Development of a Low Cost Assistive Listening System for Hearing
Contents
1. viii Classification of hearing correctness was listed as follows a correct absolutely whole word b correct only for the first syllable and vowel English equivalent example of this case would be hearing pass instead of past c correct only for the first syllable English equiv alent example of this case would be hearing cat instead of cow d correct only for the vowel English equivalent example of this case would be hearing can instead of bat ix Class teachers would remove the words perceived to be unknown to individual students from the result The experiment was shown in Figure 7 a Field Test Results The raw results for individual students are shown in Table 1 Summarised results in percentage are shown in Table 2 b Field Test Result Analysis From Table 2 our ALS receives approximately 6 higher whole word correctness than school ALS Overall correctness percentage of school ALS is higher than our ALS by approximately 2 5 These results suggest that our and school ALSs performance is similar It should be noted that hearing performance of students when wearing either our or school ALS is nearly twice as much as when not wearing ALS The result supports the obvious benefit of ALS to hearing impaired people At 10 metres original audio signal deteriorates so much that makes hearing difficult ALS helps transmit signal while maintaining quality close to the orig2. FIGURE 6 Performance characteristic test setup students speech 4 Conclusions In this work we describe the development of a low cost wire less broadcasting ALS It was designed for short range appli cation such as in the classroom for the hearing impaired students The results from audio characteristics and field test by prospective users suggest that its performance is com parable to an existing ALS system It is hence a viable alter native to existing FM ALS The ALS was also designed to address another practical issue commonly faced when students usually found using hearing aids that are incompatible with existing ALS ie no neckloop or DAI shoe in making them either not using the ALS or using it without the hearing aid The ALS in this project was designed with a built in amplifier to help the students to have the choice of using ALS alone without com pletely losing hearing aid s amplification benefits The built in amplifier was shown to have a gain of 67 3 dB a level that can be useful for students with moderate to severe hearing loss with the bandwidth that can appropriately support speech communication The bill of materials of the system costs approximately 150 US dollars for each transmitter or receiver Our system costs several times less than the commercial FM ALS currently used in our partner school Our ALS exceeds initial power target of lasting for four hours It contains one 3 7V1000mA lithium ion batt
3. All of them had severe 71 90 dB HL to profound 91 dB HL levels of hearing loss of both ears 9 They were all in primary school year five and six They were all taught orally without using sign language at school iii Three sets of 25 phonetically balanced PB Thai words were used one for each test iv Each word was played three times Students would pronounce the word they hear and write it down v Each student was accompanied by an observer who would listen and note down the words each student said This was because students may have limited spel ling skills They might have heard the word correctly Transmitter Microphone gt Amplifier Power management Charging circuit 1 2V regulator 3 3 V regulator The Scientific World Journal Transmitting circuit board Transmission control channel selection and display Transmitting module FIGURE 2 Transmitter module Receiver Receiving circuit board Receiver control channel selection and display Receiving module Power management e Charging circuit e 1 2 V regulator e 3 3 V regulator Filter and amplifier DC bias with voltage feedback amplifier Noise filter FIGURE 3 Receiver module but spelt it wrongly The observers notes would be used for analysis vi The order of word sets and devices used was randomly set for each student vii During the test SNR was measured to be between 10 and 15
4. system The system utilises a wireless digital technology communication It operates in a 2 4 GHz unlicensed industrial scientific and medical ISM radio band The internationally unrestricted ISM band allows the system to operate in most countries Due to the band wide ranging applications ISM based compo nents are readily available and at low costs Furthermore ana logue FM signal could be digitally processed to reduce noise and improve signal quality The system is designed to be short ranged and in one to many broadcasting mode It is intended to be used in a classroom for the hearing impaired students This paper is organised into four sections Section 1 contains the introduction Section 2 describes system spec ifications and architecture Section 3 reports system imple mentation Acoustic performance and field tests results and analysis are also described in Section 3 The last section com prises conclusions 2 System Specifications and Architecture 2 1 System Specifications This project has produced three previous versions of the system before achieving the final set The earlier versions were assembled on development boards temporary boxes and field cases The assembly and test results were reported in 6 The analysis and user feedbacks The Scientific World Journal FM radio signal Speaker uses transmitter Receivers with with microphone earphones headphones or neckloop FIGURE 1 An example of an FM s
