Coaxial cables meet new broadcast standards

Edited by the Electronicstalk editorial team Apr 18, 2005

SDI coaxial cables are now specified for operation at 4.5GHz, with every reel being sweep tested for return loss and certified to the new, higher frequency.

Belden CDT Electronics Division has extended the performance specifications of its SDI coaxial cables from 3 to 4.5GHz - with every reel being sweep tested for return loss and certified to the new, higher frequency.

This extension is designed to assure broadcasters, leading-edge broadcast equipment manufacturers, and the standards bodies of high performance and reliability as they begin to migrate from the existing 1080i (interlaced) SDI format to the emerging 1080p (progressive) format, protocols and equipment, which require twice the bandwidth of 1080i.

To date, Belden is the first cable manufacturer to perform sweep testing and certification to meet this emerging format.

The enhanced testing is currently being performed with three of Belden's most popular precision video coaxial cables for HDTV/SDI - Belden Brilliance 1505A, 1855A, and 1694A - and will soon be expanded to additional products.

In the 1080i (interlaced) format, the image resolution is 1920 x 1080 pixel, sent at 60 interlaced frames per second.

In the 1080p (progressive) format, the image resolution is the same but is sent at 60 complete frames per second.

Thus, in the progressive format, the complete image updates every 1/60th of a second and in the interlaced format, only half of the image updates every 1/60th of a second.

As a result, progressive images result in better image definition and quality.

Good practice requires testing to the third harmonic of the operating frequency.

For serial, single cable 1080p, that means testing to a frequency of 4.5GHz.

This 1080p format is expected to transmit serial digital video signals at 2.97Gbit/s, requiring an operating frequency of 1.485GHz.

This translates to a shorter wavelength and a greater chance that inconsistently manufactured cables will choke off the signal with a high level of return loss.

This may be caused by even the slightest irregularity in the cable manufacturing process.

A slightly bent shaft, bad bearing or uneven cable tension, for example, can cause reflection and affect the signal wavelength.

Therefore, utmost consistency and control in the manufacturing process is essential to the reliability of the cable's signal transmission at high frequencies and to minimise return loss.