SDI based video systems transmit video data frame by frame, line by line, pixel by pixel. So, to transmit one frame of video, the sender must iterate through the entire frame, pixel by pixel. This happens over a span of time. If you have a single camera running into a single monitor, it doesn't matter at what time the camera starts sending video data to the monitor. However, if you have multiple cameras running into a video switcher, then the video switcher running into a monitor, the cameras must be in sync with one another. The way the cameras and the switcher stay in sync with one another is through a sync generator that makes a reference signal. The reference signal instructs the cameras to send the first pixel of the frame all at the same time.
Think of genlock as a master clock for all the production equipment in a video system. If all pieces of equipment in the system that generate a signal receive the genlock signal, they will all be in sync. What it means to be in sync is that all of the pieces of equipment transmit the start of a video frame at the same time.
If you think about a gradient, going from black to white, in the real world, there would be an infinite number of different shades of grey in between black and white. In the digital world, we can't represent all of these shades. We have to break that infinite gradient down into steps.
Let's say we have a three camera video system. The cameras output signal to a video switcher. The video switcher outputs signal to a recorder. In this scenario we have four devices that generate SDI signal that need to be in time with one another, three cameras and the video switcher. The recorder may have an output to monitor it, but we aren't concerned with the sync of that device.
If, in our system, the cameras were not synced together, the switcher would receive video frames from the cameras at different times. This would mean that when you cut from one camera to the next, the switcher will likely cut the image in the middle of the frame. The bottom portion of the camera image will be at the top of the raster and the top portion of the camera image will be at the bottom with a black bar inbetween. This happens because the video switcher isn't in time with the cameras and the cameras aren't in time with each other.
Here, the cameras are outputting their signals at different times, so the switcher is not in sync. Click or tap the sources to change them.
If we introduce a genlock reference to our video system, the reference signal will syncronize the cameras and video switcher to transmit video as the same time. This means that the first pixel of the frame will start at the same time across all of the pieces of equipment.
Here, the cameras are outputting their signals at the same time, so all signals are in sync. Click or tap the sources to change them.
Now that our system is in sync, when we cut a camera on the switcher, the image will appear correctly. The genlock reference signal also instructs the video switcher to switch between two signals at right time. When you press 'cut' on a video switcher, the video switcher waits until the current frame is done processing and then swtiches from one input to the other. Pressing the cut button on a switcher is basically telling it to change to the desired input for the next frame of video. This is the case for starting a dissolve, turning on a keyer, etc.