Basic data encoding techniques

To determine successful receiving of data we should have consider 3 major factors, they are

•   S/N � increase of this signal to noise ratio decreases the bit error rate.
•    Data rate � increase of data rate increases the bit error rate.
•   Bandwidth � increase of bandwidth tends to increase of bit error rate.

We use data encoding to increase the success fulness of the receiving signal. There are four encoding schemes     

•   Analog data � Analog signal (telephone)
•   Analog data � Digital signal (CODEC)
•   Digital data � Analog signal (Modem)
•  Digital data � Digital signal (Digital transmitter)

Analog data � Analog signal

�         Amplitude modulation (uses in fiber optics)

            In this method we varied carrier signal�s amplitude according to the amplitude of modulating signal but the frequency remains unchanged. This is simplest form of modulation. Amplitude transmission operates in the 535 kHz to 1705 kHz frequency band used for shortwave broadcasting and some citizen band aviation services.

Frequency modulation

            In the FM carrier signal�s frequency changes according to the modulating signal and amplitude of carrier signal remains unchanged. FM is widely used for broadcasting music and speech, two-way radio systems, magnetic tape recording system and some video transmission systems. FM with sufficient bandwidth cancels out natural occurring noise.

�         Phase modulation

            Varying the angle of carrier signal according to the modulating signal happens in the PM. We don�t use PM for radio transmission because it requires complex receiving hardware and there can be ambiguity in determining the phase angle.

Analog data � Digital signal

�         Pulse Code modulation

PCM is digital scheme for transmitting analog data. First divide amplitude range into 2ⁿ levels and then assign each amplitude levels to a bit pattern of n bits. Sample the analog signal periodically and approximate each amplitude level of each sample to the most closely matching bit level. This is the standard form for digital audio in computers and various Blue-ray, DVD and various Compact Disc formats also it is used in digital telephone systems.

�         Delta modulation

DM is used for transmission of voice information where quality is not primary important. In DM we approximate analog input by staircase function. So it can only move up or down one level in each sample interval. Bit stream is produced by approximating the derivative of an analog signal rather than its amplitude.

            1 is generated if the staircase function is to go up during the next interval; a 0 is generated otherwise.

Digital data � Analog signal

�                       Amplitude shift key

In ASK we represent different binary values by different amplitude values. As example we represent two binary values by two amplitudes and multiple amplitudes for representing multiple bits. This technique is used to transmit digital signals over optical fibers.

Frequency Shift key

In this method we represent different binary values by different frequencies. The most common FSK is binary FSK (BFSK). In BFSK we represent two binary values by two different frequencies near the carrier frequency. BFSK is less prone to error than ASK.

Phase shift key

PSK uses finite number of phases, each assigned to unique pattern of binary digits. Usually each pattern encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase.

The wireless LAN standard uses variety of different PSKs depending on the data rate required

                    Quadrature amplitude modulation.   

 modulation technique is a combination of ASK and PSK. It is possible to send two different signals simultaneously on the same carrier frequency by using the two copies of the carrier frequency one shifted by 90⁰ with the respective to the other. The two independent signals are transmitted simultaneously over the same medium. QAM is used in the Asymmetric digital subscriber line (ADSL) and in some wireless standards.

            Digital data � Digital signal

�         No return to zero (NRZ)

o   NRZ-L

Presence (0) or absence (1) of voltage

NRZ-L is used for short distances between terminal and modem or terminal and computer.

o   NRZ-I

Presence (1) or absence (0) of a transition

NRZ-I is differential encoding (signal is decoded by comparing the polarity of additional signal).

�         Multilevel Binary

Uses more than two signal levels

o   Bipolar AMI(most recent proceeding 1 bit has negative voltage)

No signal (0), positive pulse (1) and negative pulse (1)

No loss of sync if a long string of ones (zeros still a problem)

No net dc component.

Lower bandwidth.

Easy error detection (pulse alternation property).

o   Pseudo ternary (most recent 0 represented by negative voltage)

No signal (1), positive pulse (0) and negative pulse (0)

�         Biphase

Transmission at the middle of bit period as clocking.

o   Manchester

o   Mid bit transition for both clocking and data.

Low-to-high (1), high-to-low (0)

The dc component of the encoded signal is not dependent of the data and therefore carries no information, allowing the signal to be conveyed conveniently by media (e.g. Ethernet) which usually do not convey a DC component.

o   Differential Manchester.

Mid bit transition only for clocking.

Transition at beginning of bit period (0), no transition at the beginning of bit period (1)

Clocking information is embedded in the signal itself.

�         Scrambling schemes