TAPE GUIDE (Jan. 1987)

Which Way to Dub ?

Q. I have two cassette decks. When I dub tapes by playing deck A and recording on deck B, the signal output is pretty strong. But if I record from B to A, the resulting tape is not very satisfactory. What is wrong ?

-Paul Wollerman, Sacramento, Cal.

A. The strength of the output signal tends to vary from one brand and model of tape deck to another. Typically it is about 0.5 V at maximum signal level, but it can go down to as little as 0.25 V and up to as much as 1V or slightly more. This may explain the difficulty you are having. If your deck B has an output level control, it should, of course, be set to maximum. Inasmuch as you are able to get satisfactory results when recording from deck A to deck B, why don't you just stay with that arrangement ?

Buzz Words

Q. My cassette deck is a fine machine, except that when I play tapes I hear a buzz in one of my loudspeakers. I switched the speaker wires on my receiver and then heard the buzz through the other speaker. I don't hear the buzz when playing radio or phono. Any suggestions ?

-Greg David, Holmdel, N.J.

A. Since you only hear the buzz when playing tapes, the trouble probably lies in the deck or in the cables connecting it to the rest of your audio system. It may merely be caused by dirt or corrosion on a plug or jack; with the receiver's volume turned completely down, try unplugging both ends of the offending channel's cable, and then plug them back in-twisting each plug slightly as you put it back into the jack. This may either cure the problem or worsen it. If the latter, replace the cable.

If re-plugging has no effect, try plugging the left-channel cable into the receiver's right-channel input, and vice versa. If the buzz now appears in the opposite speaker, this absolves your receiver and speakers. Next, switch the cables' other ends, shifting one cable from the tape deck's left output to its right one, and shifting the other cable to the left-channel output jack. If the buzz stays in the same channel, the cable should be replaced. If the buzz changes channels when you do that, the tape deck is at fault and should be taken to a qualified service shop. The problem could be in the playback head, the power supply, the internal wiring, or the playback electronics.

Playback Equalization for Non-ferric Tapes

Q. I use Type II and IV tapes, with appropriate record equalization and bias. But in playback, the music always sounds cleaner when the setting for equalization is in the " Normal" position. Will I damage my deck if I continue this practice ?

-Kirk L. McDonald, New York, N.Y.

A. Using 120-µS ("normal") playback equalization results in moderate treble boost when playing Type II and IV tapes that have been recorded for playback with 70-uS equalization. This treble boost eventually reaches about 4.4 dB at 20 kHz. The effect, to your ears, is apparently to make the sound "cleaner"; some might say "airier." No damage whatsoever will be done to your deck or tapes as a result of this practice. The burden on the tweeters of your speaker system will be increased but probably not dangerously so, unless you operate at very high levels.

Meter Mismatch

Q. I have noticed that the output level shown by my cassette deck's LED meters is somewhat lower than the input level they show. For example, if I record on chrome tape at +1 dB, as recommended by the instruction manual, in playback the indicated level is around -3 dB. Is this normal, or do I have a problem? I have tried various kinds of tape, but this has not provided a solution. Friends have suggested that I may be saturating the tape, but I encounter the same situation when recording at lower levels. Another suggestion is that my use of Dolby C NR causes the output to be suppressed. Is there any truth to this ?

-Steven Woodroof, Bedford Park, Va.

A. Possibly, your deck's manufacturer used a tape with extra-high sensitivity when making playback-meter calibrations. In such a case, tapes with average sensitivity would tend to read low in playback. A more likely explanation is that the meters are mis-calibrated, either in recording or in playback, or both. That is, they may be adjusted to show too high a level in recording--which does have the advantage of protecting against over-recording, with consequent tape saturation leading to distortion and treble loss--or too low a level in playback, or some of each.

Another explanation is that the meters may reflect the input signal after record equalization, which adds a substantial amount of treble boost. Depending on the type of program material, this treble boost may significantly affect the indicated input level.

