Updated: 03/19/2006
Digital Compression In More Detail
All commonly used digital video formats use some level of compression. Top-of-the-line digital camcorders use a "no compromise" digital 4:2:2 format, which has minimal compression, with the luma and chrominance recorded separately. This format also has a much higher digital sampling rate.
Other camcorders use a 4:1:1 format (such as DV, DVCAM, DVCPRO, etc.), which depend on a greater level of compression, resulting in some minimal compromises in video quality.
Although an initial side-by-side comparison of the 4:2:2 and 4:1:1 may reveal little or no difference, the quality advantage of 4:2:2 becomes apparent when editing, copying, and complex special effects are required.
The chart below may help in visualizing the different sampling formats. The B&W (black and white) designation shows the luma samples per given interval; the Color (chroma) designation represents the number of times the chrominance is sampled during that interval.
4:4:4 Format
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
B&W/Color
4:2:2 Format
B&W/Color
B&W
B&W/Color
B&W
B&W/Color
B&W
B&W/Color
B&W
B&W/Color
B&W
B&W/Color
B&W
B&W/Color
B&W
B&W/Color
B&W
4:2:0 Format
B&W/Color
B&W
B&W/Color
B&W
B&W
B&W
B&W
B&W
B&W/Color
B&W
B&W/Color
B&W
B&W
B&W
B&W
B&W
4:1:1 Format
B&W/Color
B&W
B&W
B&W
B&W/Color
B&W
B&W
B&W
B&W/Color
B&W
B&W
B&W
B&W/Color
B&W
B&W
B&W
The more times the color is sampled the better the picture quality; but, also, the more bandwidth involved and the more demanding the digital recording process becomes.
The 4:4:4 sampling process is associated with some of the high-end, professional equipment which we'll cover in Module 49. Quality compromises are made in anything less than 4:4:4, although it may not always be readily noticeable.
MPEG and JPEG Compression
The 4:2:2 format associated with a widely-used compression method (known as MPEG2 or MPEG 422) is one of the most popular formats in professional work. It represents a practical middle ground between optimized quality and technical practicability.
To reduce editing problems, jpeg compression is used instead of MPEG compression in some cases. This is discussed here .
Like MPEG compression, JPEG compression is not without resulting artifacts. The two photos below show different levels of JPEG compression. The first photo has minimal compression; the second is compressed to such a degree that important detail is lost. You might also notice little "globs" or "blocks" of data, especially in the bottom right-hand area of the overly compressed photo.
Additional pictures showing the effects of video compression can also be found in this module .
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