EoW July 2007

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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. [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. References

Cu Shield

CCA Shield

IRE

MHz

IRE

-0.89

0.50

-0.91

-1.60

1.00

-1.66

-2.86

2.00

-2.87

-3.79

3.00

-3.68

-4.27

3.58

-4.10

-4.71

4.20

-4.49

Table 5 : Multiburst measurement RG 59 95% 1,000 ft (305 metres) ▲

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 ). 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. Chrominance to luminance

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. 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. Positive numbers indicate

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EuroWire – July 2007