JNTU B.Tech II DATA COMMUNICATION Previous Question Paper (March - 2022 - R13)

Pulse Code Modulation (PCM) is a method used to digitally represent sampled analog signals. The process involves three primary stages: sampling, quantizing, and encoding. First, the analog signal is sampled at regular time intervals, typically at or above the Nyquist rate (twice the highest frequency component) to ensure signal reconstruction. Second, each sample is quantized, meaning its continuous amplitude value is mapped to the nearest level from a predefined set of discrete values. Third, these quantized values are converted into a binary code (encoding) for digital transmission.

Converting an analog signal into a digital signal introduces several types of noise and distortion:

• Quantization Noise: This is the most significant error, arising from the difference between the actual analog sample value and the nearest discrete quantization level. It appears as a rounding error and is often modeled as additive white noise.
• Quantization Distortion: If the signal amplitude exceeds the range of the quantizer, clipping occurs, leading to severe signal distortion.
• Sampling Jitter: This occurs when the sampling clock is not perfectly periodic. Variations in the timing of samples lead to errors in the reconstructed signal, particularly at high frequencies.
• Aliasing Noise: If the input analog signal is not band-limited by a low-pass filter before sampling, frequency components above half the sampling rate (Nyquist frequency) will fold back into the baseband, creating irreversible interference.
• Aperture Effect: This occurs because the sample-and-hold circuit takes a finite amount of time to capture the sample, effectively acting as a low-pass filter that attenuates higher frequency components.