Digital Control Systems Waseem Gulsher BS (Evening) 13 Nov, 17 Interfacing with Analog Devices Lecture – 9 Digital Control Systems Waseem Gulsher

PCM Decoder

Digital Signal Processing Basics To recover an analog signal from a digitized signal we follow the following steps: We use a hold circuit that holds the amplitude value of a pulse till the next pulse arrives. We pass this signal through a low pass filter with a cutoff frequency that is equal to the highest frequency in the pre-sampled signal. The higher the value of L, the less distorted a signal is recovered. 1.#

Figure 4.27 Components of a PCM decoder

Digital Signal Processing

Digital Signal Processing Basics Digital signal processing converts signals that naturally occur in analog form, such as sound, video and information form sensors, to digital form and uses digital techniques to enhance and modify analog signal data for various applications. A digital system processing system first translates a smoothly varying analog signal into a series of discrete levels. This series of levels follows the variations of the analog signal and resembles a staircase ,as illustrated in Fig 1.This is accomplished by sample and hold circuits. 1.#

Digital Signal Processing Basics An original analog signal (sine wave) and its “stairstep” approximation. 1.#

Digital Signal Processing Basics Next, the stair step approximation is quantized into binary codes that represent each discrete step on the stair steps by a process called analog-to-digital conversion (ADC). The binary codes are then applied to Digital signal processor (DSP) which performs various operations on the incoming data ,such as removing unwanted interference, increasing the amplitude of some signal frequencies etc. 1.#

Digital Signal Processing Basics After the DSP processes the signal, it can be converted back to a much improved version of the original analog signal by digital to analog converter (DAC). Fig 2 shows a basic block diagram of a typical DSP system. 1.#

Digital Signal Processing Basics Fig 2 Basic block diagram of a digital signal processing system. 1.#

Conversion Analog to Digital

Filtering and Sampling Sampling is the process of taking a sufficient number of discrete values at points on a waveform that will define a waveform. Sampling converts an analog signal into a series of impulses, each representing the amplitude of the signal at a given instant in time. Before a signal can be sampled, it must be passed through a low-pass filter to eliminate harmonic frequencies . 1.#

Holding the Sampled Value After filtering and sampling ,the sampled level must be held constant until the next sample occurs. This is necessary for the ADC to have time to process the sampled value. This sample and hold operation results in a stair step waveform that approximates the analog input waveform, as shown in fig 3. 1.#

Holding the Sampled Value fig 3 Illustration of a sample-and-hold operation. 1.#

Analog to Digital Conversion Analog –to-digital conversion is the process of converting the output of the sample and hold circuit to a series of binary codes that represent the amplitude of the analog input at each of the sample times. The sample and hold process keeps the amplitude of the analog input signal between sample pulses. Fig 4 illustrates the basic function of a ADC converter. 1.#

Analog to Digital Conversion Figure 4 Basic function of an analog-to-digital (ADC) converter (The binary codes and number of bits are arbitrarily chosen for illustration only). The ADC output waveform that represents the binary codes is also shown.

Quantization The process of converting an analog value to a code is called quantization. The ADC converts each sampled value of the analog signal to a binary code. The Quantization operation is illustrated in fig 5. 1.#

Quantization Figure 5 Sample-and-hold output waveform with four quantization levels. The original analog waveform is shown in light gray for reference.

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