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Worm's eye view of the six-bay FM antenna for W65H,
Hartford, Conn. Feedlines from transmitter are visible
parallel to the tower.
eifically, an AM transmitter Deeds a "band" of frequencii twice as great as the bighe t tone it is to tran mit. Thus in the crowded broadcast band, only 10 kilocycles are available for each station and therefore the bighesl tone which can be transmitted is about 5000 cycles. But tho pitch of the sound sent by the FM transmitter is determined by the number of times each second that the frequency is shifted, mid therefore is limited only by human hearing ability which is 15 to 20 thousand cycles. Beyond these basic limitations is that of the audio system of the set itself. The reproducing ability of the audio system is mainly a matter of price and is not confined to either the FM or AM type of receiver.
DYNAMIC RANGE
A rather new term "dynamic range" is being used to describe the ability of the FM system to reproduce the very softest to the very loudest sounds, without the compression which must be used in AM systems.
A symphony orchestra has a dynamic or volume range of approximately 100 decibels. This corresponds to a change of ten billion to one!
In the AM system, the program must be kept above the background noise level and, at the other extreme, below the peak power rating of the station. Thus weak passages have to be made louder and strong passages softer. This "compression" of the dynamic range is essential to AM operation.
Because of the absence of background noise in the FM system, the weak passages do not have to be made louder, and the strong passages only shift the frequency further, with no danger of damaging the equipment with a power overload as would occur on AM transmitters.
STATIONS AND ANTENNAS
There is some misunderstanding by the public to the effect that an expensive AM receiver should be just as good a noise-free, high fidelity performer as the same priced FM set. This question has already been answered by the fact that both the noiseless and high fidelity characteristics are fundamental to FM and cannot be duplicated in AM sets.
Customers are asking about the installation of an FM set, and especially about the antenna which will be required. {Continued on page 78)
How FM Erases Noise
Chart shows the combining of carrier, sound, and noise components in AM and FM systems, and how the noise is erased from FM systems by limiting carrier at the receiver.
CARRIER
SOUND
SOUND ON CARRIER
A M
SOUND 8 STATIC ON CARRIER
RADIO
WHAT RECEIVER DOES
WHAT COMES OUT OF THE SPEAKER
F M
RADIO
OCTOBER, 7947
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