Digital communication

1. Digital Communication – Pulse
Code Modulation (PCM)

2. Learning Outcome & Outline
• Understand the importance and operation of PCM
• Objective of PCM
• Block Diagram
• Sampling – Quantization – Encoding
• PCM Standards
• Bit Rate & Bandwidth
• Advantages & Disadvantages
• Applications

3. Objective of PCM
• Convert analog signal into digital for transmission
• Steps: Sample → Quantize → Encode → Digital signal

4. PCM Block Diagram
• Transmitter: LPF → Sampler → Quantizer → Encoder
• Transmission Path with Repeaters
• Receiver: Regeneration → Decoder → Reconstruction Filter → Output

5. Process of PCM
• 1. Sampling: fs ≥ 2fm (Nyquist Rate)
• 2. Quantization: Approximating sampled values
• 3. Encoding: Convert to binary form

6. Quantization Example
• Quantization levels → quantization error
• Example: Error = ±0.4V, Resolution = 1/(2^n)

7. PCM Standards
• 8000 Hz – Telephony
• 44100 Hz – Audio CD
• 48000 Hz – DAT
• 96000/192000 Hz – DVD/BluRay

8. Bit Rate & Bandwidth
• fs ≥ 2fm (Nyquist Rate)
• Bit rate = v × fs
• Bandwidth ≈ bit rate / 2

9. Example (Voice Signal)
• Voice bandwidth: ≤4kHz → fs = 8kHz
• v = 8 bits/sample → Bit rate = 64 kbps
• Bandwidth ≈ 32 kHz

10. Improving Quality
• Increase sampling rate (fs)
• Increase number of levels (v)
• Balance quality, bit rate, and bandwidth

11. Advantages of PCM
• Less noise interference
• Uniform transmission quality
• High SNR for given bandwidth
• Easier encryption and security
• Simple regeneration
• Easy multiplexing and channel dropping

12. Disadvantages of PCM
• Requires large bandwidth
• Attenuation effects
• Quantization error
• Complex circuitry

13. Applications of PCM
• Telephone audio transmission
• Audio CDs
• Satellite transmission systems
• Other digital communication applications