TAPE GUIDE (Jun. 1985)

Input-Output Mismatch

Q. I seem to have several problems with my cassette deck. First of all, when I record with equal input levels on both channels, on playback the left channel is 1 to 2 dB lower than the recording level, while the right channel is 3 to 5 dB lower than the recording level. Also, when the tape is played in the reverse direction, there is an additional loss of about 3 dB in both channels.

-Joe Faiferlick, Fort Dodge, Iowa

A. The difference between input and output levels can be accounted for by tape sensitivity, that is, the amount of tape output for a given signal input.

Various brands and types of tape differ in sensitivity. The difference between the left and right channels can be due to mis-calibration of the meters in re cording or playback, or to differences in gain between the left and right channels in recording or playback. The difference in output between playback in the forward direction and in the reverse direction might be due to differences in output between ,the upper and lower gaps of the head.

It seems that the problems you de scribe are small enough to live with, at least until some other problem with your deck requires a trip to a repair shop.

Playback Misreading

Q. I recently had my old tape deck repaired but still have a problem with it. It records at one level but plays back at significantly higher levels, mostly in the red zone of the meters. I record at about the middle of the meter dials, but playback puts the meter needles out of sight. Also, the right channel seems worse than the left, which produces distortion to my ears. I have tried four different brands of tape, but all pro duce the same result. The problem existed before the repairs were made, and the service technician checked the meter calibration and said every thing was okay. I have called other service shops, but they say that be cause my deck is 10 years old it's really not worth repairing. Should I give in and buy a new deck?

-Eugene M. Forbes, Lake Ariel, Pa.

A. Most tape decks have meter-calibration potentiometers, both for re cording and for playback, although less expensive decks may have fixed resistors for this purpose. I have no way of knowing what the situation is in your case. In any event, it appears that your meters are calibrated to read too low in recording, too high in playback, or a combination of these. From your statement that you hear distortion on the worse-adjusted channel, I surmise that the meters read too low in recording. If so, this should not be too difficult for a qualified technician to set to rights.

On the other hand, very good cassette decks, giving far better performance than your present unit, can be obtained today for less than $300.

Considering that and your deck's age, it may well be unwise to put any more money into repairs. Even if your present problem is solved, there is a good chance of other problems soon developing in a deck so old.

Meter Calibrations

Q. I've purchased a cassette calibration tape which contains a 315-Hz test tone at 0 dB DIN. This tape reads +2 dB on my new cassette deck and +4 on my old one, whose 0 VU mark was calibrated at 160 nWb/m. When tapes recorded on the new deck are played on the old deck, the meters of the latter read 2 to 4 dB lower than when I originally recorded them.

Shouldn't the meters of the old deck read +2? Should +7 dB on the new deck correspond to +5 dB DIN? How should I interpret the different readings of the meters?

-Curtis P. Jeffries, New York, N.Y.

A. A number of cassette decks are calibrated so that 0 VU on the meters corresponds to Dolby level, which is 200 nWb/m. A number of others are calibrated so that 0 VU corresponds to the DIN (and IEC) standard reference level, which is 250 nWb/m, or about 2 dB higher than Dolby level. Therefore, your deck correctly reads +2 dB (relative to 0 VU) when playing a DIN calibration tape. The DIN level is about 4 dB above 160 nWb/m, so your old deck correctly reads +4 dB when playing the test tape. So far, so good.

But your problem, if I follow correctly, is that after recording a tape on your new deck, you get a reduced reading, rather than an increased reading, when playing the tape on your old deck. This may be due to a mismatch between the record and playback readings of one of your decks, probably the new one. When a tape recorded on the new deck is played back on it, does the playback reading match the record reading? If the playback reading is lower, there is your answer.

Such a mismatch could be due to mis calibration of the meter or to variation in sensitivity among brands and types of tape.

If record and playback readings match on your new deck, the reduced reading on your old deck--not a very seriously lower one--could be due to the way the meters of this deck react to program material rather than steady tones. Or, again, mis-calibration of the meters or differences in tape sensitivity could account for the situation.

Yes, +7 dB on your new deck should correspond to +5 dB DIN. As already pointed out, your deck reads +2 dB relative to 0 VU DIN, and there fore +7 dB relative to +5 dB DIN.

(This assumes that the meters of your new deck are truly logarithmic so that they accurately represent level differences in dB.)

While on the subject, it should be pointed out that, in some decks, 0 VU corresponds neither to Dolby nor DIN level but approximately to the recording level that produces 3% harmonic distortion. Depending on the brand of tape and how the deck is set up with respect to bias and equalization, the 3% harmonic distortion level is typically about 2 to 6 dB higher than the DIN 0 VU level for Type I and Type II tapes; it is about 3 to 8 dB above DIN 0 VU level for Type IV tapes. Of course, this kind of meter calibration is employed only in decks with peak-reading meters. And, finally, in some decks the 0 VU level corresponds to a level below 0 VU DIN, sometimes as much as 5 dB below. Apparently the purpose here is to provide a very good safety margin against over-recording, to prevent distortion and treble loss.

So, what is the recordist to do? What is he to make of these various calibrations and interpretations? I suggest that you experiment with recording levels, using the tape of your choice. Start with levels that cause peaks to drive the record-level meters to about 0 VU.

