978-1-4244-5858-5/10/$26.00 ©2010 IEEE ICALIP2010 378

Digital Piano Keyboard Based on PVDF Piezoelectric Film

Shupeng Liu1, Lin Chen1, Xiangming Zhu1, Lili Dai1, Yanfei Xin2

1. Institute of Biomedical Engineering, School of Communication and Information Engineering, Shanghai University, Shanghai, 200072 P.R.China; 2. Zhejiang Institute of Medicine Sciences,

Hangzhou, Zhejiang 310013 P.R.China Liusp@shu.edu.cn

Abstract

Digital piano, is an electronic musical instrument for synthesizing traditional piano timbre. With the development of digital signal processing and music synthesis algorithm, the music performance of digital piano is rivaling the traditional piano. This paper use a novel piezoelectric material—Polyvinylidene Fluoride (PVDF) to capture the strength, velocity and duration of keystrokes to substitute the hammer and string of traditional piano. Timbre with different intension, duration is synthesized according to the keystroke information captured by the piezoelectric material.

1. Introduction

Electronic musical instrument without strings, give the sound by the oscillation of different kinds of electronic units by electro-acoustic technology. Adjusting frequency, electronic musical instruments are able to obtain half-scale music of “twelve average rate”, and generate the other half-scale after dividing its frequency. Regarding timbre, it can be changed as the distribution of oscillation wave and overtone adjusted.

Electronic musical instruments can be divided into two categories: the imitation of traditional musical instruments and the musical synthesizer. The former musical instruments include electronic organ, electronic piano and electronic guitar. The latter is a kind of acoustic instrument than a kind of musical instrument. It can simulate traditional musical instruments, and can compose and play music according to user intention with computer. It has following characteristics: broad range, changeable timbre and music design.

When you play traditional acoustic piano, the hammer connected to keyboard hits the string and sets it into vibration, together with the reverberation effect of wooden soundboard, then the sound comes out. The track of hammer’s movement is linear but lay

difference in the velocity of movement. The variation of piano sound, including tune, timbre and loudness, originated from the velocity of hammer. To be concluded, the kinetic energy of hammer determines the sound of piano. A grand piano with a suitable environment could have fantastic effect, but the acoustic piano has some inherited shortcomings: bulky, inconvenient and expensive.

Digital piano replaces traditional string with timbre sampling and digital timbre record. When digital piano works, it sounds as key strikes digital contact material rather than hammer hits the string. Digital piano consists of mechanical device, oscillator, digital filter, timbre library and microphone. Oscillator, equivalent to string or reed in musical instrument, devoted to generate sound. It generates electric audio signal through stored timbre library. Digital matched filter’s polish makes sound more beautiful and pleasant. Mechanical instruments refer to keyboard, key and pedal by the variation of tone, timbre and loudness.

One of the most important qualities of piano is the flexibility of keyboard. The digital piano keyboards without string and hammer cannot match the traditional one in the flexibility. Manufacturers try to create a realistic tactile. Roland FP-5C achieves this purpose by adapting “advanced musical hammer movement” technology. But piano keyboard made of conductive rubber defects in its signal acquiring part, which is composed by a number of electric contact materials. As the velocity of hammer is calculated through acquiring connection time difference between distinct position contacts, piano’s performance could be degraded. PVDF piezoelectric film can honestly capture the information of keystrokes and guarantee the performance of piano.

Since the velocity of keystroke and the variation of timbre of digital piano are not as rich as traditional one, digital piano can’t compare with traditional piano in the aspect of controlling timbre. Electro-acoustic engineers try to synthesize the sound of grand piano through research in digital signal process [1][2]. There are a

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number of different techniques that are used for music synthesis, including additive synthesis, subtractive synthesis, frequency modulation (FM) synthesis, wavetable synthesis, physical modeling [3] and reconstructive phrase modeling [4]. In this paper, we will focus on the design of piano keyboard using PVDF Piezoelectric Film.

