AUDIOCLINIC (Dec. 1988)

Nighttime AM Reception

Q. Why is it that some AM stations are not very strong after sunset, and others, not even present during the day, are strong after sunset?

-Richard Harding, Peabody, Mass.

A. The effect of sunlight in the portion of the radio-frequency spectrum used to transmit AM broadcast signals is such that, during daylight hours, these signals are absorbed by the ionosphere. After sunset, the nature of the atmosphere changes, and these signals are not absorbed to the same ex tent. If the signals are not absorbed, they will travel farther and be heard at night.

This can result in problems after sunset. Just tune around the dial, and you'll discover that many frequencies which had no stations on them during the day are so overpopulated that there is nothing but a jumble of sound.

Even when you are near a desired AM station, reception may be blurred by distant stations occupying the same frequency.

The scenario just described would be worse if steps were not taken to alleviate the situation. Let's assume that stations A and B share the same frequency. Station A interferes with station B at night. Station B may be interfering with station A, as well. To avoid the problem, they "protect" one another. This is done by changing the pat tern in which their waves are radiated.

In daylight hours, these stations may radiate their signals equally in all directions. At sundown, each station adjusts its radiation pattern so that it becomes directional.

If you happen to live south of station A, and if that station causes interference to listeners of station B (located south of you), you will notice that the signal strength of station A drops suddenly at sunset. Because station B protects against interfering with points to its north, you may never even know it is sharing the frequency with station A.

Noisy Alternator

Q. I have a problem with my car stereo. Despite what I believe is the right installation and grounding of the equipment, and even the isolation of the equipment from the rest of the electrical system, I hear a whining, variable pitch on acceleration. This happens despite the installation of a noise suppressor, and I can hear the noise with the gain turned down. It is most apparent when other parts of my 1980 Honda Accord LXi's electrical system are in use (particularly head lights, brake lights, or the air conditioner).

-Richard Borts, St. Louis, Mo.

A. You say that you installed a noise suppressor, but some suppressors are used to cover noise from ignition systems while others are designed for use with alternators. Because of the description you gave, I believe that the noise you hear is produced by the alternator. If you do not have a suppressor on the alternator, have one in stalled as a first step.

If you have wired your stereo equipment to the battery, did you wire both the ground and "hot" wires directly to the battery? You should do so. It is never a good idea to rely on the body of the car to carry ground to the battery. It may also be that you relied on the body to carry speaker grounds.

Wire the speakers directly to the equipment.

(Editor's Note: My installer, Tony 'gel of Stratford Service in New York, suggested some additional possibilities.

He says Hondas often have wiring harnesses running just above the dash board's radio slot. One or more wires are probably radiating interference into your stereo system. These should be checked, one by one, for interference; many installers have "sniffer" devices that detect nearby signals. The guilty wires should be cut, lengthened, and rerouted. If your system has a separate amplifier, try low-level noise filters, which are designed to be wired or plugged into the signal cables between the system's head unit and the amplifier. -I.B.)

Possible Overload

Q. When I am listening to FM stereo with headphones, I can hear what sounds like another station in the back ground during quiet passages. I can only hear this with headphones. Is it the result of some shortcoming in my stereo system's tuner?

-Joseph S. Barbera, Buffalo, N.Y.

A. Do you hear the background "station" only when listening to FM? If it turns out that you hear it when listening to other sources, chances are the problem is caused by radio frequencies getting into the equipment via an interconnect cord or other means.

We'll assume, however, that you hear this background station only when listening to FM stereo, as you said. There can be a couple of potential sources of the sound.

The first possibility is that your tuner is being overloaded by a strong signal.

If you can hear a signal when tuning the dial-and this signal is heard at several points on the dial, in addition to the proper one--you probably have FM overload. To cure this, you'll need to place an attenuator between the antenna lead-in and the antenna input terminals.

The other possibility is not at all related to overload. The sound you hear may not be that of a background station, but the audio of an SCA sub-carrier. These subcarriers are broad cast along with the programs you expect to find on your FM stations, and are designed for special applications, such as background music for stores and restaurants. If the subcarrier is modulated more heavily than is appropriate, listeners to the main FM pro gram will hear a faint trace of the SCA subcarrier signal.

It also is possible that your tuner is misaligned somewhat, especially in its i.f. stages. If this is so, nonlinearities will be introduced and will result in the SCA signal being heard as described.

Relative Power of Phones and Speakers

Q. When I listen to my system via loudspeakers, the power level meter shows a reading of "1." To listen via headphones at what I believe to be the same loudness, I have to turn up the volume such that the meter shows al most "3." Can you explain this? It is my understanding that headphones re quire much less power than loud speakers.

