Video Sample Rate and Bit Depth

The video sample rate of a digital video format determines how often the light intensity of each video line is sampled.

Sample rate
Description
74.25 MHz
HD video luma (Y′) sample rate.
37.125 MHz
HD video chroma (CBCR) sample rate. This is half of the luma sample rate, used for 4:2:2 HD video.
14.3 MHz
Early NTSC digital video recorders sampled video at exactly four times the frequency of the color subcarrier signal (3.58 MHz x 4). This is the origin of the 4 in color sample ratios such as 4:2:2.
13.5 MHz
This is the sample rate for the luma (Y′) channel for SD digital video. This sample rate was chosen to work with both NTSC and PAL digital video. The 4 in 4:2:2 is now represented by this sample rate.
6.75 MHz
This is the sample rate for the color difference channels in 4:2:2 video. This is half of 13.5 MHz.

Color Sample Ratio

Color sample ratio refers to the ratio of luma (Y′) samples to each color difference sample (CB and CR). For example, 4:2:2 color sampling means that for every four pixels of Y′ data stored, only two CR samples and two CB samples are stored. By reducing the number of chroma samples, less color detail is recorded and less bandwidth is required for storage and transmission. Because we are less sensitive to color detail than we are to luma detail, subsampling the chroma signal can be considered perceptually lossless. In absolute terms, chroma subsampling can make processes like chroma keying much more difficult.

Sample ratio
Description
4:4:4
Each R, G, and B channel, or each Y′, CB, and CR channel, is sampled at the same rate. Maximum color detail is maintained.
4:4:4:4
Full sample rate for each color channel, plus a fourth alpha channel at the full sample rate.
4:2:2
The color channels are subsampled so that the color resolution is halved. For example, the first pixel in a line contains Y′, CB, and CR samples. The next pixel contains only a Y′ sample. This pattern repeats. Most professional video formats use 4:2:2 color subsampling.
4:2:2:4
4:2:2 sample rate for each color channel, plus an alpha channel at the full sample rate.
4:1:1
The color is subsampled so that the color resolution is quartered. The first pixel in a line contains Y′, CB, and CR samples. The next three pixels only contain Y′ samples. This pattern repeats.
4:2:0
This ratio indicates that the CB and CR channels are subsampled both horizontally (as in 4:2:2) and vertically. This reduces color resolution in both the horizontal and vertical dimensions compared to 4:2:2, which only reduces horizontal chroma resolution.
There are several methods for locating CB and CR samples relative to Y′ samples, yielding several different 4:2:0 formats.

The following table shows a list of color sample ratios used in various digital video formats.

Sample ratio
Video formats
4:4:4
HDCAM SR
Most RGB computer graphics files (implicit)
4:2:2
Digital Betacam, D-1, D-5, DVCPRO HD, DVCPRO 50, and HDCAM SR
3:1:1
HDCAM
4:1:1
NTSC DV, NTSC DVCAM, and DVCPRO
4:2:0
PAL DV, PAL DVCAM, DVD, and HDV

Bit Depth

The number of bits used per sample determines how accurately the sample is stored, as well as how much intensity variation is possible within the signal. For example, a video signal with a bit depth of only 1 bit can have a value of either 0 or 1, resulting in only black or white pixels. Two bits per sample results in four possible values: 00, 01, 10, or 11, or any of four shades of gray (or some other color) per sample.

Most digital video formats use a minimum of 8 bits per color channel, or 256 gradations of intensity. RGB images are traditionally described by the total bits used per pixel (8 bits per channel x 3 channels = 24 bits). 32-bit RGB images usually have 24-bit color plus 8 more bits for an alpha channel.

Note: Still images using 16 bits per color channel, or 48 bits per RGB pixel, are becoming more common. However, most video formats use 8 or 10 bits per color channel.

Video signal bit depth is usually described per channel. For example, DV and DVCPRO HD use 8 bits per color component (in other words, 8 bits for Y′, 8 bits for CB, and 8 bits for CR). Other formats, such as D-5, use 10 bits per component. This provides 1024 possible gradations instead of 256, which means much more subtle variations in intensity can be recorded.

In fact, 8-bit Y′CBCR video does not use all 256 codes to represent picture information. Black is stored as code 16 and white is code 235. Codes 1–15 and 236–254 are retained for footroom and headroom, respectively. These areas allow for quick spikes in the signal caused by filtering in analog-to-digital conversions and, in the case of white levels, can prevent clipping for highlights that may exceed 235 (white). Levels above 235 are sometimes referred to as super-white levels. For more information about super-white levels, see Rendering and Video Processing Settings.

Internally, Final Cut Pro can do pixel calculations using 32-bit floating-point precision, which allows for very accurate calculations without rounding errors. This leads to much more accurate color reproduction when applying filters and compositing layers of video. This is especially important when you are going to show your movie on film or broadcast-quality video monitors. In Final Cut Pro, the Video Processing tab in the Sequence Settings window allows you to choose the rendering bit depth for a sequence. For more information, see Rendering and Video Processing Settings.

Perceptual Coding and Gamma

The limited number of brightness steps in 8-bit digital video requires efficient use of the 256 available codes. Because perception of brightness follows a power law function, humans are more sensitive to absolute intensity changes in dark areas. In other words, the amount of light required to make a perceptual shift in brightness increases exponentially. Therefore, a gamma correction is applied to video so that the step between each code represents a perceptual shift in brightness. Without this gamma correction, the darker areas would appear to abruptly jump from one brightness level to the next (“banding”) and white levels would waste many codes with imperceptible brightness shifts. This gamma correction is reversed by video monitors so that the viewer sees the original light intensity of the image. For more information about gamma, see Rendering and Video Processing Settings.