The process of converting binary data into a sequence of bits of the digital signal is known as Line coding. It is also known as digital PAM formats.
Need of Line coding:
We always come across different types of data such as text, numbers, graphical images, audio, and video. These all data are stored in computer memory in form of a sequence of bits. As shown below, line coding converts bit sequences into digital signals.
There are several techniques for converting an analog signal to a digital signal. However, digital data can also be obtained from sources such as computers. The information obtained from such a source is inherently discrete. When this type of discrete signal is transmitted over a band-limited channel, it becomes dispersed. That is, the pulses spread out and overlap each other, causing distortion. This distortion is known as inter-symbol interference. To avoid this, we should not send the discrete signal while it is on the transmission medium. Instead, this data is converted into a Pam format or line code compatible with the baseband channel before being sent over a baseband communication channel. Line codes refer to the various pulse formats that are used depending on the situation.
Properties of line codes:
1. No DC Component:
- Every communication system such as a cable system does not allow the transmission of a dc signal over them.
- So because of that line signal must have a zero average value.
2. Self-clocking (synchronizing capability):
- Symbol or bit synchronization is necessary for every digital communication system.
- A sufficient amount of zero crossings must occur in the line code waveform for the receiver to be in synchronization.
- It implies that a transition must always occur after a predictable interval; this property is referred to as the inherent synchronizing or cloaking characteristic.
3. Bandwidth compression:
- The bandwidth of the line code should be as small as possible.
- Comparatively to the other codes, the multilevel course requires less.
4. Differential encoding:
- For communication systems where the transmitted waveform occasionally experiences an inversion, differential encoding is helpful.
- The polarity of the encoded waveform is flipped during differential encoding without impairing the ability to identify data.
5. Noise immunity:
- The selected line code should have a very high noise immunity (ability to minimize the effects of noise).
- This is necessary to have a minimum number of errors introduced due to noise.
6. Minimum crosswalk:
- The crosswalk should be minimized which is transmitted present between adjacent channels.