Hack 32. Record a Television Screen Without Flicker

If you are shooting a scene in which a television or computer monitor is visible, you might notice a flicker occurring on the screen. You can remove, or at least reduce, the flicker by making a few minor adjustments.

It's not uncommon to be watching a video and notice a television screen or computer monitor in the background flickering. The flicker is caused by the screen's presentation of moving images, through a process called refreshing, being at a different rate than you are recording. Depending on your situation, you might be able to remove the flicker completely, or at least minimize it so that it isn't a distraction for your viewers.

3.8.1. Recording a Television Screen

Shooting a television screen is actually easier than shooting a computer monitor. Because your television and video camera are recording at the same frame rate, you'll probably notice a slow, crawling flicker. The flicker occurs because of the slight difference between when the television displays a frame and when your camera captures it.

Within North America, a 60Hz current powers each television set, while Europe uses a 50Hz current. If you do a quick calculation, you'll notice that NTSC runs at 30 fps and PAL runs at 25 fps. Coincidence? No. The frame rate is directly tied to the electrical current. For a technical explanation, visit http://en.wikipedia.org/wiki/Interlace.

In simpler terms, when you turn on your camera, it begins its frame rate. This frame rate begins at a different time than the frame rate of your television. The fraction of a second difference causes the flicker to appear. Figure 3-19 shows a television screen mid flicker, as recorded using a Panasonic PV GS-120 MiniDV camera. Notice the large grayish band running across the top of screen; it is part of what causes the flicker to occur.

The solution to reducing, and usually removing, the flicker is to turn off your camera's image stabilization. For example, the Panasonic PV GS-120 has a feature called Electronic Image Stabilization (EIS). By turning EIS off, the camera will record a flicker-free television screen. Figure 3-20 shows a television screen as recorded using a Panasonic PV GS-120 with EIS turned off.

Figure 3-19. A flickering television screen

Figure 3-20. Turning off image stabilization to remove the flicker

When using a camera that records 24p, such as the Panasonic AG-DVX100, you need to change your frame rate from 24 fps to the appropriate rate (e.g., 29.97 fps for NTSC) in order to record a flicker-free image. This is in addition to turning off any image stabilization.

3.8.2. Recording a Computer Monitor

Capturing a screen of a computer monitor is only slightly more difficult, because televisions refresh based on the electrical current, which is the same for every television. Computer monitors, on the other hand, refresh at different rates, based on the user's preferences and the computer's capability.

The refresh rate of the monitor will reflect in the visible speed of flicker. Put another way, the higher the refresh rate, the faster the flicker will appear. Figure 3-21 shows a frame of video, recorded of a monitor set to refresh at 85Hz.

Figure 3-21. A monitor refreshing at 85Hz, causing a fast flicker

To reduce the speed of the flicker, you should change the monitor to a lower refresh rate that is a multiple of your frame rate. So, if you are recording at 29. 97 fps, you should set the monitor to refresh at 60Hz, 90Hz, or 120Hz. (Yes, I'm rounding; monitors don't offer rates like 59.94Hz.) Figure 3-22 shows a frame of video, recorded from a monitor set to refresh at 60Hz.

Figure 3-22. A monitor refreshing at 60Hz, causing a slow flicker

As when dealing with the flicker from a television screen, if you turn off the camera's image stabilization feature, you will dramatically reduce the presence of the flicker. However, because the refresh rates are slightly different than you need, you will still see a slight flicker, which I feel is more of a slow scan. Figure 3-23 shows a monitor at 60Hz with the video camera's image stabilization feature turned off. At 60Hz and image stabilization off, there is still a slight flicker. Notice the small light blue line about two-thirds of the way down the screen.

Figure 3-23. A slight flicker with image stabilization turned off

Even though you (more than likely) can't completely remove the flicker while shooting a computer monitor, being able to reduce it to an almost imperceptible level will help keep your viewer's attention.