Digital signal processing is a branch of communications engineering and deals with the generation and processing of digital signals with the help of digital systems. In a narrower sense, its focus is on the storage, transmission and transformation of information in the sense of information theory in the form of digital, discrete-time signals. It has diverse and far-reaching applications in today’s world and has a strong influence on almost all areas of life, as it is one of the technical foundations of the digitalization of all modern communication technology and consumer electronics. This is also known as the digital revolution.
Nowadays, almost all transmission, recording and storage methods for image and film (photo, television, video) and sound (music, telephony, etc.) are based on digital processing of the corresponding signals. Digital signal processing enables a variety of digital data conversion and editing modes, such as audio and video compression, non-linear video editing, or photo image editing. In addition to many other industrial applications, digital signal processing is also used in measurement and control technology and in medical technology, such as magnetic resonance imaging. These developments are a consequence of the rapid progress of digital and computer technology (information technology) in recent decades. With the introduction of the music CD at the beginning of the 1980s, the above-mentioned “digitalization” began to influence people’s everyday lives, which is most evident today in the universal spread of versatile, multimedia-capable smartphones.
Digital signal processing is based on electronic components, such as digital signal processors (DSPs) or high-performance microprocessors, corresponding memory elements and interfaces for signal input and output. In the case of programmable hardware, the algorithms for signal processing can be supplemented by additional software that controls the signal flow. Digital signal processing offers possibilities and processing possibilities that are not possible at all or only with great effort in the analogue circuit technology that was common in the past.
The methods of digital signal processing are much closer to mathematics, such as the subfields of number theory or coding theory, than to classical electrical engineering.
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(Image credit: signalprocessingsociety.org, IEEE Signal Processing Society)
The digital processing of a signal always follows the analog → digital scheme → processing → analogue. The changes to the signal are made exclusively in the digital domain. Using an audio CD as an example, the procedure will be explained.
In a sound recording, the sound pressure is converted into an analogue alternating voltage via a microphone. This alternating voltage is converted into a sequence of digital values with the help of an analogue-to-digital converter.
For audio CDs, a sampling rate of 44.1 kHz, i.e. the signal is sampled 44,100 times per second, a word width of 16 bits, i.e. the sampled, continuous value is mapped to one of 65,536 discrete values. In an intermediate step, the digital sound signal can now be processed, e.g. filtered or provided with effects, before it is saved.
To store the sound signal, the individual values are written to the audio CD in order. In order to play back the sound recording later, the data is read from the CD and converted back into a continuous alternating voltage by a digital-to-analogue converter. This is then transmitted to the speakers or an amplifier.
In contrast to conventional filter systems in telecommunications, which have to be implemented individually in hardware, with digital signal processing, any filter can be easily switched on or off in “real time” (e.g. for decoding) with the help of software. Depending on the performance of the system, any number of filters and complex filter curves and even phase shifts depending on other parameters can be generated in “real time” and thus the origin signal can be processed. Therefore, digital signal processing by DSPs is much more effective than conventional filter systems (e.g. noise reduction of analog signals).
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