EoW July 2007

100 / 120

EuroWire – July 2007 98
english
Four primary test parameters were
chosen, chrominance/luminance gain
inequality, chrominance/luminance delay
inequality, line time waveform distortion,
and insertion gain. In a purely passive
test configuration, such as this, all gain
measurements are actually measuring loss.
Minimum acceptable broadcast studio
quality specifications per ANSI T1.502 are
included as a reference only and are not an
indicator of a pass or fail criteria.
These specifications were set as an
overland transmission standard for NTSC
video to be received for broadcast over the
air and are more stringent than the typical
security video requirements.
A fifth set of measurements were taken
from an FCC multi-burst test signal. This is
a basic colour bar test pattern with results
shown as signal level at a given frequency
and is a function of the cable attenuation.
A brief description of the test effects for
the parameter is included before each of
the following test data tables.
Chrominance refers to the colour
information in a composite video
signal and is typically centred on 3.58
MHz. Luminance is the black and white
information and varies in frequency from
below 0.5 MHz to 4.2 MHz
Chrominance to luminance gain inequality
errors most commonly appear as
attenuation or peaking of the chrominance
information and show up in the picture as
incorrect colour saturation (see Table 1).
Chrominance to luminance delay
distortion will cause colour smearing
or bleeding, particularly at the edges of
objects in the picture. It may also cause
poor reproduction of sharp luminance
transitions.
If the delay is extreme, ghosting can
appear, distorting the image significantly.
This distortion is created by transit time
delays that vary across a given length of
cable as a function of frequency and is
usually measured in nanoseconds.
Positive numbers indicate that
chrominance information arrived after
luminance information, and negative
numbers indicate that chrominance
information arrived before luminance
information (see Table 2).
Line time waveform distortion produces
brightness variations between the left
and right sides of the screen. Horizontal
streaking and smearing may also be
apparent.
Line time distortion is apparent in low
frequency picture detail. This distortion is
caused by tilt on the line time (between
zero and 64 microsecond) pulses as shown
in Table 3.
Insertion gain is a measure of DC gain
(or loss) across a device under test which
can be seen in Table 4.
Multi-burst losses (shown in Table 5) are a
function of cable attenuation. Attenuation
losses that vary with frequency can cause
a number of picture effects including
loss of resolution, blurring, loss of colour
saturation, picture distortion, and even
failure of picture monitors to correctly
synchronise on either colour or luminance.
Screening attenuation values for the
copper clad aluminium braided cable
shield are very similar to the copper shield
material. Only slight variations can be seen
in the two cable designs.
Video test results for the copper clad
aluminium shielded coax are equivalent
to the copper shielded material with
only slight test data variation for the two
designs.
These similarities are seen independent
of the length of the cable under test.
Conclusions
Traditionally only copper shields have
been used for base band NTSC security
video applications.
Concerns relating to the low frequency
components of the video waveform
are usually cited when other metals or
bimetallic materials are considered for as
conductors for these applications.
Copper clad aluminium can be used to
replace solid copper fine wire in coaxial
cable shielding.
No detrimental effects were found in
shielding performance or video trans-
mission performance. Weight savings and
the resultant savings in material, shipping
cost, handling and installation can be
realised without electrical performance
loss in security video applications. n
Acknowledgements
Special thanks to Sandie Bollinger,
Robert Broyhill and David Wilson, all
of CommScope, who performed the
shielding measurements shown above.
The author wishes to acknowledge the
input and support of Brad Gilmer of Gilmer
and Associates in the video performance
measurements and evaluation.
References
[1] ANSI Standard T1.502-2004, System M-NTSC
Television Signals – Network Interface
Specifications and Performance Parameters;
[2] IEC 62153 Metallic communication cable test
methods – Part 4-4: Electromagnetic compatibility
(EMC) – Shielded screening attenuation,
test method for measuring of the screening
attenuation as up to and above 3 GHz;
[3] Matick R E Transmission lines for Digital and
Communications Networks (1969) McGraw-Hill Inc.
CommScope Inc
1100 CommScope Place SE
Hickory, Claremont,
NC 28603, USA
Fax: +1 800 982 1708
Email:
Website:
Table 5: Multiburst measurement RG 59 95% 1,000 ft (305 metres) ▲
Cu Shield CCA Shield
MHz IRE IRE
0.50 -0.91 -0.89
1.00 -1.66 -1.60
2.00 -2.87 -2.86
3.00 -3.68 -3.79
3.58 -4.10 -4.27
4.20 -4.49 -4.71