Noise generator
A noise generator is a special type of signal generator in the form of an electronic circuit that generates noise as a random signal fluctuation. Noise generators are used in various fields, such as in measurement technology for generating test signals, in electronic music and speech synthesis , and as a basis for generating random numbers as required in cryptography . There are also virtual noise generators in the form of mathematical equations that are only used for signal processing in software and do not output a physical signal. These are often also deterministic number generators.
Sources of noise
Various types of noise sources are used to generate the noise, which differ among other things in terms of the type of spectral distribution of the noise, the bandwidth of the noise signal, the technical feasibility and the avoidance of predictable signal components, for example interferences from periodic signals such as the network frequency . Some sources of noise, such as the radioactive decay of unstable atomic nuclei, can theoretically be used as ideal sources of noise, but are of little importance in practical applications because of the dangers involved. Only in special applications such. B. They are used as rubidium standards .
Thermal noise sources
Thermal noise sources , for example noise voltage tapped from special heated resistors , deliver normally distributed white noise . Since the thermal noise voltage delivers only very small signal voltages, for example at room temperature of 20 ° C at a resistor with 1 MΩ and a bandwidth of 1 kHz an effective voltage of 4 µV, and this noise voltage is caused by other periodic and undesirable signal components such as fluctuations in the supply voltage is superimposed, good thermal noise sources represent electrically complicated structures in practice.
Diodes
Noise sources based on diodes , which in this application are also referred to as noise diodes , are technically easier to manufacture than thermal noise sources . With appropriate generator circuits, the frequency range can range from 20 Hz to 40 GHz and is usually not spectrally white, but is described by what is known as shot noise .
Alternatively, but with the same principle, the base-emitter path of bipolar transistors can be used instead of the diode in the generator . The oldest design of diode noise sources are tube diodes with tungsten filaments, which are operated in saturation.
The signals are then amplified and spectrally filtered in filters depending on the application, or the noise signal is processed by analog-digital converters for further signal processing. With this, spectrally white noise can also be produced to a good approximation in technically used frequency ranges.
Electrical resistances can also be used in this way.
Gas discharge lamp
For technically very high-frequency noise sources in the range of microwaves up to frequencies around 200 GHz, gas discharge tubes are used, which are housed in a waveguide to decouple the noise signal . Applications of these noise generators are in the field of high frequency measurement technology.
Glow discharge
The glow discharge , for example in the form of glow lamps , can be used for very low-frequency noise in the lower Hz range .
Digital noise generators
With the help of microcontrollers and digital hardware, noise generators can now be generated very easily.
Noise generators in audio technology
White noise contains the same power density for every frequency interval, which results in an identical volume or acoustic energy (dB / Fq) when output via a loudspeaker over the entire frequency range . The pink noise is spectrally weighted and can be obtained from white noise using filters. With acoustic output, pink noise contains the same loudness perception (dB / semitone) per note interval. This means that pink noise sounds a little duller than white noise.
To generate the signals, either classic analog circuits or mathematical functions are used, the signals of which are output when required. Often pseudo-random number generators are used here , which can generate perfectly symmetrical noise.
White noise is e.g. B. used in speech synthesis for the simulation of sibilants , pink noise z. B. for a surf noise. In order to obtain realistic sounds, the basic forms of white or pink noise must be filtered accordingly. This corresponds to a spectral shaping of the noise signal, e.g. B. with one or more voltage controlled filters as an analog solution or FIR filters in digital form.
literature
- Rudolf Müller: Rauschen (= semiconductor electronics . Volume 15 ). 2nd, revised and expanded edition. Springer, Berlin a. a. 1990, ISBN 3-540-51145-8 .
- Daniel von Grünigen: Digital signal processing. With an introduction to continuous signals and systems. 5th edition. Carl Hanser Verlag, Leipzig 2014, ISBN 978-3-446-44079-1 .
- Dieter Görrisch: Jammers from VHF to microwave. Finally no more conspiracy theories. 2nd Edition. Francis Verlag, Poing 2006, ISBN 3-7723-4127-6 .
- Andreas Friesecke: The audio encyclopedia. A reference book for sound engineers. Saur, Munich 2007, ISBN 978-3-598-11774-9 .
Web links
- RPG100 . (PDF; 129 kB), techn. Data sheet of a true random generator based on the noise source of semiconductor diodes (English) accessed on July 1, 2011.
Individual evidence
- ↑ Christoph Caspari: Resistance: Noise from resistances. In: elektronikinfo.de. January 18, 2018, accessed July 13, 2020 .
- ↑ Noise generator and PRBS generator with microcontroller. Retrieved August 23, 2020 .
- ↑ Digital noise generator in the FPGA - Mikrocontroller.net. Retrieved August 23, 2020 .
- ↑ Stefan Stenzel: A New Shade of Pink - Audio Noise Generator. Waldorf Music, May 3, 2014, accessed July 13, 2020 .
- ↑ Various Authors: DSP Generation of Pink Noise. Music DSP ORG, 1999, accessed July 13, 2020 .