Fundamentals of Radio.
It has been found that the most desirable period of reverberation for
a radio studio is from 0.8 to 1.2 seconds. When the reverberation period
is greater than this, the studio is "live" and sounds persist too long. When it is less than this, the studio is "dead" and sounds die out too
soon. Singers complain that their voices seem to go out into the "dead"
room and do not come back. In order to create certain effects, studios are
now being built with "live ends" and "dead ends." The live end is one
in which the walls are hard -surfaced and flat, built for the purpose of reflecting sounds. The deliberate
purpose of this arrangement is to
introduce one relatively loud reflection into the microphone and
help the naturalness of the pickup.
An orchestra is placed with its back
to the live end, which acts as a
shell reflector. The presence of
many people in a studio will tend
to deaden it, since each individual's
clothing absorbs the sound. Therefore it becomes necessary to provide means of varying the amount
of sound -absorbing materials upon
the walls in order that the reverberation period of the room may
be kept right. In modern studios
there are sliding panels which permit the sound -absorbing material
to be moved to one side and a hard
surface which will reflect sound to
be exposed. With the development
of frequency -modulated broadcasts, studios will require special
acoustic treatment for frequency
modification
Reverberation should not be confused with echo. An echo is the return
of a sound by reflection after a short period of silence. Since the shortest
interval of silence that the ear can detect is %g second, it follows that,
for an echo to be present, there must be a difference of at least 70 feet
between the rate length of the sound reaching the listener directly and
that returning by reflection. Reverberation is the successive return of the
sound by reflection at intervals too short for the ear to detect so that the
sound seems to be continuous as its intensity decreases.
In an acoustically treated studio the announcer speaks to a microphone. His words are carried by sound waves from his mouth to the microphone. These sound waves travel at approximately 1100 feet a second.
Each note in his voice causes air vibrations or sound waves. Each sound wave has its own frequency, that is to say, the number of vibrations set
in motion per second. When these notes arrive at the microphone they
cause the sensitive face of that microphone to respond at like frequencies
and thus change the sound wave into electrical impulses.
There are three general types of microphones in current use in broadcasting stations today. These microphones, which are m tnufactured by
more than 70 concerns, have many different trade names but fundamentally they are either crystal, velocity, or dynamic microphones. The
crystal microphone is constructed of Rochelle -salt crystals about %oo
inch thick. The sound waves hit these crystal slabs and cause them to
vibrate and bend apart; the vibration sets up a weak voltage, which
varies with the sound pressures upon the crystals. This type of microphone is very rugged, not easily damaged, and comparatively inexpensive.
It may be constructed so that it is either directional or non-directional,
being responsive to sounds coming from all directions. It has very good
tonal quality but is more frequently used for public-address and recording
work than for actual broadcasting. Figure 1 is an inexpensive crystal
microphone manufactured by the Brush Development Company of Cleveland, Ohio; Fig. 2 is the all-purpose Brush crystal microphone.
These microphones are protected by grills to prevent wind disturbanbes when
they are used outdoors. Figure 3 is a non-directional crystal microphone,
also manufactured by Brush, of the diaphragm crystal character. Microphones of this type are very satisfactory for both speech and music.
The velocity type of microphone is frequently called the "ribbon
mike," and justly so, because its operation depends upon the vibration of
a very thin corrugated duraluminum ribbon suspended between the poles of a strong magnet
When the ribbon is set into motion by sound vibrations, small electric currents are developed in it which are then further
amplified. The ribbon microphone is equally sensitive on the two opposite
sides which represent the broad faces of the ribbon, while it is comparatively insensitive on the other two edges. It is an excellent type of microphone to be used for a quartet or to be placed in the center of an orchestra.
The duraluminum ribbon is hung in the bottom of a V-shaped trough. The
result is that speakers do not talk across this microphone, but into the
trough. The velocity type of microphone (Fig. 4) is manufactured by
Radio Corporation of America and is of the standard broadcasting type
Smaller velocity microphones are macle for public-address and recording
work.
