4:4:4 Versus 4:2:2 Color Sampling
Digital video is described by several parameters, including its resolution (ie. 1920 x 1080), color space (ie. REC 709), color sampling (ie. 4:4:4), and bit depth (ie. 10-bit). Higher rates of color sampling and higher bit depths result in more accurate color and more total colors contained in the video signal. The SDI I/O on Sound Devices PIX 240i and PIX 260i recorders support 12-bit 4:4:4 video over 3G-SDI and 10-bit 4:2:2 video over HD-SDI. For many applications, 4:4:4 color is a significant benefit, extending the usability of the recorder.
Human vision is more sensitive to differences in image brightness (luminance or luma) than to color differences (chroma). Sub-sampling of the color information takes advantage of these human perceptual differences to reduce the data rate of digital video. Color sub-sampling measures (samples) less color information than luminance information. The more aggressive the color sampling method, the more potential for perceptible color reduction or inaccuracy. Color sub-sampling, and the reduction of a video data rate by using more aggressive color sub-sampling, is independent of video codec.
What the 4’s Mean in 4:4:4
Color sampling is described by three numbers with separated by colons, for instance 4:4:4 and 4:2:2. While the specifics of what the individual values in the triplet mean is outside of this discussion (see more on color sub-sampling here, PDF), it is important to know what the results are with different sub-sampling methods. 4:4:4 indicates full, continuous RGB color sampling. With 4:4:4 color, luminance (brightness) and chroma (color) are sampled at the same “resolution”, resulting in an RBG color palette as big as the video sampling rate. 4:4:4 is a benefit in numerous applications, including compositing, color correction, and multi-generational editing.
4:4:4 is the biggest container available for color in video. For HD rate signals, it requires either dual-link HD-SDI (2 x 1.5 Gb/s data rate) or 3G-SDI (3 Gb/s data rate). While lower color space signals can be represented in a 4:4:4 container, when a 4:4:4 signal is sent to an input that only supports 4:2:2 color, a data mismatch results and no video is transferred.
Less than 4:4:4
While 4:4:4 is full RGB color, color sub-sampling such as 4:2:2, 4:1:1, and 4:2:0 reduce the color data. The chart below shows the amount of color information reduction with each of these sub-sampling methods.
|Color Sampling System||Data Rates of Digital Video||Notes|
|4:4:4 (RGB)||3G-SDI connection (3 Gb/s uncompressed for HD) at 30 fps; File data rate of 330 Mb/s for Apple ProRes 4444 file||Full color sampling. Luma and chroma are sampled at the same rate, resulting in full RGB. 4:4:4 has no color compression. Apple ProRes 4444 is a 4:4:4 color sampling codec, supported on the PIX 240i and PIX 260i recorders.|
|4:2:2||HD-SDI (1.5 Gb/s) for 30 fps; File data rate of 220 Mb/s for Apple ProRes 422HQ file||50% reduction in color information versus 4:4:4, sub-sampling chroma by two. Recorded, compressed ProRes files result in an overall data rate reduction of 33% versus full RGB (4:4:4) encoding. Apple ProRes LT, ProRes, and ProRes HQ are 4:2:2 codecs.|
|4:1:1||HD-SDI used for interconnection; File data rates range from 1 Mb/s to 100 Mb/s depending on codec||75% reduction in color encoding versus RGB color. The color encoding topology used in DV video.|
|4:2:0||HD-SDI used for interconnection; File data rates range from 1 Mb/s to 100 Mb/s depending on codec||75% reduction in color encoding versus RGB color. The color encoding system used in many MPEG codecs.|
When to Use 4:4:4
Many high-budget, hour-long episodic shows are shot on ARRI ALEXA cameras and record high-definition Apple ProRes 4444 files. This material is subject to extensive, precision color grading. Because the ALEXA camera outputs 4:4:4 recording to a 4:4:4 capable codec (Apple ProRes 4444), this material has extensive latitude for color correction. It holds up very well in post production.
Chroma keying benefits greatly when recording material on stage with 4:4:4 color. Keying systems use the narrow color spectrum of a “green screen” or “blue screen” background to remove the background from the subject. Accurate color yields better transitions between subjects and backgrounds than when shot with less color data.
When 4:4:4 is Overkill
To the human eye, a 4:4:4 original video file is indistinguishable from that file output at 4:2:2. Because of this, not all applications benefit from remaining in 4:4:4. As shown in the chart above, 4:4:4 is twice the uncompressed data rate of 4:2:2 and an Apple ProRes 4444 signal is 50% more than a 4:2:2 Apple ProRes HQ signal. Recording a file with 4:4:4 color sampling has no benefit unless the source device outputs 4:4:4. For instance, while the PIX 240i can record Apple ProRes 4444 files from HD-SDI sources, recording from a camera with a 4:2:2 output in Apple ProRes 4444 offers no additional color data in the resulting file.
Numerous production video cameras record to internal MPEG-based codecs which use 4:1:1 or 4:2:0 color sub-sampling. Many of these cameras have uncompressed video outputs (on HD-SDI or HDMI) that offer 4:2:2 color sampling. In these applications, recording to a PIX 220i or PIX 240i in 4:2:2 offers a noticeable improvement in the file and its ability to be color corrected.