TAPE GUIDE (Oct. 1986)

Tape Timing

Q. Can you explain why the tape counter of my cassette deck is not related to actual elapsed time? How can I use this counter to know whether I have enough tape left to record, say, a four-minute selection? How can I tell how much time is left at any given point on the tape?

-Barry Sussman, Summit, N.J.

A. In most cassette decks the tape counters are tied in with rotation of the take-up reel, and of course the number of rotations of the reel per minute of tape time keeps changing with the amount of tape on the reel. It is possible to have the counter show elapsed time, but this is a much more expensive and difficult proposition and is limited to high-priced decks.

I suggest that you invest some time in developing a graph (preferably) or a table which relates elapsed time to readings of the counter. Starting with the counter at zero, allow 60 seconds to pass, and note the counter reading;

allow another period of time to pass, say 60 seconds again, and again note the counter reading. Continue to do so throughout the reel. If you find 60-sec and intervals too laborious, use two- or three-minute intervals instead. If you put your findings into graph form, you can more easily rely on even longer intervals, say five minutes each, to give you the desired information.

The advantage of a graph is that you can connect your plotted points to form a smooth curve, thereby facilitating interpolation between the readings you have taken. Such a graph will probably be accurate to within substantially less than one minute, good enough for most purposes. Remaining time would be simply the difference between elapsed time and total time. Of course, you'll need a separate graph (or table) for each cassette length-C-90, C-60, and so forth.

Power During Demagnetization

Q. I have a question about demagnetizing my cassette deck. The instructions say to shut the power off before demagnetizing. I want to use a cassette-type demagnetizer instead of a wand. But my deck's transport is sole noid-operated, so the power must be on in order for the heads to come for ward into contact with the demagnetizer. I don't want to damage my cassette deck. Can you help me?

-Frank Scanarano, Wantagh, N.Y.

A. If you use a cassette-type de magnetizer, of course the power must be on in order to actuate it. There is no danger to your deck in doing so. How ever, you must be sure that the main volume of your audio system is all the way down, because the demagnetizer will generate sound in the playback head, and this could be destructive to your speakers.

Slipping Tape

Q. During playback of tapes that I have recorded recently or as much as five years ago, the tape slows down at various places until a soprano sounds like a bass. If I put the tapes through fast wind and rewind several times, they play normally for a while, but eventually slow down again. They act the same way on other decks. My deck is cleaned and demagnetized regularly, and I have been using a head lubricant. Could that be leaving a residue which makes the tapes adhere to themselves?

-Otis Shackelton, Santa Monica, Cal.

A. Apparently your deck is absolved, because the problem occurs on other decks, so the tapes appear to be at fault. Do tapes recorded on other decks play properly on yours? If so, the difficulty focuses all the more sharply on your cassettes. The problem could be poorly made cassettes, an off-brand, the low-price line of a reputable brand, or a poor batch of any brand. It might be, as you ventured, that your head cleaner has left a residue on the tapes which causes them to stick. Or, possibly, the lubricated heads were not allowed to dry sufficiently, so that lubricant got on the tapes, making it difficult for the capstan and pinch roller to get adequate grip on them.

Tape Shredding

Q. Recently I was given a prerecorded cassette as a gift. The accompanying literature says that it was re corded with Dolby B noise reduction and Dolby HX Pro. How is this possible? Further, whenever I play the tape with Dolby B on, it leaves "shreds" on the record/playback head, which I have to clean off with a cotton swab and isopropyl alcohol. But if I play the tape with the Dolby B off, it doesn't shred as much. What is causing this, and how can I correct it?

-Kevin Ruppenthal, Clarks Summit, Pa.

A. In recording, the Dolby circuit senses the amount of high-frequency content in the audio signal and varies the treble boost accordingly; the less the treble content, the greater the boost. (In playback the reverse hap-

pens: The less the treble content, the greater the treble cut, to restore flat response.) The same treble-sensing circuit can be used for Dolby HX Pro, so that as the treble content increases, the amount of bias current supplied to the record head decreases. The treble portion of the audio signal plus the current from the bias oscillator together constitute the appropriate amount of bias for recording.

I see no possible connection between the tape "shredding," as you call it, and activation of the Dolby circuit. Tape shredding is a physical phenomenon related to the cassette and the transport mechanism, whereas the Dolby process is purely electronic. It seems that only coincidence is at work here. If you discover otherwise, I would very much like to hear from you again.

Cassette Versus Open-Reel

Q. I think that too much is being made of how good cassettes are, while nothing is said about open-reel. I have an 18-year-old Ampex AG-500 which I believe will outperform any cassette deck when taping live music. Yet many equipment reviews suggest that a new cassette deck will do better than old open-reel decks.

-Donald Bisbee, Columbus, Ohio

A. If you are talking about relatively inexpensive open-reel decks of yester day versus relatively expensive cassette decks of today, I believe the cassette will rival or outdo open-reel. If you are talking about a superior open-reel deck of yesterday, such as your Ampex, versus the average cassette deck of today, you are probably right-the open-reel will win.

Misaligned Cassette Door

Q. I recently purchased a cassette deck and a few days later noticed that the door is out of alignment. It is about 1/16 inch too far out on one side and the same distance too far in on the other. Will this cause any loss in sound quality because the tape may not be running properly with respect to the heads?