2. Digital Piano Model Based on PVDF Piezoelectric Film

A piano keyboard based on PVDF piezoelectric film is proposed in this study. It places a 50um thick film on the key. The electric lead of piezoelectric film is wired out to signal processing unit from keys’ fixed end. This design not only avoids the disadvantage of controlling timbre by the connection between keys and conductive rubber, but also doesn’t affect the mechanical structure of traditional acoustic piano. As piezoelectric film is very soft, the distortion resistance can be neglected.

Fig.1 shows player hits keys and sets piezoelectric film into distortion, and the output of electric signal is dominated by keystroke’s strength, duration and velocity. This signal can be converted into voltage signal by electric amplifier, and it is further processed by BF-533 [5] after it is converted into digital signal by AD7794.

The main assignments of BF-533 processor: Confirm tune according to keystroke’s signal source; Confirm tune strength by keystroke’s signal; Confirm tune phase by keystroke’s signal duration; Enhance

synthesized tune by adjusting output charge and frequency of keystroke’s electric signal.

Output the detail of sound by audio chip AIC23B integrated on BF-533.

The structure diagram of digital piano is shown in Fig.2:

Keyboard equipped with PVDF-Sensor: It is designed to deliver mechanical movement of keys to PVDF piezoelectric film making it generate electrical signal. In order to facilitate A/D transforming circuit, it is necessary to adapt electric amplifier to modulate signal.

BF-533 digital audio frequency experimental development system: BF-533 digital audio frequency experimental development system is adapted to achieve digital signal’s reception, storage, process and control. Besides the core software design would also be implemented by this system.

Speaker: Digital audio signal modulated by synthesized overtone should be transformed into analog audio signal, and then outputted by speaker.

To verify the effectiveness of this program, twelve keys are installed on the digital piano model. Each functional unit is represented respectively in the following. A. Keyboard equipped with PVDF-sensor

Fig.3 shows the structure of upright piano, number 9 represents ivory key and number 10 represents ebony key. The red part is rotate axis by which the force of key could be transmitted to box. The blue part indicates the location of PVDF.

Figure 1. The design diagram of digital piano based on PVDF piezoelectric film and BF-533

Figure 2. Structure diagram of digital piano

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Figure 3. Upright piano structure diagram

Fig.4 shows the ivory key’s structural diagram of this program. Keystroke makes the key rotate around the red support. PVDF piezoelectric film is fixed between the red support and key, and it would generate charge if there is deformation. The charge could be transmitted to amplifier circuit through brown line. B. Signal amplifier circuit and A/D converter circuit

1) The piezoelectric character of PVDF: Fig.5 shows the PVDF piezoelectric film. As piezoelectric material bears mechanical stress, charge with opposite polarity but the same quantity appears on the surface. It functions as capacitance, and its capacity can be calculated using (1):

tStSC ra // 0��� �� (1)

S is the area of polar plate ( 2m ), t is the thickness of crystal ( m ), � is the permittivity of piezoelectric crystal, � is the relative permittivity of piezoelectric crystal and 0� is permittivity of vacuum. As unlike charge accumulates in the board, voltage can be calculated by (2):

aa CqU /� (2)

And q(C) is the accumulated electric charge, aC (F)

is the capacitance of the board, aU is the voltage of inter-plate.

2) Electric amplifier: Preamplifier need to have high input resistance, otherwise sensor’s signal would leak and measure error is inevitable. Charge amplifier

is a high gain amplifier with deep negative feedback. Fig.6 shows its schematic diagram:

Output voltage can be represented by (3): ])1(/[ 0 FcaOsc CACCqAU ����� (3)

Figure 4. Key structure

Figure 5. PVDF piezoelectric film

Figure 6. Piezoelectric sensor and charge amplifier circuit.

