Modulation and Coding Techniques in Digital Communications

Practice flashcards covering digital communications fundamentals, PCM, sampling, quantization, and related concepts.

What is the subject of digital communications?

Transmission of information in digital form from a source to one or more destinations, where the channel characteristics significantly influence system design.

Name three advantages of digital over analog communication.

Less sensitivity to distortion/noise/interference, more reliable and easier to design, and easier to implement processing (encryption, compression) with flexible hardware and easier multiplexing.

What are the basic elements of a digital communication system?

Source, input transducer, transmitter, channel, receiver, output transducer (with functions like modulation, filtering, and amplification between blocks).

What is the role of the source encoder?

Removes redundant information from the digital stream to achieve compression and efficient use of the channel.

What is the role of the channel encoder?

Introduces controlled redundancy to enable error detection and correction at the receiver, increasing reliability.

What does the digital modulator do?

Maps the binary information sequence into signal waveforms suitable for transmission over the channel.

What does the digital demodulator do?

Processes the channel-corrupted waveform to produce estimates of transmitted data, which are then passed to the channel decoder.

Name three common sources of channel impairments.

Additive thermal noise, man-made noise (e.g., ignition noise), and atmospheric noise (e.g., lightning discharges).

What is the goal of the receiver in a digital communication system?

To extract the message signal and mitigate channel degradations, often using amplification, filtering, and decoding stages.

What is DSP in digital communications?

Digital Signal Processing: processing of analog signals using digital methods (e.g., filtering, amplitude equalization, phase shifting), with easier storage and flexible data rates.

List advantages of digital transmission.

Better noise immunity, easier error detection/correction, compatibility with time-division multiplexing (TDM), smaller/digital ICs, and support for DSP.

What does PCM stand for and what is its basic idea?

Pulse Code Modulation; digitizes analog signals by sampling, quantizing, and encoding into binary codes.

What are the basic steps in PCM encoding?

Analog source → filtering → sampling → quantization → encoding (then regeneration/decoding and reconstruction at the receiver as needed).

What does Nyquist sampling theorem require for PCM sampling?

The sampling rate fs must be at least twice the maximum input frequency fa (fs ≥ 2fa); otherwise aliasing occurs.

What is aliasing (foldover distortion) in PCM?

Distortion that occurs when the sampling rate is too low (fs < 2fa), causing higher-frequency components to fold into the baseband.

What is a sample-and-hold circuit?

A nonlinear circuit that samples and holds the input voltage; can introduce aperture error and additional spectral components if not designed properly.

What is the difference between natural sampling and flat-top sampling?

Natural sampling preserves the natural shape of the sample; flat-top sampling uses a hold circuit and introduces aperture error due to finite pulse width.

What is aperture error?

Error caused by the finite width of the sampling pulse, causing the sampled amplitude to deviate during the sample period.

What is quantization in PCM?

Rounding the continuous analog sample to the nearest discrete amplitude level, introducing quantization error.

What is a PCM code and how are bits arranged in a sign-magnitude PCM code?

An n-bit binary code representing a quantized sample; MSB is the sign bit, and the remaining bits represent magnitude (two’s complement form is another common scheme).

What is folded binary code in PCM?

A sign-magnitude-like arrangement where the codes in the bottom half mirror the top half, with zero represented by two codes (+0 and −0).

What is the quantization interval (quantum)?

The magnitude difference between adjacent quantization levels (the step size).

What is resolution in PCM?

The smallest nonzero voltage step that can be decoded by the DAC (equal to the quantum size).

What is quantization error (Qe) or quantization noise (Qn)?

The difference between the original sample value and the quantized value; appears as noise in the reconstructed signal.

What is dynamic range in PCM and how is it calculated?

DR = Vmax / Vmin (or Vmax / resolution); in dB, DRdB = 20 log10(Vmax/Vmin) ≈ 20 log10(2^n − 1); represents usable signal range.

How many decibels of dynamic range are roughly gained per additional magnitude bit in PCM?

Approximately 6 dB per magnitude bit for larger n (beyond very small bit counts).