-Joe A. Budz, Hudson, Ohio

A. I really don't know if it will help to change the alignment of the door which presumably holds the cassette and therefore determines its alignment with respect to the heads. It may be that whatever position the cassette now takes puts it in proper alignment with the heads.

To determine whether you are now getting good response, tape some interstation noise from your FM tuner. (I assume you have a tuner or can borrow one.) Compare the tape playback with the FM noise by switching be tween the two. If the tape playback compares well with noise particularly at the high end, realignment is probably not needed.

Checking Azimuth

Q. What equipment and procedures are needed to check and adjust azimuth alignment?

-Michael J. DeVoge, Conneaut Lake, Pa.

A. One needs a high-quality test tape and an audio voltmeter; if the deck has separate record and play back heads, a signal generator is also needed. If the deck has mechanical VU meters rather than electronic ones, so that one can read fine differences in signal level, and if the meters can read playback level, one may use these in stead of a separate audio voltmeter.

The procedure is to play a high-frequency tone from the test tape, usually in the range of 12 to 15 kHz, and adjust the playback-head azimuth for maxi mum output as determined by your meter. The azimuth alignment which gives maximum output on the left channel may not be quite the same as that which produces maximum output on the right. In that case, one has to find a compromise position.

A warning is in order. It is possible to get false azimuth peaks which are lower than the true azimuth peak output, and this is more likely when adjusting azimuth on the basis of a very high-frequency tone. Therefore, some test tapes provide a lower test tone, per haps around 5 kHz, for making a preliminary adjustment of azimuth. After the preliminary adjustment, which brings one into the vicinity of the final setting, one fine-tunes azimuth on the basis of the very high-frequency tone, such as 12 kHz.

If the deck uses the same head for recording and playback, nothing more is required. If there is a separate record head, one records a high-frequency tone produced by a signal genera tor and simultaneously plays it, mean while adjusting the record-head azimuth for maximum output in playback.

As before, one should beware of false peaks in adjusting azimuth. And one should again recognize that it may be necessary to find a compromise set ting that treats both channels equally.

Mixed Dolby

Q. I have discovered that by record and playing back in Dolby C / can greatly reduce noise.

What is happening?

-Mark A. Marinello, New Orleans, La.

A. Dolby B NR accomplishes about 10 dB of noise reduction, and Dolby C about 20 dB. Thus, if you record with Dolby B and play back with Dolby C, you achieve an extra 10 dB or so of noise reduction. What is happening is that the treble cut in playback with Dolby C exceeds the treble boost sup plied in recording with Dolby B. How ever, the Dolby B/C combination also reduces treble response in playback.

This loss may or may not be offensive, depending on the listener and on the program content. In the case of pro gram material with excessive brilliance (supplied by some recording engineers) or speakers with an excessive high end, the treble cut may even be desirable.

Conversion to 15 ips?

Q. We have a Crown SX 700 Pro-Series open-reel tape recorder which has hardly been used. It is still like new, and a beautiful piece of equipment. Would it be worthwhile to modify it from 7 1/2 to 15 ips?

-John and Fran Miletiz, Ottawa, Ill.

A. I doubt that it is worthwhile to go from 7 1/2 to 15 ips for home purposes, considering the excellent results obtainable at 7 1/2 ips, and considering the effort and expense which you would have to incur. Not only would mechanical changes be required, but it would also be necessary to modify the record equalization and perhaps the bias.

Further, keep in mind that the standard NARTB speed is 7 1/2 ips; this suggests how well one can do at this speed.

The most significant improvement that can be made is in signal-to-noise ratio. Very little such improvement would come from converting to 15 ips-perhaps 3 dB or so. There would also be an improvement in headroom (protection against tape saturation at high frequencies when recording at high levels), but again not all that much. You can achieve a vast improvement in S/N-virtually total silence in the background-by using a noise-reduction unit such as dbx's.

Adjusting Bias

Q. My cassette deck has adjustable doesn't have tones for calibrating it. However, I have a mixer-calibrator with 400-Hz and 8-kHz test tones that I can feed to the deck. What is the proper procedure for calibrating bias?

-Steve Snarr, Springfield, Ohio

A. Recording at a level of about -20 dB, adjust bias so that when 400-Hz and 8-kHz tones of equal amplitude are fed into the deck, these tones are also of equal amplitude in playback. If you have a three-head deck, you can measure the amplitude of the 400-Hz and 8-kHz outputs by looking at your deck's meters or by connecting an audio voltmeter to the deck's outputs. If you have a two-head deck, making your adjustments will involve a trial-and-error procedure of making a re cording, rewinding the tape, and playing it back to test the effect of each bias change.

After you have completed the above procedure, copy a good phono disc or CD and compare the tape with the source. If highs seem excessive, in crease the bias slightly. If highs seem insufficient, decrease the bias slightly.

Another good source for comparison purposes is FM interstation noise. Do not be afraid to trust your ears in adjusting bias. In fact, some audiophiles consider this the best of all ways to adjust bias.

(Audio magazine, Oct. 1986, HERMAN BURSTEIN)

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