§  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 90DEG 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]