TAPE GUIDE (Nov. 1988)

Bass Loss in Duplicating

Q. In order to duplicate my collection of prerecorded tapes so 1 could use them in my office, I recently purchased a second cassette deck. In duplicating tapes, however, I seem to lose bass, whereas the high end appears to be faithfully reproduced. Is this bass loss inherent in dubbing, or can I remedy the situation? I do not have a problem with bass response when recording from discs on my new deck.

-Paul Burstein, Milwaukee, Wisc.

A. If you lose bass response when dubbing from tapes but not from albums, this suggests that the fault lies in the deck being used to play the original tape. Conceivably, there also may be some loss occurring in the recording deck. It could be that the loss is sufficiently small in each deck so as not to be noticeable, but that the sum of the losses becomes apparent.

In playback, you could compensate for the loss by using the bass control of your audio system, or by using an equalizer. Alternatively, an equalizer could be used in recording.

Most preamplifiers (or integrated amplifiers or receivers) do not permit one to apply equalization to the signal fed to a tape deck for recording. If yours is the rare exception, you could apply bass boost to the signal being recorded in this manner. If not, you'll find that many equalizers can be switched to affect the frequency balance of the signals at their tape-output jacks.

Test Frequencies

Q. Why is 400 Hz used for level adjustments and 15 kHz for other adjustments?

-Wayne J. Jernigan; San Diego, Cal.

A. In cassette recording, for the purpose of having a reference such as Dolby level (200 nWb/m) or DIN level (250 nWb/m), it is desirable to employ a frequency that not only corresponds approximately to the point where we find maximum energy in typical audio material, but is free from the substantial treble boost which occurs in recording. A frequency in the neighborhood of 400 Hz (used by Dolby) or 315 Hz (DIN) meets these requirements.

The Dolby reference is used for matching the recording and playback levels-essential in order to avoid treble aberration. The DIN reference is employed for measuring S/N ratio and obtaining figures for THD at various recording levels. Also used for these two purposes is the recording level which produces 3% THD at 400 or 315 Hz. Use of these frequencies, rather than one that is much lower or higher, produces a more realistic representation of what we hear.

Bias adjustment, to obtain substantially flat response, is achieved in some decks by using two frequencies. One is 15 kHz or thereabouts, and the other is much lower. Bias is adjusted for equal playback response at both frequencies. In the case of azimuth adjustment, a test tape with a high frequency is needed because azimuth becomes more critical as frequency rises. Therefore, 15 kHz or so is used for azimuth adjustment.

DynamicRange

Q. I recently acquired a CD player and have questions about taping CDs.

I have heard that the great dynamic range of CDs may pose problems for taping. Are there any special amplification or other requirements presented by CD players? I also heard recently of "digital-ready" tape. Can you enlighten me as to whether it would be helpful in taping CDs?

-Stuart Munro; Melrose, Mass.

A. With a good cassette deck that incorporates Dolby C and/or dbx noise reduction, and with high-quality tape of any type, generally there should be no problem in satisfactorily taping CDs.

Seldom is the dynamic range of music in excess of about 70 dB. (In fact, when the range does become this great, it tends to become a nuisance in the home and, especially, in the car.

Soft passages are difficult to hear without the loud passages becoming thunderous.) With Dolby C NR, a good deck can achieve a signal-to-noise ratio, and therefore a dynamic range, of around 70 dB. With dbx NR, this figure goes up into the 80s and sometimes gets close to 90 dB. If a deck has only Dolby B NR, there may be a problem at times, depending on the source material, inasmuch as the S/N ratio is then around 60 dB. I don't believe that CD players really present new problems with respect to amplification or anything else. "Digital ready" tapes for analog decks fall into the category of marketing hyperbole.

High-quality tapes are already able to cope with CDs.

Left-Channel Decline

Q. Recently I have had a problem with my cassette deck when recording from any source. If I set both the left and right record-level controls to the same point, in playback the meters seem to show a relative drop in level of the left channel. The meter for the right channel shows the same reading in playback as in recording. Therefore, in recording, I have to adjust the left level control to a higher setting in order to get equal playback readings for both channels.

I sent the deck to a factory service center, and it came back with a note saying that they found nothing wrong. I know, however, that the problem didn't exist 2 1/2 years ago, when I first got the deck. Apart from this problem, I am very happy with the quality of my recordings. Thinking that perhaps the head is getting old, I have considered having it changed. If I do, will this new head match the azimuth of the original?

-A. Harrison; Nashville, Tenn.

A. The gain of the left playback channel may have dropped for some reason, such as a change in the characteristics of a transistor, potentiometer, resistor, etc. It could also be that gain has dropped in the left recording channel at a point following that from which the signal is fed to the record level meter. Another possibility is a change in the value of a component that affects the reading given by the left meter in playback. Least likely is the possibility that the tape head's left channel has changed its characteristics. You probably have nothing to gain by changing the head, and a good deal to lose in the way of cost and possible head misalignment. Note that azimuth alignment takes place after the head has been installed, and is done by means of a test tape bearing a high-frequency tone. The head is adjusted for maximum output of this tone in playback.

I would opt to live with the problem, making the necessary adjustments in recording or playback, provided the decline in the left channel is less than about 3 dB. However, if the left channel's decline is serious, you should get the deck to an authorized service shop which has a good reputation. Be prepared to show clearly and briefly that the problem does exist--don't simply describe it. Not all service shops are eager to spend time watching the customer demonstrate the problem. But that is your right.

A Formula Explained

Q. In the October 1987 issue, you state that the turnover frequency of an RC (resistance-capacitance) circuit can be obtained from 1=159,155/uS.

Where does this come from?

-Charles Warwick; Anaheim, Cal.

A. We start with the formula for impedance (capacitive resistance) of a capacitor:

1/[Z 2 π fC]

where Z is impedance, π is 3.1415927, f is frequency in Hz, and C is capacitance in farads. (For the derivation of this formula, see an advanced physics textbook.) Using µF (microfarads, or millionths of a farad) instead, and inserting the value of we obtain:

Z= 1,000,000 / [2 x 3.1415927 x f x C]

= 159,155 / fC

In an RC circuit, the turnover frequency f occurs when R and Z are equal. At this frequency, the amplitude response will have risen or fallen, depending on the circuit, by 3 dB. Thus, at I we find:

159,155 / fC = R Transposing terms, we get:

f=159,155/RC

where f is frequency in Hz, R is resistance in ohms, and C represents capacitance in µF. The product of R x C, or simply RC, is called a time constant and is given in (microseconds) when C is in µF. RC denotes the time required for a d.c. current to charge C through R to 63.2% of the charge ultimately reached by C (as explained in physics textbooks). Whatever the individual values of R and C, so long as their product remains the same, the charging time stays the same. (For example, charging time is the same if R is 100 and C is 10, or if R is 20 and C is 50.) Hence the term time constant.

( Audio magazine, Nov. 1988, HERMAN BURSTEIN)

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