I doubt that the situation has anything to do with Dolby noise reduction.

Fortunately, your problem is not a serious one, and should not be difficult to live with.

Why Not AFM ?

Q. I have read about the great frequency response and signal-to-noise ratio that Hi-Fi VCRs achieve by means of AFM (audio-frequency modulation). Could this system be used in regular cassette decks ?

-Tony Perkins, Los Angeles, Cal.

A. A frequency-modulation system, if it is to achieve a signal-to-noise ratio commensurate with high fidelity, must have a bandwidth of several hundred kHz. Such bandwidth is available in a VCR, which goes out to several MHz, whereas the bandwidth of an analog cassette deck is in the neighborhood of only 20 kHz.

Unknown Tapes

Q. I have obtained a number of open-reel tapes, many of them no longer in their original boxes. Since I cannot identify them and therefore am not sure of their quality, I don't know whether to use them; I don't want to risk damaging the deck. Is there any way of identifying the brand and type, or at least the quality, of recording tape of unknown manufacture ?

-Russell L. Kallen, New York, N.Y.

A. I do not know of any way to identify tape by brand. You can check for quality by using the tapes and noting how faithfully they reproduce sound. A tape cannot hurt the deck if it falls short in terms of electronic performance--that is, if it exhibits high distortion, poor signal-to-noise ratio, or inadequate frequency response. There is also probably no chance of hurting your deck if a tape falls short mechanically, if you play a reel only partway through to see how it performs. If there is squeaking, it is likely that the tape is not well lubricated, and it should not be used. Also, check for excessive oxide shedding on the heads, capstan, pressure roller, etc. If you see this, the tape should not be used.

Record Level Imbalance

Q. When I am recording from FM or phono, one of the meters of my tape deck usually reads 3 or 4 dB lower than the other meter. Should I use the deck's balance control so that the meters give the same reading? Or should I keep the balance control in the middle position?

-Ted Wirth, Cleveland, Ohio

A. There may be a problem in your record gain control or in the calibration of your record level meters. Or there may be no problem; it may simply be that there is imbalance between the two channels of the signal source.

To determine whether your record level indicators are out of balance, feed the same signal-say, interstation FM noise-into both channels of your tape deck. Adjust gain on each channel, with the aid of the balance control if necessary, so that the reading is the same on each indicator as you record.

Play the tape with the playback gain controls (if any) at maximum, and check by ear whether the levels of the two channels are approximately even.

If there is a decidedly noticeable difference, this suggests that one of the indicators is not properly calibrated.

Through trial and error, you can ascertain how much mental compensation is needed in reading that record level indicator. If the record level indicators are not mis-calibrated, use the balance control to achieve equality between channels when recording, provided you are pretty sure that the signal source requires balance.

All the foregoing assumes that the outputs of your tape deck are of equal level when the recorded signals on the two channels are equal. To check this, a test tape would be desirable. In the absence of a test tape, try using a prerecorded tape or perhaps a tape recorded on another deck known to be of high quality.

Record-Level Indicators

Q. Which are better record-level indicators--VU meters or LEDs? What do the terms peak, peak-hold, and averaging mean in relation to level indicators? What purpose is served in having a VU meter along with a peak-type LED? In setting the record level to utilize all of the available headroom, what is meant by setting the indicator so that it peaks at the midrange 3% distortion point, and where is this point located on a VU or LED scale? What does "meter resolution" mean?

-Edward Brown, Far Rockaway, N.Y.

A. I tend to prefer LEDs because, being electronic rather than mechanical, they can immediately and fully respond to signal peaks. This helps guard against recording at excessive level, with consequent distortion and treble loss. On the other hand, the VU meter is more helpful in enabling one to discern fine differences in recording level. The true VU meter is an averaging device; it reads average signal level, which can be as much as 10 or even 20 dB below sharp peaks. There are also peak-reading meters, which pretty faithfully follow sharp transients.

