Telecommunications Industry Association
TR41.1.1-06-02-005
Document Cover Sheet
Project Number
PN-3-0191 (to be published as TIA-1063)
Document Title
C-Message and Psophometric Noise Definitions
Source
VTech Communications
Contact
Stephen R Whitesell
2 Shannon Ct
Howell, NJ 07731
Distribution
TR-41.1.1
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of Document
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Phone: 732 751 1079
Fax:
Email: swhitesell@vtech.ca
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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
Historical background information is provided on the relationship between the C-message and
psophometric noise weighting curves, and revised definitions are suggested for these two terms.
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Telecommunications Industry Association
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Background
A new definition for C-message weighted noise, including reference to its relationship to the noise heard
by the user of a 500-type telephone set, was provided during the 01/18/2006 teleconference. The same
definition, with reference to the 500-type telephone deleted, was copied to the definition for psophometric
noise. This author offered to review historical copies of ITU-T P.53 Recommendation for Psophometers
to see if a more specific definition could be suggested. [Note P.53 is now published as O.41, with only a
cross-reference listed under P.53.]
The oldest version of Recommendation P.53 available to the author is the one in Volume V of the
C.C.I.T.T. Red Book published in 1960. Section A of the recommendation deals with a “psophometer for
commercial telephone circuits” and begins with the following text:
The C.C.I.T.T.,
considering
that, since the psophometer for commercial telephone circuits was specified (Directives
concerning the protection of communication lines against the interfering effects of electrical
power lines, Rome Edition, 1937, revised at Oslo, 1938), considerable progress has been made in
the construction of the subscriber’s telephone apparatus, especially so far as the smoothness of
the sensitivity-frequency characteristic is concerned,
that the “Joint Subcommittee on development and research of the Edison Electric Institute
and the Bell Telephone System” (Engineering Report No. 45) has carried out numerous tests to
determine the curve to be prescribed for the psophometric filter network in order to take account
of the improved characteristics of the subscriber’s telephone equipment,
that numerous tests and measurements made in the course of the last few years show that the
electro-acoustic characteristics of the subscriber’s telephone equipment used in Europe are very
similar to those of American equipment and that, consequently, it is unnecessary to repeat in
Europe similar tests to those described by the Joint Subcommittee,
unanimously recommends
that the weights attributed to different frequencies in the weighting network of the
psophometer used for measurements at the terminals of a commercial trunk telephone circuit
should be . . .
The version of Recommendation P.53 contained in the C.C.I.T.T. White Book published in 1969 begins
with the same introductory text. However, a new section on “correspondence with the readings of
American psophometers” is added later in the recommendation. That section begins as follows:
Information now used by the American Telephone and Telegraph Company in assessing
noise impairment is given in an article by D. A. Lewinski in the Bell System Technical Journal,
March 1964 [reference provided]. In this article, noise is expressed in terms of readings with Cmessage weighting on the 3A noise meter now used in the United States. Because the weighting
differs from that associated with the older 2B noise meter and the C.C.I.T.T. 1951 psophometer,
the relationship among measurements with these instruments is influenced by the spectrum of the
noise measured. If one milliwatt of white noise in the band 300-3400 Hz is applied to each, the
following readings are obtained:
3A noise meter (C-message weighting)...................................88 dBrn
2B noise meter (F1A weighting)..............................................81.5 dBa
C.C.I.T.T. psophometer (1951 weighting)...............................–2.5 dBm
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The text then goes on to conclude that reasonable “rounded conversion factors . . . for practical
comparison purposes” are that 0 dBm of psophometric noise equals 90 dBrn of C-message weighted noise
or, equivalently, -90 dBm of psophometric noise equals 0 dBrnC.
The added text also includes the complete reference to the Lewinski 1964 BSTJ article as noted above
and additional references to articles published by Lewinski and others. The following graph is from one
of those articles, Evaluation of Message Circuit Noise, by A. J. Aikens and D. A. Lewinski, BSTJ, July
1960.
The graph shows response vs. frequency curves for three filter networks: (A) 144-line weighting. (B) F1Aline weighting, and (C) C-message weighting. The article indicates 144-line weighting was relevant to the
desk stand (candlestick) telephone used in the 1920s and early 1930s. The F1A-line weighting was
introduced for the 302 set, which used an F-type handset with a receiver unit that produced a much
broader frequency response characteristic. This was the set commonly in use from the mid 1930s until it
was replaced by the 500-type set beginning in the 1950s.
