4.2.1: Pulse Code Modulation

Question 1

What are Pulse Code Modulation’s (PCM) three stages?

Answer 1

1. Sampling
2. Quantizing
3. Encoding

Question 2

What is Sampling?

Answer 2

The process of converting a continuous-time signal (analog) to a discrete-time signal (digital).

Question 3

What’s the relationship between Sampling Period and Frequency?

Answer 3

T(s) = 1 / f(s)

f(s) = 1 / T(s)

Question 4

What is the Nyquist Sampling Rate?

Answer 4

f(s) = 2 * f(max)

Question 5

What is Quantization?

Answer 5

The process of mapping the sampled values to a finite set of values.

Question 6

How do we find the delta between zones?

Answer 6

delta = [V(max) - V(min)] / 2

Question 7

What are the steps for Quantization?

Answer 7

1. Determine max and min values.
2. Find the delta between levels.
3. Assign quantized value of 0 to (L - 1) to the midpoint of each zone.
4. Approximate value of sample amplitude to quantized values.

Question 8

How do we determine the total number of bits, given levels?

Answer 8

N(bits) = log2(L)

Question 9

What is Quantization Error?

Answer 9

Input values to quantizer are real; output values are approximates. Output values are chosen to be the middle value in the zone; if the input values is not the middle of the zone, there is an error.

Error is between (delta/2) and (-delta/2).

Question 10

What is SNR(dB) when the Quantization Error is applied?

Answer 10

SNR(dB) = 6.02 * N(bits) + 1.76.

Question 11

What is Encoding?

Answer 11

Once a sample is quantized and the number of bits-per-sample is selected, each sample is changed into an N-bit codeword.