AUDIOCLINIC (Aug. 1988)

Subsidiary Broadcasts

Q. What is "SCA," and why must it be rejected?

-Robert Beiswinger, Townbank, N.J.

A. SCA stands for Subsidiary Communications Authorization. Under this authorization, FM stations are permitted to broadcast other programming simultaneously with the programs you hear. Yes, that's right; FM stations often broadcast two or even more programs--the one you can tune into via your regular FM tuner and at least one you cannot.

These SCA broadcasts are transmitted by means of a subcarrier riding on the main carrier. In the same way that an FM station transmits audio frequencies, it can also transmit much higher frequencies. The one most commonly employed for SCA broadcasts is 67 kHz. Like the main carrier, this 67-kHz subcarrier is frequency-modulated.

The output of the tuner's detector includes this modulated subcarrier as well as the main-program audio. Tuners designed to "hear" these broadcasts feed the 67-kHz subcarrier to another detector, whose output is the audio signal representing the SCA program To avoid interference with adjacent channels and with the main programming of the FM station, the bandwidth of SCA broadcasts is restricted to an absolute maximum of 7 kHz.

The content of these subsidiary broadcasts varies from restaurant background music to medical conferences. There are special programs for the physically handicapped which feature the reading of daily newspapers and magazines. Specialized foreign-language programs are also often carried. There are even some data transmissions on these subcarriers.

If the main programming of an FM station is stereophonic, then only one subcarrier can be used. If a station is transmitting monophonically, at least one additional subcarrier can be present, though this is rare.

If there are significant nonlinearities in the FM transmitter or in the FM tuner, it is possible for the SCA signal to be heard as a faint background to the main program. Even when such signals are inaudible, if significant 67-kHz leakage is present in the output of a tuner or receiver, that frequency may combine with the bias oscillator during tape recording. The result will be background hash which will probably ruin the recording. This is why SCA subcarriers must be rejected.

Long-Distance AM Reception

Q. I am interested in buying a new tuner with good AM and FM sections. (I need AM because I like to listen to talk shows that are transmitted from considerable distances.) What tuner should I consider?

-Wayne Warren; Grants Pass, Ore.

A. My suggestion is that you buy a tuner that has the FM performance you require. As for brands, there are many fine tuners, and, because of the subjective nature of product evaluation, I never enter this area of audio.

As for the AM aspects, buy a good, general coverage shortwave receiver. I realize that you might not be interested in shortwave reception, but there are features on these receivers that will enhance your long-distance AM reception in a number of ways: Often, there are beats between the desired signal and adjacent channel signals. The bandwidth of a good shortwave receiver can be made narrow and selective enough to reject these beats.

If the adjacent-channel signal is strong and the desired one is weak, the average hi-fi AM tuner section will not do a good job of rejecting the strong station; the shortwave set will at least give it a good try, and may be completely successful.

Bandpass tuning is another beneficial feature of some shortwave receivers. It permits a shift of the i.f., which can be extremely useful in eliminating interference from a strong, adjacent -channel station.

An AM phenomenon known as selective fading sometimes results in serious audio distortion and loss of intelligibility. Some shortwave receivers have a synchronous detector, which compensates for this fading by reconstructing the lost carrier.

Sometimes, notch filters are used in shortwave receivers to help eliminate strong beats whose frequency is too low to be eliminated by narrowing i.f. selectivity. Beats of this kind may be produced by nearby TV receivers.

Shortwave sets usually require some sort of outdoor antenna. Don't let this bother you. Good results can usually be had by clipping 10 feet of wire to one of the antenna terminals.

There are some very good portable receivers (some the size of a book) which work well, have most or all of the features mentioned, and are equipped with whip antennas. They also have the means to connect external antennas, but often this only results in serious signal overload.

Editor's Note:

The McKay-Dymek DR-33 all-wave receiver, which we reviewed in the September 1979 issue, was startlingly good, particularly when used with the company's Model DA -100 all-wave receiving antenna. Unhappily, the manufacturer is now out of business, but the units might be obtained on the used market.

-E.P.

The Third Dimension

Q. Would a reverb with adjustable depth and delay help to "open up" recorded music and make it "three-dimensional"? Is this possible with two loudspeaker systems?

-Richard Garcia; Commerce, Cal.

A. It all depends on the recording. If it already possesses considerable reverb, adding more will not be too noticeable. If the added reverb is too noticeable, you may find it overwhelming rather than enjoyable.

Adding reverb to the main stereo channels can enhance a feeling of overall space. But for me, at least, this effect is not truly three-dimensional. It would probably be better to feed the reverb into a rear-channel system, like those used when four-channel sound was popular. This would create a sense of being surrounded by sound.

Feeding those rear speakers with two separate channels of reverb/delay is the only way that you can hope for some of the three-dimensional effects in which you are interested.

If you can do so, listen to some reverb/delay units, as a part of either a two- or four-channel system. In this way, you can determine whether the effects produced by these add-on devices will enhance your enjoyment.

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

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