Video quality is a characteristic of a video passed through a video transmission/processing system, a formal or informal measure of perceived video degradation (typically, compared to the original video). Video processing systems may introduce some amounts of distortion or artifacts in the video signal, so video quality evaluation is an important problem.
Video records at 50 (Eurasia) or 60[4] (US & Japan) images per second (ips) depending on the national system used; The flicker or refresh rate on a television screen is fixed to one or the other nationally chosen standards. A technique called interlace uses persistence of vision to combine two consecutive images (or fields) to create one frame with higher detail in non-moving areas. Because the fields are exposed and displayed separately, a single TV "frame" can potentially contain motion o
r even two distinct images.
With ordinary video from video cameras, the flicker rate and the image rate are the same. However, when footage shot on 24 Hz film is shown on 60 Hz TV, each film frame is repeated for 2.5 consecutive fields to produce 60 fields per second. (see 3:2 pulldown) In countries using 50 Hz TV, 24 frame/s film is sped up by 4% to produce 25 frames (50 fields) per second.
Many modern video systems also decouple display from image update, for example, systems using LCD or plasma panels with continuous light output, or intermediate frame buffers that increase the display rate to 100 or 120 fields per second. Such implementations can occur on low-flicker purpose-built CRT TVs, but decoupling can happen inadvertently on any display connected to a HTPC.
Objective video evaluation techniques are mathematical models that approximate results of subjective quality assessment, but are based on criteria and metrics that can be measured objectively and automatically evaluated by a computer program. Objective methods are classified based on the availability of the original video signal, which is considered to be of high quality (generally not compressed). Therefore, they can be classified as Full Reference Methods (FR), Reduced Reference Methods (RR) and No-Reference Methods (NR). FR metrics compute the quality difference by comparing every pixel in each image of the distorted video to its corresponding pixel in the original video. RR metrics extract some features of both videos and compare them to give a quality score. They are used when all the original video is not available, e.g. in a transmission with a limited bandwidth. NR metrics try to assess the quality of a distorted video without any reference to the original video. These metrics are usually used when the video coding method is known.
The most traditional ways of evaluating quality of digital video processing system (e.g. video codec like DivX, XviD) are calculation of the Signal-to-noise ratio (SNR) and peak signal-to-noise ratio (PSNR) between the original video signal and signal passed through this system. PSNR is the most widely used objective video quality metric. However, PSNR values do not perfectly correlate with a perceived visual quality due to non-linear behavior of human visual system. Recently a number of more complicated and precise metrics were developed, for example UQI, VQM, PEVQ, SSIM and CZD.
The performances of an objective video quality metric are evaluated by computing the correlation between the objective scores and the subjective tests results. The latters are called Mean Opinion Score (MOS). The most frequently used correlation coefficients are : linear correlation coefficient, Spearman's rank correlation coefficient, Kurtosis, Kappa coefficient and Outliers Ratio.
When estimating quality of a video codec, all the mentioned objective methods may require repeating post-encoding tests in order to determine the encoding parameters that satisfy a required level of visual quality, making them time consuming, complex and impractical for implementation in real commercial applications. For this reason, a lot of research has been focused on developing novel objective evaluation methods which enable prediction of the perceived quality level of the encoded video before the actual encoding is performed [1].
Aside from a few configurations used in the early 1990s, computer monitors do not use interlacing. They may sometimes seem to flicker, especially in a brightly lit room. This is due to the greater likelihood that a computer monitor will occupy the viewer's peripheral vision, where sensitivity to flickering is greater. Generally, a refresh rate of 85 Hz or above (as found in most modern monitors) is sufficient to minimize flicker at close viewing distances, and all recent computer monitors are capable of at least that rate. Flat-panel Liquid Crystal Display (LCD) monitors do not suffer from flicker even if their refresh rate is 60 Hz or even lower. This is because LCD pixels open to allow a continuous stream of light to pass through until instructed by the video signal to produce a darker color; see also ghosting. CRTs by comparison create a momentary burst of light each time the electron beam strikes a particular point on the CRT.
