A known problem in digital video recording is that flickering of the image may be seen during playback if the video is recorded under lighting powered by alternating current. The alternating current has a frequency of 60 Hertz (Hz) or 50 Hz. Because power varies as the square of the current, flickering lighting will have a flicker frequency which is double the mains frequency, which is therefore 120 Hz or 100 Hz. But this 120 Hz or 100 Hz flicker frequency is not the flickering seen during video playback. The flicker during video playback may arise because of a beat effect between the constant but mismatched frequencies of the flickering light source and the frame capture rate of the sensor.
Flickering in video playing may be readily perceived if the video playback flicker frequency is below about 60 Hz. Flickering in video playing may be weakly perceived if the video playback flicker frequency is greater than 60 Hz and below 100 Hz. Flickering in video playing will not be perceived if the video playback flicker frequency is 100 Hz or more.
Digital image cameras may include a digital image sensor, including an active imaging pixel array. The digital image sensor may be programmable, through an interface. The senor may be operated in a default mode, or it may be user-programmed to control the frame size, exposure, or gain setting, for example. The pixel array may include optical black columns and rows around the edges of the array, to monitor the black level, for black level adjustment. Image data may be read out in a progressive scan. Valid image data may be next to horizontal blanking and vertical blanking.
The Pixel Clock is a high frequency pulse train that may determine when the image sensor's data lines have valid data. On the active edge of the pixel clock (which can be either the rising edge or the falling edge, depending on the camera), the digital lines should all have a constant value that is input into the image acquisition device, which latches in the data. The data changes to the next pixel value before the next active edge of the pixel clock, so that the next pixel value will get latched into the image acquisition device. During the image capture process, each pixel accumulates light for a certain time and is then read.
It is known to remove flicker by altering the frame rate of a digital video camera to equal the artificial lighting flicker rate divided by an integer. The idea is that the start of each frame will then have the same light intensity as the corresponding start of all successive frames—so that there will be no light flicker. So in the US, an iPhone6 with native 1080p filming at 60 frames per second (fps) or an iPhone5 with native 720p filming at 30 fps will exhibit no flicker when filming in lighting flickering with a flicker rate of 120 Hz, because 120/60 is exactly 2, and because 120/30 is exactly 4. But those same devices recording video in Europe, with a 100Hz light flicker frequency, may exhibit flicker during video playback because 100/60 is not an integer, and because 100/30 is not an integer. And smartphones do not provide the end-user with any native capability to alter the frame rate to reduce flicker: a typical smartphone might permit recording at one frame rate, so 30 fps, or 60 fps, and another much higher rate for slow-motion recording. But they typically do not permit the kinds of adjustment needed to reduce flicker; this kind of adjustment is however possible in a professional-grade video camera, such as the cameras from the Red Digital Cinema Camera Company.
If the formula fps*N=F*2 is valid, where fps is the frame rate, and F is the mains current frequency (50 Hz or 60 Hz), and the integer number N=1,2, . . . then there will be no visible flicker. This is because in this condition, the frame rate of a digital video camera is equal to the artificial lighting flicker rate divided by an integer.
It is also known to remove flicker by altering the shutter speed of a digital video camera to equal the artificial lighting flicker rate divided by an integer. The idea is that each frame will then include a whole number of cycles of varying light intensity—so again there will be no light flicker. So if your camera shoots at 60 frames per second, and you're in Europe with a 100 Hz light flicker, then you may set your shutter to a constant 1/100 seconds (s). Or if you are shooting at 30 fps with a 100 Hz light flicker, you can set your shutter to any of 1/100 s, 1/50 s, 1/33.3 s. But this approach is only suitable when the environment is not bright. The brighter the environment, the faster the shutter speed should be, otherwise the image will be overexposed.
A drawback of these known approaches to reducing flicker seen in video playback is that they may lead to under-utilization of device video recording capabilities. For example, for a given image resolution, a video recording device may be capable of filming at up to about 50 fps. But under 120Hz light flickering, to eliminate flicker, the frame rate would have to be reduced to 40 fps, because 120/40 is an integer, whereas 120/50 is not an integer. Of course 60 fps would also eliminate flicker, because 120/60 is an integer, but 60 fps is not achievable by the device in this example. It is desirable to fully utilize device video recording capabilities, without leading to flickering in video playback.