By: John R. Fleming III
Meaning of CTB - "Composite Triple Beat", is a peak level ratio that measures the amount
of noise distortion over a cable line. It will commonly appear if an
amplifier is in use on the cable line, and that amplifier is being
overdriven. Since the CTB is related to the line itself, it can relate to
any traffic being transmitted over the line, including cable television as
well as cable Internet. There is an acceptable CTB ratio range, but once the
ratio is rises above it, degradation in the quality of either cable
television picture or Internet signal is apparent.
“CTB is the ratio of carrier to Composite Third order
Beat, also called Composite Triple Beat. CSO the ratio of carrier to
Composite Second Order Beat. Both of these distortions are the result of
multiple carriers experiencing second and third order non-linearity and so
generating multiple distortion products.” [Matrix Test Equipment, Inc.]
“CTB is a commonly encountered manifestation of
coherent noise. When it appears, it normally signifies that somewhere
between the signal source and the subscriber, an amplifier is being
overdriven. The CTB impairment is due to the cumulative effect of hundreds
of third order intermodulation beat products.” [Ward Laboratories, Inc]
“CTB (Composite Triple Beat) - (1) A clustering of
third-order distortion products around the visual carriers in cable systems;
(2) A ratio, expressed in decibels, of the peak level of the visual carrier
to the peak of the average level of the cluster of the third-order
distortion products centered around the visual carrier; (3) The Number of
beats (distortion products) occurring in cable television broadband
transmissions” [Massmediainternational.com]
“Amplifiers have improved greatly since the phenomenon
was discovered in the ’70s, largely conquering CTB in older, smaller
systems. However, recent trends toward running higher levels and more
channels have brought CTB back to the fore, so it pays to conquer it.”
[Henry Kallina – ct-magazine.com]
“They are named composite distortions because they are
made up of a composite of discrete distortions. They fall in a narrow range
of frequencies near the carrier frequency and are measured as a group.”
[Matrix Test Equipment, Inc.]
“In multichannel systems using push-pull, PHD, and
feed-forward amplifiers, the limiting performance factor is usually
composite third order beats. The total number of beats that 'pile up' at a
single frequency can be calculated. The composite triple figures are with CW
carriers at the output level and tilt listed on the specification sheets.
Any deviation creates on a 2:1 basis. Worst-case CTB occurs at the high end
of the amplifier's passband. With TV modulated carriers, the average signal
power is reduced. Measured composite triple beat levels are at least 6dB
lower than with CW carriers, and possibly as much as 10 dB lower when
frequency offsets are considered. The composite triple beat is measured with
a spectrum analyzer of 30 kHz resolution.” [Advanced-television.com]
“Composite triple beats are distortions generated by
the addition and subtraction of three visual carriers, the harmonics of
visual carriers, or any combination of these. In standard frequency
systems, the majority of these distortions will be found within 15 KHz of
the video carriers. Since offsets of 12.5 KHz are required in the
aeronautical band, the distortions produced by combinations including these
carriers will occur farther in frequency from the visual carrier.
[Trilithic]
“The NCTA RECOMMENDED PRACTICES FOR MEASUREMENTS ON
CABLE TELEVISION SYSTEMS defines CTB as "A composite third order distortion
of the desired signals caused by third order curvature of non-linear
transfer characteristic in system equipment. It is the ratio, expressed in
dB, of the peak of the RF signal to the average level of the cluster of
distortion components centered around the carrier." [Matrixtest.com]
“The strongest and most important third order products
are the result of three frequencies. These can be expressed as:
A+/-B+/-C where A<B<C
It is valuable to further categorize these products as:
A+B+C, A+B C, A B+C, A B C where A<B<C
One distortion product generated by the third order
non-linearity is 3A, three times any frequency. There are of course N of
these products where N=number of carriers. These products are 15.6 dB weaker
than the ABC products. These products do not fall near a carrier and usually
are ignored.” [from Matrixtest.com – Matrix Test Equipment Inc.]