5. Hindawi Publishing Corporation The Scientific World Journal Volume 2013 Article ID 787656 6 pages http dx doi org 10 1155 2013 787656 Research Article Hindawi Development of a Low Cost Assistive Listening System for Hearing Impaired Student Classroom Setha Pan ngum Tharapong Soonrach Sangvorn Seesutas Anukool Noymai and Pasin Israsena Department of Computer Engineering Faculty of Engineering Chulalongkorn University Phyathai Road Patumwan Bangkok 10330 Thailand National Electronics and Computer Technology Center Rehabilitation Engineering and Assistive Technology Institute 112 Thailand Science Park Khlong Luang Pathumthani 12120 Thailand Correspondence should be addressed to Setha Pan ngum setha p chula ac th Received 29 March 2013 Accepted 7 May 2013 Academic Editors A Deveze and M Stankovic Copyright 2013 Setha Pan ngum et al This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited This paper describes the design development and tests of a low cost ALS It was designed for hearing impaired student classrooms It utilised digital wireless technology and was aimed to be an alternative to a popular FM ALS Key specifications include transmitting in 2 4 GHz ISM band with eight selectable transmission channels battery operated a
6. They were the full on gain measured via HEA FOG 50 dB setup following IEC60118 7 standard total harmonic distortion THD and its audio bandwidth These parameters would help us determine the quality of audio signal produced as the ALS was designed with a built in amplifier Full on gain will tell us the amplification gain bandwidth the range and THD the quality of the signal The test was done using the software Sound Check 8 1 and Anechoic Test Box Type 4232 The transmitter was placed in the box next to a speaker which played a sound at different test frequencies The receiver was placed next to the box The received signal from the receiver was passed on to the box microphone The signal picked up from the microphone was sent to the software for evaluation The experiment setup is shown in Figure 6 The software calculated HFA FOG 50 dB which is the high frequency average gain given input at 50 dB audio band width and total harmonic distortion THD The HFA FOG 50 dB was 67 3 dB The audio bandwidth was the range between low and high frequency cutoffs Our ALS had the bandwidth of 133 Hz 6 52 kHz THD was mea sured at three different input frequencies At the input of 500 Hz and 70dB SPL THD was 11 8 At the input of 800 Hz and 70 dB SPL THD was 13 3 At the input of 1 600 Hz and 65 dB SPL THD was 2 5 Reading from the results it can be seen that the ALS has a gain of 67 3 dB which is considerably high as int
7. e Scientific World Journal References 1 M Ross Room acoustics and speech perception in FM Auditory Training Systems Characteristics Selection and Use M Ross Ed pp 21 44 York Press Timonium Md USA 1992 2 N Lederman and P Hendricks Induction loop assistive listen ing system in Communication Access for Persons with Hearing Loss M Ross Ed pp 19 40 York Press Timonium Md USA 1994 3 M Lieske Infrared systems in Communication Access for Per sons with Hearing Loss M Ross Ed pp 41 50 York Press Timonium Md USA 1994 4 William sounds 2008 http www williamssound com 5 S M Bankoski and M Ross FM systemseffect on speech dis crimination in an auditorium Hearing Instruments vol 35 no 7 pp 8 12 1984 6 P Israsena P Dubsok and S Pan Ngum A study of low cost robust assistive listening system ALS based on digital wireless technology Disability and Rehabilitation vol 3 no 6 pp 295 301 2008 7 Microchip dsPIC33FJGPX06A X08A X10A Microchip Technology 2012 8 Aurel TX AUDIO 2 4 AE user manual Aurel s p a 2009 9 J G Clark Uses and abuses of hearing loss classification ASHA vol 23 no 7 pp 493 500 1981 datasheet The Scientific World Journal l MEDIATORS S INFLAMMATION X Journal of Diabetes Research http www hindawi Journal of Immunology Re
8. ended comparable to the rage of hearing aid amplification needed for user with moderate to severe hearing loss The bandwidth achieved also covers the targeted speech rage In terms of THD the values are relatively high compared to the range of 1 2 expected from hearing aids but still competitive compared to THD of FM products available in the markets that we also measured This implies that the quality of the audio signal would be between that of hearing aids and commercial FM products 3 2 2 Field Test The field test was designed to test our ALS in an actual operating environment which was in classrooms of hearing impaired students It was also tested by prospective users which were the hearing impaired students from our partner school In the test students would listen to separate Thai words and write them down The results would reflect their ability to hear and recognise words when using ALS For comparison each student would do three identical tests but using three different devices They would wear our ALS acommercial FM ALS currently used in the school and their own hearing aids with no ALS The test details were as follows i It was carried out in the school library The recorded words were played from a speaker The testers sat 10 metres away from the speaker The library was used because available classrooms were 8 metres long which were shorter than our ALS specifications ii Five students participated in the test