-Michael Gompper, Manchester, Conn.

A. You are quite right! Headphones require much less power than loud speakers do. If the phones were connected directly to the speaker terminals, very little volume would be needed. In fact, it would be almost impossible to adjust the volume. When the volume was properly set, you would find that the power level meter would probably show nothing, so slight is the power being used.

What accounts for your observations? There is a power-reducing circuit in the amp. The phones are connected to it, rather than to the speaker terminals. This permits better control of the volume and prevents the phones from being damaged by excessive power.

Your level indicator doesn't really read power (which involves both volt age and current) but voltage alone.

Depending on where it is in the amplifier circuit, the reading will be about the same for any given volume control set ting and input signal level. This is true whether the amplifier is feeding reasonably high power to the speakers or low power to the headphones.

Harmonics and Octaves

Q. I am confused as to what a harmonic is. What are first, second, third, etc. harmonics? What are they used for? Also, what is an octave and how does it fit in with the subject of harmonics?

-Sid Drexler, Wantagh, N.Y.

A. Mankind can, and does, deal with harmonics. They are inherent in most devices which produce sound waves.

Consider a frequency--any frequency. For illustration, we will consider a frequency of 200 Hz. When this frequency is transmitted in air, the device producing that frequency will vibrate 200 times each second. This is also true of our eardrums, which receive the sound. Because of the nature of a vibrating system, there will be other vibrations set up in the device which is producing our 200 Hz. The most common occurrence is for the device to vibrate both as a whole and as though it were really two separate halves.

Thus, in addition to the main, 200-Hz tone, this same device will also pro duce a 400-Hz tone.

The original 200-Hz tone is often referred to as the fundamental, or the first harmonic. The second frequency (400 Hz) is twice that of the fundamental and is known as the second harmonic. Other vibrations can be generated which will always be exact multiples of the original, or fundamental, frequency. Thus, the third harmonic is 600 Hz (three times the fundamental). Harmonics and octaves are related.

If the fundamental frequency is doubled, we say that the resulting frequency (the 400-Hz second harmonic) is one octave higher than the fundamental (the 200-Hz frequency we started with), What is the relation to the fundamental if we wish to produce the next higher octave? Well, it is 800 Hz be cause, in order to obtain it, we again must double the frequency. Hence, the second octave above the fundamental is 800 Hz, which is the fourth harmonic of 200 Hz, and not the third as you might have supposed.

Different vibrating systems produce different degrees of harmonic energy in relation to the fundamental. In large measure, it is by these frequencies and their relative strength that we determine the nature of a sound. The difference in sound between a clarinet and a flute is determined in this way.

The flute's output contains very little harmonic energy. The clarinet, on the other hand, is an instrument that is rich in harmonics.

When it comes to the reproduction of sound, our goal is to reproduce a tone as accurately as possible. We do not want added harmonics to be generated in a high-fidelity system. If there are harmonics already present in the original program source, so be it. We do not wish to add more of them via the sound system.

When we refer to measuring harmonic distortion, we are measuring the amount of harmonics which are introduced by the device being tested. The lower the percentage of such added harmonics, the better the device will be in this regard.

A Different Approach to European Power

Q. I believe that I have another answer to the matter of operating nominal 110-V, 60-Hz sound equipment when you are in an area which has 220-V, 50-Hz power. Why not simply produce your own 110-V, 60-Hz power? Just buy a portable a.c. generator, avail able at most hardware stores. Rather than using the noisy gasoline engine to turn the shaft, remove it and replace it with a suitable 220-V a.c. motor capable of running at 50 Hz.

I am assuming that this can be situated in a basement far enough from the listener that the noise would be inaudible. The 110-V current could be brought to the audio equipment via an extension cord, if necessary. Do you think this solution is reasonable?

-Rick Heil, Glendale, Wisc.

A. Well, yes, it is possible to do what you suggest. You must be sure that the motor turns at the speed which will permit the generator to produce 60-Hz current. This could entail using belts and pulleys properly chosen to pro duce the needed generator speed.

The contraption will be bulky and expensive. I can't prove it, but I think it will cost more than the step-down transformer usually employed. Of course, your scheme will provide 60 Hz; that is important for some devices which include synchronous or induction motors whose speed is directly dependent on power-line frequency.

Not only will you need a line from the basement to feed power into your audio equipment, but you will also need a way of turning the generator off when it is not needed.

==============

(Source: Audio magazine, Dec. 1988, JOSEPH GIOVANELLI)

= = = =