Line-Lock is a cheap, "quick and dirty" means of minimizing picture "roll" when a video monitor is switched between cameras, most commonly in CCTV security installations.

A line-locking camera roughly synchronizes its video field output to its AC power supply. When the cameras in a surveillance system are all synchronized in this manner, a video monitor doesn't have to significantly resynchronize its display for each camera as it is switched between them, thereby avoiding objectionable picture "roll".

When operating in line-lock mode, color cameras violate video industry standards, frequently causing poor performance in other video equipment built to those standards. NTSC industry standard specifications require that color subcarrier frequency be 3,579,545+10 Hz, horizontal sync frequency be 15,734.263 Hz, and vertical sync frequency be 59.94 Hz. Video equipment must strictly adhere to these frequencies in order to maintain proper phase relationships between these three signals.

Cameras operating in Line-Lock mode violate these standards by synchronizing vertical sync to the 60 Hz (not 59.94 Hz) sine wave from the power line. To maintain proper phasing between horizontal and vertical sync, these cameras further have to violate the horizontal frequency specification by running at 15,750 Hz, not 15,734 Hz.

Colorado Video equipment runs at the horizontal and vertical sync frequencies present at its input. In order to maintain industry mandated phase relationships between vertical, horizontal, and color subcarrier, the subcarrier frequency may be forced too high for some monitors to display color. Many times in this situation, some monitors will display color while others will not.

To set your camera for proper operation, find a switch or jumper on the camera that refers to “line-lock” (sometimes “line-loc”) or “external” and switch it away from that position. Proper settings are referred to as “int”, “internal”, “internal crystal” or line-lock “off”. If this setting is not available, you can install a Time Base Corrector in series at the camera output. It is also possible that running your camera on a power adapter with a DC, not AC, output will solve the problem. It may be necessary to try both polarities when using a DC adapter.