TAPE GUIDE (Oct. 1987)

HX Pro: What It Isn't

A number of readers apparently misunderstand the nature of Dolby HX Pro, for they put it in the category of a noise-reduction device.

HX Pro is not a noise-reduction system. Its purpose is to increase headroom (i.e., to maximize record level before substantial treble loss occurs) by reducing the amount of oscillator bias fed to the record head when there is substantial treble content in the program. Such treble content acts as bias, and excessive bias causes treble loss.

HX Pro can tie in with Dolby noise reduction by using the same circuit to sense the amount of high-frequency content. However, HX Pro is used only in recording, and does not affect the tape's playback compatibility. Dolby noise reduction is applied in both recording and playback, and tapes made with either Dolby B or C NR should be played back with the same NR system.

Cleaning-Fluid Warning

The "Audioclinic" column in the November 1986 issue suggested the use of WD-40 to clean dirty electrical contacts in tape decks and other audio equipment. The advice is good; any lubricant of the WD-40 type makes a fine cleaner and anti-oxidant. One word of warning, though: WD-40 is flammable. Spray-happy consumers sometimes find themselves amazed at what can happen if WD-40 gets into high-current electrical contacts such as power switches or nearby motor brushes. So be careful!

-Greg Clugston; Orange, Cal.

Reading Meters

Q. I have found that recordings I've made based on true (average-reading)

VU meters have been better than those using peak-reading meters. The latter are noticeably noisier. Perhaps in making them I was relying too much on seeing that the music didn't peak over 0 dB. Using the various types of noise reduction, how much over 0 dB can the music peak without producing appreciable distortion?

-George James Jew; Corvallis, Ore.

A. I am surprised to learn that you have obtained better results with a true VU meter than with a peak-reading meter. Most audiophiles have the opposite experience. Is it possible that your peak-reading device was mis-calibrated, causing you to record at too low a level and therefore to not realize the signal-to-noise potential of your tape deck? On many or most decks, 0 dB on a peak-reading record level meter denotes DIN level (250 nano-webers per meter). This is several dB below the maximum permissible recording level, customarily taken to be that which, at 315 Hz, produces 3% harmonic distortion on the tape. The difference would typically be about 2 to 5 dB; in other words, one can usually record safely at a level about 2 to 5 dB above 0 dB. However, tapes differ in the amount of signal they can accept before reaching 3% distortion. They also differ in their sensitivity, that is, in the amount of signal output they deliver for a given signal input. Accordingly, the amount that one can go over 0 dB in recording will vary with tape type and brand.

Sometimes the meter's 0-dB point corresponds to Dolby level, which is about 2 dB below DIN level. This increases the extent to which one can exceed 0 dB in recording. Or, as previously suggested, the meter may be calibrated so that 0 dB corresponds to a still lower level.

When using dbx noise reduction, one can usually go a greater distance above 0 dB than when using Dolby B or C-to +10 VU or so.

All in all, one is well advised to experimentally determine how high a recording level one can safely employ, depending on the particular tape one chooses and on whether dbx or Dolby NR is used.

Cleaning Misadventure

Q. I have a Sony Walkman WM-800. The manual says to clean the heads with denatured alcohol. With the deck in play mode, I cleaned the playback head with 70% isopropyl alcohol. Following the cleaning, the deck exhibited very poor sound quality. After much trouble-shooting by technicians in a service shop, it was determined that the head was permanently ruined. No one seems able to explain the cause, however. Is isopropyl alcohol too strong for such application? Could the magnetic properties of the head be altered if it is cleaned while current is passing through it?

-Tom Whang; Glendora, Cal.

A. It is a rare occurrence for a head to be harmed by application of isopropyl alcohol. Normally, the stronger, 91 % isopropyl alcohol is recommended; therefore, the strength of the solution you employed doesn't seem to be at fault. I doubt, too, that cleaning the head with power on would cause your problem.

The head gap has a filler, and this may have been affected by the alcohol. Try cleaning the head once more, but this time allow at least five minutes of drying time before putting the deck into operation.

You should report your problem to the manufacturer. Possibly your unit had a defective head to begin with. In any event, have the manufacturer advise you as to what cleaning fluid to use in the future.

Microseconds

Q. Every time I buy a tape I see the term "µS." What does this mean?

-Varick Williams, Hartford, Conn.

