Advantages of digital communication; source and channel encoders

Advantages of digital over analog communication

1. Noise immunity: only discrete levels are sent; regenerative repeaters remove noise at each hop, so noise does not accumulate over long distances.
2. Error detection and correction using channel coding gives very low error rates.
3. Security: data can easily be encrypted.
4. Easy multiplexing and integration: voice, video and data share the same network (TDM, ISDN, Internet).
5. Flexible, cheap hardware: VLSI and DSP chips, stable with temperature and ageing.
6. Storage and processing: digital data can be stored, compressed and processed easily.
7. Trade-off between bandwidth and SNR is more efficient.

Importance of the source encoder

The source encoder represents the source output with as few bits as possible by removing redundancy.

• Reduces bit rate, so less bandwidth, power and storage are needed (e.g. speech from 64 kb/s PCM to 8 kb/s with a vocoder; text with Huffman coding).
• Its efficiency is measured against source entropy $H$; the average code length $\bar L\ge H$.
• Example: in Huffman coding, frequent symbols get short codes and rare symbols get long codes.

Importance of the channel encoder

The channel encoder adds controlled redundancy so that errors caused by noise can be detected or corrected at the receiver.

• $k$ information bits become an $n$-bit codeword, code rate $r=k/n<1$.
• It lowers the bit error rate for the same power, or allows lower transmit power for the same BER (coding gain).
• Examples: parity check (detection), Hamming (7,4) code (single-error correction), CRC, convolutional and turbo codes.

Thus source coding makes transmission efficient, while channel coding makes it reliable.