TAPE GUIDE (Oct. 1988)

Loss of Bias

Q. I recently acquired an audio sys tem with a double cassette deck.

About four weeks later, the recording--only section of the deck began to exhibit serious distortion, particularly on high-frequency material, and excessive sibilance. I tried cleaning the heads, demagnetizing them, and changing tape brands, but with no improvement. Before I take the deck in for service, I would appreciate your suggestions and opinion as to whether the cause might be a bias misadjustment.

-Matt Beam, Elgin, Ill.

A. The likely cause of your problem is insufficient bias current to the record head. Bias is a high -frequency current, usually in the vicinity of 100 kHz, which is fed to the record head along with the audio signal. Its purpose is to reduce distortion and to maximize the amount of signal recorded on the tape. Bias, unfortunately, also acts to reduce treble response; this reduction, however, is compensated by treble boost in re cording. Thus you can appreciate that a partial or complete loss of bias will result in excessive distortion and in excessive treble response, which produces annoying sibilance.

A Problem Cassette

Q. I have a strange problem with just one cassette: It plays back slowly on my deck. Originally, it played correctly, but soon it began to play slow.

Stranger still, it plays correctly on another deck that I own and on my brother's deck. (According to tests I have made, when the tape plays slow it re quires about an extra two seconds per minute.) What is causing this?

-Wade Marshall, Chicago, Ill.

A. Inasmuch as your difficulty occurs only with one cassette, the fault appears to be in the cassette rather than in the deck. There may be a defect in the cassette, such as poor slip sheets, which produces a resistance to tape motion. Your principal tape deck may have less driving power than the other decks you mention, and thus cannot overcome this unusually high resistance to tape motion. I would be inclined to throw away the offending cassette and forget about the problem, unless of course it occurs again with other cassettes. Then, you would need help from a service shop.

CD Compression

Q. I am interested in copying CDs onto cassettes for use in my car. I understand that I must use some compression of the signal to get the best results since the original dynamic range is so great. How do I go about doing this?

-Marvin Shuster, North Brunswick, N.J.

A. You may need a compressor. A few such units are listed in Audio's Annual Equipment Directory (October issue) and Stereo Review's Buyers Guide.

Not all CDs require compression, however. Some, particularly those from older analog originals, are based on master tapes that were already com pressed to meet the limitations of the LP medium. Furthermore, the program material may have limited dynamic range, obviating the need for compression. Much popular music, particularly rock, has a very limited dynamic range-sometimes as little as 20 or even 10 dB--in contrast to classical music's dynamic range of 50, 60, or 70 dB (or even more on rare occasions).

An alternative to using a compressor is to ride gain while recording a cassette. This assumes intimate familiarity with the program material so that you can judiciously lower or raise the re cording level as the program material requires. And it is a tedious process.

Channel Imbalance

Q. Is there an intrinsic reason why most of the phonograph albums that I use to make tapes produce consistently higher readings on the left channel than on the right? In recording, should the readings be equal?

-Neil Brooks, Kalamazoo, Mich.

A. The higher level of the left channel may be due to an imbalance be tween the two channels of your magnetic phono pickup or the channels of your preamp. An imbalance in the re cord section of your tape deck or in the calibration of its record-level meters also could be the source of your channel balance problem.

If the left and right channels appear to have about equal levels in playback, despite unequal readings when re cording, leave things as they are. If the left channel comes out louder in play back, set your levels for about equal readings when recording.

Coming to Terms with Terms

Q. What do the terms slew rate, damping factor, and headroom mean?

-Allen Polmann, Laurel, Md.

A. Slew rate refers to the rapidity with which an amplifier's output can change in response to a very sharp change in the input signal. It is usually measured in volts per microsecond.

When an amplifier cannot handle the fastest signal changes--particularly high-frequency transients--slew-rate limiting is said to occur. This tends to produce intermodulation distortion, with consequent changes in the character of the sound.

