9-3   HDTV

HDTV has been getting media attention for several years now，and if you go to an electronics store you can see a fairly good selection of HDTV sets today. If you have ever looked at one of these sets，you know that the image they display is sharper and wider 一it is more like a movie screen than it is a TV set！

HDTV has lifelike pictures and digital sound. The higher resolution produces clarity like you have never seen from a picture tube. Films retain their original width, enhancing your home theater experience. Imagine seeing more of a football field or a scenic panorama！

In analog TV，a 6 MHz analog signal carries intensity and color information for each scan line of the picture. An analog TV signal in the U. S. has 525 scan lines for the image，and each image is refreshed every 30th of a second（half of the scan lines are painted every sixtieth of a second in what is called an interlaced display）. The horizontal resolution is something like 500 dots for a color set. This level of resolution was amazing 50 years ago，but today it is rather passed. The lowest resolution computer monitor that anyone uses today has 640 X 480 pixels，and most people use a resolution like 800 X 600 or 1024 X 768. We have grown comfortable with the great clarity and solidity of a computer display，and analog TV technology pales by comparison. Many of the new satellite systems，as well as DVDs，use a digital encoding scheme that provides much clearer picture. In these systems，the digital information is converted to the analog format to display it on your analog TV. The image looks great compared to a VHS tape， but it would be twice as good if the conversion to analog didn’t happen. There is now a big push underway to convert all TV sets from analog to digital，so that digital signals drive your TV set directly. When you read and hear people talking about digital television (DTV)，what they are talking about is the transmission of pure digital television signals，along with the reception and display of those signals on a digital TV set. The digital signals might be broadcast over the air or transmitted by a cable or satellite system to your home. In your home，a decoder receives the signal and uses it，in digital form，to directly drive your digital TV set. There is a class of digital television that is getting a lot of press right now. It is called high-definition television，or HDTV. HDTV is high-resolution digital television（DTV）combined with Dolby Digital surround sound（AC-3）．HDTV is the highest DTV resolution in the new set of standards. This combination creates a stunning image with stunning sound. HDTV requires new production and transmission equipment at the HDTV stations，as well as new equipment for reception by the consumer. The higher resolution picture is the main selling point for HDTV. Imagine 720 or 1080 lines of resolution compared to the 525 lines people are used to in the United States（or the 625 lines in Europe）- it’s a huge difference！Of the 18 DTV formats，six are HDTV formats，five of which are based on progressive scanning and one on interlaced scanning. Of the remaining formats，eight are SDTV（four wide-screen formats with 16:9 aspect ratios，and four conventional formats with 4:3 aspect ratios），and the remaining four are video graphics array（VGA）formats. Stations are free to choose which formats to broadcast. The formats used in HDTV are：

u 720p  一 1280 X 720 pixels progressive

u 1080i 一 1920 X 1080 pixels interlaced

u 1080p 一 1920 X 1080 pixels progressive

‘Interlaced’ or ‘progressive’ refers to the scanning system. In an interlaced format，the screen shows every odd line at one scan of the screen，and then follows that up with the even lines in a second scan. Since there are 30 frames shown per second，the screen shows one half of the frame every sixtieth of a second. For smaller screens，this is less noticeable. As screens get larger，the problem with interlacing is flicker. Progressive scanning shows the whole picture，every line in one showing，every sixtieth of a second. This provides for a much smoother picture，but uses slightly more bandwidth.

MPEG-2＆MPEG-4

Broadcasters are having to squeeze the increased picture detail and higher quality surround sound into the same 6 megahertz（MHz) bandwidth used by analog television. Compression software，very similar to what is used in personal computing，allows this to happen. Digital TV relies on a compression and encoding scheme known as MPEG-2 to fit its stunning images into a reasonable amount of bandwidth. In each image，the MPEG-2 software records just enough of the picture without making it look like something is missing. In subsequent frames，the software only records changes to the image and leaves the rest of the image as-is from the previous frame.[1] MPEG-2 reduces the amount of data by about 55 to 1．MPEG-2 already is the industry standard for DVD videos and some of the satellite TV broadcast systems.  Compression reduces image quality from what is seen by the digital camera at the studio. However．MPEG-2 is very good at throwing away image detail that the human eye ignores anyway. The quality of the image is very good，and significantly better than traditional analog TV. The use of MPEG-2 permits an HDTV receiver to interact with computer multimedia applications directly. For example，an HDTV show could be recorded on a multimedia computer，and CD-ROM applications could be played on HDTV systems. A digital TV decodes the MPEG-2 signal and displays it just as a computer monitor does，giving it high resolution and stability. The MPEG-2 technology，however，still takes up a significant amount of bandwidth，and so satellite broadcasters have begun to embrace the newer technology of MPEG-4.[2] MPEG-4 compression technology is able to squeeze twice as much HD video into the same amount of bandwidth as MPEG-2. Others will soon likely follow the satellite broadcaster’s leads and slowly begin to embrace the newer MPEG-4 technology， making it the new standard.

HDTV Stations

There are HDTV stations "on the air" in many large cities. The first HDTV station was WRAL-HD in Raleigh，NC. The Federal Communications Commission（FCC) has mandated that all stations be capable of broadcasting HDTV by 2006. The timeline of HDTV coverage gives you an idea of what will be available in your area，and when.

