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Product Details
| ISBN-13: | 9780080560120 |
|---|---|
| Publisher: | Elsevier Science |
| Publication date: | 10/07/2008 |
| Series: | Instant Access |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 240 |
| File size: | 3 MB |
About the Author
Read an Excerpt
Digital Video and DSP
Instant AccessBy Keith Jack
Newnes
Copyright © 2008 Elsevier Inc.All right reserved.
ISBN: 978-0-08-056012-0
Chapter One
Video Overview
In an Instant
• Video Definitions
• Video Today
• Video Data
• Video Timing
• Video Resolution
• Video and Audio Compression
• Other Video Applications
• Standards Organizations
Video Definitions
Although there are many variations and implementation techniques, video signals are just a way of transferring visual information from one point to another. The information may be from a VCR, DVD player, a channel on the local broadcast, cable television, or satellite system, the Internet, cell phone, MP3 player, or one of many other sources. Invariably, the video information must be transferred from one device to another. It could be from a satellite set-top box or DVD player to a television. Or it could be from one chip to another inside the satellite set-top box or television. Although it seems simple, there are many different requirements, and therefore many different ways of doing it.
This book will cover the engineering essentials of this important technology. First we'll define some video terms and concepts.
A color space is a mathematical representation for a color. Initially video contained only gray scale, or black-and-white, information. When color broadcasts were being developed, attempts were made to transmit color video using RGB (red, green, blue) color space data, but that technique occupied too much bandwidth so other alternative color spaces were developed. They will be covered in more detail in Chapter 2.
Component video is video using three separate color components, such as YCbCr (digital), YPbPr (analog), or R'G'B' (digital or analog). Composite video uses a single signal to contain color, brightness and timing information.
Compression is an important part of video technology. MPEG stands for Moving Picture Experts Group, an international standards group that develops various compression algorithms. MPEG video compression takes advantage of the redundancy on a frame by frame basis of a normal video sequence. There are several different MPEG standards which we'll cover in later chapters.
VIDEO TODAY
A few short years ago, the applications for video were somewhat confined—analog video technology was used for broadcast and cable television, VCRs, set-top boxes, televisions, and camcorders. Since then, there has been a tremendous and rapid conversion to digital video, mostly based on the MPEG-2 video compression standard.
The average consumer now uses digital video every day thanks to continuing falling costs. This trend has led to the development of DVD players and recorders, digital set-top boxes, digital television (DTV), portable video players, and the ability to use the Internet for transferring video data. Equipment for the consumer has also become more sophisticated, supporting a much wider variety of content and interconnectivity. Today we have:
• HD DVD and Blu-ray Players and Recorders. In addition to playing CDs and DVDs, these advanced HD players also support the playback of MPEG-4.10 (H.264), and SMPTE 421M (VC-1) content. Some include an Ethernet connection to enable content from a PC or media server to be easily enjoyed on the television.
• Digital Media Adapters. These small, low-cost boxes use an Ethernet or 802.11 connection to enable content from a PC or media server to be easily enjoyed on any television. Playback of MPEG-2, MPEG-4.10 (H.264), SMPTE 421M (VC-1), and JPEG content is typically supported.
• Digital Set-Top Boxes. Cable and satellite set-top boxes are now including digital video recorder (DVR) capabilities, allowing viewers to enjoy content at their convenience. Use of MPEG-4.10 (H.264) and SMPTE 421M (VC-1) now enables more channels of content and reduces the chance of early product obsolescence.
• Digital Televisions (DTV). In addition to the tuners and decoders being incorporated inside the television, some also include the digital media adapter capability. Support for viewing on-line video content is also growing.
• IPTV Set-Top Boxes. These low-cost set-top boxes are gaining popularity in regions that have high-speed DSL and FTTH (fiber to the home) available. Use of MPEG-4.10 (H.264) and SMPTE 421M (VC-1) reduces the chance of early product obsolescence.
• Portable Media Players. Using an internal hard disc drive (HDD), these players connect to the PC via USB or 802.11 network for downloading a wide variety of content. Playback of MPEG-2, MPEG-4.10 (H.264), SMPTE 421M (VC-1). and JPEG content is typically supported.
• Mobile Video Receivers. Being incorporated into cell phones, MPEG-4.10 (H.264) and SMPTE 421M (VC-1) is used to transmit a high-quality video signal. Example applications are the DMB, DVB-H and DVB-SH standards.
There are many engineering challenges faced when incorporating video into today's product designs. Implementing real-world solutions is not easy, and many engineers have little knowledge or experience in this area. This book is a quick-start guide for those charged with the task of understanding and implementing video features into next-generation designs.
VIDEO DATA
Initially, video contained only gray-scale (also called black-and-white) information. While color broadcasts were being developed, attempts were made to transmit color video using analog RGB (red, green, blue) data. However, this technique occupied three times more bandwidth than the gray-scale solution, so alternate methods were developed that led to using Y, R–Y, and G–Y data to represent color information, where Y represents the luma (black-and-white part), and R–Y and G–Y represent color difference signals made by subtracting the Y from the red and blue components. A technique was then developed to transmit this Y, R–Y, and G–Y information using one signal, instead of three separate signals, and in the same bandwidth as the original gray-scale video signal.