The principle of the dynamic microphone is essentially that of the
dynamic loud -speaker. It consists of a diaphragm on which is mounted a
small coil of fine wire. This, vibrating in the field of a strong magnet,
generates minute electric currents proportional to the incoming sound
impulses. Its diaphragm is moved back and forth by the air or sound
waves. This causes the coil to move in a powerful magnet field and electrical impulses result. The dynamic microphone may be constructed as
either a directional microphone or a nondirectional microphone. The two most popular types at the present are the "eight -ball" and the "saltshaker" types.
Figure 6 shows the eight -ball microphone which was developed in 1939 by Western Electric Company. This is nondirectional
when it is upright as shown in the picture, but, by using a swivel and the
acoustic baffle assembly, it may be converted into a semidirectional
microphone for speech and announcing. The eight -ball is probably the
most popular of the high -quality broadcasting microphones. Figure 7
portrays the salt -shaker microphone developed by Western Electric in
1937. This is a high -quality microphone designed for general utility work
in broadcasting, including those pickups made outside of regular studios.
When upright and used in such a way that the speakers talk over the
microphone, it is nondirectional, but when used as illustrated, with the
swivel faced toward the speaker, it becomes a semidirectional microphone.
In 1939 Western Electric developed the cardioid directional microphone,
which was further perfected in 1940. This microphone (Fig. 8) is really
two microphones, a ribbon microphone and a dynamic microphone, each
of which can be used independently or in conjunction with the other.
This was the first instrument to combine not less than three pickup
characteristics in one instrument. By the use of a small switch located at
the base of the microphone, it is possible to convert this instrument into
nondirectional, unidirectional, and cardioid or heart -shaped selectivity.
Three other coverage areas designed to minimize reverberation are also
possible with this microphone; Fig. 9 shows a diagram of three of the
pickup areas for this cardioid microphone. Radio Corporation of America
makes an all-purpose microphone consisting of two ribbon -type microphones operating in a common airgap (Fig. 10). This microphone also has
the three pickups-bidirectional, nondirectional, and cardioid. The grills
or screens on all microphones are designed for protection and wind
screening.
Two interesting microphones are the machine-gun and the parabolic. The machine-gun accessory (Fig. 11) consists of a series of tubes strapped together through which sound is conveyed to a dynamic microphone
which fits into the end. This type of equipment is designed to reduce
reverberation and extraneous noises in distant pickups. The muzzles of
the tubes are directed at a speaker, soloist, or musical group at a distance
and only that sound which enters the end of the tube in a direct line is
conveyed to the microphone. For the parabolic microphone equipment
in this instance (Fig. 12), a very
large wooden chopping bowl has
been used to directionalize a distant
pickup of a band or of a speaker in
a convention. The microphone is
placed in the focal point of the concave side of the bowl. The sound is
reflected to the microphone. Equipment of this sort is used on gridirons
and in convention halls. Various
companies make parabolic reflectors
The electric impulses that are
developed in the microphone are
carried to a control board adjacent.
to the studio in which the announcer is speaking. Here the control operator blends the output of
microphones which are in use and
amplifies the volume before it is sent
out over special . telephone lines.
Special instruments calibrated in
volume units (decibels), called "VU"
by the technician, show the loudness of the programs at all times,
and it is one of the duties of the control operator to keep the loudness
within certain limits, namely, between 10 and 100 Volumes unites
equivalent to -5 to -0 decibels.
The operator also checks the quality of outgoing music and speech by listening to it to see that no distortion is present. He formerly had to
modulate sudden explosive sounds to avoid blasting; however, this is now
accomplished automatically by equipment at the transmitter.
After the program has been amplified and monitored in the control
room, it is put onto a telephone line. The electrical impulses are carried
by this telephone line at approximately 30,000
miles per second. If the program is a network
program, it is carried by these telephone lines
to the various transmitters of the stations
that compose that network throughout the
country and is put into the air by the individual transmitters of these stations. If the
program is a local one, it is sent by ttelephone
line to the station's own transmitter.
