《AS 60118.2-2007 Hearing aids Part 2 Hearing aids with automatic gain control circuits.pdf.pdf》由会员分享，可在线阅读，更多相关《AS 60118.2-2007 Hearing aids Part 2 Hearing aids with automatic gain control circuits.pdf.pdf（29页珍藏版）》请在三一文库上搜索。
1、 AS 60118.22007 IEC 60118-2, Ed. 2.0 (1983) Includes Amendment 1:1993 and Amendment 2:1997 Australian Standard Hearing aids Part 2: Hearing aids with automatic gain control circuits AS 60118.22007 This Australian Standard was prepared by Committee AV-003, AcousticsHuman Effects. It was approved on b
2、ehalf of the Council of Standards Australia on 3 November 2006. This Standard was published on 20 February 2007. The following are represented on Committee AV-003: Association of Accredited Certification Bodies Association of Australian Acoustical Consultants Association of Consulting Engineers Aust
3、ralia Audiological Society of Australia Australasian Faculty of Occupational Medicine Australian Acoustical Society Australian Chamber of Commerce and Industry Australian Council of Trade Unions Australian Industry Group Department of Consumer such devices differ basically from AGC circuits, which,
4、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 meas
5、urements of certain characteristics of hearing aids with AGC circuits that are not described elsewhere in IEC 60118-0: 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.
6、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. Standards Australia www.standards.org.au 2 3 Conditions 3.1 General
7、conditions Reference is made to IEC 60268-8: Sound system equipment, Part 8: Automatic gain control devices. Measurements other than those described herein and that are stated in IEC 60118-0 can be performed in accordance with that publication, but with AGC operating, provided the operating conditio
8、ns are stated. 3.2 Throughout this standard, all sound pressure levels are referred to 20 Pa. 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 sig
9、nal 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
10、function of the input sound pressure level for a specified frequency, both expressed in decibels on identical linear scales (Figure 1). 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 t
11、o 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 corresponding output sound pressure level difference, both expressed in decibels (Figure 1
12、). 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). 4.6 Attack time The time interval between th
13、e 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 AGC circuit stabilizes at the elevated steady-state level within 2 dB (Figure 2). 4.6.1 Attack time for the normal dynamic range
14、 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. www.standards.org.au Standards Australia 3 4.6.2 High level attack time The attack time, as defined in sub-clause 4.6, when the init
15、ial 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-
16、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 2 dB (Figure 2). 4.7.1 Recovery time for the normal dynamic range of speech The recovery time, as defined in Sub-claus
17、e 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
18、. 4.8 AGC activated frequency response The frequency response when the AGC circuit is activated by a specified AGC activating signal. 4.9 Operating frequency range of the AGC For a specified input sound pressure level, the frequency range in which the AGC threshold is reached or exceeded. 4.10 Overa
19、ll root-mean-square sound pressure level (overall r.m.s. SPL) The root-mean-square sound pressure level with measurement bandwidth equal to the frequency range covered by the one-third-octave frequency bands (see IEC 61260) from 200 Hz to 8 000 Hz. 4.11 One-third-octave band level The level of that
20、part of the signal contained within a band one-third-octave wide as defined in IEC 61260. 4.12 Auto-spectrum (power spectrum) The power spectrum of either the input signal (GAA) to or the output signal (GBB) from a hearing aid in the frequency domain. It is computed by multiplying the Fourier transf
21、orm of the signal by the complex conjugate of the Fourier transform of the same signal. 4.13 Cross-spectrum (GAB) The degree to which the same signal frequencies are mutually present in the input and output of a hearing aid. It is computed by multiplying the complex conjugate of the Fourier transfor
22、m of the input signal to the hearing aid by the Fourier transform of the output signal from the hearing aid. Standards Australia www.standards.org.au 4 4.14 Coherence A number ranging from 0 to 1 showing to what degree the output from a hearing aid is correlated to the input. Coherence for a random
23、noise test signal is reduced by non-linearity and by system noise. The coherence is calculated from the auto- and cross-spectrum averages as follows: Coherence = G GG AB 2 AABB 4.15 Synchronous analysis Analysis which is synchronized with the period of the input signal, for example with the periodic
24、ity of pseudo-random noise. 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
25、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
26、 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. 5.2 Methods of measurement The gain control is adjusted to its ma
27、ximum 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 ra
28、tio 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
29、 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, w
30、hen 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 th
