Ghosting (television)

Lore & Background

Common causes of ghosts include mismatched impedance along the communication channel, which causes unwanted reflections—a phenomenon known as ringing—and multipath distortion, where radio frequency waves take paths of different length by reflecting from buildings, transmission lines, aircraft, clouds, and other objects. RF leaks may also allow a signal to enter the set by a different path, most common in large buildings such as tower blocks or hotels where one TV antenna feeds many rooms, causing a condition called pre-echo. If the ghost appears on the left of the main picture, it is likely pre-echo, which can occur when an additional aerial is plugged into a TV already connected to a communal TV aerial or cable TV.

More severe cases of ghosting can show the ghost with a different hue than intended, as ghosting can change the phase of the color burst, confusing the television set and displaying the wrong color. Ghosting is not specific to analog transmission; it may appear in digital television when interlaced video is incorrectly deinterlaced for display on progressive-scan output devices. The mechanisms that cause ghosting in analog television may corrupt the signal beyond use for digital television, and modulation schemes such as 8VSB and COFDM seek to correct this.

Reader's Guide

Ghosting has been a persistent issue in television reception, particularly for analog broadcasts where the video signal uses AM. The phenomenon is notable because it directly degrades picture quality by superimposing a delayed replica of the image, and it can arise from a variety of environmental and system-level factors. In the 1960s and 1970s, consumer devices such as a simple resistive attenuator were sold as 'ghost eliminators' to make ghosting less visible. The problem is especially challenging in urban environments with many reflective surfaces, and in large buildings with shared antenna systems where RF leaks cause pre-echo. While the audio portion of analog TV is largely immune due to FM's capture effect, the video remains vulnerable. In digital television, ghosting can still occur from incorrect deinterlacing, and the same multipath interference that causes analog ghosts can render a digital signal unusable, driving the development of robust modulation techniques like 8VSB and COFDM. The legacy of ghosting is thus tied to the evolution of broadcast engineering, from simple attenuation fixes to sophisticated digital error correction.

Did You Know?

• Ghosting is a problem specific to the video portion of television because it uses AM; the audio portion uses FM and benefits from the capture effect.
• A 'ghost eliminator' sold to consumers in the 1960s and 1970s was a simple resistive attenuator.
• In SECAM TV, ghosting only affects the luma portion because the chrominance signal uses FM.
• If a ghost appears on the left of the main picture, it is likely pre-echo caused by an RF leak in a building with long TV downleads.