In the early days of radio it was convenient
to locate the transmitter on the same building
in which the studios were housed, but it was
soon found that this arrangement had several
disadvantages, such as too much screening of
the station's signal by large steel buildings in
the neighborhood and unsatisfactory ground
conditions. As a result, transmitters are now
usually located several miles outside the city,
where conditions are better for maximum
efficiency. The Columbia Broadcasting System
has recently built an island for its transmitter
off the shores of Long Island, New York.
The straight vertical antenna with a height equal to 0.58 of the staf on's wave length gives better results than any of the older inverted
L or T types. Some of these have small bases,
large middle sections, and then small tops, much resembling two ordinary towers fitted together base to base.
Others are straight vertical structures of uniform thickness throughout. In either type the steel structure of the tower is the actual
radiating system. A necessary part of the transmitter's radiator is the
system of ground wires that is buried in the soil around the base of the
antenna. Although never seen by the visitors to the stations, these bare
copper wires are laid out with great care at a depth of 6 to 12 inches beneath the surface in much the same pattern as the spokes of a wheel about
the hub, each wire or spoke being almost as long as the antenna itself.
The transmitter proper (Fig. 13) consists of a quartz -crystal oscillator
which generates the radio frequency (the quartz crystal to maintain the exact frequency, the number of kilocycles of the station). This crystal
oscillator is followed by several more stages of radio -frequency amplification which increase the power to a value suitable for modulation. The
speech which comes from the microphone or incoming telephone line is
amplified by a series of audio -frequency amplifiers which terminate in a
stage called the "modulator." This modulator in turn is connected to the
radio -frequency stage previously mentioned. It is at this point that the
mixing of the audio frequency and radio frequency takes place. Further
amplification follows, and the resulting power is fed into the antenna and
radiated in all directions.
This modulation or mixing process gives rise to
other frequencies in addition to the carrier frequency,
which is the frequency of the quartz crystal. These other radio frequencies, called "side bands," are located in the assigned channel on either side of the carrier
and contain the speech of the announcer whose program we are tracing
from his mouth to the radio listener. The Federal Communications
Commission limits the width of this channel to 10 kilocycles.
Every station has its own carrier wave located in the center of its
assigned channel. These carrier waves vary between 550 and 1600 kilocycles for the regular broadcast band. These waves travel at the speed of
light. All carrier waves travel at the same speed, but those having fewer
kilocycles do not oscillate so fast as those having more kilocycles. A station
operating at 550 kilocycles has a rate of oscillation of 550,000 cycles per
second for its carrier wave.
The carrier waves which are sent out by the radio station may be
divided into two categories; first, the ground wave, and second, the sky
wave. During the daytime the sky waves have no affect upon the coverage
of the station because they travel upward and are lost, but at night these
sky waves play a very important part because they go up and hit the
Kennelly -Heaviside layer and are reflected back to the earth. These reflected sky waves are evident usually only after sunset and extend the nighttime coverage of stations.
The reflected sky wave is important only
to the most powerful stations in the clear -channel classification., Such
stations can be heard ordinarily during the daytime between 100 and
200 miles by means of their ground waves, but at night, through the
medium of the reflected sky wave, they are heard at great distances
because the sky waves are not absorbed by ground conditions as the ground wave is. The sky wave is not so dependable as the ground wave of
the station, and generally this extended coverage is considered as the
secondary coverage area. It is this reflected sky wave that causes fading,
inasmuch as the fading area exists where the ground wave of the station
interferes with the reflected sky wave of the same station. Despite the
faults and unreliability of the sky wave, a very large proportion of the
radio audience depends upon sky -wave reception for its evening programs.
Local and regional stations do not benefit from their reflected sky waves
because they are located closer to one another than are clear -channel
stations and, instead of having an area cleared of interference for their
sky waves, they have merely an area in which their sky waves interfere
with those of another station upon the same wave length. If a listener to a regional or local station has his receiving set near the outside limits of
the ground wave of a local or regional station, he will find at night that
there is interference with another station because he is picking up the
sky waves from one or more stations operating on the same frequency.
Thus the coverage of a regional or local station is less at night than it is
during the daytime, and the coverage of the clear -channel station is
greater (see Fig. 14).
