1. Core 3: Communication Systems

3. Encoding and decoding analog and digital signals…  Encoding involves converting data from its original form into another form for transmission.  Decoding is the reverse process converting data from the form used for transmission back into the original form.

4. Eg. A telephone encodes the sounds from your voice into electrical pulses that can be transmitted along the telephone lines. The telephone receiving the call decodes the electrical pulses back to the sounds of your voice. The type of encoding and decoding depends on whether the original data is analog or digital forms.

5. Analog data is represented by using using continuous variable physical quantities such as voltages. Eg. A clock that displays time by hands moving smoothly around the clock is an analog device. Most natural events are in the real world such as temperature, light and pressure are in analog forms. Analog signals are pulses, usually electrical or optical, in the form of continuous wave. Sounds, images and video are naturally in analog form.

6. Digital data is represented in the form of digits and numbers. Eg. A digital clock shows time as a certain number of hours and minutes as graphics on an LCD screen. Numbers, text and other characters are naturally in digital form. IT works with data in a digital form represented as a series of 1’s and 0’s.

7.  The form of the data (either analog or digital) and the transmission signal (either analog or digital) affects the quality of the data received and the cost of transmission.  The quality of the analog data depends on maintaining the exact wave as it moves through a wire or space. If it is corrupted in any way there is no way of regenerating the wave. However, digital data is transmitted as 1’s and 0’s and it is possible to resend or regenerate the corrupted data (see checksum, partiy, CRC).

8.  To encode analog data into a digital signal it requires that the data first be converted into digital using an analog to digital converter (ADC). Similarly to encode digital data into an analog signal the data must be converted to analog data using a digital to analog converter (DAC).

9. There are 4 encoding and decoding possibilities in transmission… Analog Data to Analog Signal The wave shape of the data is encoded into the signal. When the data is analog the waveform varies continuously with the changes in the original analog data. An example is the microphone collecting analog sound waves is encoded as an infinitely variable electromagnetic wave that is decoded by a speaker and converted back to audio waves for your ear. If the signal is corrupted there is no way of restoring the original data.

10. There are 4 encoding and decoding possibilities in transmission… Digital Data to Digital Signal A series of 0’s and 1’s is transmitted by sending through a channel as a series of on and off pulses. Data transmitted in a LAN is digital data using a digital signal. There is a low error rate for this type of transmission. Digital data is encoded into a digital signal by the computer or peripheral device.

11. There are 4 encoding and decoding possibilities in transmission… Digital Data to Analog Signal Digital Signals are produced when digital data is encoded onto analog waves. To decode the wave and retrieve the encoded digital data requires the receiver to read the wave at the same intervals. The time between waves is the bit time. A modem encodes (or modulates) digital data from a PC into analog signlas for the phone line. When the signal is received by another modem it decodes (or demodulates) the analog into digital.

12. There are 4 encoding and decoding possibilities in transmission… Analog Data to Digital Signal The wave shape of the data is encoded into a series of 0’s and 1’s. This process of generating digits or numbers is called digitising. Images are digitised using devices such as scanners and sounds are digitsed using devices such as microphones (called sampling).