A. The term µS is an abbreviation for microseconds (millionths of a second) and applies to the types of equalization used in tape recording and playback. Ferric-oxide (Type I) cassette tapes use 120-µS equalization; the newer, high-bias (Type II) and metal (Type IV) cassette tapes use 70-µS equalization.

This is a rather roundabout notation, since µS is an abbreviation for a unit of time and we think of equalization more in terms of frequency than time. What it refers to is the time characteristics of the playback equalizer circuits.

Cassette playback equalization curves are fairly flat at both ends, with a long slope in the middle. This slope can be considered either a treble cut or a bass boost, since it extends through both frequency regions. Considering it as a treble cut, it commences (3 dB below maximum) at 50 Hz and levels off (3 dB above minimum) at a "turnover" frequency specified by industry standards. This turn over frequency, f, can be obtained from the relationship f = 159,155/µS. Thus, if a tape employs 70-µS playback equalization, f = 159,155/70 = 2,274 Hz; for 120-µS playback equalization, f = 1,326 Hz.

Dashed Hopes?

Q. I plan to install a stereo system in my car, but the only available place for an in-dash unit is a vertical slot instead of the regular horizontal one. The tape deck that I want to buy calls for horizontal mounting. Can I mount it vertically without developing problems with the tape transport system?

-Edwin F. Marcano; Joseph, Ore.

A. Often one invites trouble by mounting a tape deck in other than the intended position. But this is not an absolute certainty. Some manufacturers design their decks so they can operate satisfactorily in either a vertical or a horizontal position. The best approach is to query the deck manufacturer or, perhaps, an authorized service agency for the deck in question.

Filter Facts

Q. What is the purpose of a multiplex filter?

-Tom Ritchey, Carroll, Ohio

A. A stereo FM signal contains a 19 kHz pilot tone which enables the FM tuner to recover stereo information.

However, this pilot tone may cause improper action of the Dolby circuits in a tape deck, resulting in incorrect treble response; this is called mistracking.

Therefore, FM tuners frequently incorporate a filter to remove the pilot tone from the tuner's output. Cassette decks too usually incorporate a multiplex filter, in case the tuner doesn't have one or doesn't perform adequately. If Dolby noise reduction appears to be misbehaving, or if your deck emits chirps or birdies or the like, the multiplex filter should be engaged, although this means that response will not extend to 20 kHz but will drop rapidly beyond 17 kHz or so.

Bulk-Eraser Insufficiency

Q. What would cause a bulk eraser to fail to erase cassettes completely? Mine does about an 80% job.

-Ward G. Erwin, Kissimmee, Fla.

A. One factor would be an insufficiently strong magnetic field produced by the eraser. A second would be the manner in which the eraser is used: It should be brought into contact with the cassette and moved in a circular fashion all about the cassette. A third factor is the type of tape. Metal tapes are the hardest to erase, some brands more so than others, and it is an unfortunate fact that some bulk erasers simply cannot adequately erase a signal on a metal tape.

Copying CD onto Tape

Q. I have a cassette deck with Dolby B, Dolby C, and dbx noise reduction. I also have an expander and a graphic equalizer. If I use the deck to copy from Compact Disc, which NR system should I use? Should I use the graphic equalizer or bypass it? And how about the expander?

-Gines A. Martinez-Zayas; Caguas, P.R.

A. The choice is really between Dolby C and dbx NR. Ascertain, by listening, whether extreme low-frequency response and extreme high frequency response are audibly better with Dolby C NR, as is sometimes the case. Determine whether noise is audibly lower with dbx, as may be the case, depending on how loud you like your reproduced music. And see whether side effects-such as sudden bursts of hiss and sudden volume changes-are noticeably different between Dolby C and dbx NR. Based on these three considerations, you can decide for yourself which noise-reduction system gives you best net results.

Assuming that the program material on the CD has reasonably satisfactory frequency response, it is best to use the graphic equalizer only in playback.

This would be particularly true if the reason for using the equalizer were a frequency abnormality elsewhere in your system, for example a sizzling high end in your speakers. However, rf there are major deviations from flat response in the program material itself, then you should use the equalizer during recording.

The expander is best used in playback. Considering the great dynamic range of most CDs, you would hardly want to use it in recording. In fact, one may often want to use a compressor in order to hear low passages when playing at moderate level. A compressor could be used either in recording or playback.

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

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