Listen carefully to the playback. Successively increase the recording level about 1 or 2 dB at a time, until distortion and/or high-frequency saturation become noticeable in playback, and then back down 1 or 2 dB for safety.

Depending on your deck and the tape you use, you might find that you get satisfactory recordings when the meters read as high as +7 or +8 dB, and occasionally even + 10 or more.

Portable-Player Compatibility

Q. I recently purchased a personal portable cassette player which has a switch providing for playback of either " Normal" or "Metal" tapes; I assume this means the choice of either 120- or 70-uS equalization, respectively. However, the majority of my cassettes were recorded on my home deck using chromium dioxide tape, which utilizes 70-uS equalization. Therefore, when playing these cassettes with my portable unit, which playback setting should I choose, " Normal" or "Metal"? Will I damage the tapes or the playback head of my portable unit if I choose the wrong setting?

Also, my portable has an on-off switch for Dolby noise reduction, which I assume is Dolby B. But the majority of my cassettes were recorded with Dolby C. What will occur when playing the Dolby C-encoded tapes when the player is set for Dolby B? What will happen when the portable's Dolby B switch is off? In the future, should I record with Dolby B on my home deck if I plan to play the cassettes with the portable unit, or should I continue to record with Dolby C?

-David M. Lukac, Towaco, N.J.

A. Equalization of 120 and 70 uS applies only to playback. (Much different equalization is employed in recording, varying according to tape type, the cassette deck manufacturer's notions about optimal bias current for each tape type, and other factors. The required record equalization is that which, in conjunction with standard 120- or 70-µS playback equalization, will result in flat response on a record-playback basis.) No matter what happened in recording, you may apply any kind of equalization you wish in play back without causing harm to the tapes or heads.

Dolby C NR applies substantially more treble boost in recording, and substantially more treble cut in play back, than does Dolby B. If you play a Dolby C-encoded tape with the deck set for Dolby B, the effect will be emphasized treble. If a Dolby C-encoded tape is played with no Dolby decoding whatsoever, the result will be still greater treble emphasis.

If you plan to use Dolby B in decoding, you should use Dolby B in encoding, unless you find that the extra treble emphasis due to Dolby C encoding is desirable, perhaps to compensate for a treble deficiency elsewhere.

Extra-Long-Play Cassettes

Q. I intend to purchase a cassette deck and wish to get maximum recording time by using C-120 or even C-180 cassettes. The programs that I plan to record are mostly speech. Do you know of any tape brands particularly suited to my purpose?

-R. Kroenke, Mt. Shasta, Cal.

A. My experience with such cassettes is very limited inasmuch as they are not recommended for high fidelity. For this reason, and also because the policy of Audio prohibits me from recommending specific brands, I cannot answer your question directly. How ever, what experience I have had indicates that results are not as good as with the shorter tapes (C-90, C-60, etc.). Recordings don't sound as clean, and there is increased danger of the tape fouling up. However, your experience for your particular purpose may prove otherwise. When you get your deck, experiment with one or two major brands of the extra-long-playing cassettes and compare them with C-90s of the same brand.

Dolby Dubbing

Q. When copying a chromium-type tape that is Dolby-encoded, should the decks' Dolby switches be on or off? Should the tape-type switch be in the "chromium" position for both decks? When I make a copy with both decks' Dolby switches off, the tapes seem to sound fuller than with the switches on.

-Deb Murphy, Chicago, Ill.

A. When copying a Dolby-encoded tape, usually the best procedure is to play the source tape with Dolby on and to record the copy with Dolby on. How ever, the proof of the budding is in the eating. If the copy sounds better when dubbing with both Dolby switches off, then make your copies that way. In the case of a Dolby-encoded tape, the copy may sound fuller because of some loss of highs when playing with one deck and recording with another.

This would be due to mistracking, which signifies that the signal level for Dolby decoding (playback) is not matched to the signal level for Dolby encoding (recording). Such matching is more difficult to achieve when using different decks for recording and play back instead of the same deck for both operations. Yes, the tape-type switch should be in the "chromium" position on both tape decks.

Adjusting Bias

Q. I am considering the purchase of a cassette deck that offers bias adjustment but not test tones. Is there a simple and effective way to adjust a deck to match a given tape?

-Michael Deutsch, Los Angeles, Cal.

A. The easiest way to adjust bias is for flattest response, as judged by ear, when taping interstation FM noise. If the deck has separate record and playback heads, the adjustment can be made very quickly by monitoring playback while recording, and immediately comparing the playback sound with the source. If the deck uses the same head for recording and play back, it will take several trials to arrive at the optimum bias setting. That is, one records, rewinds, plays, and com pares the playback sound with the source. If bias is too low, the tape will have exaggerated treble response; if too high, treble will be deficient. One makes a tentative bias adjustment in the proper direction and repeats the procedure until the optimum setting is reached.

Do not underestimate the importance and effectiveness of your own ears as measuring instruments. Should you find that there is a range of bias (usually small) within which you achieve flat response according to your ears, use the greatest amount of bias which still maintains apparently flat response. This will tend to minimize distortion.

(Source: Audio magazine, Jun. 1985, HERMAN BURSTEIN)

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