Telecommunications Industry Association
TR41.3.3-08-08-008-L
Document Cover Sheet
Project Number
Document Title
Measurement methods for amplified phone
Source
IEEE Subcommittee on Telephone Instrument Testing
Contact
Name:
John R. Bareham, Chair
Phone:
Complete
Address:
309 Riverview Drive
Fax:
Ann Arbor, MI 48104
Email:
Distribution
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of Document
(Select one)
734-665-4224
jbareham@mich.com
TR-41.3.3, TR-41.3.10, TR-41.3.9, TR41.3
For Information
The document to which this cover statement is attached is submitted to a Formulating Group or
sub-element thereof of the Telecommunications Industry Association (TIA) in accordance with the
provisions of Sections 6.4.1–6.4.6 inclusive of the TIA Engineering Manual dated March 2005, all of
which provisions are hereby incorporated by reference.
Abstract
The text that follows is the current draft of an annex to be included in the revision of IEEE Std 269. While
the title currently refers to amplified phones, the IEEE Subcommittee on Telephone Testing has decided
to apply the principles in the annex broadly, throughout the entire document.
v1.0 – 20050426
Telecommunications Industry Association
TR41.3.3-08-08-008-L
The text below is the current draft of an annex to be included in the revision of IEEE Std 269.
Annex A
(normative)
Measurements on telephones with amplified receivers*
*Needs new title, since it has been decided to apply these concepts generally throughout IEEE 269.
A.1 Amplified telephones
Telephones with amplified receivers are often marketed as “amplified telephones.” Strictly speaking, all
telephones are amplified. Nevertheless, the terms “amplified phone” or “amplified receiver” generally are
used to describe a telephone device which has more than the usual or “standard” gain in the receive path.
These devices are generally marketed to persons with some degree of hearing loss.
Telephones with amplified receivers are often compared to ordinary telephones with respect to
amplification or gain. They are also compared with respect to frequency response characteristics such as
high-frequency boost. The use of real or “reference” telephones is specifically not recommended for these
purposes. Instead, this clause provides absolute references and metrics so comparisons can be made in
a well-defined objective manner that relates to actual face-to-face conversations.
The reference for level for comparison is 60dBSPL (unweighted) in the free field. (Under study now.
Could be any number from 59-65 dBSPL. 64dBSPL is a likely candidate.) This is approximately the
normal level of a face-to-face conversation between two people 1 meter apart, in a quiet room without
reverberation.
The reference for frequency response is a flat characteristic in the free field.
This clause applies to handset or headset receivers.
The subject of measurements on amplified phones continues to be under study. The methods in this
clause may be subject to revision.
A.2 Acoustic output level
The acoustic output level at the receiver is strongly related to loudness, so it is a simple measure of the
quantity of sound delivered to the ear. Acoustic output level is especially useful when comparing different
kinds of communication equipment to each other, or when comparing a telephone conversation to a real
face-to-face conversation.
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Telecommunications Industry Association
TR41.3.3-08-08-008-L
Receive loudness rating (RLR) is the traditional way comparisons have been done in the telephone
industry. RLR is the method generally accepted in the telecom industry for describing how the receive
sensitivity and frequency response relate to the level of signals in the network. Loudness ratings are most
valid when comparing phones of similar type, but they are less valid when comparing phones of very
different designs. RLR does relate to face-to-face conversations, but only indirectly, through the medium
of a defined “reference telephone.” It is seldom obvious to anyone outside the telecom industry how to
interpret the meaning of an RLR, or how to compare it with a level or sensitivity measurement.
To measure the acoustic output level, apply the test signal at the RETP. Measure the active total SPL at
the receiver, translated to the free field, using the translation data in IEEE 1652.
The test signal is real speech (Male conversational speech per IEEE Std. 269). The measurement shall
be averaged over the entire length of the test signal, excluding pauses or silences. The measurement
bandwidth shall be the same as the applied test signal, nominally 100-8500 Hz.
Volume control shall be set to maximum.
For an analog phone, the test signal levels at the RETP shall be:
Loud: -14 dBV with 0 km loop
Nominal: -25 dBV with 2.7 km loop
Quiet: -38 dBV with 4.6 km loop
A.3 Conversational gain
The conversational gain of a receiver expresses the difference in level between a face-to-face
conversation at 1 meter and a conversation using a telephone. The face-to-face conversation is assumed
to be in a reflection-free and quiet environment, using both ears. The telephone conversation is assumed
to use a handset or headset on one ear, in an environment which may be noisy. Conversational gain is
dependent on both the volume control setting and the incoming signal level.
By this definition, virtually all telephones have some conversational gain under some conditions. A
standard phone might have a conversational gain in the range of 0dB for low levels of incoming signal at
reference volume control setting, and up to 40dB for high levels of incoming signal at maximum volume
control setting.
An amplified phone is intended to provided high amounts of conversational gain under a wide range of
incoming signal levels. When this is achieved, a listener with a hearing loss is more likely to be able to
hear most conversations.
The conversational gain of a receiver is defined as follows:
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Telecommunications Industry Association
TR41.3.3-08-08-008-L
Conversational Gain = Acoustic output level in dBSPL (free field) – 60dBSPL (May change to
64dBSPL.)
Acoustic output level shall be measured according to A.2.
For an analog phone, the (active) test signal levels at the RETP shall be:
Loud: -14 dBV with 0 km loop
Nominal: -25 dBV with 2.7 km loop
Quiet: -38 dBV with 4.6 km loop
The conversational gain at all three test signal levels should be reported.
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Telecommunications Industry Association
TR41.3.3-08-08-008-L
A.4 Frequency response metrics
The general shape of the receive frequency response can be described in a way that relates to the overall
spectral balance of the sound at the receiver. Most phones have significant response in the middle of the
telephone band. They may or may not have significant response in the high frequency or low frequency
parts of the telephone band.
By comparing the low, middle and high-frequency parts of the response with the full bandwidth, it is
possible to approximately describe spectral balance with 3 numbers, called low frequency balance, mid
frequency balance and high frequency balance.
If a receiver has a relatively natural-sounding frequency response, all three metrics will have values near
zero (either plus or minus). If the low frequency balance value is negative by several dB, the receiver
sounds thin. If the high frequency balance value is negative by several dB, the receiver sounds dull. If the
mid frequency balance is positive by several dB, and the low and high frequency balance values are
negative, this indicates a phone with most of its response in a narrow middle part of the telecom
bandwidth. (This shape is sometimes referred to as a “haystack.”)
To calculate these frequency response metrics, the receive frequency response should be measured in
1/3rd octave band resolution, from 250-3150 Hz. Portions of the frequency response curve should be
averaged on a dB basis, in the following frequency ranges:
Wideband average
= (FR250 + FR315 + … + FR3150)/12
Low frequency average = (FR250 + FR 315 + FR 400 + FR500)/4
Mid frequency average = (FR800 + FR 1000 + FR 1200)/3
High frequency average = (FR2000+ FR 2500 + FR 3150)/3
Where FR250 = receive frequency response in the 250 Hz band,
FR315 = receive frequency response in the 315 Hz band,
Etc.
Low frequency balance = Low frequency average – Wideband average
Mid frequency balance = Mid frequency average – Wideband average
High frequency balance = High frequency average – Wideband average
A.5 Distortion
Distortion shall be measured using the PN-SDNR method.
At a minimum, the following frequencies should be tested: 400 Hz, 1000 Hz, and 2000 Hz. (tentative).
The tests should be performed at several levels, at least including -14dBV with no loop.
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