String instruments generate sound by means of vibrating strings, the strings acting as resonators in a process of converting mechanical movements into sound signals. A string at a certain length and tension may generate only a single note at a time and the sound generated by the string is determined by combination of the physical characteristics of the string and several parameters set dynamically by the performer in the process of playing the instrument. The parameters set by the performer are primarily the length of the string, determining the pitch of the sound signal. This is usually done through the selection of a certain fret on the fret-board. However, there are many more parameters such as the intensity, position and style of plucking the string, as well as other sound production methods such as striking, hammering, bending, sliding etc.
In processing of a string instrument, it is often desirable to have the ability to operate and control a synthesizer from any common string instrument. Exemplary systems dedicated for guitars are often referred to as guitar-synthesizers or midi-guitars.
Currently, the process of converting the instrument playing process on a string instrument into synthesizer control messages such as MIDI is usually achieved by pitch detection techniques. Pitch detection (such as Dame, 1997) is a method in which the output signal of a string instrument is processed and the base frequency is detected using a variety of Digital Signal Processing (DSP) techniques. After the base frequency has been detected, a control signal is conveyed to a synthesizer, which produces the desired sound.
The main drawback of pitch detection is a persistent and inevitable delay between sound generation on the guitar and frequency determination and the consequent synthesizer sound generation. This delay is inherent to all DSP techniques and is disruptive for musical performance. This delay is related to the wavelength of the sound, and is not due to the lack of computing power. It is also due to the fact that the initial period after a sound is generated (the “attack”) is a transient stage in which string motion is not yet a clean harmonic motion. One method to try to solve this problem involves timing the spacing of plucking transient pulses (Szalay, 1999). This method is still limited by the time delay caused by the propagation of the pulses along the string.
Other attempts to solve these problems are by directly determining the desired note by establishing an electrical connection to each fret in order to determine the selected fret (Young 1984, Meno 1984), by placing push-buttons under frets, or by the transmission of ultra-sound frequency sound along the string and by timing echoes, determining the selected frets (Takabayashi, 1990). These methods where abandoned with time due to various implementation, installation and performance issues.
It would be desirable therefore to have a method and system dedicated to string instruments that allows the conversion of playing on a string instrument into control signals such as MIDI, without any perceptible delays and with minimal alterations of the musical instrument.
Another aspect of string instruments is the use of pickups. Most string instruments can be fitted with pickups to convert the string's vibrations into an electrical signal which is amplified and then converted back into sound by loudspeakers. The conversion of the sound into a corresponding electrical signal also enables the recording of the sound produced as well as signal processing. Pickups for string instruments are well known in the art and usually involve electromagnetic, piezoelectric, or optical conversion principals.
One drawback of the use of electromagnetic pickups is their ability to detect only string movement and not the absolute position of a string, nor the resting position of a string. Another problem arising mainly in magnetic pickups is that due to the nature of this technique it is limited to metallic strings and sometimes the magnetic sensors are prone to crosstalk interference. Another drawback of the electromagnetic pickup is its susceptibility to external magnetic/electric field interference. Another drawback of the electromagnetic pickup is its limited frequency range which causes loss of some of the sound energy and information produced on the guitar. Optical pickups are susceptible to ambient lighting conditions, often necessitating cumbersome coverings that hinder playing and are limited to near bridge placement, where string dynamics are minimal.
Therefore, it would be further desirable to have a method and a system that enables reconstructing and reproducing the sound of a string instrument and that the conversion process from mechanical movements to an electrical signal will be of high fidelity and not prone to external interferences.
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