According to the North American Regional Broadcasting Agreement
of 1940, entered into by the United States, Canada, Mexico, and Cuba,
the 106 channels in the standard broadcast band are divided into three
principal classes-clear, regional, and local.
1. Clear channel. A clear channel is one on which the dominant station or stations render service over wide areas and which are cleared of objectionable
interference, within their primary service areas and over all or a substantial portion of their secondary service areas.
2. Regional channel. A regional channel is one on which several stations may
operate with powers not in excess of 5 kw. The primary service area of a station
operating on any such channel may be limited, as a consequence of interference,
to a given field intensity contour.
3. Local channel. A local channel is one on which several stations may operate
with powers not in excess of 250 watts. The primary service area of a station
operating on any such channel may be limited, as a consequence of interference,
to a given field intensity contour.
All countries are permitted to use all regional and all local channels
subject to power limitations and standards for the prevention of objectionable interference. The clear channels were assigned definitely to the
various countries, 63 clear -channel stations being permitted to the United
States. Twenty-four of these channels are used in conjunction with other
countries. The remaining 39 are exclusive United States channels. With
only 59 clear channels available and with the United States permitted
to operate 63 clear -channel stations, it is obvious that certain of these
stations must be located far enough apart so that interference of the sky
waves will be negligible. With only 106 channels available for broadcasting in the United States and with 881 stations operating on January 1,
1940, it is equally obvious that a great many of these stations have to be
in the same frequencies, but by placing them far enough apart so that the ground waves of regional and local stations do not interfere and that the
sky waves of clear -channel stations do not interfere, it is possible to
obtain good reception from all these licensed stations. This is achieved by
the Federal Communications Commission, which limits the power of the
various stations and the hours in which certain stations may broadcast.
Various stations are allotted a certain amount of power for broadcasting their programs. Those which have clear channels are generally
allowed 50,000 watts; those in the regional classification do not exceed,
under ordinary circumstances, 5000 watts; and those in the local category
have a maximum of 250 watts. Under ordinary circumstances a station
with 50,000 watts would be able to send its carrier wave approximately
three times as far as a station with 250 watts. However, there are factors
that determine the coverage of a station in addition to power. A station
which broadcasts upon a low frequency, as a 550 -kilocycle station, will
go farther with less effort than a station which is broadcasting upon a frequency of 1550 kilocycles, because the latter carrier wave has to oscillate so many more times in covering the same distance. In an article by
J. M. Greene, circulation manager of the National Broadcasting Company, in Printer? Ink, April 26, 1940, the following illustration explains
this : To explain why one carrier wave travels farther than the other, let us compare
them with two men, one tall and the other short, walking at the same speed along a soft, sandy beach. Each step absorbs energy and the result is that the taller
man takes fewer steps (the radio station broadcasting upon the lower frequency)
and is still going strong after the shorter man has given up (the radio station
broadcasting on the higher frequency).
A second factor which determines the coverage of a radio station is
the ground over which it passes. Various geological conditions affect the
transmission and cut down the coverage of the station. Therefore the
station which has the greatest power and the lowest number of kilocycles
and broadcasts over the best ground conditions is the one that will be
heard the farthest. Power is not the only factor in station coverage. It is
entirely possible under certain conditions for a station operating on 250
watts to have a greater coverage than one operating on 50,000 watts.
Ground conductivity alone can offset the advantages of both high power
and low frequency.
Not only do such things as power, the frequency, and ground conductivity affect the coverage and reception of programs, but man-made
conditions may affect it. Electrical disturbances caused by X-ray machines, power lines, etc., create disturbances which affect the signal
received by the broadcasting set. High steel structures surrounding the
antenna of the station's transmitter will affect its coverage.
As has been pointed out, radio signals travel farther at night by their
sky waves than they do during the daytime. Therefore, in order further to
avoid interference, the Federal Communications Commission grants
licenses to certain stations which are located close to one another to
broadcast with decreased power after sunset. More stations broadcast
from sunrise to sunset than are permitted to air programs after sunset.
