What are retrace lines on a tv?

Answer 1

Why TV Retrace Lines OccurI am no expert, but I will try to explain A standard TV screen is made up of 480 lines from top to bottom. Retrace lines will only happen on a TV with a regular Picture tube (known as a CRT), they will not happen on a LCD or Plasma screen. So before I say how retrace lines happen, lets first get little understanding of how the TV puts an image on a CRT. The lines on a CRT are made by a device in the back (known as the neck)of the picture tube called the electron gun. By use of magnetic fields controlled through a device called a Yoke mounted on the neck of the CRT. Now the electron beam emitted by the electron gun is made to go across the front of the CRT screen that is coated with a layer of Phosphor that glows when the electron hits it. When that beam starts at one side reaches the opposite side of the picture tube it has to quickly return back to that first side in order to make that next line. On a standard TV, in order to make the image of a single frame, it creates this line across the screen 240 times from top to bottom, it then returns to the top and starts down again with a second set of 240 lines that fill in between the first 240 lines to create one full frame image of 480 lines, and it does this 30 times a second. Now the retrace lines you are asking about happens when a circuit in the TV known as the Blanking Circuit, fails causing the Beam that is moving from the bottom to the top of the screen to become visible.