DIGITAL-TO-DIGITAL CONVERSION

In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed.

Topics discussed in this section:

• Line Coding
• Line Coding Schemes
• Block Coding
• Scrambling

Line Coding

• Line coding is the process of converting digital data to digital signals.
• Line coding converts a sequence of bits to a digital signal.
• At the sender, digital data are encoded into a digital signal; at the receiver, the digital data are recreated by decoding the digital signal.

Line coding and decoding

Line Coding Characteristics

• Signal Element Versus Data Element

• A data element is the smallest entity that can represent a piece of information: this is the bit.
• Signal element is the shortest unit (time-wise) of a digital signal. In digital data communications, a signal element carries data elements.
• In other words, data elements are what we need to send; signal elements are what we can send.
• Data elements are being carried; signal elements are the carriers.

Signal element versus data element

• Signal Rate Versus Data Rate

The data rate defines the number of data elements (bits) sent in 1s.The unit is bits per second (bps).

• The signal rate is the number of signal elements sent in 1's. The unit is the baud.
• The data rate is sometimes called the bit rate; the signal rate is sometimes called the pulse rate, the modulation rate, or the baud rate.
• One goal in data communications is to increase the data rate while decreasing the signal rate. Increasing the data rate increases the speed of transmission; decreasing the signal rate decreases the bandwidth requirement .
• We can formulate the relationship between data rate and signal rate as:

S=c×N×1/r

• where N is the data rate (bps); c is the case factor, which varies for each case; S is the number of signal elements; and r is the previously defined factor.

Note

Although the actual bandwidth of a

digital signal is infinite, the effective

bandwidth is finite.

• Baseline Wandering

• In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline.
• The incoming signal power is evaluated against this baseline to determine the value of the data element.
• A long string of 0's or 1's can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly.
• A good line coding scheme needs to prevent baseline wandering.

• DC Component

When the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies. These frequencies around zero, called DC (direct-current) components.

• DC components, present problems for a system that cannot pass low frequencies or a system that uses electrical coupling via a transformer.
• For example, a telephone line cannot pass frequencies below 200 Hz.

1. Self-synchronization

• To correctly interpret the signals received from the sender, the receiver's bit intervals must correspond exactly to the sender's bit intervals.
• If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might misinterpret the signals.
• A self-synchronizing digital signal includes timing information in the data being transmitted.
• This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse.
• If the receiver's clock is out of synchronization, these points can reset the clock.

Effect of lack of synchronization