In a problem of creation necessary soundings of electromusicial instruments (EMI) the major place belongs to electronic systems of processing the signals, which were received from gauges. The gauge can be the microphone, electromagnetic, piezo, etc. The devices considered in this work, are focused first of all on working together with traditional electroguitars with electromagnetic gauges, however they also can be used with other types of EMI.
Among extremely numerous family of electronic systems of processing the signals of electroguitars the most widespread are the devices which are carrying out peak restriction of an initial signal. This function can be realised by some types of devices, main of which are «Fuzz», «Distortion», «Overdrive», and also preamplifiers and power amplifiers working in a mode of nonlinear amplification. There are no principal differences between this types of devices - the differences are in types of frequency response etc. All this types can be called «drives».
The general feature for all types - peak restriction of the signal - is used for expansion of an initial spectrum of a signal. In sphere of electromusical instruments spectrum expansion in high-frequency area is a traditional way of creating new soundings, the tool of enrichment of initial sounding by the necessary overtones. Difference in overtones of sound is the reason on which the sounding of violin is not similar to a guitar's sounding even the mechanism of a sound's forming in these instruments is similar.
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Researching the spectral structure of signals in out of vacuum-tube distortions and systems on semi-conductor elements, revealing the prominent features of a spectrum of this devices and characteristic differences between them.
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forming the requirements to developed system on semiconductors from the point of view of spectrum transformation in it;
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The analysis of circuitry of both types of devices, revealing to the reasons causing differences in a spectrum of signals;;
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Working out of circuitry of new type of the solid-state systems providing spectral structure of a target signals, identical to received with help of devices on vacuum lamps;
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"Transfering" concrete lamp-based system to semiconductor element base: working out of the scheme of the semiconductor system providing spectral transformation, identical to received with help of devices on vacuum lamps;
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The analysis of results.
Prospective scientific novelty
A variety of drives on solid-state element base made for today is huge. However release of devices on vacuum lamps does not stop till now. Moreover, new models on the basis of lamp cascades are regularly issued by various manufacturers. Thus the circuitry of such devices, doesn't changes serious in comparison with traditional decisions.
The reason of this fact is the difference between spectral transformations on devices of both types. Most of musicians says that the quality of sound processing in tube devices is much better than in systems based on semiconductor.
The general target is to find a method which will help us to create a new system with the sound quality of best vacuum tube drives but small sizes and low weight.
The analysis of existing solvings
Peak restriction can be made as by traditional analogue devices, and also with help of digital signal processing. The analysis of digital prototypes is difficult. Therefore in this case only analogue prototypes are considered. The animated function chart of typical electronic system of processing the signals of EMI realising operation of peak restriction, is shown on fig. 1. On this figure 1 - string, 2 - gauge, 3 - preamplifier, 4 - clipping stage, 5 - poweramp, 6 - loudspeaker.
Figure 1 - Animated function chart of peak restriction system of EMI signals
Quantity of shots - 5; the quantity of repetition cycles of repetition - infinite; size - 28кB
Vacuum tube-based prototype
Schematic of prototype which is based on electronic tubes is shown on fig.2. Our prototype is Marshall JCM800 guitar amplifier. The spectrum of a target signal is shown of fig.3. The frequency of initial signal is 1kHz.
Figure 2 - Marshall JCM800 schematic
Figure 3 - Spectrum of a target signal
The sounding of this prototype is good. Lacks of a tube prototype are characteristic for all tube electronics as a whole. Among them the big dimensions and weight of tube devices, their high cost, short life of electronic tubes.
Prototype based on semiconductor elements
Our semiconductor-based prototype is Marshall Shredmaster, because this device is positioned by Marshall company, as cheap alternative to more expensive tube systems, in particular, JCM800. The schematic of Shredmaster is shown on fig.4.
Figure 4 - Schematic of Marshall Shredmaster
The general structure of the schematic is similar to the structure of a tube prototype. However signal restriction is made not by the cascade on the vacuum triode, but by the clipping stage on silicon diodes. The general view of the oscillogram of voltage on output of the clipping stage is shown on fig. 5. Initial signal is monoharmonic. The General view of the spectrum of output signal of system with parallel diodes clipping stage is shown on fig. 6.
Figure 5 - Oscillograms of voltages on input and output of parallel diodes clipping stage
Figure 6 - Spectrum of signal after clipping
ShredMaster has very low weight, small sizes and lower price. But the sound quality of Shredmaster is much worse than the quality of JCM800 has. Overtonic sounding structure, a spectrum of a target signal are unsatisfactory in comparison with provided with tube systems. From comparison of spectrograms it is possible to make a conclusion that a principal reason of unsatisfactory sounding of a semiconductor-based prototype is obvious difference from the tube prototype in spectral structure of a target signal: the semi-conductor prototype enriches a signal exclusively odd harmonics which are discording since 7th (sounding «roughly», «inharmoniously» in the point of view of the musical theory) accords with the basic harmonic and with each other (except octava).
The planned and already received result
During the working over work of the master it's supposed to execute high-grade research of both prototypes: working out of mathematical model of the tube cascade in a nonlinear operating mode and its check during experimental researches, experimental receiving of frequency response characteristics of active cascades and passive frequency correction filters. On the basis of the collected results requirements to the device on the solid-state element base should be formulated, allowing to receive identical nonlinear transformation (restriction) of signal and, finally, the spectral structure of target signal identifical to tube-based systems. On the basis of the formulated requirements the block diagram of the new device on semiconductor element base and circuitry of blocks will be developed.
For the purpose of researches unique system for receiving frequency response characteristics of linear quadripoles in automation mode on the basis of a synthesizer with direct digital synthesis of the frequency, described in [1] has been developed and assembled. Research of prototypes by means of this system will allow to reduce time of researching a prototypes and to raise reliability of results. Information search in a theme, in particular, search of the patents illustrating attempts to solve a problem during earlier time is going now.
End of work and is planned for December 2009.
The list of sources
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2. Кемпф В. Полевые транзисторы в устройствах "Distortion". www.rusblues.ru
3. Кемпф В. ПТ эмуляция триодного ограничения. www.rusblues.ru
4. Eric K. Pritchard. The Prejudice Against Solid State and Pritchard Amps. www.pritchardamps.com
5. Арзуманов С. Электронная обработка гитарного сигнала, Часть 1. Электрогитара www.guitar.ru
6. Войшвилло Г.В. Усилительные устройства. М. «Радио и Связь», 1983.
7. Шкритек П. Справочное руководство по звуковой схемотехнике. М. «Мир», 1991.
8. Сато Ю. Обработка сигналов. Первое знакомство. М. «Додека», 2009.
9. Кинг Г. Руководство по звукотехнике. Ленинград, «Энергия», 1980.
10. Eric K. Pritchard. The Tube Sound and Tube Emulators. dB Magazine, July/August 1994.
