Week 5 - PCM, analog-digital conversion

Question 1

What is digitization?

Answer 1

Converting analog signal to digital data.

PCM is a common method.

Question 2

What are the 3 main elements of Pulse Code Modulation (PCM)?

Answer 2

Sampling
Quantizing
Encoding

Question 3

What are the three sampling methods (PAM - Pulse Amplitude Modulation)?

Answer 3

Ideal Sampling (discrete point)

Natural Sampling (sampling with finite width pulse/high speed switch)

Flat-top Sampling (Similar to natural sampling but the sample value is discrete and held over the duration of pulse).

Question 4

What is the restriction on sampling rate?

Answer 4

To reproduce the original analog signal, we must sample at x2 the highest frequency.

Why? To capture the peak and trough of each cycle.

Question 5

f, 2f, 4f Which of these would be oversampling and undersampling? (f = highest frequency of analog wave)

Answer 5

f - under-sampling
4f- over-sampling

2f - required sampling rate

Question 6

Why can’t we tell what the minimum sampling rate is for a bandpass signal given the bandwidth?

Answer 6

Because the lowest frequency is non-zero, we cannot assume the highest frequency = bandwidth.

Sampling rate = 2*highest frequency

Question 7

What is Quantization?

Answer 7

Mapping signal amplitudes to discrete values. (Approximation)

Question 8

How do we determine quantization levels?

Answer 8

(Max voltage - min voltage) / levels

Question 9

What is the relationship between normalized amplitude and normalized quantized values?

Answer 9

The amplitudes are divided into quantization levels. Therefore, each quantization level represents a range of amplitudes.

Question 10

How do we determine normalized quantized values?

What is the relationship between normalized quantized values, normalized PAM values, and normalized error?

Answer 10

Say we have 8 levels. Our quantization levels would range from 4 to -4 (to represent a sine wave)

The midpoint of each level would be a normalized quantized value.

Any normalized PAM values within that level will assume the normalized quantized value.

The difference in value between the two is the normalized error.

Question 11

What is a quantization code?

Answer 11

Refers to each quantization level starting from 0.

Question 12

How is quantization error related to the number of levels?

Answer 12

More levels means less error and more accurate signal recovery.

Question 13

Quantization error changes the SNR of a signal (T or F)

Answer 13

True

Question 14

What is the issue with uniform quantization?

Answer 14

Quantization levels are evenly spaced, this may not be practical for music for example where amplitudes are concentrated near zero.

Question 15

What is PCM Encoding?

Answer 15

Converting each quantization level into an encoded word (1010)

Question 16

Why might digitization increase the amount of bandwidth we need?

Answer 16

The more bits we use to represent each quantization level, the more bandwidth we need.

Question 17

Why is baud rate <= bit rate?

Answer 17

S = N/r

Question 18

What is a carrier signal?

Answer 18

A high frequency simple signal from the sending devices that acts as a base for information.

Question 19

What is modulation/shift keying?

Answer 19

Modifying amplitude, frequency, or phase of the carrier signal to convey digital data.

Question 20

What kind of signal does ASK create?

Answer 20

Composite non-periodic

Question 21

What is the bandwidth for ASK?

Answer 21

Bandwidth is proportional to the signal rate (baud) S = N / r * case factor

Question 22

How many bits per signal element are there in binary and multi-level ASK?

Answer 22

log(base2) # amplitudes

Question 23

In FSK the frequency of the modulated signal is constant for the duration of a signal element (T or F)

Answer 23

True

Question 24

What is the bandwidth for FSK?

Answer 24

The bandwidth(s) are centered around the various frequencies.

Question 25

How many bits per signal element are there in binary/multi-level FSK?

Answer 25

Same as ASK log(base)2 #frequencies

Question 26

Why is PSK less susceptible to noise than ASK?

Answer 26

Noise affects amplitude.

Question 27

Which analog to digital modulation technique requires more sophisticated hardware?

Answer 27

PSK Phase-shift keying

Question 28

What is the bandwidth required for PSK?

Answer 28

Same as ASK, bandwidth centered around carrier signal frequency. Requires less BW than FSK.

Question 29

What is binary PSK?

Answer 29

Uses an in-phase, and out of phase wave for the signal element/ Offset by 180 degrees or 1/2 cycle

Question 30

What is Quadrature Amplitude Modulation (QAM)?

Answer 30

Combines ASK and PSK. Dominant method of digital to analog modulation One in phase carrier and one quadrature carrier offset by 90 degrees. Why? Same bandwidth as ASK and PSK. Using phases of two carriers AND amplitude allows for more distinct values.

Question 31

What does binary ASK look like on constellation diagram?

Answer 31

Two dots on the x axis representing a single in-phase carrier at two distinct positive values

Question 32

What does binary PSK look like on a constellation diagram?

Answer 32

Two dots on the x-axis representing a single in phase carrier. Each dot will be a positive and negative amplitude respectively to represent the 180 degree phase shift.

Question 33

What does a QAM look like on a constellation diagram?

Answer 33

You have an I component (in-phase) and a Q component (90 degrees offset). Each constellation point on the quadrant correlates to a specific amplitude, phase of the in-phase carrier, and phase of the quadrature carrier.

Question 34

What is Amplitude Modulation (AM)?

Answer 34

Carrier amplitude is modulated in response to the input analog signal. Bandwidth of modulated signal is twice that of input (think of how noise affects amplitude)

Question 35

What is the bandwidth of an AM station and guards?

Answer 35

10kHz To find the total # of stations we would take entire frequency range and divide by 10kHz. Then divide the result by 2.

Question 36

What is the bandwidth of FM station and guards?

Answer 36

200kHz The bandwidth of the input is equal to the modulated signal because we are altering frequency in response to input signal amplitude.