Interference can affect image clarity on analogue CCTV systems, producing rolling lines, "hum" bars, ghosting, excessive grain or other visual artifacts. The following list contains the most common causes of interference and fixes to mitigate adverse effects.
Loose or Damaged BNC Connectors
Cause: Loose twist-on/crimp BNC connectors, oxidized centre pins, or a stray strand of outer shielding braid touching the centre conductor inside the plug.
Symptom: Sudden image flickering, colour loss, or rolling static lines that clear up or worsen when physically wiggling the cable at the back of the DVR or camera.
Fix: Cut off poor fittings and re-terminate using high-quality 75-ohm compression BNC connectors.
Low Voltage & Underpowering
Cause: Excessive voltage drop over long runs, damaged cables, poor DC termination or thin wire gauges, exacerbated at night when IR LEDs activate.
Symptom: Complete picture cutout at night, IR LED strobing, camera stuck in B&W mode, flickering, or heavy video noise/grain.
Fix: Ensure voltage at the camera remains within 12V DC ±10% under full IR load. Upgrade to a thicker DC wire gauge or move power supply closer to the camera.
Power Supply Ripple
Cause: Cheap brick power supplies with passive 1-to-4 splitter cables lacking quality DC smoothing/filtering under load, allowing AC to "ripple" into the DC cables.
Symptom: Rolling noise or flickering that appears or worsens specifically when additional cameras are plugged into the same power supply.
Fix: Replace passive power splitters with a dedicated multi-channel fused Power Supply Box with isolated outputs.
Water Ingress & Moisture in Connectors
Cause: Rain or condensation penetrating non-weatherproofed outdoor BNC joints, creating a resistive bridge between the core wire and shield.
Symptom: Rolling noise, washed-out colour, severe picture fuzz, or total signal dropouts during or immediately following rain.
Fix: Protect outdoor connections inside IP-rated junction boxes, wrap BNC joints in self-amalgamating rubber tape, and form drip loops on cable runs.
Ground Loop Current
Cause: Differing voltage potentials between camera and DVR ground points (e.g., camera mounted to metal) creating stray currents on the ground loop.
Symptom: Stray currents on the coaxial cable shield cause dark horizontal "hum bars" rolling slowly up or down the screen.
Fix: Isolate the camera body using plastic/insulated junction boxes, power equipment from a common AC circuit (same outlet or PSU), or install a passive BNC Ground Loop Isolator.
AC Mains Induction ("AC Bleed")
Cause: Coaxial cable running parallel to 230/240V AC power lines, inducing 50Hz/60Hz magnetic (EMF) noise.
Symptom: Wavy diagonal lines, rolling horizontal bands, or low-frequency picture distortion.
Fix: Separate coax from power cables by at least 200mm, cross AC lines strictly at 90-degree angles, use solid-copper RG59 and avoid CCA (copper-clad aluminium) coaxial cable.
Cable Impedance Mismatch & Signal Reflection
Cause: Using 50-ohm RF cable/connectors instead of 75-ohm hardware, a crushed/kinked dielectric layer, or unterminated pass-through (signal splitter) ports.
Symptom: Double outlines ("ghosting"), edge ringing around sharp objects, loss of picture sharpness, or high-contrast image smearing.
Fix: Ensure all coax, BNC connectors, splitters, and pass-through equipment strictly maintain a 75-ohm impedance path.
High-Frequency Radio Interference (RFI)
Cause: Cables transmitting HD analogue (30–100MHz) absorbing high-frequency noise from sources such as Variable Frequency Drives (VFDs powering HVAC systems, Elevators, industrial conveyor belts), industrial arc welders, two-way radios, noisy LED drivers, fluorescent light ballasts, or switching power supplies.
Symptom: Fine "snow," static grain, or complex herringbone/zigzag patterns across the image.
Fix: Use quality 75-ohm RG59/RG6 with solid copper shielding or enclose cable runs in grounded metal conduit. Maintain at least 300mm of clearance from parallel high-voltage/VFD lines, and apply ferrite cores/beads where needed. Avoid CCA (copper-clad aluminium) coaxial cable.
Split-Pair Wiring on UTP/Balun Systems
Cause: Reverse polarity or using two wires from different twisted pairs (e.g., Solid Blue + Solid Orange) instead of a matching twisted pair when running video baluns over Cat5e/Cat6.
Symptom: Severe background noise, colour bleeding, image darkening or complete breakdown of common-mode noise rejection over distance.
Fix: Ensure the two wires of a single twisted pair maintain matching positive and negative polarity at both ends. Check the positive and negative terminals of the video balun connect strictly to the two wires of a single twisted pair (e.g., Solid Blue + White/Blue).
Internal Camera Hardware Failure
Cause: Aging smoothing capacitors on the camera circuit board or a failing image sensor module.
Symptom: Persistent noise, colour distortion, or rolling lines that remain even when the camera is bench-tested with a 1-metre patch cable directly at the DVR.
Fix: Replace the camera unit.