AV Technology - June 2008 - (Page 68)

Video Codecs video codec is a combination of hardware and/or software that creates a binary stream of data that represents the video and audio captured by a camera. Encoders differ from capture devices primarily in what they are intended to create as output. A capture card usually creates a binary stream that will be stored as a file. An encoder usually creates a stream of data that is to be transferred to a second device. This second device has various names such as set-top box or decoder, but it essentially reverses the process carried out by the encoder and re-creates the representation of the scene picked up by the camera. Video codecs basically do three things. If the camera is analog, they sample the output signal. The rate at which this is done is referred to as the sampling rate. Each sample is then converted into a certain number of bits (often 8) during the analog-to-digital conversion process (A/D). This is called quantization. Finally, the codec must do compression on the resulting bit stream because it is usually too much information to be efficiently transmitted. Before any of this processing occurs, the image must be captured by the camera, whether it’s a pan-tilt-zoom camera, a videoconferencing camera, or a digital camcorder. In the U.S., the technique is based on the same process devised in the 1940s when only black and white images were possible. The camera sensor makes scans left to right to create lines, and then scans verti- A cally to record about 480 lines per scene. This creates video frames at the rate of 30 frames per second. The number of positions on the line where the samples are gathered is usually 720 in standard definition TV. Each sampling point on a line is one pixel and corresponds to a position on the output device (video monitor or TV) that will be energized. In monochromatic (black and white) TV, only the brightness of the sampled position needs to be represented. However, when color was added, the chroma value was also captured, which consists of a representation of the amount of red, blue, and green measured by the camera. Through a clever mathematical computation, it was determined that these four signals could be derived by the receiver if only three were transmitted. You can think of the three — Y, Cr, and Cb — as brightness, the red signal, and the blue signal. SAMPLING Imagine the image seen by the camera as being covered with a grid consisting of 480 horizontal lines and 720 vertical lines. The camera could take a sample of brightness and chroma at the center of each square. If each sample were represented with 8 bits for brightness and 10 bits for each of the three colors (10 bits per color is common in the industry), the WHY IS THE BRIGHTNESS SAMPLED MORE OFTEN THAN THE COLOR? OUR EYE IS MORE SENSITIVE TO CHANGES IN BRIGHTNESS THAN IT IS TO CHANGES IN COLOR. resulting bit stream for 30 frames/second would exceed 400 Mb per second! This would clearly not be appropriate for a single standard definition (SD) TV signal. Consequently, the CCIR-601 recommendation adopted in 1982 allowed for a sampling technique called 4:2:2 in which the luminance signal is sampled at 720 positions on each line, but each chrominance signal is sampled at alternate positions 360 times per line. An additional sampling technique called 4:2:0 modifies this procedure and samples the chrominance on every other line vertically. These techniques are called color subsampling. So, why is the brightness sampled more often than the color? Our eye is more sensitive to changes in brightness than it is to changes in color. Therefore, the picture seems more accurate to our mind if we have more data about brightness than about color. These techniques (and many others) were first specified by the Motion Picture Experts Group (MPEG). This group’s mission was, and continues to be, to standardize compressed audio and video. The MPEG methods also account for our sense of motion. Since we are more sensitive to horizontal motion than to vertical motion, more information is transferred to the receiver about horizontal movement. The general category of methods that are based on the way we perceive things is called perceptual coding. Color subsampling and motion estimation are examples of perceptual coding. Such sampling and quantization creates 68 | AV TECHNOLOGY | june 2008 www.avtechnologyonline.com http://www.mrpodium.com http://www.avtechnologyonline.com http://www.mrpodium.com

Table of Contents Feed for the Digital Edition of AV Technology - June 2008

AV Technology - June 2008 Contents Precedent Corporate: Global Standardization Government: Strategic Technology Planning Education: It’s All Just a Click Pre-Empting Projector Breakdown Dealing With Technophobes Keeping Ahead of Technology Obsolescence Understanding Video Codecs Rules of Engagement Network, See & Learn Sound From the Heart Sportsbook Makes an Impact Upgrading an AV Image Product Review: Polycom’s RealPresence Experience HD Tech Horizons: A True AV Networking Standard? Product Spotlight: Digital Media Servers; Flat Panel New Products Ad Index AV MO: Calculating Fiber Optic Transmission Distance

AV Technology - June 2008

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