Today, even though there are many ways of representing color video (R G B , YIQ, YCbCr, YPbPr, YUV, and others are covered in Chapter 2), they are still all related mathematically to RGB.
S-Video was developed for connecting consumer equipment together (it is not used for broadcast purposes). It is a set of two analog signals, one grayscale (Y) and one that carries the analog R–Y and B–Y color information in a specific format (also called C or chroma). Once available only for S-VHS, it is now supported on most consumer video products.
Although always used by the professional video market, analog RGB video data has made a temporary comeback for connecting high-end consumer equipment together. Like S-Video, it is not used for broadcast purposes.
Insider Info
A variation of the Y, R–Y, and G–Y video signals, called YPbPr, is now commonly used for connecting consumer video products together. Its primary advantage is the ability to transfer high-definition video between consumer products. Some manufacturers incorrectly label the YPbPr connectors YUV, YCbCr, or Y(B-Y) (R-Y).
Digital Video
The most common digital signals used are RGB and YCbCr. RGB is simply the digitized version of the analog RGB video signals. YCbCr is basically the digitized version of the analog YPbPr video signals, and is the format used by DVD and digital television.
Technology Trade-offs
There is always the question of "What is the best connection method for equipment?" For DVD players and digital cable/satellite/terrestrial set-top boxes, the typical order of decreasing video quality is:
1. HDMI (digital YCbCr)
2. HDMI (digital RGB)
3. Analog YPbPr
4. Analog RGB
5. Analog S-Video
6. Analog Composite
Some will disagree about the order. However, most consumer products do digital video processing in the YCbCr color space. Therefore, using YCbCr as the interconnect for equipment reduces the number of color space conversions required. Color space conversion of digital signals is still preferable to D/A (digitalto-analog) conversion followed by A/D (analog-to-digital) conversion, hence the positioning of HDMI RGB above analog YPbPr. The computer industry has standardized on analog and digital RGB for connecting to the computer monitor.
VIDEO TIMING
Although it looks like video is continuous motion, it is actually a series of still images, changing fast enough that it looks like continuous motion, as shown in Figure 1.1. This typically occurs 50 or 60 times per second for consumer video, and 70–90 times per second for computer displays. Special timing information, called vertical sync, is used to indicate when a new image is starting.
Each still image is also composed of scan lines, lines of data that occur sequentially one after another down the display, as shown in Figure 1.1. Additional timing information, called horizontal sync, is used to indicate when a new scan line is starting.
The vertical and horizontal sync information is usually transferred in one of three ways:
1. Separate horizontal and vertical sync signals
2. Separate composite sync signal
3. Composite sync signal embedded within the video signal
The composite sync signal is a combination of both vertical and horizontal sync.
Insider Info
Computer and consumer equipment that uses analog RGB video usually uses technique 1 or 2. Consumer equipment that supports composite video or analog YPbPr video usually uses technique 3. For digital video, either technique 1 is commonly used or timing code words are embedded within the digital video stream.
Interlaced vs. Progressive
Since video is a series of still images, it makes sense to simply display each full image consecutively, one after another.
This is the basic technique of progressive, or noninterlaced, displays. For progressive displays that "paint" an image on the screen, such as a CRT, each image is displayed starting at the top left corner of the display, moving to the right edge of the display. The scanning then moves down one line, and repeats scanning left-to-right. This process is repeated until the entire screen is refreshed, as seen in Figure 1.2.
In the early days of television, a technique called interlacing was developed to reduce the amount of information sent for each image. By transferring the odd-numbered lines, followed by the even-numbered lines (as shown in Figure 1.3), the amount of information sent for each image was halved. Today, most broadcasts (including HDTV) are still transmitted as interlaced. Most CRT-based displays are still interlaced while LCD, plasma, and computer displays are progressive.
FAQs
Given the advantage of interlacing, why bother to use progressive?
With interlace, each scan line is refreshed half as often as it would be if it were a progressive display. Therefore, to avoid line flicker on sharp edges due to a too-low frame rate, the line-to-line changes are limited, essentially by vertically lowpass filtering the image. A progressive display has no limit on the line-to-line changes, so it is capable of providing a higher-resolution image (vertically) without flicker.
VIDEO RESOLUTION
Video resolution is one of those "fuzzy" things in life. It is common to see video resolutions quoted as "720 × 480" or "1920 × 1080." However, those are just the number of horizontal samples and vertical scan lines, and do not necessarily convey the amount of useful information.
For example, an analog video signal can be sampled at 13.5MHz to generate 720 samples per line. Sampling the same signal at 27MHz would generate 1440 samples per line. However, only the number of samples per line has changed, not the resolution of the content.
(Continues...)
Excerpted from Digital Video and DSP by Keith Jack Copyright © 2008 by Elsevier Inc.. Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
1: Introduction 2: Introduction to Video 3: Color Spaces 4: Video Signals Overview 5: Analog Video Interfaces 6: Digital Video Interfaces 7: Digital Video ProcessingWhat People are Saying About This
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