L2 Digital/Analog

Question 1

Distinguish between digital and analog systems

Answer 1

Analog systems:

• Continuous, proportional measurement
• Use continuous variables that can take infinite number of possible values (usually real numbers)
• usually uses a mechanical system (e.g. mercury thermometer)
• Electrical analog systems will convert this measurement into a proportional voltage or current (exact voltage matters)

Digital systems:

• discrete measurement
• Use digitalised variables that can take a finite number of distinct values (usually binary numbers)
• Uses a high/low voltage to transmit info as bits (exact voltage doesn’t matter - just if it’s high or low)

Question 2

How are binary quantities represented

Answer 2

Represented by a range of voltage in a circuit

• 0 = low voltage range
• 1 = high voltage range
• Inbetween 0 and 1 is a range of invalid voltages

Question 3

What are the advantages of digital systems over analog systems?

Answer 3

1. Design: Easier to design
2. Information storage: is easy
3. Accuracy and precision: Easier to maintain (signal doesn’t deteriorate because of temperature, humidity, imperfections in CD etc.) BECAUSE less affected by noise: just need to distinguish between high and low (error control coding helps even more with this)
4. Integration: higher degree of integration bc more digital circuitry can be fabricated on IC chips
5. Programmability: More varied and complex operations can be programed
6. Well-developed mathematical theory

Question 4

What are digital circuits designed to do?

Answer 4

1. Accept input voltages that are within the defined 0 and 1 ranges
2. Process the input signals in a predictable way
3. Produce output voltages that fall within the prescribed 0 and 1 ranges

Question 5

Outline the processing of a voice signal in a mobile phone

Answer 5

• Microphone: Voice is picked up by analog microphone
• A/D converter: turns voice to digital signal
• Digital processing: amplifies signal and reduces background noise
• D\A convertor: Digital signal is combined with high frequency carrier radio wave (either using AM or FM)
• Antenna: sends encoded radio wave to cell tower

Question 6

What is analog to digital conversion (AD conversion)?

What are the steps of AD conversion?

What is bit rate?

Answer 6

An analog waveform is converted to a series of 0s and 1s

Sampling:

• Measure the continuously varying analog signal at regular intervals of Ts seconds
• Keep these samples and discard rest of the signal
• Sampling rate must be faster than rate that signal varies so you don’t lose too much information

Quantisation

• Map actual sampled value to nearest quantisation level (a finite number of voltage levels)
• Each quantisation level is mapped to a unique bit string (00, 01, 10 etc.)
• N-bits gives 2^N unique bit strings and 2^N possible quantisation levels
• Bit rate (bits/sec) = sample rate (samples/sec) x number of bits per sample used in quantisation (bits/sample)
  
  • how fast the AD conversion is happening

Question 7

One method for transmitting digital/analog information

Answer 7

Amplitude modulation (AM)

• Carrier wave is of higher frequency than the information being transmitted
• Carrier wave’s amplitude is changed to transmit the info
• This produces an “amplitude envelope” -> which is the same as the signal being transmitted
• Analog AM: you are sending analog information - specific amplitude values matter!
• Digital AM: specific amplitude values don’t matter as much - you can have a range of amplitude corresponding to 0 and 1