Audioclinic (Oct. 1977)

Tuner Selectivity

Q. Please explain to me selectivity, i.f. rejection, and image rejection.

-Warren Laliberte, Ozone Park, N.Y.

A. By the term selectivity we refer to the ability of an FM tuner to reject stations which are nearby in frequency to the desired signal. Generally a tuner will not have tremendously good selectivity between the desired channel and the immediately adjacent channel, but this is not a problem because stations occupying adjacent channels are never assigned to the same listening area. Therefore what we usually are concerned with is "alternate channel selectivity," which is given in terms of the ratio of the desired signal to one removed by the distance of one FM channel, and not the adjacent one.

Every tuner or receiver has what is known as an i.f. (intermediate frequency) amplifier and all incoming signals are altered so they can enter the i.f. amplifier. The operating frequency of this amplifier is 10.7 mHz, which is a popular one used by overseas transmission for all kinds of information, voice, facsimile, and teletype traffic. If the tuner is poorly designed, some of these signals will find their way into the i.f. system and be heard along with the desired signal. The ability of the tuner to reject these signals is a measure of its i.f. rejection.

If you own an inexpensive FM portable radio you have, probably, at one time or another heard aircraft signals from planes, or perhaps from the control tower. These signals are not supposed to be there, and such undesired signals are called "images." The ability of a tuner or receiver to reject these images is a measure of the image rejection of the equipment.

Four-Channel FM

Q. Please enlighten me as to why it is taking the FCC so long to make a decision on broadcasting four channel sound.

- Gene Lake, Viriden, III.

A. The FCC has received a number of proposals regarding four-channel broadcasting. The FCC does not have a large staff of engineers and each proposed system must be carefully considered with a view towards learning which one is actually better. Also, the implications for interference with the SCA services are becoming increasingly important.

Any proposed system which appears to have possibilities must then be tested on FM stations to see how it works out in practice. One thing which must be carefully considered is the possibility of one station interfering with another by the unexpected sidebands produced by a four channel system. All these things take time.

Any four-channel system will be more complex, not only to broadcast but to be kept in adjustment by station personnel. It is highly likely that the means for this monitoring do not exist at present, but it would have to be designed, tested, and then built.

Cable Distortion

Q. If I want to have a headphone extension outlet in another room with, say 50 feet of cable, between the stereophone output of my receiver and the extension socket, would there be any loss of signal or signal distortion? I've heard that similar arrangements using extremely long cables can greatly lower signal strength.

- Vion Sandor, New York, N.Y.

A. I see no problem with running this length of extension to feed your stereophones. By using no. 16 gauge wire for this purpose the results should be excellent. However, longer lengths or multiple outlets will probably require a 70-V system.

Speaker systems require a lot of driving power and any resistance in the interconnecting cable will steal some of this power. Furthermore, the greater the resistance in the cable, the less effective the amplifier will be in terms of maintaining accurate control of speaker cone motion. But, these conditions are not involved with headphones.

(Source: Audio magazine, Oct. 1977; Joseph Giovanelli)

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