Sec.  68.316   Hearing aid compatibility: Technical requirements.

   A telephone handset is hearing aid compatible for the purposes of this
   section  if  it complies with the following standard, published by the
   Telecommunications Industry Association, copyright 1983, and reproduced by
   permission of the Telecommunications Industry Association:

   Electronic Industries Association Recommended Standard RS–504 Magnetic Field
   Intensity Criteria for Telephone Compatibility With Hearing Aids

   [ Prepared by EIA Engineering Committee TR–41 and the Hearing Industries
   Association's Standards and Technical Committee ]

   Table of Contents

   List of Illustrations

   1  INTRODUCTION

   2  SCOPE

   3  DEFINITIONS

   4  TECHNICAL REQUIREMENTS

   4.1  General

   4.2  Axial Field Intensity

   4.3  Radial Field Intensity

   4.4  Induced Voltage Frequency Response

   Appendix A—Bibliography

   List of Illustrations

   Figure Number

   1  Reference and Measurement Planes and Axes

   2  Measurement Block Diagram

   3  Probe Coil Parameters

   4A  Induced Voltage Frequency Response for receivers with an axial field
   that exceeds −19 dB

   4B  Induced Voltage Frequency Response for receivers with an axial field
   that exceeds −22 dB but is less than −19 dB

   Magnetic Field Intensity Criteria for Telephone Compatibility With Hearing
   Aids

   (From EIA Standards Proposal No. 1652, formulated under the cognizance of
   EIA TR–41 Committee on Voice Telephone Terminals and the Hearing Industries
   Association's Standards and Technical Committee.)

   1  Introduction

   Hearing-aid users have used magnetic coupling to enable them to participate
   in  telephone  communications  since  the 1940's. Magnetic pick-ups in
   hearing-aids have provided for coupling to many, but not all, types of
   telephone handsets. A major reason for incompatibility has been the lack of
   handset magnetic field intensity requirements. Typically, whatever field
   existed had been provided fortuitously rather than by design. More recently,
   special handset designs, e.g., blue grommet handsets associated with public
   telephones, have been introduced to provide hearing-aid coupling and trials
   were conducted to demonstrate the acceptability of such designs. It is
   anticipated that there will be an increase in the number of new handset
   designs in the future. A standard definition of the magnetic field intensity
   emanating from telephone handsets intended to provide hearing-aid coupling
   is needed so that hearing-aid manufacturers can design their product to use
   this field, which will be guaranteed in handsets which comply with this
   standard.

   1.1  This  standard is one of a series of technical standards on voice
   telephone terminal equipment prepared by EIA Engineering Committee TR–41.
   This document, with its companion standards on Private Branch Exchanges
   (PBX), Key Telephone Systems (KTS), Telephones and Environmental and Safety
   Considerations (Refs: A1, A2, A3 and A4) fills a recognized need in the
   telephone  industry  brought about by the increasing use in the public
   telephone network of equipment supplied by numerous manufacturers. It will
   be  useful  to anyone engaged in the manufacture of telephone terminal
   equipment and hearing-aids and to those purchasing, operating or using such
   equipment or devices.

   1.2 This standard is intended to be a living document, subject to revision
   and updating as warranted by advances in network and terminal equipment
   technology and changes in the FCC Rules and Regulations.

   2  Scope

   2.1  The purpose of this document is to establish formal criteria defining
   the magnetic field intensity presented by a telephone to which hearing aids
   can couple. The requirements are based on present telecommunications plant
   characteristics at the telephone interface. The telephone will also be
   subject to the applicable requirements of EIA RS–470, Telephone Instruments
   with  Loop  Signaling  for  Voiceband  Applications  (Ref: A3) and the
   environmental requirements specified in EIA Standards Project PN–1361,
   Environmental and Safety Considerations for Voice Telephone Terminals, when
   published (Ref: A4).

   Telephones which meet these requirements should ensure satisfactory service
   to  users of magnetically coupled hearing-aids in a high percentage of
   installations, both initially and over some period of time, as the network
   grows and changes occur in telephone serving equipment. However, due to the
   wide  range  of customer apparatus and loop plant and dependent on the
   environment in which the telephone and hearing aid are used, conformance
   with this standard does not guarantee acceptable performance or interface
   compatibility under all possible operating conditions.

   2.2  A telephone complies with this standard if it meets the requirements in
   this standard when manufactured and can be expected to continue to meet
   these requirements when properly used and maintained. For satisfactory
   service a telephone needs to be capable, through the proper selection of
   equipment  options,  of  satisfying the requirements applicable to its
   marketing area.

   2.3  The standard is intended to be in conformance with part 68 of the FCC
   Rules and Regulations, but it is not limited to the scope of those rules
   (Ref: A5).

   2.4  The signal level and method of measurement in this standard have been
   chosen to ensure reproducible results and permit comparison of evaluations.
   The measured magnetic field intensity will be approximately 15 dB above the
   average level encountered in the field and the measured high-end frequency
   response will be greater than that encountered in the field.

   2.5  The  basic  accuracy  and reproducibility of measurements made in
   accordance with this standard will depend primarily upon the accuracy of the
   test equipment used, the care with which the measurements are conducted, and
   the inherent stability of the devices under test.

