Need of Modulation – Until the process of superimposing a low-frequency (long-wave) voice information component on a high-frequency (short-wave) carrier signal was perfected, the most widely used form of communications was a system based on the transmission of a continuous-wave (CW) signal. With this system, the signal was interrupted periodically (Morse code) to produce a coded message. The CW system required tremendous training and expertise on the part of the persons involved in transmitting and receiving the messages, and therefore the field was limited to a few experts.
With the development of Need of Modulation, a whole new era of communications evolved, the results of which can be seen all around us today. We will now examine the process of modulation in more detail.
There are two alternatives to the use of a modulated carrier for the transmission of messages in the radio channel. One could try to send the (modulating) signal itself, or else use an unmodulated carrier. The impossibility of transmitting the signal itself will be demonstrated first.
Although the topic has not yet been discussed, several difficulties are involved in the propagation of electromagnetic waves at audio frequencies below 20 kilohertz (20 kHz) (see Chapters 8 and 9). For efficient radiation and reception the transmitting and receiving antennas would have to have lengths comparable to a quarter-wavelength of the frequency used. This is 75 meters (75 m) at 1 megahertz (1 MHz), in the broadcast band, but at 15 kHz it has increased to 5000 m (or just over 16,000 feet)! A vertical antenna of this size is impracticable.
There is an even more important argument against transmitting signal frequencies directly; all sound is concentrated within the range from 20 Hz to 20 kHz, so that all signals from the different sources would be hopelessly and inseparably mixed up. In any city, the broadcasting stations alone would completely blanket the “air,” and yet they represent a very small proportion of the total number of transmitters in use. In order to separate the various signals, it is necessary to convert them all to different portions of the electromagnetic spectrum. Each must be given its own frequency location. This also overcomes the difficulties of poor radiation at low frequencies and reduces interference. Once signals have been translated, a tuned circuit is employed in the front end of the receiver to make sure that the desired section of the spectrum is admitted and all the unwanted ones are rejected. The tuning of such a circuit is normally made variable and connected to the tuning control, so that the receiver can select any desired transmission within a predetermined range, such as the very high frequency (VHF) broadcast band used for frequency modulation (FM).
Although this separation of signals has removed a number of the difficulties encountered in the absence of Need of Modulation, the fact still remains that unmodulated carriers of various frequencies cannot, by themselves, be used to transmit intelligence. An unmodulated carrier has a constant amplitude, a constant frequency and a constant phase relationship with respect to some reference. A message consists of ever-varying quantities. Speech, for instance, is made up of rapid and unpredictable variations in amplitude (volume) and frequency (pitch). Since it is impossible to represent these two variables by a set of three constant parameters, an unmodulated carrier cannot be used to convey information. In a continuous-wave-modulation system (amplitude or frequency modulation, but not pulse modulation) one of the parameters of the carrier is varied by the message. Therefore at any instant its deviation from the unmodulated value (resting frequency) is proportional to the instantaneous amplitude of the modulating voltage, and the rate at which this deviation takes place is equal to the frequency of this signal. In this fashion, enough information about the instantaneous amplitude and frequency is transmitted to enable the receiver to recreate the original message