loop systems – t-coil

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Loop systems – T-coilConny Andersson

IEC 60118-4

Conny.Andersson@edin.se

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Loop System – Wireless system

Receiver = T-coil in Hearing Instrument (HI)No current consumption

Transmitter = Loop Amplifier and Loop

World Wide Standard

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Input

Acoustic and T-coil inputAcousticOutput

switch

MT

HearingInstrument

Loop Level:Fixed @ 400mA/mfor all signals

Speech level=70dB@1m distance

130dB

Gain +60dB

Coil Sensitivity130dB

Same output level for M and T-position

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Relative T-coil sensitivitySwitching between M and T will give

approximately the same output level

It could be stated differently:0dB difference between M and T-position

Related to acoustic level: The acoustic output is the same for T and M-position with a 70dB acoustical input signal

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International Standards, IEC

Hearing Instrument (HI) Many parameters are fitted for the

individual. Can not be standardized

Hearing Loop system All parameters are standardized

InternationalElectrotechnicalCommission

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ANSI S3.22-2009American National Standard Institute has

many abbreviations for T-coil functions3.4 Terms relating to measurements with inductive inputs3.4.1 high frequency average SPL for an inductive telephone simulator (HFA-SPLITS). The high frequencyaverage of the SPLITS levels (see Clause 3.4.5). The abbreviation for this term is HFASPLITS.3.4.2 high frequency average SPL in a vertical magnetic field (HFA-SPLIV). The high-frequencyaverage of the SPLIV levels (see Clause 3.4.6). The abbreviation for this term is HFA-SPLIV.3.4.3 relative simulated equivalent telephone sensitivity (RSETS). Difference in decibels obtainedby subtracting the RTG + 60 dB SPL from the HFA-SPLITS. The abbreviation for this term is RSETS.3.4.4 relative test loop sensitivity (RTLS). Difference in decibels obtained by subtracting the RTG +60 dB SPL from the HFA-SPLIV. The abbreviation for this term is RTLS.3.4.5 sound pressure level for an inductive telephone simulator (SPLITS). Sound pressure leveldeveloped in the coupler by a hearing aid with the gain control at the RTS when the input is themagnetic field generated by a Telephone Magnetic Field Simulator (refer to Clause 3.2.5). Theabbreviation for this term is SPLITS.3.4.6 sound pressure level in a vertical magnetic field (SPLIV). Sound pressure level developed inthe coupler by a hearing aid with the gain control at the RTS when the input is a 31.6 mA/m sinusoidalalternating magnetic field parallel to the vertical reference and the input selector of the hearing aid setto T-position. The abbreviation for this term is SPLIV.

Good News, ANSI S3.22 committee has decided to make T-coil specification mandatory for quality test inspection

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Standardized Parameters

Magnetic Field Strength, 400mA/m

Coverage area/space (sitting and standing),Should have minimum variations (+-3dB)

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Field distribution – level/coverage

Variation less than +-3dB

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Standardized Parameters (hum)

Magnetic field strength from electrical appliances

Received directly by the T-coilNOT by the hearing loop system

Less problematic due to stronger regulations of electrical environment

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Hearing Instr. frequency response

Mic

T-coil

Hearing instrument, microphone/telecoil response

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Often reduced low freq. T-coil

Mic

Tele-coil

Reduced low frequency sensitivity

25dB

Hearing instrument, microphone/telecoil response

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HI has large variation in freq resp

Mic

Tele-coil

Hearing intrument, microphone/telecoil response Hearing instrument, microphone/telecoil response

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Loop frequency response is flat

Mic

Tele-coil

Hearing instrument, microphone/telecoil response

Within +-3dB +-3dB

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Loop specifications is regulated

It is therefore impossible to judge the full quality of a loop system by using a Hearing Instrument

Measurements are mandatory

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Vertical Field Strength

The specifications for most of the parameters for loops are for vertical directions

This corresponds to HI manufacturers placing the T-coil in vertical position

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-3dB (@45°)-3dB (@45°)

Conventional loops - tilt angel

0dB=full level

Silence!

Vertical field

Silence!

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Loops/T-coil with TWO directions

full levelfull level

Vertical AND Horizontal Field0dB=full level

-3dB (@45°)-3dB (@45°)

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Conclusion and a `T-coil wish`Increased low frequency responseHum harmonics removerDefault sensitivity should follow the standard

of 400mA/m (125ms, RMS) If the sensitivity is set higher in the Hearing

Instrument there is a strong risk for high distortion, clipping/saturation when entering a correct installed Loop System.A weak loop must not be compensated by increasing the T-coil sensitivity with a decreased Signal to Noise ratio.

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Conclusion and a `T-coil wish`Several channels and modulated/coded

loop system.

One “standard loop system for world wide use in theatres etc

Other channels for school and domestic use. Decreased Signal to Noise ratio.

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Thank you for your attention!

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Thank you for your attention!

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Input

Acoustic and T-coil inputAcousticOutput

switch

MT

HearingInstrument

Loop Level:Fixed @ 400mA/mfor all signals

Speech level=70dB@1m distance

130dB

Gain +60dB

Coil Sensitivity130dB

Same output level for M and T-position

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Input

Acoustic and T-coil inputAcousticOutput

switch

MT

HearingInstrument

Loop Level:Fixed @ 100mA/mfor all signals

Speech level=70dB@1m distance

130dB

Gain +60dB

Coil Sensitivity130dB

Same output level for M and T-position

Long-term signals:

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Input

Acoustic and T-coil inputAcousticOutput

switch

MT

HearingInstrument

Loop Level:Fixed @ 31.6 mA/mfor all signals

Speech level=60dB@1m distance

120dB

Gain +60dB

Coil Sensitivity120dB

Same output level for M and T-position

Long-term signals: