Brief Description of Video and Image Formats


Digital Video

D1 -  is a component format. The extremely high cost of D1 recorders limits their use to only the very elite production facilities that incorporate many special effects with multiple layering of the video signal. Such special effects layering does not degrade the image quality in the pure D1 digital environment. While most professional grade digital video is sampled at a rate of 4:2:2, or 4 samples of Y, 2 of (R-Y), 2 of (B-Y), D1 is 4:2:2:4. This 4:2:2:4 sampling permits an alpha (transparency, or linear keying) channel to be recorded. (Sampling Ratio 4:2:2:4; max. Data Rate= 270Mbps; Bits Per Sample= 8 or 10; Compression= none)

D2 - This is a composite format, but the quality is so high that signal degradation due to the mixing of video information is kept to a minimum. D2 is not a pure digital format, as the inputs and outputs are standard analog composite ports. Although this may degrade the digital signal somewhat, it does offer the advantage of integrating D2 with other existing equipment. Professional composite recorders use 4:0:0, because they sample the composite picture-imbedded colors and all-4 times, then 4 more times, etc. Since there are no color components, to sample, the other numbers are zero. (Sampling Ratio 4:0:0; max. Data Rate= 143Mbps; Bits Per Sample= 8; Compression= none).

D3 - D3 is also a composite format like D2. (Sampling Ratio 4:0:0; max. Data Rate= 143Mbps; Bits Per Sample= 8; Compression= none)

D5 - This is a component format rather than composite. D5 is the newest digital format and its common use and acceptance have yet to be determined. All digital formats on this page sample the Y component at 13.5 MHz but D5 is switchable to 18 MHz for HDTV. (Sampling Ratio 4:2:2; max. Data Rate= 170Mbps; Bits Per Sample= 10; Compression= none)

Digital Betacam - This is the most commonly used video format in post production due to excellent picture reproduction and reasonable cost.(Sampling Ratio 4:2:2; max. Data Rate= 90Mbps; Bits Per Sample= 10; Compression= 2:1)

DVCPRO50 - Panasonic's  DVCPRO50 use two DV codecs, thereby doubling the data rate to 50 Mbps.  (Sampling Ratio 4:2:2; max. Data Rate= 50Mbps; Bits Per Sample= 8; Compression= 3.3:1*)

DV (DV25) - This is the lesser of the 'low-end' DV formats. MiniDV consumer cameras are the least desireable. I will only give specs for the more ‘professional’ forms of DV25.

DVCAM**:  (Sampling Ratio 4:1:1 –NTSC, 4:2:0 -PAL; max. Data Rate= 25Mbps; Bits Per Sample= 8; Compression= 5:1*).

DVCPRO**: (Sampling Ratio 4:1:1 –NTSC, 4:1:1 -PAL; max. Data Rate= 25Mbps; Bits Per Sample= 8; Compression= 5:1*).

*The DV compression formats (DV, DVCAM, and DVCPro) are like MJPEG, in that they compress only a single frame at a time. They are both intra-frame compression formats.  MPEG, on the other hand, is an inter-frame compression format.  Several frames at once are considered while performing the encoding operations. Another thing in common with all three formats is that they use the Discrete Cosine Transform to convert the pixel data into a form that is easier to compress with less artifacts. DV and MJPEG are different though.  DV uses a more sophisticated way of packing the encoded bits than does MJPEG. Also there is more flexibility in DV to handle complex and bland areas of the frame so that encoding is more efficient.  As a result DV, on average, gives you better image quality than does MJPEG when both are compressed at the same bitrate.

**I have read conflicting technical data on the DV formats, but I believe the info above is correct. Generally, DVCAM and DVCPRO run the tape faster and use wider track areas to improve picture quality.


 

RGB vs. YUV

There are many possible color space definitions, but I will address those commonly used in video production. Digital imagery often uses the red/green/blue color space, known simply as RGB. Intensity-chromaticity color spaces, YUV and YIQ, are used for television broadcast. In addition to kYUV219, Final Cut Pro contains two different RGB colorspaces, kRGB219 and kRGB255. kRGB255 is the standard computer color space, the same as used by Photoshop, After Effects and most other computer graphics applications. kRGB219 is a restricted color space, designed to clamp values to match YUV broadcast standards.

Avid's Meridien systems keep the signal in YUV throughout, except for the Symphony Color Correction, which uses 10 bit RGB to avoid clipping and rounding errors. A common problem solved with color correction is color shift that happens during the processing of film stock. This problem is typically a non-linear shift in the CMY color space that film stock uses. It can only be solved in RGB space, the inverse of CMY. You can't fix these problems in YUV space. Older Avid products that use the ABVB take a YUV input and efficiently convert it to RGB, then convert again to YUV on output to tape.

 

Most graphics applications work in RGB and many which work with broadcast video regularly move between color spaces without any degradation of the image. The bottom line is that when the image goes to broadcast, it will be at the most restrictive of the possible color spaces, YUV or YIQ.

 

Red, Green, Blue (RGB): RGB is an additive color model that is used for light-emitting devices, such as CRT displays (note: CMY is a subtractive model that is used often for printers). RGB can be thought of as three grayscale images (usually referred to as channels) representing the light values of Red, Green and Blue. Combining these three channels of light produces a wide range of visible colors. All color spaces are three-dimensional orthogonal coordinate systems, meaning that there are three axes (in this case the red, green, and blue color intensities) that are perpendicular to one another. The red intensity starts at zero at the origin and increases along one of the axes. Similarly, green and blue intensities start at the origin and increase along their axes. Because each color can only have values between zero and some maximum intensity (255 for 8-bit depth), the resulting structure is a cube. We can define any color simply by giving its red, green, and blue values, or coordinates, within the color cube. These coordinates are usually represented as an ordered triplet - the red, green, and blue intensity values enclosed within parentheses as (red, green, blue). Where the best video formats are sampled at 4:2:2, RGB graphics are sampled at 4:4:4, but it is not practical to broadcast the much information, particularly when much of it is beyond our ability to perceive. YUV (Y’C B C R) reduces the amount of information required to reproduce an acceptable video image.

Luminance-Chrominance (YUV & YIQ): Broadcast TV uses color spaces based on luminance and chrominance, which correspond to brightness and color. These color spaces are denoted as YUV and YIQ. The YUV space is used for the PAL broadcast television system used in Europe and the YIQ color space is used for the NTSC broadcast standard in North America. The two methods are nearly identical, using slightly different equations to transform to and from RGB color space. In both systems, Y is the luminance (brightness) component and the I and Q (or U and V) are the chrominance (color) components. These are the variables that are changed by the brightness, color, and tint controls on a television.

The principle advantage of using YUV or YIQ for broadcast is that the amount of information needed to define a color television image is greatly reduced. However, this compression restricts the color range in these images. Many colors that appear on a computer display cannot be recreated on a television screen due to the limits of the YUV and YIQ standards.

For a clear and more technical comparison of RGB and YUV read 422-RGB Conversions, a whitepaper by Charles Poynton.


Terms

HSB (Hue, Saturation, and Brightness) - this color space is convenient for doing color correction work. HSB is also called HLS (Hue, Luminance, and Saturation ). Hue is the position of a color along the spectrum. Hue can be described as any particular color like red, or green. Luminance is the brightness of the image on a gradient scale from white to black. Raising the luminance channel can turn dark maroon into a bright red. Saturation is the amount of color (chrominance) which also moves on the gradient scale from white to black. Lowering the saturation can turn your bright red into a pink color.

YIQ - is a color space derived from the NTSC television color standard. Y is the luminance channel, I is the red-cyan channel (in-phase), while Q is the magenta-green channel (quadrature).

YUV - is a color space derived from the PAL television color standard. Y is the luminance channel, U is the blue channel, and V is the red channel.

CMYK - is a color space derived from the color separations used in commercial print shops. The standard inks for printing are Cyan, Magenta, Yellow, and Black. Thus, CMYK.

RGB - is the color space, which uses a number for each color. One for red, one for green, and one for blue. This number can be used to describe a precise color in the spectrum. An example of this would be, 8 bits defines 256 color scale, 16 bits defines 65,536 colors, and 24 bit defines 16.7 million colors. If you add the bits together you get the color depth of that image.

There are a few image formats that include yet another number which defines an alpha channel. This is known as an RGBA file. There is another important factor in all this as well, the color depth of an image can be described by the total number of bits, and by the number of bits in each channel. Most common DV formats use 8 bits per sample, which results in 256 levels of image brightness. This is quite satisfactory, but a few of the higher end formats use 10 bits per sample, permitting far more gradations in brightness yielding smoother pictures with a greater signal-to-noise ratio. A 10 bit image has four times the information of an 8 bit image.


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