9. ery When fully charged it can operate up to ten hours In school usage students can wear them in class all day and the teachers can charge them overnight The 2 7 cm x 7 8 cm x 2 7 cm dimension makes our ALS fit into student s shirt pocket or snap to teacher s belt comfort ably Each unit weighs just over 100 grams We anticipate that it could be made lighter by changing plastic material for casing The current material is PVC which is strong but heavy 5 Further Work We are in the process of assembling a set of two transmitters and ten receiver units They would be given to our partner school for long term use We plan to keep usage and main tenance record for at least six months This should provide data of our ALS robustness and durability which is our main concern From observation there seems to be much wear and tear with children users Furthermore there are additional chances of the system being accidentally exposed to moisture and water This would severely test our system robustness and durability Acknowledgments The authors would like to thank Dr Siriparn Sriwanyong for providing PB word sets He also kindly gave a valuable advice on field test setup and procedures The authors would also like to thank teachers and students of Kanchanapiseksompoj School who kindly and enthusiastically participated in all of our field tests This project is funded by The National Science and Technology Development Agency NSTDA Th
10. inal signal It can be seen from Table 1 that hearing ability of students varies greatly even though they study at a comparable level Student 5 got 43 whole words correct while student 4 only got 15 correct One teacher added that recognising separate words is difficult for hearing impaired students The stu dents had better understanding of sentences through context cognition The notes from the observers are not reported here because the observers were mostly unable to recognise The Scientific World Journal 5 TABLE 1 Field test results by students Student 1 Student 2 Student 3 Student 4 Student 5 No Our School No Our School No Our School No Our School No Our School ALS ALS ALS ALS ALS ALS ALS ALS ALS ALS ALS AILS ALS AIS ALS Correctness results Whole word 10 12 6 3 ll 14 3 8 17 7 4 4 7 24 12 First syllable and vowel 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 First syllable only 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Vowel only 0 2 2 l 0 3 l 2 l 0 0 2 l 0 3 Unknown word s 4 2 2 2 4 2 4 2 2 0 0 0 0 0 0 FIGURE 4 Transmitter and receiver PCBs TABLE 2 Summarised results Correctness percentage No ALS Our ALS School ALS Whole word 26 09 50 43 44 54 First syllable and vowel 0 0 0 84 First syllable only 0 0 0 Vowel only 2 61 3 42 10 92 FIGURE 5 Transmitter and receiver casing FIGURE 7 Field test the words that students pronounced It is observed that only skillful listeners such as teachers could understand individual
11. ircuit and the 1 2 V and 3 3 V regulator circuits The receiver module is shown in Figure 3 The receiving circuit board contained the same micropro cessor and microcontroller as the transmitting circuit board They performed the reverse tasks by basically selecting the channels and demodulating signal Before reaching a listener the signal was improved by filtering noise and amplified Filtering was done by a simple low pass filter The amplifier circuit output was adjusted by user rolling switch The user could then select desirable output volume The power man agement circuit was the same as in the transmitter 3 System Implementation and Testing 3 1 System Implementation Since the system was designed for actual usage by primary school children robustness and durability were the main concerns All the circuits were inte grated into a single two sided printed circuit board PCB Moderately thick PVC casing was used to house the assembly The case had width length and thickness 2 7 centimetres 7 8 centimetres and 2 7 centimetres respectively The trans mitter and receiver PCBs are shown in Figure 4 The finished transmitter and receiver in casing are displayed in Figure 5 3 2 System Testing Performance characteristics and user tests were done on the system to evaluate its performance 3 2 1 Performance Characteristics Test The system was tested for three key performance characteristics prior to the field test on users
12. nd chargeable pocket size and ranged up to thirty metres Audio characteristics and user tests show that it is comparable to a commercial system currently employed in our partner school The results also show that wearing an ALS clearly improves hearing of hearing impaired students Long term usage by school children will be monitored to evaluate the system robustness and durability 1 Introduction Assistive listening systems ALSs are devices that help hearing impaired people by reducing the effects of ambient noise and distance on sound travel Sound progresses from a source such as a speaker through air to listener s ears During travel sound quality deteriorates Longer distance reduces amplitude and surrounding noise interferes with the original sound This combination causes hearing difficulty to normal listeners and the effects are more significant to hearing impaired people An ALS generally consists of a transmitter and a receiver The transmitter picks up sound near the source usually by a microphone It then converts it into electronic signal The signal is transmitted to the receiver through wires or by wireless transmission protocols The receiver sends the signal directly to a hearing aid or converts it back to sound for the user Since the signal is transmitted electronically the adverse effects of distance and background noise are eliminated The sound quality that a listener hears is close to the sound quality fr
