation on signal peaks unless gain (record level) were momentarily reduced during those peaks. This reduction could be done automatically by electronic de vices, such as limiters, or manually, by riding gain. The latter practice requires accurate knowledge of the music, preferably by having its score before one's eyes, so that gain can be reduced and restored at just the right moments.

However, with signal-to-noise ratios of 70 dB or greater now common on tape decks, riding gain seems both unnecessary and undesirable.

(Editor's Note: If you must ride gain, ride against the music-that is, slowly lower the gain while the music is getting louder, and raise it while the music is getting softer. If done properly, this enables you to maintain an only slightly diminished sense of the music's dynamics while ensuring that the signal will never fall into the noise floor or reach the overload level. Don't wait until the signal gets too high or low and then give the gain control a too -late jerk. -I.B.)

Speed Adjustment

Q. I am thinking of synchronizing the speeds of the two transports in my dubbing deck, with each other and with the deck in my car. At present their speeds are ever so slightly different. They are probably within specifications, but the difference is notice able to me. Is there a way to adjust the speed of a cassette deck?

-T. Veitch, address withheld

A. Some decks provide an internal speed adjustment, but many, regrettably, do not. Often there are other means of adjusting speed, such as changing a rotating part in the drive mechanism for one of slightly greater or smaller diameter. I have no way of knowing whether speed adjustment is readily feasible for your particular decks. Query the decks' manufacturers, or ask at the service shops in your area.

Dubbing via Videocassette

Q. I would like to copy some cassettes that were recorded with Dolby noise reduction. Since I do not own a second cassette deck, I would like to use my Hi-Fi VCR and then dub back to my cassette deck. Would this yield acceptable results? In what position should I set the Dolby switch on my cassette deck to make the final recording?

-Rod Davidson, Alliance, Neb.

A. Your dubbing project is entirely feasible and should yield excellent results. Play the original tape with Dolby decoding on as you record onto video tape. Then play the videotape and re cord onto cassette with Dolby encoding on, using Dolby B or C NR, as you prefer. Best results in terms of low noise and extended treble response are usually obtained with Dolby C NR.

Audibility of Distortion

Q. I am planning to purchase an outboard noise -reduction system to be used with a very high-quality cassette deck. The specifications for the NR unit state that its total harmonic distortion is 0.1%. Is this amount of distortion significant? More important, will it be audible?

-Keith B. Dant, Saginaw, Mich.

A. On a single tone, a listener with a highly trained ear might be able to detect distortion of 0.1%; on program material, it is very unlikely that he would be able to do so. Tests have indicated that distortion must reach about 5% on program material before listeners be come aware of it. In sum, for all practical purposes, unless one confines listening to test tones, distortion of 0.1% is inaudible and therefore insignificant.

Bias Basics

Q. How are audio and bias signals processed to make recording possible? Is the bias frequency standardized, or does the deck manufacturer optimize it for each deck model? In terms of bias, what is the purpose of the HX Pro recording process, and how does it work?

-Joseph J. Ferrier, Brooklyn, N.Y.

A. In tape recording, the relation ship between the signal fed to the re cording head and the magnetization level produced on the tape is not linear. In other words, changes in the magnetization level do not accurately reflect changes in the signal when that signal is very small or very large, creating distortion at those points. There is, however, a range of signal levels for which the relationship is linear. The addition of a bias signal, in proper proportions, shifts the audio signal up into this linear area. (This is explained more fully in a feature article on HX Pro in the August 1984 issue.) The only "processing" involved in re cording is the mixing of the audio and bias signals. This usually occurs just before they are fed to the recording head, though some decks have used a separate bias head located just across the tape from the recording head.

The bias frequency is not standardized and is left to the manufacturer; the choice will depend in part on the characteristics of the record head. It is universally recognized that the bias frequency should be at least five times the highest audio frequency to be reproduced, in order to avoid audible beats between the audio and bias signals. Therefore, the bias frequency is at least 75 kHz, and often 100 kHz or higher.

It has been found that the high-frequency portion of audio signals tends to behave in the same manner as bias generated by the bias oscillator.

Hence, if there is considerable high-frequency content in the audio signal, the tape may be over-biased and cause treble loss in the signal recorded on the tape. HX Pro senses the amount of high -frequency content in the audio signal and correspondingly adjusts the bias current fed to the record head.

Thus, the tape is better protected against treble saturation, and head room is increased. A If you have a problem or question on tape recording, write to Mr. Herman Burstein at AU DIO, 1515 Broadway, New York, N.Y. 10036. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Source: Audio magazine, Aug. 1987, HERMAN BURSTEIN)

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