The FCC mandate affects broadcasters, cable companies and consumers in significant ways：

l Consumers have to buy new equipment，either a set-top box（to convert digital signals to analog signals) or a whole new TV set.

l Broadcasters have to spend a considerable amount of money to switch to HDTV. They have to buy new cameras, new titling and editing equipment, new tape machines, new rigs for their news vans – it’s a big investment.

l Cable operators have to convert all of their equipment and all of their set-top boxes.

l Communities need to agree to have new towers built for broadcast channels.

The station decides which DTV format it will transmit. For example，cable operators may push for 720p so they can fit more HDTV channels onto the cable. A clear pattern has yet to emerge in the industry.

How is HDTV Different？

The usual National Television Standards Committee (NTSC) analog TV screen in the U.S. has 525 scan lines，with 480 actually visible. The usual TV has an effective picture resolution of about 210，000 pixels. In the highest resolution digital TV formats，each picture contains about 2 million pixels. This means about 10 times more picture detail on the HDTV screen！

The typical TV show uses 35-mm film (or is recorded direct-to-video using NTSC equipment）．In the case of film，the broadcaster converts it to an analog TV signal for broadcasting. Standard 35-mm film has an aspect ratio of 1.37 : 1，meaning it is 1．37 times as wide is it high. A conventional TV screen has a 4:3（1.33 : 1）aspect ratio，so the conversion is easy.

To deal with HDTV’s new standards，broadcasters will need to get all new equipment，such as cameras，remote broadcast units，control rooms，cables，and sound equipment. This is because digital TV has：

l Wider images

l Much more detailed pictures

l 5. 1 channel CD-quality Dolby Digital (AC-3）surround sound

l The ability to send data directly to a screen or to a PC as a download（The actual HDTV transmission is based on a 19.3-Mbps digital data stream.）

The aspect ratio（width to height）of digital TV is 16:9（1.78 : 1），which is closer to the ratios used in theatrical movies, typically 1．85：1 or 2．35:1．Currently broadcasters must either pan and scan the image（crop the full picture of the film down to 4:3，eliminating part of every scene in the process）or letterbox it（present the full picture only on the middle part of the screen，with black bars above and below it）．With a 16:9 screen，panning and scanning a theatrical movie doesn’t remove so much from the original picture and letterboxing doesn’t block out so much of your screen.[3]

What Do I Need to Do?

When local over-the-air television stations moving to digital，you need to check your television sets. If your TV receives signals via cable，satellite or the internet，you will not be affected by the transition. You will continue to receive your existing television services. You do not need to buy additional equipment or subscribe to additional TV services.

If your TV receives signals over-the-air using an outdoor antenna or “Rabbit Ears”, you may be affected by the transition to over-the-air digital television. If your television is equipped with a digital tuner，you will be able to continue viewing local stations that have switched to digital. The majority of television antennas that are currently used for watching analog signals will continue to work with digital signals. If your television does not have a built-in digital tuner，you have three options：

l Install a digital-to-analog converter box. A digital-to-analog converter box is a unit that sits on top of or near your television and that picks up over-the-air digital signals to convert them for display on a standard analog television. However，the HDTV shows you see will look no better than DVD on your analog TV 一 you will get none of the resolution and format benefits of a real HDTV set.

l Buy a Digital Television. Today’s HDTV sets come in several forms. Be sure any television receiver you purchase has input jacks that match the connectors on the VCR，cable box，DVD player and video game console you currently own. For many years，you will have to straddle the digital/analog fence, using, for example, an analog VCR on your digital TV. At the moment, there are no “standards” for what connections will appear on the back of an HDTV set. Therefore you should look for composite, S-video and component video as a minimum set of analog jacks so you can use your existing analog equipment with the new set.

l Many early purchasers will have to “go back” to a traditional outside UHF         television antenna to receive the over-the-air (OTA) HDTV signal. The HDTV transmission system is an eight-level vestigial sideband（VSB）technique that uses UHF channels. Your antenna rotor setting for reception of HDTV signals will be easy to adjust. You either have a picture or you do not - there can not be a snowy image with digital technology. There also will not be any "fringe area” reception.

l Subscribe to Cable，Satellite，or an Internet-protocol（IP）Television Service.

WORDS AND PHRSES

picture tube 电视机的显像管  

panorama   全景画

stunning    了不起的,出色的,漂亮的

VHS       家用录像系统

HDTV     高清电视

progressive scanning    顺序扫描,步进扫描,逐行扫描

interlaced scanning     隔行扫描

SDTV    标清电视

VGA     视频图形阵列

mandate     授权

set-top box    置顶盒

theatrical   剧场的

tuner   无线电收音机

vestigial sideband（VSB）残留边带

rotor     旋转器

fringe area    电视接收边缘区

NOTES

[1] In each image，the MPEG-2 software records just enough of the picture without making it look like something is missing. In subsequent frames，the software only records changes to the image and leaves the rest of the image as-is from the previous frame.

每幅图像中， MPEG-2的软件仅仅记录足够的信息使图像看起来没有任何信息丢失。在后续的帧中，软件仅记录图像的变化部分，其余部分的图像与前面帧的图像相同。

[2] The MPEG-2 technology，however，still takes up a significant amount of bandwidth，and so satellite broadcasters have begun to embrace the newer technology of MPEG-4.

然而，MPEG-2技术仍然需要相当数量的带宽，这使得卫星广播公司开始接受新的MPEG-4的技术。

[3] With a 16:9 screen，panning and scanning a theatrical movie doesn’t remove so much from the original picture and letterboxing doesn’t block out so much of your screen.

16:9的屏幕，平移和扫描一部剧场电影不会从原画面删去过多的信息，而且简单的宽屏效果不会过多的遮挡屏幕。