SIGNAL

Analog data-   Analog data is human voice.  When somebody speaks, a continuous wave is created   in the air.  This can be captured by a microphone and converted to the analog signal.

Digital data-   Digital data is data stored in memory of the computer in the form of 0s to 1s.  It is usually converted to a digital signal.  When it is transferred from one position to another position.

Analog and digital Signal-   Signal can be analog and digital.  Analog signal can have any value in range.  Digital signals can have only a limited number of value.

                              

Periodic Signals-   A periodic signal consists of a continuously repued pattern.  The period of signal (T) is expressed in seconds.

                                      

                                                  Example of Periodic and aperiodic signal

Aperiodic Signals-   An aperodic or nonperiodic signal has no repetitive pattern.

Amplitude-   Amplitude refer to height of signal.  The unit for amplitude depend on the type of the signal.  For electrical signal, the unit is normally volts ampere or watts.

Sound level	Type of Sound
40db	normal speech
90db	lawn mowers
110db	shotgun blast
120db	jet engine taking off
120db+	rock concerts

Frequency-  The frequency of a periodic function is the number of complete cycles that can o cccur per second,  Frequency is denoted with a lower-case f.  It is defined in terms of the period, as follows:

Period of frequency-   Period refers to the amount of time in seconds, a signal needs to complete one cycle.  Frequency refer to the number of period in  one second.

Unit of Period-   Period is expressed in seconds the communication industry uses five unit to measure period second (s) millisecond, microsecond, nanosecond and picoseconds.

Unit of Frequency-   Frequency is hertz, after the German Physicist Heinrich Rudolf Hertz.  The communications industry uses five units to measure frequency. Hertz, Kilohertz, Megaherth, Gigahertz and Terahertz.

Unit of Frequency-  

Frequency	1 mHz (10−3)	1 Hz (100)	1 kHz (103)	1 MHz (106)	1 GHz (109)	1 THz (1012)
Period (time)	1 ks (103)	1 s (100)	1 ms (10−3)	1 µs (10−6)	1 ns (10−9)	1 ps (10−12)

Digital Signal-   In addition to being represented by an analog signal, data can also be represented by a digital signal.

             For example, A “1” can be encoded as a positive voltage and a “0” as a zero voltage.

Bit Interval and Bit Rate-   Bit interval and bit rate are used to describe digital signals.

        The bit interval is the time required to send one single bit.

        The bit rate is the number of bit interval per second.  This means that the bit rae is the number of bits sent in one second usually expressed in bits per second (bps).  

           

Example of bit rate and bit interval

Problems of using Voice Channels for Digital Transmission
A digital signal is comprised of a number of signals. Specifically, the signal is represented as follows,

signal = f + f3 + f5 +f7 +f9 +f11 +f13 ....f(infinity)

This means a digital signal has a base frequency, plus another at three times the base frequency, plus another at five times the base frequency etc. f3 is called the third harmonic, f5 the fifth harmonic and so on. The third harmonic is one third of the amplitude of the base frequency (called the fundamental frequency), the fifth harmonic is one fifth the amplitude of the fundamental and so on.

In order to send a digital signal across a voice channel, the bandwidth of the channel must allow the fundamental plus third and fifth harmonic to pass without affecting them too much. As can be seen, this is what such a signal looks like, and is the minimum required to be correctly detected as a digital signal by the receiver.

Lets consider sending a 2400bps binary digital signal down a voice channel rated with a bandwidth of 3.1KHz. The base frequency of the digital signal is 1200Hz (it is always half the bit rate), so the fundamental frequency will pass through the channel relatively unaltered. The third harmonic is 3600Hz, which will suffer attenuation and arrive severely altered (if at all). The fifth harmonic has no chance of passing the channel.

In this case it can be seen that only the base frequency will arrive at the end of the channel. This means the receiver will not be able to reconstruct the digital signal properly, as it will require f3 and f5 for proper reconstruction. This results in errors in the detection process by the receiver. Baud Rate-The baud rate indicates the number of bits per second That are transmitted. For example: 300 baud means hat 300 bits are trans mitted each second (abbreviated 300bps).  Assuming asynchronous communication, which requires 10 bits per character, this translates to 30 characters per second (cps).  For slow rates (below 1200 baus), you can divide the baudby 10 o see how many characters per second are sent.