   3  Definitions

   This section contains definitions of terms needed for proper understanding
   and application of this standard which are not believed to be adequately
   treated  elsewhere. A glossary of telephone terminology, which will be
   published as a companion volume to the series of technical standards on
   Telephone Terminals For Voiceband Applications, is recommended as a general
   reference and for definitions not covered in this section.

   3.1  A telephone is a terminal instrument which permits two-way, real-time
   voice communication with a distant party over a network or customer premises
   connection. It converts real-time voice and voiceband acoustic signals into
   electrical signals suitable for transmission over the telephone network and
   converts received electrical signals into acoustic signals. A telephone
   which meets the requirements of this standard also generates a magnetic
   field to which hearing-aids may couple.

   3.2  The telephone boundaries are the electrical interface with the network,
   PBX or KTS and the acoustic, magnetic and mechanical interfaces with the
   user. The telephone may also have an electrical interface with commercial
   power.

   3.3  A hearing aid is a personal electronic amplifying device, intended to
   increase the loudness of sound and worn to compensate for impaired hearing.
   When equipped with an optional inductive pick-up coil (commonly called a
   telecoil), a hearing aid can be used to amplify magnetic fields such as
   those from telephone receivers or induction-loop systems.

   3.4  The  reference  plane is the planar area containing points of the
   receiver-end of the handset which, in normal handset use, rest against the
   ear (see Fig 1).

   3.5  The  measurement plane is parallel to, and 10 mm in front of, the
   reference plane (see Fig 1).

   3.6  The reference axis is normal to the reference plane and passes through
   the center of the receiver cap (or the center of the hole array, for handset
   types that do not have receiver caps).

   3.7  The measurement axis is parallel to the reference axis but may be
   displaced from that axis, by a maximum of 10 mm (see Fig 1). Within this
   constraint, the measurement axis may be located where the axial and radial
   field intensity measurements, are optimum with regard to the requirements.
   In a handset with a centered receiver and a circularly symmetrical magnetic
   field, the measurement axis and the reference axis would coincide.
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   4  Technical Requirements

   4.1   General .

   These criteria apply to handsets when tested as a constituent part of a
   telephone.

   4.1.1  Three parameters descriptive of the magnetic field at points in the
   measurement plane shall be used to ascertain adequacy for magnetic coupling.
   These three parameters are intensity, direction and frequency response,
   associated with the field vector.

   4.1.2  The procedures for determining the parameter values are defined in
   the IEEE Standard Method For Measuring The Magnetic Field Intensity Around A
   Telephone Receiver (Ref: A6), with the exception that this EIA Recommended
   Standard does not require that the measurements be made using an equivalent
   loop of 2.75 km of No. 26 AWG cable, but uses a 1250–ohm resistor in series
   with the battery feed instead (see Fig 2).

   4.1.3  When testing other than general purpose analog telephones, e.g.,
   proprietary  or  digital  telephones,  an appropriate feed circuit and
   termination shall be used that produces equivalent test conditions.

   4.2   Axial Field Intensity .

   When measured as specified in 4.1.2, the axial component of the magnetic
   field directed along the measurement axis and located at the measurement
   plane, shall be greater than −22 dB relative to 1 A/m, for an input of −10
   dBV at 1000 Hz (see Fig 2).

   Note: If the magnitude of the axial component exceeds −19 dB relative to 1
   A/m, some relaxation in the frequency response is permitted (See 4.4.1).

   4.3   Radial Field Intensity .

   When measured as specified in 4.1.2, radial components of the magnetic field
   as measured at four points 90 ° apart, and at a distance ≥16 mm from the
   measurement axis (as selected in 4.2), shall be greater than −27 dB relative
   to 1 A/m, for an input of −10 dBV at 1000 Hz (see Fig 2).

   4.4   Induced Voltage Frequency Response .

   The frequency response of the voltage induced in the probe coil by the axial
   component of the magnetic field as measured in 4.2, shall fall within the
   acceptable  region of Fig 4A or Fig 4B (see 4.4.1 and 4.4.2), over the
   frequency range 300-to-3300 Hz.

   4.4.1  For receivers with an axial component which exceeds −19 dB relative
   to 1 A/m, when measured as specified in 4.1.2, the frequency response shall
   fall within the acceptable region of Fig 4A.

   4.4.2  For receivers with an axial component which is less than −19 dB but
   greater than −22 dB relative to 1 A/m, when measured as specified in 4.1.2,
   the frequency response shall fall within the acceptable region of Fig 4B.
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   Appendix A—Bibliography

   (A1) EIA Standard RS–464, Private Branch Exchange (PBX) Switching Equipment
   for Voiceband Applications.

   (A2)  EIA  Standard RS–478, Multi-Line Key Telephone Systems (KTS) for
   Voiceband Applications.

   (A3) EIA Standard RS–470, Telephone Instruments with Loop Signaling for
   Voiceband Applications.

   (A4) EIA Project Number PN–1361, Environmental and Safety Considerations for
   Voice Telephone Terminals.

   (A5) Federal Communications Commission Rules and Regulations, part 68,
   Connection of Terminal Equipment to the Telephone Network.

   (A6)  IEEE  Standard, Method for Measuring the Magnetic Field arould a
   Telephone Receiver. (to be published)

   [ 49 FR 1363 , Jan. 11, 1984, as amended at  61 FR 42187 , Aug. 14, 1996]