13. ne would not be needed Selectable frequency channels were included to avoid interference from nearby classrooms When interference occurs a teacher can tell all students to switch to different channels Normal classroom length is six to eight metres The 30 metre operating range is placed in the specifications to meet a teacher s suggestion that it should be long enough to cover outdoor class For practical purpose the system has to be portable Students put them in their shirt pockets and teachers tuck The Scientific World Journal them on their belts so our system size was designed to meet this requirement 2 2 System Architecture From the above specifications the system architecture was designed as shown in Figure 2 The signal from a condenser microphone was modified with an analogue amplifier circuit The amplified signal was sent to a dsPIC33FJ128GP708A 16 bit low power microcon troller 7 The microcontroller was connected to a user chan nel selector switch It then sent control signal to the trans mitting module accordingly All user switches and displays such as status LEDs and seven segment LED for channel display were controlled by this microcontroller Transmission was done by U3 TX Audio 2 4 AE processor which applied frequency shift keying FSK modulation 8 The processor could transmit at eight different frequencies between 2400 and 2483 5 MHz The power management circuit consisted of analog charg ing c
14. om the sound source Hence an ALS essentially maintains a signal to noise ratio SNR of the sound source ALS could be used together with a hearing aid The hearing aid function is to amplify the sound to compensate for a user s hearing impairment A hearing impaired person has difficulty differentiating the sound source from surrounding noises 1 ALS helps hearing impaired people to participate in social activities with normal hearing people such as in classrooms or living rooms For example by placing a transmitter near to a televi sion a hearing impaired person can watch TV programs with normal hearing family members Without ALS the hearing impaired person would have to turn up TV volume so much that other family members might find it disturbing Signal transmission between transmitter and receiver through wire obviously limits ALS practical use and porta bility Personal ALSs therefore rely on wireless transmission Current ALSs employ one of these three technologies induc tion loop IL 2 infrared IR 3 and frequency modulated FM radio systems Of the three systems FM is the most popular The sound signal is frequency modulated onto a radio frequency carrier wave The wave is sent from a transmitter to a receiver which is set to the same frequency The receiver then demodulated the carrier wave to retrieve the original sound signal The technology is the same as radios where radio stations are transmitters and h
15. ousehold radios are receivers The original sound signal can reach the user by earphone headphone or through a hearing aid via a neckloop or digital audio input DAT connector An example FM system from Williams Sound Corporation is shown in Figure 1 4 FM systems were initially employed in broadcasting applications such as in classrooms for hearing impaired stu dents 5 Current advance in microelectronics enables robust and portable FM systems to be used as a standalone point to point personal device For instance a user can place a portable transmitter near a television set With the receiver connected to a hearing aid the user can enjoy watching TV programs with the best SNR for sound As with FM radios FM ALSs have to operate within approved frequency bands The United States Federal Com munications Commission FCC allocated 72 76 MHz fre quency band for ALS usage Subsequently 216 217 MHz was also allowed for ALS However there is no specific frequency band for ALS globally FM ALS has advantages over the IL system in terms of lower transmitting power and longer range It also requires no line of sight between transmitter and receiver as compared to IR However FM ALSs can be subject to signal interference and bad reception FM radio users probably have experienced similar problems which adversely affect sound quality For the above reasons this paper describes our develop ment of a low cost wireless ALS broadcasting
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17. ystem 4 from those results were used to design and assemble the final system For practical classroom of the hearing impaired usage the system was designed to meet the following specifications i transmits in broadcast mode at 2 4 GHz ii uses chargeable battery that lasts at least 4 hours half day usage iii is chargeable via portable charge switching same as mobile phone charging iv can work with both internal and external micro phones v has eight selectable frequency channels vi has working range of up to 30 metres vii has to fit into a shirt pocket and could be tucked onto a belt As mentioned earlier the 2 4GHz is the international frequency band allocated for scientific and medical usage A device can transmit in this band without having to seek government permission The partner school currently uses commercial FM sys tems Teachers carry transmitters and students carry receivers with them during class Students return their devices to teach ers during lunch and at the end of school day for charging Hence the specifications are designed to meet their usage in terms of ease of charging and battery life In class teachers speak into an external microphone which is connected to the transmitters However internal microphone was added to the specification for flexibility of usage For example a transmitter can be placed in front of a television or radio In this case an external micropho
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