Hearing aids -- Part 2: Hearing aids with automatic gain control circuits

Gives uniform methods for specifying dynamic and static performance characteristics of hearing aids with automatic gain control circuits together with the relevant methods of measurement for these characteristics.

Hörgeräte -- Teil 2: Hörgeräte mit automatischer Verstärkungsregelung

Appareils de correction auditive -- Partie 2: Appareils de correction auditive comportant des commandes automatiques de gain

Donne des méthodes homogènes pour spécifier, aussi bien en régime transitoire qu'en régime permanent, les caractéristiques des appareils de correction auditive comportant des circuits de commande automatique de gain, ainsi que les méthodes de mesure de ces caractéristiques.

Hearing aids - Part 2: Hearing aids with automatic gain control circuits (IEC 60118-2:1983 + A1:1993)

General Information

Status
Published
Publication Date
30-Jun-2004
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Jul-2004
Due Date
01-Jul-2004
Completion Date
01-Jul-2004

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SLOVENSKI SIST EN 60118-2:2004
STANDARD
julij 2004

Hearing aids - Part 2: Hearing aids with automatic gain control circuits (IEC 60118-

2:1983 + A1:1993)
ICS 11.180.15 Referenčna številka
SIST EN 60118-2:2004(en)

© Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

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NORME CEI
IEC
INTERNATIONALE
118-2
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
1983-01
Appareils de correction auditive
Deuxième partie:
Appareils de correction auditive comportant
des commandes automatiques de gain
Hearing aids
Part 2:
Hearing aids with automatic gain control circuits
© IEC 1983 Droits de reproduction réservés — Copyright - all rights reserved

Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in

utilisée sous quelque forme que ce soit et par aucun any form or by any means, electronic or mechanical,

including photocopying and microfilm, without permission in
procédé, électronique ou mécanique, y compris la photo-

copie et les microfilms, sans l'accord écrit de l'éditeur. writing from the publisher.

International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http: //www.iec.ch
CODE PRIX
Commission Electrotechnique Internationale
PRICE CODE
International Electrotechnical Commission
IEC McKinyHapo.nHaa 3neKrporexHHVecnaa HoMHCCHa
vigueur
Pour prix, voir catalogue en
• •
For price, see current catalogue
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— 3 —
118-2 © IEC 1983
CONTENTS
Page
FOREWORD 5
PREFACE 5
Clause
1. Scope 7
Object 7
3. Conditions 9
4. Explanation of terms
4.1 Automatic gain control (AGC)
4.2 Steady-state input/output graph
4.3 Lower AGC limit or AGC threshold
9
4.4 Compression ratio (between specified input sound pressure level values)
4.5 Dynamic output characteristics
4.6 Attack time
4.7 Recovery time
Steady-state input/output graph 5.

5.1 Graph showing the relation between input sound pressure level and output sound pressure

level
5.2 Methods of measurement
Dynamic output characteristics 6.
Characteristics to be measured 6.1
6.2 Methods of measurement
7. Non-linear distortion
7.1 Transients
7.2 Harmonic distortion
15
7.3 Intermodulation distortion

Effect on steady-state and dynamic performance with respect to variation in battery or supply

voltage
FIGURES
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118-2 © IEC 1983 — 5 —
INTERNATIONAL ELECTROTECHNICAL COMMISSION
HEARING AIDS
Part 2: Hearing aids with automatic gain control circuits
FOREWORD

1) The formal decisions or agreements of the IEC on technical matters, prepared by Technical Committees on which all the

National Committees having a special interest therein are represented, express, as nearly as possible, an international

consensus of opinion on the subjects dealt with.

They have the form of recommendations for international use and they are accepted by the National Committees in that

sense.

3) In order to promote international unification, the IEC expresses the wish that all National Committees should adopt the text

of the IEC recommendation for their national rules in so far as national conditions will permit. Any divergence between the

IEC recommendation and the corresponding national rules should, as far as possible, be clearly indicated in the latter.

PREFACE TO THE FIRST EDITION

This standard has been prepared by IEC Technical Committee No.29: Electroacoustics.

Work was started at the meeting held in Moscow in 1974.