Damping factor applies to power amplifiers and refers to the ability of a power amplifier to "short-circuit" any oscillation that a speaker tends to make on its own. In the absence of damping, an audio signal fed to a speaker might be somewhat blurred by the speaker's tendency to vibrate on its own when excited by the sudden application or removal of a signal.

However, a well-designed amplifier has a very low impedance compared with that of most speakers--sometimes less than one-hundredth as much. In this case, the amplifier virtually short-circuits the voltages generated by the speaker's motions, limiting its ability to vibrate on its own. (This may be compared to short-circuiting an electric generator, thereby limiting its ability to turn.) Headroom refers to the ability of a tape system to record high frequencies at a high level without running into distortion and treble loss due to tape saturation. The amount of headroom, or the safety margin against saturation, depends on the deck, the tape, and the noise -reduction system used. Type IV (metal) tape offers more headroom than the other types, Dolby C NR offers more than Dolby B NR, and dbx NR provides more than Dolby C NR. The deck manufacturer's decision on how much bias and record treble boost to use are interrelated with objectives for treble response, distortion, and S/N ratio--all of which affect headroom.

Dubbing Strategies

Q. I have just purchased a second cassette deck. When dubbing from one deck to the other, which should I use to play and which to record?

-Phil Brewer, Williamsburg, Ky.

A. As a general rule, it is wisest to play on the same deck that was used to record it. If there is a discrepancy in azimuth alignment between the two decks, you are apt to do better by playing a tape on the deck used to re cord that tape. However, if you are copying a prerecorded cassette, you will have to determine by ear which deck gives you the best playback.

If you are dubbing your own old tapes, be sure when playing them to use the same noise-reduction system that was employed in recording. For example, if these tapes were recorded in Dolby B NR, play them in Dolby B NR. You may copy them in whatever noise-reduction format you wish Dolby B, Dolby C, or dbx.

Three Heads vs. Two

Q. I have a 1984 three-head deck and a 1976 two-head deck. I plan to retire the older deck and purchase a new one for dubbing. Should I buy a two- or three-head model?

-John De Rosa, Mattapan, Mass.

A. Generally, three-head decks tend to be of higher quality than two head models, although there are exceptions. In a three -head deck, the record and playback heads (the erase head makes three) each can be de signed to best serve their functions, making for lower distortion in recording and better treble response in play back. A record/playback head (in a two -head deck) must necessarily be a compromise. Still, good engineering and construction can minimize the dis-advantages to the point where one might not notice them.

A three-head deck has the further advantage of permitting simultaneous record and play, thereby enabling one to immediately check the quality of a recording, and facilitating adjusting bias level (when the deck permits) for optimum performance.

A two -head deck has an advantage with respect to azimuth alignment, which is orientation of the head gaps so that they are at a right angle to the long dimension of the tape. Any azimuth error it recording is cancelled in playback when the same head is used for both recording and playback. In a three-head deck, however, it is all too easy for there to be a slight difference in azimuth between the record and playback heads, resulting in treble loss in playback. (This problem can be avoided by careful azimuth alignment on the part of the factory or a service agency.)

All in all, I tend to vote for a three-head deck.

Making the Right Connections

Q. Please explain the procedure for transferring open -reel tape recordings to videocassettes via a Hi-Fi VCR. How do you make all the input and output connections for this transfer?

-Richard C. Emery, Dearborn, Mich.

A. One way to accomplish such a transfer is to feed the open -reel deck's outputs directly into the audio inputs of the VCR. (A Hi-Fi VCR has inputs and outputs for audio as well as video.) Another way is to feed the open-reel deck's audio outputs into the tape inputs of your preamp (or integrated amp or receiver), and to feed the tape outputs of the preamp (etc.) to the audio inputs of the Hi-Fi VCR. (These VCRs have a control for adjusting record level.) Hi-Fi VCR manuals are usually clear on how to make these connections.

( Audio magazine, Oct. 1988, HERMAN BURSTEIN)

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