Lesson 1/Learning Event 12
(3) C-Message weighting. This is noise weighting used in a test set
to measure noise on a line that is terminated by a 500-type or similar
subset.
C-message is the most widely used type of weighting in military
voice communication.
(4) Flat weighting.
The amplitude-frequency characteristic of a
noise-measuring set is flat over a specified frequency range, which must be
stated in descriptions of the equipment. The range is usually controlled by
an input filter.
(5) Psophometric weighting.
This is a type of noise weighting
established by the Consultative Committee for International Telephone and
Telegraph (CCITT) for use in a noise measuring set known as a psophometer.
The shape of this characteristic is virtually identical with that of FlA
weighting.
Psophometric weighting is used mostly in Europe and is seldom
employed in the United States.
b. Units of noise measurement.
The units that follow are used to
express weighted and unweighted circuit noise.
These units include terms
used in American as well as international practice.
(1) DBa (dBrn adjusted). DBa (dBrn) is the noise level in a circuit
having F1A-line or HAl-receiver weighting.
0 dBa is noise power of 3.16
picowatts (-85 dBm), equivalent to 0 dBrn (-90 dm or 1 picowatt with C-
message weighting) adjusted to F1A weighting.
NOTE:
A 1-milliwatt 1,000-Hz tone will read +85 dBa.
However,
the same power in white noise, randomly distributed over a
3-kHz band (nominally 300 to 3,300 Hz) will read 3 dB less,
or +82 dBa.
(2) DBa (F1A). DBa (F1A) is the noise level measured on a line by a
noise-measuring set having F1A-line weighting.
(3) DBa (HA1).
Weighted noise power across the receiver of a 302-
type or similar subset measured by a test set having HA1 receiver weighting
is identified as dBa (HA1).
(4) DBa0.
The power level of noise can be referred to the signal
level at a particular point in the circuit. In this case, the signal level
becomes the reference (0). Noise measured in dBa with respect to this point
is called dBa0.
NOTE:
It is preferred to convert circuit noise readings from dBa
to dBa0, as this makes it unnecessary to know or state the
relative transmission level at point of actual measurement.
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