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Digital Hearing aids and fm radio systems fm Advantage procedures
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Digital Hearing aids and fm radio systems fm Advantage procedures

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  1. Digital Hearing aids and fm radio systems fm Advantage procedures David Evans, MD, Connevans Limited.{click here to jump to practical section} V 2003.2

  2. Introduction • Digital hearing aids are here to stay! • Different procedures for setting up & testing digital hearing aids with fm systems is necessary – fmAdvantage • The fmAdvantage protocols can be used with any testbox but using testboxes with DSP test signals is preferable

  3. Do digital hearing aid users benefit from using an fm system in class? • YES !

  4. Practical advantage • We are about to look at graphs of hearing aid output when used with an fm system …. however …. do not forget the obvious, remember the benefit of a wireless microphone • Whatever the distance and whichever direction the tutor faces the fm system is providing consistency of sound – no DSP aid alone can match that

  5. An audio demonstration • Lets listen to a hearing aid alone recorded through a coupler .... … and then with an radio aid to improve the quality of sound • Play demo • Which sound would you prefer?

  6. Meanwhile we had better return to the plot?

  7. fmAdvantage • Motivated by the wish to ensure better fmAdvantage in classrooms • Motivated by the wish to maximise the benefits of fm use by utilising the way the DSP hearing aids work to obtain a beneficial s/n ratio • Lets consider WDRC aids Wide Dynamic Range Compression

  8. WDRC aids • How does a WDRC aid cope with the different sound levels from its own microphone and from an fm radio system?

  9. Design of DSP hearing aids H.A. MIC INPUT A/D DSP D/A REC. > > > F.M. INPUT FM signal enters before any DSP, in parallel with the microphone input The loudest input will drive the compressor

  10. DSP WDRC hearing aids … • Many have no volume control • The aid is programmed to automatically adjust it’s gain to present a comfortable sound range • Different gain is programmed for different frequency slices or bands • Different compression for different input levels

  11. Child’s voice & classroom sound Child’s voice & classroom sound Listening level Background Noise Background Noise DSP WDRC aid listening … Relative levels

  12. Child’s voice & classroom sound Child’s voice & classroom sound fm – teacher’s voice Listening level Background Noise Background Noise DSP WDRC aid listening … Relative levels

  13. Child’s voice & classroom sound fm – teacher’s voice Listening level Background Noise DSP WDRC aid listening … Relative levels

  14. Child’s voice & classroom sound fm – teacher’s voice Listening level Background Noise DSP WDRC aid listening … Relative levels

  15. Child’s voice & classroom sound fm – teacher’s voice Listening level Background Noise DSP WDRC aid listening … Relative levels

  16. Child’s voice & classroom sound Listening level Background Noise DSP WDRC aid listening … Relative levels

  17. Child’s voice & classroom sound Listening level Background Noise DSP WDRC aid listening … Relative levels

  18. Child’s voice & classroom sound Listening level Background Noise DSP WDRC aid listening … Relative levels

  19. Child’s voice & classroom sound Child’s voice & classroom sound Listening level Background Noise Background Noise DSP WDRC aid listening … Relative levels

  20. Child’s voice & classroom sound Child’s voice & classroom sound Listening level Background Noise Background Noise DSP WDRC aid listening … Relative levels Repeat demo

  21. The result • Better hearing aid use as a result of • Reduced background noise • Improved sound consistency • Improved clarity • Providing a good listening experience

  22. The future • The future will surely see TOD's with hipro boxes and NOAH software on their laptops • Is there no limit to their expertise?

  23. For the moment TOD’s will need good information from the clinic – how have the hearing aid programmes been set? • Also needed is a stetoclip tester with a variable attenuator to listen & confirm what happens in which situation • It is important to understand and demonstrate to a user how their hearing aid & fm system works in different situations

  24. Listening tests • Must be carried out in each different teaching situation • Noisy rooms … playground … quiet rooms … music rooms … lunchtime? • What hearing aid programme is intended for which situation?

  25. New test procedures • Who will it involve? • Teachers of the Deaf, Educational Audiologists, hospital Audiologists and sometimes clinicians • Are adult and paediatric requirements different? • Yes, children’s hearing aids require regular checks of their performance over time

  26. Historically ……….. • Historically radio aid systems were balanced for equal output • 65db and 75db inputs were used to balance the radio aid in a testbox • And then we turned the volume up a bit anyhow! • The fmAdvantage procedure should let you set it correctly

  27. Traditional equal output balancing approach (65/75) Linear hearing aid with fm system At the Child’s ear Teacher’s voice Child’s neighbour’s voice 110dB SPL fm and noise advantage lost 75dB SPL 65dB SPL 105dB SPL 60dB SPL 60dB SPL Classroom noise Classroom noise Can the child hear the teacher clearly? Teacher Classroom noise Child

  28. From now onwards • Move from ‘Equal output’ approach to ‘fm Advantage’ approach - we want the fm system to have an advantage • But as we do not want to overload linear hearing aids they will require a different protocol to non linear • But what is the fmAdvantage approach?

