Download Analog-to-Digital Conversion Techniques and more Exams Communication in PDF only on Docsity! Data Communication & Computer Networks: Analog-to-Digital Lec.10 1 Prepared By: Eng. Omar M. Hussien University of Anbar / College of Computer ANALOG -TO - DIGITAL CONVERSION The techniques described in previous lecture convert digital data to digital signals. Sometimes, however, we have an analog signal such as one created by a microphone or camera. We have seen that a digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data. In this section we describe pulse code modulation techniques (PCM). After the digital data are created (digitization), we can use one of the techniques described to convert the digital data to a digital signal. Pulse Code Modulation (PCM) The most common technique to change an analog signal to digital data (digitization) is called pulse code modulation (PCM). A PCM encoder has three processes: 1. The analog signal is sampled. 2. The sampled signal is quantized. 3. The quantized values are encoded as streams of bits. Sampling The first step in PCM is sampling. The term sampling means measuring the amplitude of the signal at equal intervals. The analog signal is sampled every Ts s, where Ts is the sample interval or period. The sampling process is sometimes referred to as pulse amplitude modulation (PAM). We need to remember, however, that the result is still an analog signal with nonintegral values. In PAM , the original signal is sampled at equal intervals as shown in figure below . Data Communication & Computer Networks: Analog-to-Digital Lec.10 2 Prepared By: Eng. Omar M. Hussien University of Anbar / College of Computer Sampling Rate One important consideration is the sampling rate or frequency. What are the restrictions on Ts?. According to the Nyquist theorem, to reproduce the original analog signal, one necessary condition is that the sampling rate must be at least twice the highest frequency in the original signal. So if we want to sample telephone voice with a maximum frequency (4000 Hz) , we need sampling rate at (8000) sample per second. Data Communication & Computer Networks: Analog-to-Digital Lec.10 5 Prepared By: Eng. Omar M. Hussien University of Anbar / College of Computer How many Bits per sample? After we found the sampling rate, we need to determine the number of bits to be transmitted for each sample. This depends on the level of precision needed. The choice of L, the number of levels, depends on the range of the amplitudes of the analog signal and how accurately we need to recover the signal. If the amplitude of a signal fluctuates between two values only, we need only two levels; if the signal, like voice, has many amplitude values, we need more quantization levels. Example: A signal is sampled, each sample requires at least 12 levels of precision (+0 to +5 , - 0 to -5), how many bits should be sent for each sample? Sol: We need 4 bits, 1 bit for the sign and 3 bits for the value; a 3 bits value can represent 23 = 8 levels (000 to 111) Which is more than what We need. A 2 bits value is not enough since (22 = 4). A 4 bits value is too much because (24 = 16). Data Communication & Computer Networks: Analog-to-Digital Lec.10 6 Prepared By: Eng. Omar M. Hussien University of Anbar / College of Computer PCM is actually made up from four separate processes, PAM, Quantization, binary encoding and Line encoding. figure below shows the entire process.
