A hum eliminator or isolation transformer can be inserted at points within an installation at which it is observed that 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.
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.
Video Hum Eliminators - Analogue
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.
HEC hum eliminators 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 over the full bandwidth, 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 and 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. Four, threaded, mounting holes (6-32 x 0.5 inch deep) are provided on the rear face of the units.
HEC-2000-H (Horizontal), HEC-2000-V (Vertical) - Video Hum Eliminator, 1 channel
Video signal format: Composite. Impedance: 75 ohms. Bandwidth: DC to 30MHz. Hum reduction: 60dB typical, 50Hz or 60Hz. Insertion loss: 0.75dB at 10MHz, 1.20dB at 20MHz, 1.50db at 30MHz. Return Loss: Better than 20dB. Dimensions: 120 x 94 x 53mm. Weight: 1.9kg.
HEC-3000 - Video Hum Eliminator, 3 channel
Three independent hum-reduction channels in a single unit. Specification as HEC-2000-V except:
Isolation between channels: 70dB typical. Dimensions: 189 x 120 x 78mm. Weight: 4.54kg.
HEC-4000 - Video Hum Eliminator, 3 channel with sync isolation transformer
Three channels of hum reduction and one channel of transformer isolation in a single unit, suitable for RGB, or YUV plus sync systems.
Video signal format: Component plus sync. Impedance: 75 ohms. Bandwidth (RGB): DC to 30MHz. Hum reduction: 60dB typical, 50Hz or 60Hz. Insertion loss: 0.75dB at 10MHz, 1.00dB at 20MHz, 1.25db at 30MHz. Bandwidth (Sync): 20Hz to 6MHz. Return Loss: Better than 20dB. Isolation between channels: 80dB typical. Dimensions: 189 x 120 x 78mm. Weight: 3.4kg.
HEC-5000 - Video Hum Eliminator, 100MHz Extended Bandwidth HDTV, 5 channel
Five extended-bandwidth hum-reduction channels in a single unit. Each channel has a bandwidth of 100MHz, making it suitable for most computer generated HDTV video applications and many video projection systems. The five, fully-isolated, channels may be used independently or to accommodate RGB or YUV plus H.Sync and V.Sync.
Video signal format: Composite or component plus H.sync and V.sync. Impedance: 75 ohms. Bandwidth: DC to 100MHz. Hum reduction 60dB minimum per channel. Insertion loss: 0.2dB at 100kHz, 0.3db at 10MHz, 0.8 at 30MHz, 1.2db at 100MHz. Return Loss: 20dB minimum. Isolation between channels: 70dB typical. Dimensions: 189 x 120 x 78mm. Weight: 2.72kg.
Video Isolation Transformer
Isolation transformers are most suited to applications where there are large earth potential differences, greater than 20 volts, between sections of an installation.
They are especially useful in outside broadcast or temporary installations, or between buildings where different power supplies and earth points are present. Breaking the ground connection in the video transmission line will eliminate the 50Hz or 60Hz hum currents in the ground loops. The VIT- 75 video isolation transformers provide true electrical isolation, with no DC path and 500V DC breakdown voltage between input and output, while passing the video signals completely unaltered. (Note: provision should be made for circuits such as camera power or control, requiring transmission of DC through the video coax line.)
Allen Avionics video isolation transformers are housed in extremely rugged, waterproof, high impact ABS or die-cast metal cases. BNC input and output connectors are located on the top, or front face and the VIT-75-3 has stainless steel hoop guards either side of the connectors for protection during transit and rigging. Four threaded mounting holes are provided on the rear face of the units.
VIT-75 - Video Isolation Transformer, 1 channel
Video signal format: Composite. Impedance: 75 ohms. Bandwidth: 20Hz to 6MHz. Video isolation: 100M ohms minimum. Insertion loss: 1.0dB maximum. Return Loss: 20dB minimum. Case: High impact ABS. Mounting holes: 8-32 x 1.5 inches. Dimensions: 95 x 67 x 38mm. Weight: 0.45kg.
VIT-75-3 - Video Isolation Transformer, 3 channel
Three independent video isolation transformer channels in a single unit. Specification as VIT-75 except;
Isolation between channels: 60dB typical. Case: Die cast metal. Mounting holes: 6-32 x 1.5 inches. Dimensions: 120 x 94 x 27mm. Weight: 1.14kg.
Many other hum blockers, isolators, transformers, video noise eliminators, attenuators and delay lines are available from Allen Avionics, to assist with the removal of unwanted interference, synchronisation and equalisation in video and audio systems. Formats include SDI-HD, Component and composite video, CATV, S-VHS and analogue audio. Packaging options include stand-alone modules, card and rack frame enclosures and discrete PCB mounting components.