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More Selective TV Sets
Q. My color Tv set is being fed by a rotating, high-gain, highly directional antenna, 66 feet above the ground.
Distant stations are often adversely affected by adjacent-channel interference caused by strong local stations.
Why are not Tv set makers able to eliminate this adjacent-channel interference?
Great strides have been made in this respect where the FM band is concerned. Crystal filters, such as are used in Heath and other tuners, allow one to listen to a weak station even when it is next to a strong local one. It is possible to apply similar circuits to a Tv tuner so as to accomplish similar results for the video and audio performances of TV sets? If so, why are not the manufacturers making use of these filters?
- Chester J. Alkema, Grand Rapids, Mich.
A. There is no technical reason why television receivers cannot be made more selective, and, at the same time, produce pictures with even more definition than is presently the case.
However, economic considerations play a role here. Because competition among manufacturers is keen and costs are measured in tenths of a cent, a manufacturer would have to be pretty sure that these circuit changes would really be important to the consumer before he would decide to include them.
As it is, you are one of the very few who is interested in having such a set. Hence, you will either have to do some design work of your own or simply enjoy your present set.
Another problem regarding selectivity is one which involves the front end. Very often, amateur radio operators are blamed for creating interference to television receivers. True, the emanations of some of these transmitters do find their way into television sets, but it is not the fault of the operator of the station. It is the fault of poor front-end design of the television receiver which allows this type of interference to enter the receiver.
This is the kind of problem that most television users would not understand as a design problem, and, therefore, would not complain to the manufacturers of the sets.
Q. I want to learn how to construct a voltage divider to put between my cartridge and preamplifier in order to cut down first-stage overload.
-R. B. Taylor, Sellersville, Pa.
A. You can make a voltage divider which will reduce the output from your cartridge sufficiently to prevent overloading of the early stages of your equipment.
There are two resistors needed.
Their total ohmic value, when they are connected in series, is 47 k ohms so that it will match the cartridge's requirements. You probably will need to reduce the signal output to perhaps 6 dB below its present level. Therefore the value of each resistor should be 23.5 k ohms. However, there is no such value made in standard resistance sizes. Therefore, 22 k ohms is the closest obtainable value. (There will be no degradation of sound quality as a result of this slight discrepancy.) See Fig. 1.
Place the two voltage dividers (one for each channel) in a metal container.
Mount the input and output connectors to the box. This arrangement is useful in eliminating unshielded resistors and their leads.
It might be convenient to have a method whereby any voltage between zero and the full output capability of your cartridge can be fed to the input of your preamplifier. This can be accomplished through the use of a 50-k ohm potentiometer. See Fig. 2.
The 50-k ohm resistance value of this pot is close enough to the recommended 47-k ohm cartridge termination that there will be no degradation of sound quality.
As with the circuit of Fig. 1, this potentiometer should be mounted in a metal box.
If you have a VTVM and a test record, the two potentiometers needed for stereo should be adjusted to provide equal voltages between their wipers and ground.
Regardless of which of the two circuits you choose, connect a ground wire between the metal box in which the circuit is mounted and your preamplifier.
Q. What is meant by "background music"?
-Mr. M. Reich, Bronx, N.Y.
A. Background is that music which you often hear in restaurants, banks and other establishments. It is carried on ordinary FM stations, probably the very ones to which you listen all the time. You cannot hear it at home, however, because it is transmitted on what is known as a subcarrier. The main FM carrier is modulated by the audio you hear, plus some stereo information which you cannot hear on a monophonic receiver. Furthermore, there is a 67-kHz carrier which also frequency modulates the main carrier.
This carrier, in turn, is frequency modulated, and contains the background music service.
In order to receive this background music programming, the FM signal is first detected in the usual way. The regular program is then filtered out of the resulting audio and all that is left is the recovered 67-kHz subcarrier. This carrier is then passed into circuitry which detects its FM just as is done when detecting the FM with which you are familiar.
Transformers in Solid-State Amplifiers
Q. It appears that most, if not all, of the latest solid-state amplifiers have no output transformers, driver, or inter stage transformers.
Do you feel there is any advantage in performance of either having driver transformers or not having them? Would their presence deteriorate transient responses?
- Leonard Drasin, Jamaica, N.Y.
A. I do not think driver transformers impede good transient response. They are not required to handle much power. Further, because of the low impedances associated with solid-state circuitry, these transformers will not have sufficient turns to cause severe losses of highs because of distributed capacitance.
The use of driver transformers solves some design problems, especially that of phase splitting.
Q. Several FM stereo tuners I have seen have both a center-of-channel tuning meter and a signal-strength meter. If properly aligned, should the two meters agree with one another as to the point of best tuning? If not, which one should be relied on?
- Leonard Drasin, Jamaica, N. Y.
A. Theoretically, center channel meters both should give proper readings at the same time. In other words, when the center-of-channel meter shows proper tuning, this reading should coincide with the maximum signal point on the signal strength meter. I noticed, however, that in many cases the readings will correspond at one particular signal strength -- that used when aligning the tuner.
A signal which is stronger or weaker than this one will produce disagreement between the meter readings.
I am not altogether sure of the reason why this discrepancy should exist.
My personal guess would be that it has something to do with current flowing in the detector circuit and the changes in diode conductivity with varying signal levels feeding this circuit.
In any case, when the two meters disagree, rely on the center-of-channel meter reading.
Of course, if the readings are in violent disagreement, it might indicate that you must have your tuner aligned.
Relay Protection for Solid-State Amps
Q. Why don't manufacturers use relays as aids in protecting solid-state circuits from overload?
A. When transistor circuits are overloaded, they often burn out with lightning speed. Therefore, in order to protect such devices, the protective circuit must sense the potential danger even faster than it can act to cause the damage.
Being mechanical devices, relays are not fast acting. Therein lies the basic reason why they are not used in solid-state amplifier protective circuits. Further, relays are not completely reliable in other ways. Oxidation of their contacts can make them useless. Failure of a restoring spring can also destroy their usefulness.
(Source: Audio magazine, Feb. 1970)
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