As aC cC can be omitted compared with FC ,

and 0A >>1, the output voltage is given as (4):

Fsc C

qU �� (4)

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As piezoelectric sensor combines with electric amplifier, its LF error and cutoff frequency are only decided by parameter FR and FC of feedback circuit.

FC is determined by voltage output. The elevation of

FC can promote piezoelectric sensor’s LF, but it

decreases SCU when overdone. Before A/D sampling, the dynamic range of the voltage signal should be adjusted to satisfy A/D converter’s dynamic scope.

3) A/D converter and its realization: A/D converter AD7794 is adapted to realize analog voltage’s digitalization. AD7794 has following characters: low power consumption and perfect simulation of input terminal. And it can be used in LP signal’s measurement. Fig.7 shows the flow chart of AD7794.

Figure 7. Flow chart of AD7794

C. The design of piano tune restore softwareextraction of fundamental tune, loudness and phase 1) Fundamental tune: Each key of keyboard

represents a fundamental tune in the model. According to the consecutive sampled passage n, pressed key and fundamental tune’s frequency can be identified.

2) Loudness: Loudness is determined by the charge’s output of PVDF piezoelectric film, it also can be defined as the maxim value of consecutive output of PVDF piezoelectric film. Fig.8 shows output wave correspondent to distinctive keystroke strength.

Fig.8-(a) is 2V per grid, Fig.8-(b) and Fig.8-(c) are 5V per grid. Although there is baseline drift in Fig.8-(a), the wave output still has a good result. It functions as DSP receives a set of digital signal sampled continuously by AD7794, and keeps sampled value in buffered area in one-dimensional array. After acquiring

a maximum value of this sampled sequence through continuous comparison, following tune can be calculated by it.

3) Phase: Phase is dependent on the velocity of the keystroke. The tune is strong and short when hit fast otherwise it would be soft and gentle. Fig.9 shows the output wave correspondent to distinctive keystrokes.

It is obvious that different keystrokes mean different output time. As M multiplied with sT , M is the

number of effective sampled data of Fig.7 and sT is interval time of sampling, duration of keystrokes can be calculated. And then the duration time functions as a parameter to control the calculation of following timbre.

(a)

(b)

(c) Figure 8. Output wave correspondent to distinctive keystroke strength

Figure 9. Output wave correspondent to distinctive keystroke velocity.

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3. Conclusion

In this paper, PVDF piezoelectric film is adapted to capture keystroke’s message. It can accurately and honestly reflects the details of keystroke, which guarantees the authenticity of timbre.

The next work, wavetable synthesis algorithm, a high-fidelity audio performance would be used to decompose or synthesize the timbre. Timbre with different spectrums can be obtained from the shift of fundamental frequency. With the combination of fundamental tune’s frequency, loudness and phase, sound can be synthesized through processing audio wave stored in the processor.

Acknowledgment

This work is supported by Shanghai Leading Academic Discipline Project and STCSM (S30108 and 10DZ2210900).

References

[1] S. Yim, Ding Yinong and E.B. George, “A real-time MIDI music synthesis system using sinusoidal modeling on a TI TMS320C32 digital signal processor,” IEEE First Workshop on Multimedia Signal Processing, June 1997, pp. 469-474 . [2] J.A. Gibbons, D.M. Howard and A.M Tyrrell, “FPGA implementation of 1D wave equation for real-time audio synthesis,” IEE Proceedings: Computers and Digital Techiniques, vol.152, no.5, September 2005, pp. 619-631. [3] J.Rauhala, H.M Lehtonen and V. Valimaki, “Toward next-generation digital keyboard instruments,” IEEE Signal Processing Magazine, vol 24, March 2007, pp. 12-20. [4] Eric Lindermann, “Music Synthesis with Reconstructive Phrase Modeling,” IEEE Signal Processing Magazine, vol. 24, March 2007, pp. 80-91. [5] Analog Devices. ADSP-BF533 Black-fin Processor Hardware Reference. 2004.