Historically, the most important application for electronics has been in the communication of information. It was the goal of information dissemination, exchenge, manipulation, and storage that fostered developments in telegraphy, telephony, radio, television, and computers. Accordingly a brief overview of communication systems may sever to introduce concepts that will be found useful in the study of modern electronics.
A Radio Broadcast System
We shall first consider a simple radio broadcast system. Here the information is speech and music, originating from a human source, a record player, or a tape recorder. After transduction and amplification we obtain electrical signal of reasonable strength that are ready for transmission to remotely located listeners. However, a problem arises if one attempts to transmit these signals directly. The problem is twofold : First, since the signals are in the audio-frequency range their wavelengths are quite long and hence the transmitting antenna required to convert the signal to electromagnetic radiation would have to be very long. Second, if all radio stations were to transmit the same frequency band (audio), the remote listener would find it impossible to distinguish between the broadcasts received from the various stations.
A solution to this problem is provided by the process of modulation which allows one to shift the frequency band to be transmitted from the audio range to a location at a much higher frequency [in a range know as the radio frequency (RF) range]. In this way the required antenna is of moderate length. Furthermore, since each of the radio stations in a given geographical area is assigned a different location in the radio-frequency band for its transmission, the listener can easily select a desired station by simply tuning the receiver to the frequency location at which this particular station is broadcasting.
Telephone communication system have undergone tremendous development in the last few years. Many of these developments have been a direct result of advance in microelectronics. Currently, the telephone system is used to communicated not only speech but also television programs and digital data. Such data communication take place between computer terminals located at widely separated locations and a central computer, as well as between two or more computers. Familiar example of data communication systems include system used in banks and for airline reservations. Perhaps less glamorous but extremely useful is the ability dial up a computer from one’s home, connect the telephone via an acoustic coupler to a computer terminal, and then proceed to enter one’s programs on the terminal. This arrangement was employed for typing and editing a good part of the manuscript for this book.
Advances in microelectronics are also allowing the addition of “intelligence”capabilities to the telephone set. As an example, “smart” telephone that “remember” frequently dialed numbers and that allow the user to reach any one of such numbers by pushing a single button are currently available.
Let us briefly examine the telephone communications system. For the sake of simplicity, let us restrict outselves to speech communication. In order to make it possible to use a single telephone cable for the transmission of many separate conversations (separated in what is called telephone channels) the bandwidth allotted for each channel is limited to about 4 kHz (actually, from 300 to 3400 Hz). It has been established that this relatively narrow bandwidth is sufficient for the transmission of speech signal is fed to a low-pass-filter whose passband extends to about 4 kHz. This low-pass-filter eliminates the higher-frequency components, thus avoiding the possibility if the various channels carried on the same cable “running into one another”
To carry a number of telephone channels on a single cable, modulation is used to shift the spectrum of each channel to a different frequency band. Such a system is know as a frequency division multiplexing (FDM) system. At the receiving end the channels are separated using filtering and demodulation.
Another more recent approach to multiplexing a number of telephone channels on a single cable is use of time division multiplexing (TDM). In TDM the signals from the channels to be multiplexed (say 12 channels) are sampled in sequence. That is, a sample is taken from Channels 1, then a sample taken from Channel 2, and so on to the last channel, Channel 12. Then a new round of sampling is started and the process is periodically repeated. The samples are transmitted on the same cable in the same order in which they were obtained. Thus samples 1, 13, 25, etc. belong to the signal of Channel 1, and so on.