1) The document describes an experiment on pulse-code modulation (PCM) using an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). 2) The objectives are to demonstrate PCM encoding and decoding, examine how sampling rate relates to analog signal frequency, and study the effect of low-pass filtering on the DAC output. 3) The conclusion is that PCM converts analog voltages to binary codes using an ADC for encoding and a DAC for decoding, and the sampling rate must be at least twice the highest analog signal frequency to retain all information.

1. NATIONAL COLLEGE OF SCIENCE & TECHNOLOGY
Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite
EXPERIMENT 2
DIGITAL COMMUNICATION OF ANALOG DATA USING
PULSE-CODE MODULATION (PCM)
Ramos, Mary Grace C. September 20, 2011
Signal Spectra and Signal Processing/BSECE 41A1 Score:
Engr. Grace Ramones
Instructor

2. OBJECTIVES:
1. Demonstrate PCM encoding using an analog-to-digital converter (ADC).
2. Demonstrate PCM encoding using an digital-to-analog converter (DAC)
3. Demonstrate how the ADC sampling rate is related to the analog signal
frequency.
4. Demonstrate the effect of low-pass filtering on the decoder (DAC) output.

3. SAMPLE COMPUTATION:
Sampling frequency
Frequency
Cut-off frequency

4. CONCLUSION:
Pulse-code modulation (PCM) consists of the conversion of a series of sampled
analog voltage levels into a sequence of binary codes, with each binary number that is
proportional to the magnitude of the voltage level sampled. Translating analog voltages
into binary codes is called A/D conversion, digitizing, or encoding. The device used to
perform this conversion process called an A/D converter, or ADC.
It is observed after the experiment that the measured sampled frequency has the
same value as the frequency generator. But the sampling frequency is more than twice
the value of the sampling frequency. While, as compared to the Nyquist frequency, it is
twice the highest value of the sampling frequency.
After filtering, there is no change observed in the DAC output. The filtered
output is observed to be closely similar to the representation of the analog input
waveshape.
The ADC conversion time puts a limit on the sampling rate because the next
sample cannot be read until the previous conversion time is complete. The sampling
rate is important because it determines the highest analog signal frequency that can be
sampled.
In order to retain the high-frequency information in the analog signal acting
sampled, a sufficient number of samples must be taken so that all of the voltage changes
in the waveform are adequately represented. Because a modern ADC has a very short
conversion time, a high sampling rate is possible resulting in better reproduction of
high frequency analog signals.