Selecting The Proper SDI Cable

Choosing the right SDI cable in professional video applications is critical in ensuring a reliable performance. The signal cabling selection is too often neglected by users, and it is important to understand that not all SDI cables are created equal, and each have their own targeted applications. With increasing bandwidth and large range of video formats it is important to properly understand the influence cable selection has on an video system.

Theatrixx' xVision Video Converters, when used with the proper cable, allow 3G-SDI signal transmission for distances up to 125 meters.  The most common SDI format nowadays is 3G-SDI (2.97 Gbit/s), which can carry a full 1080P signal at 60Hz. This format requires double the bandwidth of its predecessor HD-SDI, which clocked in at 1.485 Gbit/s, and therefore, a cable with higher bandwidth is required, which is why Theatrixx offers two choices of cables depending on the required bandwidth.

The most popular and most economical option is Belden's 1694 RG6 cable. Belden offers this cable in two main variants, the 1694A targeted at fixed or semi-permanent applications, and the 1694F that is much more flexible (but comes with a slight range penalty) for temporary applications. Theatrixx recommends this type of cable for HD-SDI applications (720P, 1080i or 1080P at 30Hz).

For higher-bandwidth applications that still require a 125 meters transmission distance, Theatrixx offers the Vector SC-DZ type RG7 cable. With similar dimensions and flexibility as Belden's 1694F offering, the Vector cable is an attractive solution to replace bulky and heavy legacy RG7 cables.

Another important factor to consider when planning long cable runs is the amount of junctions needed. For example, while a single, continuous 125 meters RG7 cable will be able to transmit 3G-SDI without issues, 5 x 25 meters RG7 sections connected together with couplers may not have the same results. Here?s why : A typical 75 Ohms BNC connection has an insertion loss of 1dB. BNC couplers therefore add 2dB of loss. Good quality RG7 cable (like the Theatrixx Vector SC-DZ) will have loss of 0.1667db/meters. So a 125 meters run will have total losses of 22dB (20 dB cable + 2 connectors at 1 dB each), while a 5 x 25 meters assembly will have total losses of 30 dB (20 dB cable + 4 couplers at 2 dB each + 2 connectors at 1 dB each). Since the maximum acceptable signal loss of most professional SDI equipment is typically 30 dB, the continious 125 meters will have no issues, while the 5 x 25 meters assembly may to be able to properly transmit the signal. It is therefore important to select the right cable type and length for the application and minimize the amount of junctions for an optimal performance. 

If even longer runs must be accomplished, Theatrixx offers two solutions: For simple SDI links, the xVision Dual SDI Repeater amplifies and fully reclocks incoming SDI signal, preparing them for another 125 meters transmission. Since most cable runs that long are usually doubled for redundancy, the SDI Repeater can accept up to two input signals simultaneously. 

For extra-long cable runs of over 300 meters, the xVision Fiber Converters can seamlessly converter copper-SDI signals into optical-SDI format. With the single-mode option, this enables a 3G-SDI signal to reach distances of several kilometers. Since the converters use standard optical-SDI topology, they are compatible with existing video SFP-based outputs and inputs on switchers, scalers, projectors and video wall controllers.