There are other instances where stations share time, one station being permitted on the air for part of the day and another one for the balance of
the day. These limitations permit the licensing of a greater number of
stations.
Also in an effort to decrease interference between stations, directional
antennas are sometimes installed. Under normal circumstances a vertical
antenna will radiate almost equally well in all directions, but it is possible
by proper modification to directionalize the radiation from an antenna.
The bulk of the station's power may be sent in one certain direction, as is
done in radio airway beacons, or it may be kept from radiating in that
direction and left free to traverse all the others.
The carrier frequency and side band (sometimes called "side frequencies ") come through space to be picked up by the aerial of the
receiving set. Radio waves travel through the air at the speed of light,
approximately 186,000 miles per second. If the announcer in a prize fight
is talking to a person located in the 25 -cent seats 500 or 600 feet away
from the ring, and to a microphone, you who are listening to the program
500 to 600 miles away will hear his voice over the radio before it will be
heard by the man who has paid his quarter. These radio waves, picked up
by the aerial, are changed into electrical impulses (of the same frequency
as the radio waves), which are conveyed to apparatus which tunes the set
to the frequency of the station. After suitable amplification these impulses
go into a detector in which the speech of the announcer, in the form of
electrical impulses of the same frequency as developed by the microphone,
is extracted from the carrier and side bands. Thence these impulses are
further amplified and conducted to a voice coil mounted in a magnetic
field. This voice coil is attached to the paper cone of the loud -speaker.
The impulses cause the voice coil and hence the cone to vibrate. The
vibrations of the cone result in sound waves just like those that were
projected by the announcer in the studio (see Fig. 15).
The phraseology I have used in this explanation (channels, bands) is
that used by technicians, specialists in electrical engineering and physics.
However, it does give rise to a misconception on the part of the layman.
In reality there are no definite layers in the air. Possibly a better illustration to use in connection with broadcasting is that there are two stations,
one represented by a red light and the other by a green light. When these
stations are broadcasting, both lights are illuminated and the air about hem is filled with red and green rays representing their radio frequencies.
Both colors are everywhere just as their radio waves fill the air. Your
receiving set is a filter which picks out only the red rays or only the green
rays as you tune that filter (receiving set) to the station to which you
desire to listen. The red rays do not go in a definite pathway or band, but,
go everywhere, up and down and around the light which is the antenna
of the station. If the red or green light were made brighter and dimmer
according to some prearranged code, while the color was not changed,
and the person watching the lights could interpret that message through
the medium of a code, he would be using the light rays just as the receiving
set picks up radio waves. The intensity of the signal is varied by the
sound wave w
Following the same illustration, if the code system sent out by the
light is built around a change in the color of the light instead of in the
brightness, we have the new frequency -modulation or staticless radio idea
illustrated. In other words, the frequency of transmission is changed back
and forth as a code. The receiving set under these circumstances must be
sensitive to frequency changes or, in the illustration, to color changes.
The frequency -modulation system possesses several important advantages over the older method of amplitude modulation which makes its
employment desirable. Man-made and natural interference is reduced by
this method of broadcasting. Nearby stations do not cause annoying
interference because of the wide band of frequencies needed by this
system. To date the only real interference that is noticeable is that caused
by automobiles. Frequency modulation is a short-wave form of broadcasting and covers a restricted area. Because of the high frequency of
this type of broadcasting, there is room for a great many more stations
than now exist in the broadcasting band. The frequency -modulation
transmitter, furthermore, is much simpler than the amplitude -modulated
transmitter and less expensive to operate; receivers can be serviced by
regular radio -service men and can be combined with receivers for the
regular broadcast band. At the present time seven companies are man in- facturing frequency -modulated receiving sets. While only 17 stations are
operated at the present writing, there are hundreds of applications filed
with the Commission for such stations. There are two major. reasons for
not changing at once to this method of broadcast. Radio sets in existence
today will not receive frequency -modulated radio waves, and, at the high
frequency at which these transmitters are employed, the radio waves
have begun to take on some of the properties of light and will not go very
far beyond the horizon. In the majority of instances such stations cover a radius of only about 25 to 50 miles. The width of the new FM channels
has been adjusted to permit high-fidelity transmission, making more
noticeable the high frequencies in the reception with the result that the
listener has to be trained to appreciate these frequencies rather than to
rely upon the lower tonal qualities of regular broadcast. However,
frequency -modulated sets have tone control just as do regular receiving
sets.
