Text 6. Detection

The progress by which the signal being transmitted is reproduced from the radio-frequency currents present at the receiver is called detection, or sometimes demodulation. Where the intelligence is transmitted by varying the amplitude of the radiated wave, detection is accomplished by rectifying the radio-frequency currents. The rectified current thus produced varies in accordance with the signal originally modulated on the wave radiated at the transmitter and so reproduces the desired signal. Thus, when the modulated wave is rectified, the resulting current has an average value that varies in accordance with the amplitude of the original signal. In the transmission of code signals by radio, the rectified current reproduced the dots and dashes of the telegraph code and could be used to operate a telegraph sounder. When it is desired to receive the telegraph signals directly on a telephone receiver, it is necessary to break up the dots and dashes at an audible rate in order to give a note that can be heard, since otherwise the telephone receiver would give forth a succession of unintelligible clicks. The detection of a frequency-modulated wave involves two steps. First, the wave is transmitted through a circuit in which the relative output obtained from the circuit depends upon the frequency. The circuit output is then an amplitude-modulated wave since, as the frequency of the constant-amplitude input wave varies, the output will vary correspondingly. The resulting amplitude-modulated wave is then rectified.

Modulation

The transmission of information by radio waves requires that some means be employed to control the radio waves by the desired intelligence. One way to do this, termed amplitude with is to vary the amplitude of the radiated wave in accordance with the intelligence to be transmitted. In radio telegraphy, this involves turning the radio transmitter on and off in accordance with the dots and dashes of the telegraph code. In radio-telephone transmission by amplitude modulation the radio-frequency wave is varied in accordance with the pressure of the sound wave being transmitted.

Similarly in picture transmission, the amplitude of the wave radiated at any one time is made proportional to the light intensity of the part of the picture that is being transmitted at that instant. Intelligence may be transmitted by other means than by varying the amplitude. For example, one may the amplitude constant and vary the frequency that is radiated in accordance with the intelligence, thus obtaining frequency modulation. Frequency modulation has many advantaged and is widely used in very high-frequency communication systems.

Text 7a Mystery

“Something is very wrong,” says the detective.

“I know!” says Ms. Gervis. “It is wrong that someone has stolen from me!”

The detective looks around Ms. Gervis’ apartment. “That is not what I am talking about, ma’am. What is wrong is that I do not understand how the robber got in and out.”

Ms. Gervis and the detective stand in silence. Ms. Gervis’ eyes are full of tears. Her hands are shaking.

“The robber did not come through the window,” says the detective. “These windows have not been opened or shut in months.”

The detective looks at the fireplace. “The robber did not squeeze down here.”

The detective walks to the front door. He examines the latch. “And since there are no marks or scratches, the robber definitely did not try to break the lock.”

“I have no idea how he did it,” says a bothered Ms. Gervis. “It is a big mystery.”

“And you say the robber stole nothing else?” asks the detective. “No money, no jewelry, no crystal?”

“That’s right, detective. He took only what was important to me,” Ms. Gervis says with a sigh. “There is only one thing I can do now.”

“And what is that?” the detective asks with surprise.

“I will stop baking cakes,” Ms. Gervis says. “They are mine to give away. They are not for someone to steal.”

“You can’t do that!” says the detective with alarm. “Who will bake those delicious cakes?”

“I am sorry. I do not know,” says Ms. Gervis.

“I must solve this case immediately!” says the detective.