Comparison of the F1A and C-message weighting curves in the above figure with the psophometric and
C-message weighting curves shown in Figure A.1/O.41 of current Recommendation O.41/P.53, suggests
that the psophometric curve is the same as the F1A curve. This assertion is supported by the statement
quoted above that the C-message waiting curve “differs from that associated with the older 2B noise
meter and the C.C.I.T.T. 1951 psophometer,” which seemingly equates the curves for those two devices.
The 2B noise meter definitely used the F1A weighting curve.
This view is further supported by the “considerings” at the beginning of the recommendation. It is likely
that these statements about the improved “smoothness of the sensitivity-frequency characteristic” and the
similarity between American and European telephone equipment making it “unnecessary to repeat in
Europe similar tests to those described by the Joint Subcommittee” were written when the “C.C.I.T.T.
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1951 psophometer” was standardized. The following additional information is contained in the 1960 and
1969 versions of Recommendation P.53 concerning this subject:
The psophometer, which was standardized by the XVIth Plenary Assembly of the C.C.I.F. for
relatively stable circuit noise measurements, consists, for use at the end of an international
telephone circuit (see above), of a filter network which takes account of the characteristics of a
fairly modern type of telephone set used in the United States of America together with the mean
characteristics of the national telephone network of that country.
The C.C.I.F was the predecessor of the C.C.I.T.T. It was formed in 1925 and merged with the C.C.I.T.
(established in 1924) to create the C.C.I.T.T. in 1956.* While I have not been able to definitely confirm
that the XVIth Plenary Assembly of the C.C.I.F. occurred in 1951, it is reasonable to expect that it did if
we assume Plenary Assemblies were held about every two years, with perhaps a couple of extra ones
thrown in when they were getting started. Thus, the “fairly modern type of telephone set used in the
United States” was most likely the 302 set, which was used from the mid 1930s through the 1950s and
which showed “considerable progress . . . so far as the smoothness of the sensitivity-frequency
characteristic is concerned.” Hence there was no reason for the filter characteristic of the psophometer to
be different from the F1A filter characteristic of the 2B noise meter used at that time because “the electroacoustic characteristics of the subscriber’s telephone equipment used in Europe are very similar to those
of American equipment.”
In summary, C-message weighting is intended to reflect the user perception of the electrical noise on the
telephone line as it would be perceived by a listener using a 500-type telephone set, whereas
psophometric weighting is more representative of user perception by a listener using older telephone sets
available in America and Europe in the 1935 to 1955 time frame. Given that here is not really that much
difference between the two curves, I’m not sure it is worth highlighting the fact that adopting
psophometric weighting apparently represents a step backward in time.
However, in creating the definitions related to noise meters, I believe it is important to distinguish
between the weighting curve, which is a frequency response characteristic, and the weighted noise
measurement, which has units like dBrnC or dBmp. In that regard, I would like to propose the new and
revised definitions shown below.
Proposal for Revised Definitions
C-message weighting – a frequency weighting characteristic used for measurement of electrical noise in
telephone circuits that reflects the spectral content perceived by a listener using a North American 500type telephone set. See Annex A of TIA-470.110-C for specification of the weighting characteristics.
C-message weighted noise – electrical noise measured on the telephone line using C-message weighting
and expressed in dBrnC, where 0 dBrnC = -90 dBm for a 1000 Hz sine wave.
Psophometric weighting – a frequency weighting characteristic used for measurement of electrical noise
in telephone circuits that reflects the spectral content perceived by a listener using a telephone set of the
type in common usage in North America and Europe at the time the weighting characteristic was
developed. See Annex A of TIA-470.110-C for specification of the weighting characteristics.
Psophometrically weighted noise – electrical noise measured on the telephone line using psophometric
weighting and expressed in dBmp, where 0 dBmp = 0 dBm for an 800 Hz sine wave.
* C.C.I.F. – Comite Consultatif International de Téléphonique, or International Telephone Consultative Committee
C.C.I.T. – Comite Consultatif International de Télégraphique, or International Telegraph Consultative Committee
C.C.I.T.T. – Comité Consultatif International de Télégraphique et Téléphonique, or International Telephone and
Telegraph Consultative Committee
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