A peak-hold meter will stay at the peak level for a brief period so that the eye can follow it.

Considering the inability of a true VU meter to follow signal peaks, its indications have to be interpreted, and one's interpretation may be incorrect. A true VU meter is usually calibrated so that 0 VU corresponds to about 1% harmonic distortion. This provides a safety margin of about 6 to 8 dB with respect to the 3% harmonic-distortion level, which is generally considered the maximum compatible with high-fidelity reproduction. That is, when the meter reads 0 VU, the peak signal level may be about 6 to 8 dB higher, producing about 3% distortion. But with some signals, the peak level may be a good deal higher still, so that interpretive skill is necessary when setting the record level. The actual amount of distortion will vary with type and brand of tape.

Experimentation is advisable in finding the highest level at which one can safely record without noticeable distortion and treble loss.

It should be mentioned, however, that the true VU meter, whose characteristics (such as needle reaction speed) are precisely specified, is not the only kind of averaging meter. So it is useful to experiment with each new meter to see how its actions may best be interpreted. This is one reason why a VU or other averaging meter is sometimes accompanied by an LED which flashes when signal peaks reach a preset danger level; the LED guards against misinterpretation of the meter and consequent over-recording.

Peak-reading devices are calibrated so that the 3% harmonic distortion level on the tape corresponds to a point near 0 VU on the device. This point may actually be as low as 0 VU, but is usually somewhat higher, such as +3 dB. The signal level which causes distortion to reach 3% will vary somewhat with the type and brand of tape. Once more, experimentation is advisable in order to find at how high a level one can record with a given tape.

In theory, the zero point on either average--or peak--reading meters should indicate the 3% distortion point-by inference in the case of the averaging meter, and directly in the case of the peak-reading meter. In practice, however, meters are not always calibrated to a specific standard.

If zero on the meter is at or near the Dolby calibration mark (for which there is a standard), the deck's maker has chosen a zero point that will allow safe recording well above "0," though at the expense of apparently higher noise levels. If the meter's zero is well above the Dolby mark, the deck's maker preferred to maximize apparent S/N by reducing the amount of headroom above "0." The actual performance of both decks could be the same, but you would need to set the recording level to different points on each meter to attain that level of performance.

Meter resolution refers to the ability to make fine distinctions in recording or playback level. Here one is best off with a true meter, i.e., with a needle, rather than all-electronic "meters" made up of LEDs or spots on an LCD or other display. Some such meters have many fine calibration marks, but their apparent resolution is illusory--the calibration marks do not turn on one by one, but several at a time.

Some Basics

Q. I am planning to buy my first tape deck, but there are several terms I find confusing. What is bias ? Is it true that different brands of tape decks are biased for different brands of tapes ? Is bias changeable in a repair shop ? Also, what is equalization ? And what is MOL ? I have seen specifications such as MOL equals +7 dB.

-Tad Defiler, Fairfield, Conn.

A. Bias is a high-frequency current, usually at least 75 kHz and often over 100 kHz, that is supplied to the record head along with the audio signal. The relative magnitude is roughly 10 to 1.

Its purpose is to maximize the amount of signal recorded on the tape, thereby maximizing signal-to-noise ratio, and to minimize distortion. Without bias, the recorded signal would be very weak and very distorted. The amount of bias required varies with tape type. Type I (ferric oxide) requires the lowest bias level; Type II (chromium dioxide and ferri-cobalt) requires about 50% more bias than Type I, and Type IV (metal particle) requires still more, perhaps twice as much as Type I. Type Ill (ferrichrome), which requires slightly more bias than Type II, is rarely seen in the U.S. anymore.

Cassette decks today usually provide switchable bias for Types I, II, and IV. Theoretically, all brands of a given type should conform to an industry standard and therefore take the same bias. But in practice, there are some differences from brand to brand of a given type. Therefore, manufacturers align their decks on the basis of specific brands of each tape type, and often they will recommend that these specific brands be used. Some decks provide for user adjustment of bias, some automatically adjust bias, and some provide no adjustment. Any competent repair shop can adjust bias to achieve optimum performance with respect to a specific type and brand of tape.

