Analog and Digital Transmission: Signal Characteristics, Modems, and Modulation
Computers think exclusively in discrete binary digits (0s and 1s), but the natural physical world (sound, light, telephone copper wires) operates in continuous analog waves.
1. Analog vs. Digital Signals
Analog vs Digital Signals
〰️ Analog Signal
- Continuous sine wave that varies smoothly over time.
- Can take an infinite number of values within a range.
- Examples: Human voice, mercury thermometer, AM radio music.
⎍⎍ Digital Signal
- Discrete square pulses representing binary 0s and 1s.
- Takes only two fixed voltage levels (e.g., 0V = Bit 0, +5V = Bit 1).
- Examples: Computer CPU, RAM, USB flash drive data.
2. What is a MODEM? (MOdulator - DEModulator)
A Modem is a hardware device that converts signals between digital and analog forms:
[ Computer (Digital) ] ➔ [ MODULATOR ] ➔ (Analog Wave over Telephone/Cable) ➔ [ DEMODULATOR ] ➔ [ Computer (Digital) ]- Modulation (Sender side): Converts digital binary data (square pulses) into an analog carrier wave (sine wave) so it can travel down telephone copper lines.
- Demodulation (Receiver side): Converts the incoming analog wave back into digital 0s and 1s so the receiving computer can read it.
3. Digital Modulation Techniques (ASK, FSK, PSK, QAM)
When sending digital bits (0s and 1s) over an analog carrier wave:
Formula: s(t) = A \sin(2π f t + φ) An engineer can vary three basic properties: Amplitude (A), Frequency (f), or Phase (φ):
Basic Digital Modulation Techniques
1. ASK (Amplitude Shift Keying)
Bit 1 = High amplitude wave; Bit 0 = Zero/low amplitude. Highly vulnerable to noise!
2. FSK (Frequency Shift Keying)
Bit 1 = High frequency (fast waves); Bit 0 = Low frequency (slow waves). Highly noise-resistant!
3. PSK (Phase Shift Keying)
Change the phase angle (e.g., 180° flip) to represent a change between 0 and 1
4. QAM (Quadrature Amplitude)
Combines ASK + PSK to transmit multiple bits (4, 8, 16 bits) in a single wave cycle!
💡 DAVV Exam Tip
Question (10 Marks): “Explain the difference between Analog and Digital transmission. What is a Modem? Describe ASK, FSK, and PSK with waveforms.”
- Draw the continuous sine wave (Analog) vs square pulse (Digital).
- Explain Modem: Modulator + Demodulator.
- Draw the 3 modulation waveforms for bit sequence
1 0 1 1 0.