  29. What is the fmAdvantage approach for WDRC aids? • 65db input level for the hearing aid • 65db also for the fm system • By balancing with a 10dB lower level into the transmitter than traditionally the receiver output is set 10dB higher • The result is a 10dB+ fmAdvantage

  30. Two questions Q: Is this for all hearing aids? A: No – there is a new proposal for linear aids too Q: Has this been tried? A: Earlier wave paediatric sites and Connevans users have used the fm advantage since last year. PC Werth now advise the same fmAdvantage protocol

  31. Noise reduction aids • No DSP NR aid actually recognises speech • Their logic is … • Speech is not constant • Thus constant is not speech • Thus constant is noise and the aid will reduce the gain of that frequency slice

  32. When placed in a testbox • What happens is ….. • Hearing aid hears constant tone • Constant tone = noise • Hearing aid turns itself off • Oh dear!

  33. Hearing aid test tones • Puretone • Composite tone • Nowa new DSP composite tone • The DSP tone is best described as a ‘chirpy sound’ which randomly comes and goes before the aid can respond

  34. How may a DSP aid react? • The following video clip demonstrates how a DSP aid with noise reduction might react in a test box. • The first curve shows the aid response with a DSP composite test signal. • The second curve shows the aid turning itself down in the presence of a constant composite test signal.

  35. Next slide (click on picture to repeat)

  36. Issues • How about the American ASHA guidelines? We believe the fmAdvantage procedures to be better • Measuring distortion cannot be done with a DSP aid in a testbox • A realistic listening test is essential

  37. The fm Advantage protocols and procedures

  38. fm Advantage • Digital or analogue ? • Whether the hearing aid is digital or analogue is not important, the relevant factor is whether the hearing aid is operating in linear or non linear mode

  39. Linear • 'Linear' is when an SPL change at the input is equally reflected at the output. i.e. a 5dB change at the input gives a matching 5dB change at the output. • Hearing aids with either output limiting compression, linear peak clipping or soft peak clipping are regarded as being linear for the purposes of these procedures.

  40. Linear test levels • 65dB SPL for the hearing aid • 70dB SPL for the fm radio system hopefully less people will then need to turn up the volume! 70dB is chosen to minimise the risk of distortion in a linear aid • 80dB SPL – the reality part of the test, will we get an fm advantage?

  41. Non linear • 'Non linear' is when an SPL change at the input is not reflected at the output. i.e. 9dB change of input does not give a 9dB change of output • Hearing aids with WDRC or full range compression are 'Non Linear'

  42. Non linear test levels • 65dB SPL for the hearing aid • 65dB SPL for the fm radio system • 80dB SPL – the reality part of the test, will we get an fm advantage?

  43. How many fmAdvantage procedures? • 4 in all • 2 for hearing aid direct input • 2 for hearing aid telecoil input

  44. Direct input • 1 - Linear analogue aids and digital aids operating in linear mode • 2 - Non linear analogue aids and digital aids operating in non linear mode

  45. ‘T’ Telecoil input • 3 - Linear analogue aids and digital aids operating in linear mode using telecoil ‘T’ pickup for use with an inductive neck loop • 4 - Non linear analogue aids and digital aids operating in non linear mode using telecoil ‘T’ pickup for use with an inductive neck loop

  46. The fm Advantage procedures … A practical demonstration {jump to test steps}

  47. Please do not panic! • We are not doing all 4 today • Please refer to the printed copy for all the procedures …. or …. • The fm Advantage procedures are on the Connevans website • The fm Advantage procedures are also on the Connevans CD • You can also register for automatic future email updates fmadvantage@connevans.com

  48. Today’s demonstration • Procedure 2 - Non linear analogue aids and digital aids operating in non linear mode Using a WDRC (digital) aid and a DSP stimulus

  49. Objective To ensure that the sound from the radio system transmitter has an advantage over the general room noise picked up by the hearing aid and that the overall system functions as intended

  50. The practical advice bit before you start ……… • Understand the different programmes programmed on a particular hearing aid • Ask which hearing aid programme is intended for fm use? • Carry out realistic listening tests – not just in a quiet room