04. Pulse Code Modulation

pulse code modulation (PCM)

It is essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream.

instantaneous samples

PCM is essentially analog-to-digital conversion of a special type where the information contained in the____ of an analog signal is represented by digital words in a serial bit stream.

FALSE (inexpensive)

PCM is relatively expensive digital circuitry that may be used extensively.

PCM

It is derived from all types of analog sources that may be merged with data signals and transmitted over a common high-speed digital communication system.

TRUE

[TRUE or FALSE] In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeated, where the input consists of a noisy PCM waveform.

TRUE

[TRUE or FALSE] The noise performance of a digital system can be superior to that of an analog system.

appropriate coding techniques

The probability of error for the system output can be reduced even further by the use of _____.

• sampling

• quantizing

• encoding

3 Basic Operations of PCM Signal (HINT: SQE)

flat-top PAM signal

Sampling operation generates a ______.

quantizing operation (quantization)

It approximates analog values by using a finite number of levels.

approximates

Quantizing operation _____ analog values by using a finite number of levels.

• uniform quantizer

• quantization error

• quantized PAM signal output

3 Steps of Quantizing Information (HINT: UQQ)

quantized PAM signal

PCM signal is obtained from the _______ by encoding each quantized sample value into a digital word.

• sampling

• quantization

• encode

3 Functions of Analog-to-digital Converter (ADC)

sampling

This make the signal discrete in time.

TRUE

[TRUE or FALSE] If the analog input has a bandwidth of W Hz, then the minimum sample frequency such that the signal can be reconstructed without distrotion.

W Hz

If the analog input has a bandwidth of ____, then the minimum sample frequency such that the signal can be reconstructed without distrotion.

quantization

This makes the signal discrete in amplitude.

quantization

This rounds off to one of q discrete levels.

encode

It maps the quantized values to digital words that are v bits long.

v bits long

It maps the quantized values to digital words that are _____.

TRUE

[TRUE or FALSE] If the Nyquist sampling theorem is satisfied, then only quantization introduces distortion to the system.

analog-to-digital conversion

continuous in amplitude

The output of a sampler is still ______.

TRUE

[TRUE or FALSE] Each sample can take on any value

infinite

The number of possible values in sampling is _____.

TRUE

[TRUE or FALSE] To transmit as a digital signal, we must restrict the number of possible values.

quantization

It is the process of “rounding off” a sample according to some rule.

quantization error

uniform quantizers

Most ADCs use ______.

equally spaced apart

In uniform quantization, the quantization levels are _____.

uniform quantizers

They are optimal when the input distribution is uniform. When all values within the dynamic range of the quantizer are equally likely.

dynamic range

Uniform quantizers are optimal when all values within the _______ of the quantizer are equally likely.

sampling timing

quantized 5-level

quantized 10-level

33 bits

How many bits were used to encode a signal?

M possible signal levels

The output of the quantizer is one of __________.

n-bit binary word

If we want to use a binary transmission system, then we need to map each quantized sample into an _________.

M = 2ⁿ

Formula in Finding Quantization/Signal Levels

n = log₂(M)

Formula to find the n-bit binary word.

encoding

It is the process of representing each quantized sample by a v bit code word.

one-on-one

In encoding, the mapping is _____ so there is no distortion introduced by encoding.

TRUE

[TRUE or FALSE] Some mappings are better than others.

gray code

This gives the best end-to-end performance.

TRUE

[TRUE or FALSE] The weakness of Gray codes is poor performance when the sign bit (MSB) is received in error.

most significant bit (MSB)

the sign bit is also known as the _____.

one bit position

With gray codes, adjacent samples differ only in _____.

number of quantization levels

In M = 2ⁿ, M is the?

number of bits per sample

In n = log₂(M), n is the?

• counting or ramp (Maxim ICL7126 ADC)

• serial or successive approximation (AD 570)

• parallel or flash encoders (CA3318)

3 Popular Techniques Used to Implement ADC Encoding Operation (HINT: CSP)

Maxim ICL7126 ADC

example of an ADC in counting or ramp

AD 570

example of a serial or successive approximation

CA3318

example of a parallel or flash encoders

to generate PCM word

What is the objective of practical PCM circuits?

serialized

Parallel digital output obtained needs to be _____ before sending over a 2-wire channel.

parallel-to-serial converters

Serializing parallel digital output is accomplished by _____.

Serial Input-Output (SIO) chip

example of a parallel-to-serial converter

• UART

• USRT

• USART

Examples of Serial Input-Output Chips (HINT: 3Us)

TRUE

[TRUE or FALSE] The spectrum of the PCM signal is not directly related to the spectrum of the input signal.

bandwidth

The _____ of (serial) binary PCM waveforms depends on the bit rate R and the waveform pulse shape used to represent the data.

R = nf_s

formula for bit rate R

number of bits in the PCM word (M=2^n)

In R = nf_s, what does n represent?

sampling rate

In R = nf_s, what does f_s represent?

B_PCM(min) = R/2 = nfs/2

For no aliasing case (fs≥2B), the minimum bandwidth of PCM is?