Telecommunications Industry Association
TR41.3.5-13-04-006
Document Cover Sheet
Project Number
ANSI/TIA-PN-470.122
Document Title
Issue with “Stepped” Speakerphone Frequency Response Templates
Source
Stephen R Whitesell as Consultant to Microsemi Corporation
Contact
Stephen R Whitesell
2 Shannon Ct
Howell, NJ 07731-8919
Distribution
TR-41.3.5
Intended Purpose
of Document
(Select one)
X
Phone: 732 751 1079
Fax:
Email: swhitesell@ieee.org
For Incorporation Into TIA Publication
For Information
Other (describe) -
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 October 2009, all
of which provisions are hereby incorporated by reference.
Abstract
This contribution discusses an issue raised by James Bress of AST Technology Labs during the February
2013 TR-41.3.5 meeting in Orlando. Jim pointed out that products that actually met the frequency
response requirements could appear to fail when plotted on the “stepped” frequency response template
with the 1/3 octave center frequencies connected by straight lines.
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Telecommunications Industry Association
TR41.3.5-13-04-006
Background
The speakerphone response templates shown in the initial draft of ANSI/TIA-PN-470.122 (contribution
TR41.3.5-13-02-005-MR2) use the same “stepped” approach as in published speakerphone standards
ANSI/TIA-470.120-C and TIA-920.120-A. They actually come from the latter document. These
templates were plotted in steps to emphasize the fact that the requirements apply to the response averaged
over 1/3 octave bands.
During the TR-41.3.5 discussion in the February meeting, there was agreement that the send and receive
frequency response data is either (1) to be measured in 1/3 octave bands or (2) have band averaging
applied to determine the 1/3 octave levels if measured in narrower (e.g., 1/12 octave bands). James Bress
of AST Technology Labs pointed out that most test equipment determines the 1/3 octave levels correctly
and then plots them as “dots” at each 1/3 octave center frequency connected by lines. In doing so, it is
possible for a line connecting the levels at adjacent 1/3 octave frequencies to cross the corner of a “step”
and give the appearance of a failure even though the two levels actually meet the requirement.
Illustration for Send Frequency Response
The following figure illustrates how this could happen for a send measurement. An arbitrarily drawn
response “curve” is shown in red and connects values at the center frequencies of 1/3 octave bands that
are all within response limits. The data points were chosen to be near the lower limit at low frequencies
and near the upper limit at high frequencies so that it is not possible to get a better “fit” by shifting the
template up or down. Specific data points in the mid frequencies are not relevant to this discussion and
were omitted. Note how the curve “nicks” the corners of the stepped template (shown in gray).
However, if the template is drawn by connecting the center frequencies for each 1/3 octave band (shown
in black), then the response curve is shown within limits, thus avoiding the appearance of failures.
Send Frequency Response Template Problem
20
Relative Response (dB)
10
0
-10
-20
100
1000
Frequency (Hz)
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Telecommunications Industry Association
TR41.3.5-13-04-006
During the discussion at the February meeting, there seemed to be agreement to expand the table listing
the limits to include values for each 1/3 octave center frequency accompanied by a note making a point
that the limit applies to the average value of the signal in the 1/3 octave band whose center frequency is
shown. There also seemed to be agreement to show the template as lines connecting the 1/3 octave center
frequency limits (i.e. the black line template in the above figure).
Taking this approach results in a template with slightly “wavy” lines for the upper limit as shown in the
figure below (gray curve). This is apparently an artifact from the original decision to show the template
in steps and choosing to keep the step sizes in whole dBs. I am proposing to “straighten” the limit curves
by adjusting the limits slightly (black curve). The table following the figure provides the original 1/3
octave limits that were taken from TIA-920.120-A (list expanded to show limit for each individual 1/3
octave frequency) and the proposed modified limits. Two proposals are included for the low frequency
upper limit. One keeps the same endpoint value at 100 Hz and the other minimizes the average difference
between the original and proposed values. The modified values are shown in red, with the additional
modified values for the alternative low frequency proposal shown in blue. There are no changes to the
lower limit.