The first draft was discussed at the meeting held in Gaithersburg in 1976. As a result of this meeting, a draft, Document

ix Months' Rule in September 1976.

29(Central Office)107, was submitted to the National Committees for approval under the S

The following countries voted explicitly in favour of publication:
Spain
Australia Germany
Belgium Italy Sweden
Switzerland
Canada Japan
Netherlands Turkey
Czechoslovakia
Denmark Norway United Kingdom
United States of America
Egypt Romania
South Africa (Republic of)
France
PREFACE TO THE SECOND EDITION

This second edition comprises the first edition, issued in 1979, and editorial amendments which are due to the issuing of IEC

Publication 118-0 (1983).
Other IEC publications quoted in this standard:

Publications Nos. 118-0: Hearing Aids, Part 0: Measurement of Electroacoustical Characteristics.

268-8: Sound System Equipment, Part 8: Automatic Gain Control Devices.
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118-2 © IEC 1983 7
HEARING AIDS
Part 2: Hearing aids with automatic gain control circuits
1. Scope

1.1 This standard applies to the hearing aids of any type with automatic gain control (AGC) circuits.

This standard gives uniform, methods for specifying dynamic and static performance characteristics of

hearing aids with AGC circuits together with the relevant methods of measurement for these charac-

teristics.

This standard is confined to a description of the different characteristics and the relevant methods of

measurement. It does not attempt to specify performance requirements.

1.2 This standard includes devices which have compression and/or limiting properties with respect to the

envelope of the input signal. Devices which control the long-term average output level are also

included.

a) AGC is employed to obtain compression, or the reduction of the dynamic range of the sound at the

output, with the object of preserving the integrity of the input waveform.

b) AGC circuits instead of clipping devices are often used for limiting purposes.

A limiting effect occurs when the input/output characteristic flattens out at higher input levels.

Limiting action is mainly used as a means of preventing excessive output sound from the hearing aid

from reaching the listener's ear.
1.3 This standard does not include:
a) Expanders.

Clipping devices, which cut off the signal peaks above a certain level; such devices differ basically

from AGC circuits, which, in a steady state, tend to preserve the waveform of the input signal.

Note. – An AGC circuit with very short recovery time may cause considerable distortion, especially in the low-frequency

range. This should be given special attention.
2. Object

2.1 The purpose of this standard is to facilitate measurements of certain characteristics of hearing aids with

AGC circuits that are not described elsewhere in IEC Publication 118-0: Hearing Aids, Part 0:

Measurement of Electroacoustical Characteristics, and which are considered necessary for a physical

description of the function of the automatic gain control.
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118-2 © IEC 1983 — 9 —

2.2 In general, the methods of measurement recommended are those which are considered to be the most

directly related to the characteristics. This does not exclude the use of other stated methods which will

give equivalent results.
3. Conditions
3.1 General conditions

Reference is made to I E C Publication 268-8: Sound System Equipment, Part 8: Automatic Gain

Control Devices.

Measurements other than those described herein and that are stated in I E C Publication 118-0 can

be performed in accordance with that publication, but with AGC operating, provided the operating

conditions are stated.

3.2 Although a pure-tone input signal of 1600 Hz or 2500 Hz when appropriate, is specified for various

measurements throughout this standard, it is intended that pure-tone signals of other frequencies or

signals of other spectral compositions may be used in addition, where they would provide important

information.
3.3 Throughout this standard, all sound pressure levels are referred to 20 µPa.
4. Explanation of terms
4.1 Automatic gain control (AGC)

A means in a hearing aid by which the gain is automatically controlled as a function of the magnitude

of the envelope of the input signal or other signal parameter.

Note.— Throughout this standard, reference is made to the use of acoustic inputs. However, where appropriate, additional

measurements may be made with an electromagnetically induced input.
4.2 Steady-state input/output graph

The graph illustrating the output sound pressure level as a function of the input sound pressure level

for a specified frequency, both expressed in decibels on identical linear scales (Figure 1, page 16).

