This lecture will teach us about the different types of Information source in communication systems. There are many sources of information that are used in the communication system. we will cover them one by one in every detail.
before discussing the information sources, we will know the definition of the signal. what are a signal and the classification of signals?
Signal and Classification of Signals
A signal may be defined as the single-valued function of time. Time plays the role of an independent variable. This means that at every instant of time, the signal has a unique value.
Classification of Signals:
- The signal may be classified as under:
- Now let us discuss the sources one by one.
Information source in communication system
- Some of the important sources of information in the communication environment may be listed as under:
| 1. Speech | 2. Music | 3. Pictures | 4. Computer data |
- A source of information is basically a signal which carries the information.
1. Speech:
- Speech involves the transfer of information from the speaker to the listener. Such a transfer of information takes place in the following three stages:
- Production
- Propagation and
- Perception
- The band of frequencies considered to be essential for speech communication is 300 Hz to 3100 Hz. This band is utilized for commercial telephonic communication.
2. Music:
- The music signal is originated from the instruments such as the piano, violin, flutes, etc. The musical note may last for a short time or for a longer time depending upon the instruments being used for its origination.
- Music signal has the following two possible structures:
- The melodic structure consists of a time sequence of sounds, whereas the harmonic structure consists of a set of simultaneous sounds. The music signal is a bipolar signal and requires a bandwidth of about 15 kHz. hence the channel bandwidth required for the transmission of the music signal is much larger than that required for the transmission of the speech signal.
3. Pictures:
- The picture can be either static or dynamic. An example of a static picture is a picture sent by a fax machine and that of a dynamic picture is one produced on T.V. The T.V. camera produces a dynamic picture and the picture tube reproduces it at the receiving end. The electrical equivalent of a dynamic picture is produced at the output of the camera and it is called the typical video signal.
- The bandwidth required for this video signal is 0 to 5 MHz.
- The video signal consists of a luminance signal (Y) and a chrominance signal. The luminance signal conveys the brightness information while the chrominance signal conveys the color information. Along with them, synchronization signals ( horizontal and vertical) are also transmitted. These are used by the receiver to synchronize its vertical and horizontal scanning rates with those at the transmitter.
4. Computer Data:
Personal computers are used for electronic mail, the exchange of software, and sharing of resources. The text matter transmitted by a computer is encoded using the American standard code for Information Interchange (ASCII). Each character in ASCII is represented by seven data bits. Hence total 27 = 128 characters can be represented using ASCII.
The Table gives a complete listing of ASCII characters. The seven data bits are ordered starting with the MSB b7 and LSB b1. An extra eight-bit b8 is used as a parity bit for the purpose of error detection. When ASCII data are transmitted river these ports, a start bit ( which is set to 0) and a stop bit ( which is set to 1) are added as shown in the figure below.
ASCII Code Set
Control symbols used in ASCII code set
Important Point: Computer-generated data and television signals are both wide-band signals. In that their power content occupies a wide range of frequencies. Another important characteristics of data communication between personal computer is burstiness, which means that information is usually transmitted from one terminal to another in bursts with silent period between bursts.
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