Send Frequency Response Template Proposal
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Relative Response (dB)
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-10
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100
1000
Frequency (Hz)
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Telecommunications Industry Association
TR41.3.5-13-04-006
Send Frequency Response Limits
1/3 Octave
Center
Frequency
(Hz)
TIA-920.120-A
Lower
Limit
(dB)
Upper
Limit
(dB)
Proposal
Lower
Limit
(dB)
Upper
Limit
(dB)
Alternate Proposal
Lower
Limit
(dB)
Upper
Limit
(dB)
100
2.0
2.0
0.0
125
2.0
2.5
1.0
160
2.0
3.0
2.0
200
2.0
3.5
3.0
250
-8.0
4.0
-8.0
4.0
-8.0
4.0
315
-6.0
4.0
-6.0
4.0
-6.0
4.0
400
-4.0
4.0
-4.0
4.0
-4.0
4.0
500
-4.0
4.0
-4.0
4.0
-4.0
4.0
630
-4.0
4.0
-4.0
4.0
-4.0
4.0
800
-4.0
4.0
-4.0
4.0
-4.0
4.0
1000
-4.0
4.0
-4.0
4.0
-4.0
4.0
1250
-4.0
5.0
-4.0
4.5
-4.0
4.5
1600
-4.0
5.0
-4.0
5.0
-4.0
5.0
2000
-4.0
6.0
-4.0
5.5
-4.0
5.5
2500
-4.0
6.0
-4.0
6.0
-4.0
6.0
3150
-4.0
7.0
-4.0
6.5
-4.0
6.5
4000
-6.0
7.0
-6.0
7.0
-6.0
7.0
5000
-8.0
6.0
-8.0
6.0
-8.0
6.0
6300
4.0
5.0
5.0
8000
4.0
4.0
4.0
Receive Frequency Response Proposal
Similar considerations apply for the receive frequency response. Although no arbitrary response curve is
included, the first figure below shows the stepped response template taken from TIA-920.120-A (gray
curve) and the corresponding template with the 1/3 octave center frequency limits connected with straight
lines (black curve). The second figure below shows a proposal to “straighten the limit lines by slightly
modifying the 1/3 octave limit values. Finally, the table following the second figure provides the original
limit values taken from TIA-920.120-A and the proposed modified limits with the modified values shown
in red.
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Telecommunications Industry Association
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Recieve Frequency Response Template Problem
20
Relative Response (dB)
10
0
-10
-20
100
1000
Frequency (Hz)
10000
Recieve Frequency Response Template Proposal
20
Relative Response (dB)
10
0
-10
-20
100
1000
Frequency (Hz)
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Telecommunications Industry Association
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Receive Frequency Response Limits
1/3 Octave
Center
Frequency
(Hz)
TIA-920.120-A
Lower
Limit
(dB)
Upper
Limit
(dB)
Proposal
Lower
Limit
(dB)
Upper
Limit
(dB)
100
4.0
3.0
125
4.0
3.5
160
4.0
4.0
200
4.0
4.5
250
-9.0
5.0
-9.0
5.0
315
-7.0
5.0
-7.0
5.0
400
-5.0
5.0
-5.0
5.0
500
-5.0
5.0
-5.0
5.0
630
-5.0
5.0
-5.0
5.0
800
-5.0
5.0
-5.0
5.0
1000
-5.0
5.0
-5.0
5.0
1250
-5.0
5.0
-5.0
5.0
1600
-5.0
5.0
-5.0
5.0
2000
-5.0
5.0
-5.0
5.0
2500
-5.0
5.0
-5.0
5.0
3150
-6.0
5.0
-5.5
5.0
4000
-6.0
5.0
-6.0
5.0
5000
-9.0
4.0
-9.0
4.5
6300
4.0
4.0
8000
4.0
3.5
Note to Accompany Tables
The following text is suggested as a note to follow both the send and receive frequency response limit
tables:
Note: The values shown apply to the signal level averaged over the 1/3 octave band
associated with the given center frequency. If the measurement is made using bands
narrower than 1/3 octave, see IEEE Std 269 for determining the frequencies of the band
edges and, hence, the sub-bands to be used in determining the average signal level to be
associated with each 1/3 octave band center frequency.
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