31、ese characteristics will depend on test frequency as well as on such factors as signal level, control settings and battery voltage. www.standards.org.au Standards Australia 5 6.2 Methods of measurement 6.2.1 Dynamic output characteristics for speech levels At the maximum setting of the gain control
32、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
33、 level by 25 dB. The pulse length shall be at least five times longer than the attack time being measured. If more than a single pulse is applied, the interval between two pulses shall be at least five times the longest recovery time being measured. Note 1 - This test may be carried out at various c
34、ontrol settings as stated in Sub-clause 5.2. Note 2 - If lower gain control settings are applied, the method of obtaining these settings shall be clearly specified. Note 3 - The loudspeaker employed for the measurement of dynamic output characteristics as in Clause 6 must be sufficiently free of tra
35、nsient distortion so that test results are not appreciably affected. Note 4 - The output signal should be monitored on a device such as an oscilloscope, the time constants of which are considerably shorter than those being measured. Note 5 - When very short response times are to be measured, the res
36、ponse time of the source shall be reported. Note 6 - For half-wave rectifying AGC circuits, the attack time is dependent upon the polarity of the first half wave of the test signal after the onset of the modulating square wave envelope. Depending upon the polarity, a shorter or longer attack time wi
37、ll occur. This is best demonstrated in the case of an instantaneous rise in the amplitude occurring at a zero crossing of the test signal. 6.2.2 Dynamic output characteristics for high level input At the maximum setting of the gain control an input signal of 1600 Hz or 2500 Hz when appropriate with
38、a sound pressure level of 60 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 40 dB. The pulse length shall be at least five tim
39、es the attack time observed. If more than a single pulse is applied, the interval between two pulses should be at least five times the longest recovery time being measured. Note - See Notes 1 to 6 of sub-clause 6.2.1. 7 Non-linear distortion 7.1 Transients The signal may be distorted during the atta
40、ck time and recovery time by transients as well as by unwanted low-frequency modulation caused by instabilities. The effect of these phenomena on the listener is not sufficiently understood to allow a recommendation for measuring transient distortions to be made. 7.2 Harmonic distortion 7.2.1 Charac
41、teristics to be specified The purpose of this test is to determine the harmonic distortion as a function of the input sound pressure level after steady-state conditions have been reached. 7.2.2 Methods of measurement Harmonic distortion is measured in accordance with the test procedure described in
42、IEC 60118-0. Note - This test may be carried out at various control settings as mentioned under sub-clause 5.2. Standards Australia www.standards.org.au 6 7.3 Intermodulation distortion Intermodulation distortion is measured in accordance with the test procedure, described in IEC 60118-0. 8 Effect o
43、n steady-state and dynamic performance with respect to variation in battery or supply voltage In accordance with IEC 60118-0, sub-clause 7.8, it is recommended that the change in the following performance parameters be tested with respect to variation in battery or supply voltage: steady-state input
44、/output graphs as mentioned in Clause 5, dynamic-output characteristics, attack and recovery times as mentioned under Clause 6, and non-linear distortion as mentioned in Clause 7. 9 AGC activated frequency response of hearing aids with a single channel AGC circuit in operation using pure tone signal
45、s 9.1 Introduction 9.1.1 Measurement below the AGC threshold It is important to know the frequency response of hearing aids when the automatic gain control is active. Measurements carried out in accordance with 7.4 of IEC 60118-0 can be employed, provided that the input signal levels at all measurin
46、g frequencies are below the AGC thresholds. 9.1.2 Measurement above the AGC threshold When measurements are taken above the AGC threshold with moderately slow scanning speeds and the attack and decay times in current use, they can be considered at any frequency as steady-state measurements. From the
47、 set of comprehensive frequency response curves obtained using this method of measurement, a steady-state input/output graph can be constructed for any frequency. 9.2 General conditions Throughout this standard all sound pressure levels specified refer to 20 Pa. When appropriate, sound pressure leve
48、l will be abbreviated SPL. NOTE - Throughout this standard, reference is made to the use of acoustic input. However, when appropriate, additional measurements may be made with electromagnetic or electric inputs in accordance with IEC 60118-1 and IEC 60118-6. Test results obtained by the substitution
49、 method (see 4.2 of IEC 60118-0) shall be considered basic. 9.3 Test equipment The test equipment shall comply with clause 5 of IEC 60118-0. Figures 3 and 4 give schematic illustrations of the measuring equipment. A tracking bandpass filter, centred at a centre frequency fc, shall be inserted in the measuring system. The 3 dB bandwidth shall not be greater than 10 % of the centre frequency, and the 20 dB bandwidth shall not be greater than 20 % of the centre frequency. For f