BASIC PRINCIPLES OF FREQUENCY MODULATION 1944 U.S. WAR DEPARTMENT FILM FM RADIO 86794 Made during WWII by the War Department, this official training film describes what frequency modulation (FM) is and how to use it in reference to radio communications. It also provides the comparison between FM and AM radio as far as what advantages FM has over AM. Originally restricted, the film was released for public educational use in 1949. It opens with U.S. soldiers playing ping pong while listening to AM radio (:28). When one of the men begins to use an electric razor, the radio gets staticky and one of the men switches the radio to FM so they can still listen to the music clearly (1:02). FM was a major step forward in radio and as guests tuned into a radio programs now they were listening to what the microphone was picking up (1:24). AM radio had its drawbacks in the sense that whenever it was used for communication in the battlefield (2:06) it would pick up static which would mess with the signal. Tank tracks made reception near impossible (2:13) as well as a slew of other war materials and natural occurrences such as thunderstorms (2:25). FM was able to remove this issue (2:34) and the film turns to breaking down the proponents of AM radio (2:40). A diagram follows, pointing to the oscillator (2:49) which generated the carrier wave that is fed into the amplifier (2:57). At this point it carried no message and for AM, intelligence was pressed upon the wave through a microphone (3:13). The intelligence is converted into electrical energy which goes into the modulator (3:24) and is then amplified. The intelligence is radiated by the antenna (4:06). The process of amplifying the waves is depicted (4:41) as it moves through the IF Amplifier and then the demodulator (4:55) which changes the vibrations back into audio frequency (5:03). From here it enters a speaker and becomes sound again (5:14). AM worked well in perfect conditions (5:38) unless there were any electrical interferences. A demonstration follows where a lighting strike interferes with the wave (6:00) which makes the wave fuzzy and the intelligence indiscernible. The film shows the difference between AM and FM (7:22). AM carried the wave through increasing or decreasing the strength of the carrier whereas FM did the same job by changing the frequency of the carrier this way the static would only affect the amplitude and not the frequency (7:43). A few of the types of FM transmitters used by the army follow (7:59). The FM oscillator is broken down in diagram next (8:21) with the rest wave pointed to. Capacitor microphones could change AM to FM waves (9:01) and this was done by placing one parallel with the capacitor and the tank circuit (8:57). With this addition, the frequency now varied at an audio rate (9:41). A diagram follows pointing to a line which represents the rest frequency of the oscillator (11:14) and how it varies when sound is impressed on the capacitator mic (11:34). As example shows how the frequency affects the rate of change (11:43) as well as how the volume or loudness of the frequency determines the amount of deviation (13:13). Due to the potential of the deviation to interfere with other transmitters (13:26), the army set a maximum deviation of 40 kilocycles (13:32) on both sides of the rest frequency. Guard bands were also provided as there must be some sort of separation between the channels (14:10) and 10 kilocycles are allotted on either side. In addition to what the film has already shown, the army also used two other systems to get FM called radiance tube and phase modulation (15:04). There are three main differences between the AM and FM receivers and these are the band pass (15:40), the frequency modulation receiver (16:03) and the amplitude variations. A diagram of the AM receiver follows (16:33) and how these variations can be changed to turn it into an FM receiver including the addition of the limiter (16:57) and the discriminator (17:21). The limiter is broken down in diagram (18:00) and how it clips off any variations on the positive and negative portions of the wave (18:49). After the variations have been clipped off, the wave ends are squared and this leads to distortion of the signal (24:42). The plate circuit handles this problem by smoothing out any irregularities or sharp corners of the wave (24:49). A simplified diagram of the discriminator follows (25:23). The wave from the discriminator is fed into AF amplifiers and comes out of the speakers in the same way which an AM receiver does (27:57). This film concludes (28:19).

Dr. Klaus Volkamer Vortrag beim Drei-Länder-Wasser-Symposium der DGEIM - 12.11.2013 in Lindau am Bodensee