2 - Converting analog to digital signals and vice versa

Abstract

Most of the signals encountered in engineering applications are analog. To process analog signals using digital techniques, the analog signals must first be converted into digital signals. Digital processing of analog signals proceeds in three stages: the digitization of analog signal takes place, the appropriate digital signal processing (DSP) algorithms process the digitized signal, and the results or outputs of the processing are converted back into analog signals through interpolation. This process is called digital-to-analog (D/A) conversion. Quantization or discretization of the sampled signal amplitude converts an analog signal into digital form. In practice, because the quantization process takes a finite amount of time, the sampled signal amplitude has to be held constant during this time. The sampling process is usually performed by a sample-and-hold circuit. It is desirable that the replicas of baseband spectrum be removed. As the sampling rate is sufficiently high, low-pass filtering can isolate the baseband spectrum. A low-pass filter that removes the remaining replicated spectra is also known as an image-rejection filter.