TAPE GUIDE (Jan. 1990)

Dubbing Dolby NR

Q. Should the Dolby NR circuits of both decks be switched on when dubbing from deck to deck? I have heard many different theories on this.

-Mark Greenblatt, Los Angeles, Calif.

A. The less one tampers with the signal, the better the eventual results are apt to be. Therefore, in theory, it is desirable to dub a Dolby-encoded tape with the Dolby circuits off. However, to maintain substantially flat response, it is necessary to equate the output and input levels of the two decks with reference to Dolby level, which can be a tricky business.

In practice, the wisest course is to use Dolby decoding in playback of the original tape and Dolby encoding in recording the dubbed tape. In this way, one is most likely to preserve correct Dolby level and to achieve flattest response, particularly in the treble region. I suggest you experiment with both modes of dubbing and let your ears decide which gives the most satisfactory results.

Tape Drag

Q. Do you get many complaints about tape drag, that is, speed being slightly slow on cassette decks? I have always had a problem with this, even though I had my decks checked and was always told their speed is okay. If I take up all the slack in a tape, this sometimes helps for a while.

-Jimmy Edwards, Greenville, N.C.

A. Yes, I get a fair number of complaints about cassette decks being off speed. These involve situations where different decks are used for recording and playback. Tape drag can be due to a number of factors: Excessive holdback torque by the supply hub; insufficient take-up torque by the take-up hub; a glazed pressure roller, which cannot grab the tape adequately; insufficient pressure by the roller; a capstan that somehow has gotten lubricant on it and therefore cannot grab the tape, and some brands and types of highly polished tapes, which have a tendency to slip.

Usually a tape deck whose speed is off by no more than 0.5% is considered to be operating at proper speed. Some service shops may even consider speed within 1% as proper operation and hence may put their okay on a deck that, to a highly sensitive ear, is somewhat slow. However, I must repeat, this can happen only if different decks are used for recording and playback or if a deck has changed its speed characteristic over time, so that the speed in playback differs from the original speed in recording. When the same deck is used for recording and playback, and if its speed has not changed, any speed error which occurred during recording is cancelled by the same error during playback.

Dolby Symbol

Q. The fluorescent level meters on my cassette deck have the Dolby double-D symbol between the 0 and 4 dB lines. Please explain the meaning of this symbol and what it signifies for recording.

-Victor S. Zupancic; Kirkland Lake, Ont., Canada

A. Dolby level corresponds to 200 nWb/m (nanowebers per meter) on the tape at 400 Hz. This is nearly 2 dB below DIN level (250 nWb/m at 315 Hz), which is generally considered close to the maximum level at which one can safely record in the vicinity of 300 to 500 Hz without running into excessive distortion. In most program material, maximum amplitude tends to occur in the range of roughly 300 to 500 Hz; hence if you are safe at 400 Hz, you tend to be safe at all other frequencies.

Recording at a level no higher than Dolby level usually provides a bit of safety margin against recording overload. If you want to push your recording level to the maximum feasible point in order to extract the last dB of signal-to-noise ratio from your deck, you can probably record so that peaks read no higher than +4 or +5 on your deck's meter. However, you should experiment to determine whether this is a safe practice so far as your ears are concerned.

If you have the necessary test facilities, you can use the deck's Dolby level indications to check for correct Dolby NR tracking. First, record a 400-Hz tone from a test disc or oscillator, with your deck's level meters set at the Dolby-level mark. If this tone also reads at the Dolby mark in playback, your deck is properly adjusted for correct Dolby NR decoding of the recordings you make on that tape formulation. (Other tapes may require different sensitivity settings on the deck; some decks can adjust themselves for these differences automatically.) You can also use the Dolby-level marks to determine whether your deck is properly calibrated for tapes made on other decks using Dolby NR. This, however, requires an accurately made test tape containing a 400-Hz tone recorded at a measured level of 200 nWb/m.

Crosstalk from Opposite Tracks

Q. When I listen to cassette tape through my headphones, between selections I think that I can hear material being played backward from the other side of the tape. Why?

- Richard Harding, Peabody, Mass.

A. If you count tracks from top to bottom of a cassette tape, then tracks 1 and 2 are respectively for the left and right channels, while tracks 3 and 4 are respectively for the right and left channels in the opposite direction of tape travel.

The position of the playback head should be such that its two gaps exactly correspond to tracks 1 and 2 in one direction, and to tracks 4 and 3 in the other direction. A downward shift in the position of the head may cause the lower gap to pick up a bit of the information on track 3, particularly at lower frequencies, where the magnetic field tends to spread out beyond the track (fringing). If the crosstalk that you hear is not particularly strong, I would be inclined to leave things as they are if everything else is working well. If you have a technician try to eliminate the crosstalk by shifting the head, there may be an adverse effect on azimuth alignment, with consequent loss of treble.

If your deck has separate record and playback heads, it is possible that they are not correctly aligned with respect to each other in terms of vertical position, resulting in crosstalk.

If yours is a reversing deck whose playback head has two sets of gaps, it is possible that crosstalk occurs within the head owing to imperfect isolation between the upper set of gaps (for one direction of tape travel) and the lower set (for the other direction). Perhaps the only thing you can do about this is to try the costly step of replacing the head, although the new head may perform no better than the old one.

Sweet Spot

Q. My cassette deck is nearly 10 years old and suffers from a problem that, at first, seemed to be azimuth misalignment but unfortunately is not. If I play a well recorded tape and skew the tape slightly (the deck is an open design that enables me to do this), I get my sweet spot. As soon as I let the tape go, there is a dramatic loss in high-frequency response, and the tape sounds very dull.

I took the deck to a fairly competent shop, and they made an azimuth adjustment in addition to demagnetizing and cleaning the heads, but the deck behaves the same as before. Physical inspection seems to indicate that the heads are in good condition. Please give me some leads as to what may be wrong with my deck.

-Andy Valiente, Winter Park, Fla.

A. If moving the tape with respect to the playback head restores high-frequency response, this suggests either improper azimuth alignment or poor tape-to-head contact. Apparently, you have ruled out azimuth misalignment, but are you sure? Did the shop align azimuth with respect to a standard tape or with respect to your tapes? Your tapes may have been recorded with a head that was not in standard alignment-that is, not with its head gaps at a perfect right angle to the tape's long dimension. Playing such tapes with a head in standard alignment would result in treble loss. You might check the situation with another repair shop that is more than just "fairly competent." Poor tape-to-head contact may be due to a flaw in the deck's mechanism that prevents the cassette's pressure pad from doing its job. Perhaps insufficient torque by the take-up hub contributes to the problem, or it may be due to inadequate "grab" by the capstan and pressure roller. A worn, grooved head also may interfere with good contact.

(Source: Audio magazine, Jan. 1990, HERMAN BURSTEIN)

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