Classification of Signals
The signals may be classified as under :
Fig.1 : Classification of signals
Let’s now discuss these sources one by one in detail.
Speech involves transfer of information from the speaker to the listener and such a transfer of information takes place in three stages such as :
The band of frequencies required for the speech communication is 300 Hz to 3100 Hz.This band is utilized for the commercial telephonic communication.
Music signal is originated from the instruments such as the piano, violin, flute etc. The musical note may last for a short time or for a longer time depending upon the instrument being used for its origination.
Music signal has two possible structures such as :
(i) Melodic structure
(ii) Harmonic structure
The melodic structure consists of a time sequence of sounds, whereas the harmonic structure consists of a set of simultaneous sounds.
Music signal is a bipolar signal and it requires a bandwidth of about 15 kHz.
Hence, the channel bandwidth required for the transmission of music signal is much larger than that required for the transmission of speech signal.
The picture can be either static or dynamic. For example the picture sent by fax machine is a static picture and picture that is produced on T.V is an example of dynamic picture.
The T.V. camera produces a dynamic picture which is reproduced at the receiving end by the picture tube.
The electrical equivalent of dynamic picture is produced at the output of the camera and it is called as the typical video signal.
Figure 2. shows a typical video signal.
Fig.2 : Video signal
The bandwidth required for this video signal is 0 to 5 MHz.
The video signal consists of a luminance signal (Y) and chrominance signal. The luminance signal conveys the brightness information while chrominance signal conveys the colour information. Along with them the synchronizing 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, exchange of software, and sharing of resources.
The text transmitted by a computer is encoded using 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 seven data bits are ordered starting with the MSB (b7) and LSB (b1). An extra eighth bit (b8) is used as a parity bit for the purpose of error detection.
When ASCII data are transmitted over these ports, a start bit (which is set to 0) and a stop bit (which is set to 1) are added as shown in figure 3.