In order that all the frequencies may be broadcast under this system,
it is essential to have equipment, such as microphones and studios, which
will carry all the frequencies which are not being carried at the present by
the amplitude system. Telephone lines are being developed to carry
these frequencies so that frequency -modulated programs will be satisfactorily broadcast over a network of stations connected by telephone
lines. It is also possible for such frequency -modulation broadcasts to be
rebroadcast by radio relays established over a prescribed area. The
F.C.C. has not yet ruled as to whether such relayed networks will be
permitted. In making available such channels for frequency modulation,
the government has set aside hands for educational purposes exclusively
which adjoin the bands for commercial purposes, with the result that all
the research that is conducted by commercial stations will be advantageous to the educational broadcasters. Furthermore, all receiving sets
built to receive commercial frequency modulation will also be constructed
to receive programs in the educational bands.
The band between 4e,000 and 50,000 kilocycles is set aside to accomodate both commercial and educational FM stations. These FM stations
can operate upon the same channel without objectionable interference
with much less mileage separation than is possible for the standard broadcast station. FM has the ability to exclude all but the strongest signal;
consequently the service range of such stations, though limited, will in
many cases be greater than that obtained in the primary service area of
comparable standard broadcasting stations. As the commission intends to
grant licenses upon the basis of coverage without consideration of power,
the coverage of the FM station will be substantially the same both day
and night.
Facsimile.
Facsimile is the reproduction of an original picture or page of printing.
It was first used commercially as wirephoto service and consisted of the
sending of photographs by telephone methods. The same method of transmitting pictures and copy from radio stations to the home is now being
used in the field of broadcasting.
Facsimile broadcasting equipment consists of sending and receiving
instruments. The sending equipment utilizes the photoelectric cell or eye
20 Handbook of Broadcasting
to scan in orderly fashion all the line elements of material placed in the
scanning machine. The photoelectric eye receives more or less reflected
light, depending upon whether the subject matter is black or shades of
gray or white. It transfers these light variations into electrical impulses
which are amplified by conventional amplifiers and passed to a transmitter
suitable for the transmission of voice or music. These electrical impulses
are sent through the air just as a regular broadcast is but they are retained
as impulses by the receiving set instead of being converted into sound
waves.
At the receiving point some form of printing mechanism (an electric
pen or scanner) is necessary to scan the receiving paper in exact juxtaposition with the sending point, and this reconstructs a large number of dots
or lines across the page in exactly the same relative position and in the
same density as they appeared upon the original, in a sense half -toning
or screening the picture. The scanning machine can operate as high as
125 lines to an inch.
The equipment can be used on any wave length on which the transmitter is broadcasting; the normal broadcasting band or the ultra -high - frequency band. Technically the major difference in these two is that
facsimile is permitted to be broadcast on the normal broadcasting band
only in the early -morning hours, while all ultra -high -frequency stations
are allowed to broadcast facsimile at any time of the day or night. The
standard bands would make facsimile more suitable to rural coverage
while the short waves would be used primarily for metropolitan coverage.
The coverage of facsimile would be the same as the coverage of the station
over which it is broadcast if the regular band were used in the early - morning hours. Up to the present time only experimental licenses have
been granted by the F.C.C.
Facsimile is a very interesting device and like a great many other
scientific devices its value will depend upon the ingenuity of the various
groups employing it. It is being experimented with by various newspapers,
who feel that it will be confined to a bulletin or headline type of news
rather than used for the lengthy story. It is possible to use facsimile in
connection with such programs as the cooking school, in which speech
could be combined with the sending of a facsimile of the recipe. Fashion
talks could be given in the same way. It is possible that facsimile might
be used in extension teaching as an educational medium.
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