Equalization is a system of frequency alteration in recording and playback, without which a tape deck's record/play response would exhibit severe bass and treble losses and high frequency noise. In recording, equalization consists largely of treble boost, while playback equalization consists largely of bass boost. The two are not precisely complementary, but are designed so that the combination of recording and playback equalization and the frequency alterations imposed by the tape will add up to flat response.

Playback equalization is standardized, with one curve for Type I tapes (called the 120-uS curve) and another (70-uS) for all other types. In recording, a different equalization is used for each type, and, in some decks, for each tape brand and formulation. Recording EQ is designed so that the signal coming from the recorded tape will, after standard playback EO, have flat frequency response within stated tolerances.

The term MOL denotes maximum output level-the highest level that can be recorded on the tape, at which point the tape is "saturated" and no higher level can be recorded. The standard recording level, DIN 0 dB, is the signal necessary to produce a recorded signal level of 250 nanoWebers per meter at 33 Hz. A tape with an MOL of + 7 would refer to a recorded level 7 dB above 250 nWb/m.

Specification Trade-Offs

Q. I'm about to purchase a cassette deck and have two in mind, priced about the same. My first choice has all the features I'm looking for, but its frequency response is rated at only 30 Hz to 21 kHz, ±3 dB, with metal tape. The second deck, with most of the features I want, doesn't have as high a signal-to-noise ratio, nor does it have dbx NR, but its rated frequency response is 20 Hz to 20 kHz with metal tape. Would the difference between 20 and 30 Hz be noticeable in normal listening (I listen primarily to jazz), or am I splitting hairs ? And will the first deck be able to record any music at 20 Hz ?

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

A. Assuming that the second deck's frequency response is also specified within ±3 dB limits, it would be only in extremely rare instances-such as, perhaps, with a very large organ that goes down to 16 Hz-that you might find a noticeable difference between a deck with response no more than 3 dB down at 30 Hz and one no more than 3 dB down at 20 Hz. Most music has extremely little content below 30 Hz, and still very little below 50 Hz. In the case of jazz, it's extremely unlikely that you would notice a difference between the two decks on the basis of 30versus 20-Hz response.

If a deck is only 3 dB down at 30 Hz, it will usually provide some useful response at 20 Hz, unless it incorporates a sharp bass cutoff to avoid problems when taping phono records that are appreciably warped. Quite possibly, response at 20 Hz might be down only about 4 or 5 dB. In that case, a bass control or equalizer used in playback could give you what you want at the extreme low end.

Another, and very important, consideration is the response of your loudspeaker. Most speakers start going rapidly downhill below about 40 or 50 Hz. It is a rare speaker, and ordinarily a very expensive one, that maintains substantial response to 30 Hz, much less 20 Hz.

In any event, the performance of the two decks you're considering might be more comparable than you think. You say that the first deck's 30-Hz response is ±3 dB, but give no tolerance for the second one's 20-Hz response. It could well be that it too is down 3 dB at 30 Hz and down still further (6 dB or more) at 20 Hz, and the first deck might have comparable response at 20 Hz. Similarly, the first deck's greater S/N figure might be due only to its having dbx NR; how do the two decks' noise figures compare when using Dolby B NR (which I presume both have) ? If you're making tapes only for your own use, the dbx edge might be significant; if you often share tapes with others who do not have dbx, performance with Dolby B NR might be more significant. Also, the degree of significance depends on the noise levels in question: A 5-dB difference looms a bit larger when comparing 60- and 65-dB ratios than when comparing 70- and 75-dB ratios.

All in all, I think you are splitting hairs; the ultimate test consists of listening. Give each of the decks a hearing and decide which one is superior.

If they sound the same, select the one that meets your needs in terms of features and price.

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

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