Encoding is the **process of converting data into a format required for** a number of information processing needs, including Program compiling and execution.

Mechanisms for digital-to-digital encoding fall into the following categories:

Digital encoding is of three types-

1. Unipolar

2. Polar

3. Bipolar

## A. Unipolar digital encoding

**1.** Unipolar encoding is very simple and primitive. **2.** Unipolar encoding uses only one polarity. **3.** In unipolar encoding, all the signal levels are on one side of the time axis, either above or below.**a. NRZ (Non-Return-to-Zero) :** **i. **In a non-return-to-zero (NRZ) scheme the voltage defines bit 1 and the zero voltage defines bit 0. **ii.** Since the signal does not return to zero at the middle of the bit it is called NRZ.

## B. Polar digital encoding

**1.** The polar encoding uses two voltage levels: one positive and one negative. **2. **By using both levels, the average voltage level on the line gets reduce in most of the polar encoding. **3.** NRZ encoding includes two methods: nonreturn to zero, level (NRZ-L), and nonreturn to zero, invert (NRZ-I). **a. NRZ L ****i. **In NRZ-L encoding, the level of the signal depends on the type of bit it represents.**ii.** A positive voltage usually means the bit is a 0, and a negative voltage means the bit is a 1 (or vice versa). iii. Thus, the level of the signal is dependent on the state of the bit represents.**b. NRZ-I: ****i. **In NRZ-I, an inversion of the voltage level represents a 1 bit. **ii. **It is the transition between a positive and a negative voltage that represents a 1 bit. **iii.** A 0 bit is represented by no change. **iv. **NRZ-I is superior to NRZ-L due to the synchronization provided by the signal change each time a 1 bit is encountered.

**c. Return to Zero (RZ):** **i.** In NRZ encoding problem occurs when the sender and receiver clocks are not synchronized. **ii.** The receiver does not know when one bit has ended and the n.next bit is starting. **iii. **The solution to this problem is a return-to-zero (RZ) scheme. **iv.** RZ scheme uses three values: positive, negative, and zero. **v.** In RZ, the signal changes not between bits but during the bit.

**d. Biphase:** **i.** In bi-phase encoding, the signal changes at the middle of the bit interval but does not return to zero.**ii.** Instead, it continues to the opposite pole. **iii.** These mid-interval transitions allow for synchronization. **iv.** There are two types of biphase encoding: Manchester and differential Manchester.

### 1. Manchester:

**i. **Manchester encoding uses the inversion at the middle of each bit interval for both synchronization and bit representation. **ii.** A negative-to-positive transition represents binary I and a positive-to-negative transition represents binary 0.

### 2. Differential Manchester

**i. **In differential Manchester, the inversion at the middle of the bit interval is used for synchronization. **ii.** Also, the presence or absence of an additional transition at the beginning of the interval is used to identify the bit. **iii. **A transition means binary 0 and no transition means binary 1. **iv. **To represent binary 0 and binary 1 manchester requires two signal changes.

## C. Bipolar digital encoding

**1.** In bipolar encoding, we use three levels: positive, zero, and negative. **2. **The voltage level for one data element is at zero, while the voltage level for the other element alternates between positive and negative. **3.** There are two variations of bipolar encoding: AMI and pseudo ternary. **a. Alternate Mark Inversion (AMI) :****i.** In AMI the word ‘mark’ means 1. So AMI means alternate 1 inversion. **ii.** A neutral zero voltage represents binary 0. **iii.** Binary Is are represented by alternating positive and negative voltages. **b. Pseudo ternary:****i. **A variation of AMI encoding is called pseudo ternary. **ii.** In pseudo ternary the 1 bit is encoded as a zero voltage and the 0 bit is encoded as alternating positive and negative voltages.

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