SDI and HD-SDI signals suffer less from extraneous noise and low-frequency hum problems than do analogue video signals, however, within complex broadcast installations the potential exists for ground-loops and potential differences between grounds. These hum eliminators and isolators may be used to mitigate the errors thus caused.
Hum is the term for low-frequency noise induced by alternating currents at 50Hz or 60Hz in AC mains power lines. There are two main sources of hum and two types of hum-elimination available from Allen Avionics. Hum can appear on video signals because it has been induced in a ground-loop, or introduced because of a real voltage potential difference (p.d.) between the grounds on the transmitting and receiving equipment. In many installations, equipment racks are powered, unavoidably, from different supplies. This can easily lead to induced hum on video signals. These effects are often seen where signals are fed from floor to floor within buildings and can be very significant when they are passed from building to building. Probably the worst environment is the outside broadcast where multiple generators or AC power cables may be in close proximity to the equipment racks or video cables, inducing tens of volts of ripple on the coaxial cable screens.
Ground loops and noise will not immediately introduce impairments in the picture but will worsen jitter in the signal which will compound with cable-attenuation, poor re-clocking, and signal source instabilities, to push the signal into digital errors. Unlike analogue signals, digital signals do not degrade gracefully but are subject to a 'cliff effect' where the transition from acceptable picture to no picture is quite sudden.
The hum eliminator can be inserted at points within the installation at which it is observed the jitter performance degrades. Applications include sports stadiums and large arenas where it is very difficult to know where different earth potentials exist. Identification of the problem areas is undertaken by simplifying the circuits as much as possible, until satisfactory performance is obtained, and then re-introducing additional elements until the offending equipment is found which will indicate where a loop eliminator should be inserted.
Video Hum Eliminators - HD-SDI - 1.5G
Hum eliminators work best in circumstances where interference is caused by small differences in ground potential, up to 20 volts, or by induced currents in long cable runs caused by the close proximity of power or lighting cables. They provide significant impedance to the induced currents, but pass the video signals completely unaltered. It appears to be standard coaxial cable to the video signal, but to the unwanted ripple, it appears to be a choke and hence provides attenuation. This is achieved by the use of magnetic materials with responses tailored to this application.
In the unlikely event that a small amount of hum is still present with a hum eliminator inserted, a second unit may be connected in series, either adjacent to the first or at another point in the video circuit.
Allen Avionics video hum eliminators are designed for life on the road; stand-alone single channel and isolated three channel or five channel units are housed in extremely rugged, waterproof, die-cast metal cases. BNC input and output connectors are located on the top, or front face and are protected from damage in transit or during rigging by stainless steel hoop guards. Three and five channel units are also available in a 1U rack mount case.
Video signal format: SD-SDI to SMPTE 259M between 143Mb/s and 360Mb/s, and HD-SDI to SMPTE 292M up to 1.485Gb/s. Impedance: 75 ohms. Bandwidth: 3.0GHz. Return loss: Better than 15dB to 1.5GHz. Insertion loss: Equivalent to approximately 6m of Canford SDV-L, Draka 1.0/4.8AF or Belden 1694A. Ground loop isolation: better than 60dB.
Dimensions and weights:
|1 channel, 120 x 94 x 53mm, 1.35kg
|3 channel, 189 x 120 x 78mm, 4.1kg
|3 channel rackmount, 1U, 4.1kg
|5 channel, 216 x 89 x 102mm, 5.90kg
|5 channel rackmount, 1U, 5.9kg