4.3 Lower AGC limit or AGC threshold

The input sound pressure level which, when applied to the hearing aid, gives a reduction in the gain

of 2 ± 0.5 dB with respect to the gain in the linear mode (Figure 1).
4.4 Compression ratio (between specified input sound pressure level values)

Under steady-state conditions, the ratio of an input sound pressure level difference to the corre-

sponding output sound pressure level difference, both expressed in decibels (Figure 1).

4.5 Dynamic output characteristics

The output sound pressure envelope shown as a function of time when an input sound signal of a

predetermined frequency and level is modulated by a square envelope pulse with a predetermined

pulse amplitude (Figure 2, page 17).
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118-2 © IEC 1983
- 11 —
4.6 Attack time

The time interval between the moment when the input signal level is increased abruptly by a stated

number of decibels and the moment when the output sound pressure level from the hearing aid with the

dB (Figure 2, page 17).
AGC circuit stabilizes at the elevated steady-state level within
4.6.1
Attack time for the normal dynamic range of speech

The attack time, as defined in Sub-clause 4.6, when the initial input sound pressure level is 55 dB and

the increase in input sound pressure level is 25 dB.
4.6.2 High level attack time

The attack time, as defined in Sub-clause 4.6, when the initial input sound pressure level is 60 dB and

the increase in input sound pressure level is 40 dB.
4.7 Recovery time

The time interval between the moment when the stated input signal level is reduced abruptly to a

level a stated number of decibels lower after the AGC amplifier has reached the steady-state output

under elevated input signal conditions, and the moment when the output sound pressure level from the

hearing aid stabilizes again at the lower steady-state level within dB (Figure 2).

4.7.1 Recovery time for the normal dynamic range of speech

The recovery time, as defined in Sub-clause 4.7, when the initial input sound pressure level is 80 dB

and the decrease in input sound pressure level is 25 dB.
4.7.2 High level recovery time

The recovery time, as defined in Sub-clause 4.7, when the initial sound pressure level is 100 dB and

the decrease in input sound pressure level is 40 dB.
5. Steady-state input/output graph

5.1 Graph showing the relation between input sound pressure level and output sound pressure level

The graph shall have the input sound pressure level as abscissa and the output sound pressure level as

ordinate, both expressed in decibels on linear scales having divisions of identical size.

Note.— In the input/output graph of an AGC device, different portions may be distinguished:

— Below the lower AGC limit the slope is essentially 45° (linear amplifier mode).

— Above this limit, the graph curves over in a portion having a decreasing slope, often followed by another portion

having a nearly flat slope (AGC mode).

— At very high input levels, a flat or sloping portion may be followed by a portion with a steeper slope, generally

due to saturation of the AGC circuit.
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1983 —
118-2 © IEC 13 —
5.2 Methods of measurement

The gain control is adjusted to its maximum setting. Any adjustable gain control after the AGC-loop

shall be adjusted in such a manner that overload of the hearing aid is avoided.

An input sound signal of frequency 1600 Hz or 2500 Hz when appropriate, is applied at the lowest

possible level consistent with an adequate signal-to-noise ratio of preferably more than 10 dB. The

input sound pressure level is increased up to 100 dB in sufficiently small steps, and the corresponding

output sound pressure level is measured after steady-state conditions have been reached. The graph is

plotted with the input sound pressure level as abscissa and the output level as ordinate, as described in

Sub-clause 5.1.

Where separate adjustable controls exist, such as AGC, gain or output controls, which will influence

the shape and other characteristics of the steady-state input/output graph, it is recommended that

input/output graphs be plotted, when useful, for various additional stated setting of such controls.

6. Dynamic output characteristics
6.1 Characteristics to be measured

The purpose of this test is to determine the dynamic characteristics of the AGC circuit, particularly

attack and recovery times. It should be emphasized that all these characteristics will depend on test

frequency as well as on such factors as signal level, control settings and battery voltage.

6.2 Methods of measurement
6.2.1 Dynamic output characteristics for speech levels

At the maximum setting of the gain control an input signal of 1600 Hz or 2500 Hz when appropriate

with a sound pressure level of 55 dB is applied. Any adjustable gain control after the AGC loop shall be

adjusted in such a manner that overload of the hearing aid is avoided.

This signal is modulated by a square envelope pulse raising the input level by 25 dB. The pulse length

shall be at least five times longer
...

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