Digital television, HDTV, MPEG, DTV: Confused? You have a right to be! The purpose of this page is to clarify, for the average consumer, the basics of digital TV and television signals in general.
Television has been an important part of our culture for over fifty years. Digital TV (DTV) is transforming television with new flexible capabilities and an evolution in the quality of the television signals received by the TV consumer.
Digital television has many advantages over the old analog TV signal that has been the standard for many years. Digital television capabilities can produce a clearer picture with much greater detail and can also produce a wide screen picture more like movie theater screens. We can finally watch a movie at home with more of the film's original wide screen ratio left intact. The film's original wide screen visual content will not have the sides cut off in order to make it fit into a 4:3 TV screen.
Knowledge about television signals will help you to understand digital television, regardless of whether you use satellite, cable, or an off-air antenna to receive TV signals in your home.
Basic Television Concepts
Some rudimentary concepts and terminology about television will help you to understand the changes that are happening today via digital television.
Until recently, all television programming signals were strictly analog. Of course in the early days of television, analog TV broadcasting and reception were technical marvels. When color TV came along, engineers figured out how to add color to the existing TV signal standards without cutting out all the people that were still watching television on black and white TV sets.
In short, the analog TV signal standard which dates back to 1941 is an old technology with many limitations.
Radio Frequency Bandwidth
Television video and sound signals are transmitted by radio waves. The consumer may receive television programming that is riding on off-air antenna, cable, or satellite TV frequencies.
Bandwidth refers to the frequency range of a transmission channel. In other words, how wide is the radio signal that is of interest. Generally, a wider bandwidth means increased transmitted information capacity. Information can be used to generate sound, video, or data for consumer electronic devices such as televisions and radios.
A simple example of the information capacity of a signal can be found in a comparison between AM and FM radio station bandwidths. The bandwidth of an AM radio signal is 10 KHz. This small AM bandwidth means that the full 20Hz - 20KHz range of human hearing can't be transmitted by an AM radio station. FM radio stations transmit signals with a bandwidth of 75 KHz. The larger FM bandwidth is able to reproduce the full 20Hz - 20KHz range. That is the main reason that music sounds better on FM than it does on AM radio.
Of course a signal that carries both picture and sound information requires a much larger bandwidth than AM or FM radio. The bandwidth of standard analog off-air or cable television is 6 MHz. The picture information in this 6 MHz bandwidth, is in the form of a very complicated analog waveform with limited resolution.
Digital television can squeeze more picture information into a given bandwidth. The amount of picture information available in the old analog television signal is much less than is possible with DTV.
Video Resolution and Scanning
In order to put a picture on a television screen or monitor, an electron beam "paints" the screen. The electron beam generated by the television's electronics moves systematically across the TV screen. The screen consists of small picture elements called pixels. The pixels "glow" a color as the electron beam passes over them. The small and narrow electron beam starts at the top left of the screen and moves horizontally across the screen to produce one line. The electron beam continues in this fashion starting at the left of the screen and painting another horizontal line until the screen has been filled. When the screen has been filled this is called a frame. Of course all this happens very quickly and we perceive a solid picture on the screen.
Video resolution is the ability to reproduce fine detail. As you might expect, the more picture information that is transmitted, the better the video resolution. Video resolution is improved when the number of lines transmitted to be "painted" on the screen is increased. Resolution is also increased when their is more information transmitted for each line.
Television Broadcast Standards
A television broadcast standard defines the television signals that are transmitted by TV program providers. Television program providers can use over the air broadcasting, cable, or satellite to transmit their programs. Regardless of how a program provider transmits it's signal, the television broadcast standard is what dictates the television signal that is riding on over the air, cable, or satellite frequencies.
Not all the countries in the world use the same television standards as the United States. These broadcast standards are what dictate the type of television receiver that is manufactured in your part of the world. A television set that works in the US will not work in France because the French use a different broadcast standard than the US ( you may insert your own jokes about the French in this space ). This page covers television standards for the US and most of North and South America.
NTSC The Original
The NTSC (National Television Standards Committee) with it's analog signal has been the long standing broadcast standard of US television. NTSC television is the plain vanilla type of color TV signals that we have received in the US for many decades. The original black and white ( monochrome ) NTSC became a standard in 1941. In 1953 NTSC added color to the existing TV signal standards. It was quite a technical feat to add color, but still allow reception by black and white television sets. Color TV sets were very expensive for a long time.
NTSC is broadcast with approximately 480 active scan lines. Each line has about 440 pixels. The approximate total number of pixels transmitted in NTSC therefore is 480 (lines) X 440 (pixels per line) = 211,200 pixels.
NTSC Aspect Ratio
Aspect ratio refers to the width and height of a television screen. NTSC television signals are limited to a 4:3 aspect ratio. This 4:3 ratio means that for every 4 units of width there will be 3 units of height. For example, if a screen is 4 inches wide it will be 3 inches tall, if the screen is 4 feet wide it will be 3 feet tall. The screen's width will always be the first number expressed in an aspect ratio.
NTSC Interlaced Scanning
NTSC video signals use interlaced scanning. When the electron beam paints lines on the screen it doesn't paint line 1, then 2, then 3. In interlaced scanning all of the odd lines are scanned 1,3,5 etc. When the bottom of the screen is reached then the electron beam goes back up to the top left of the screen and paints the even lines. Each of the interlaced scans produces half of the picture, which is called a field. A field takes about 1/60 of a second to scan even or odd video lines. An NTSC frame of video is made up of both fields. The frame rate of NTSC is 29.97 frames per second (Fps). This frame rate is faster than the international film rate of 24 Fps that is standard for the world's movie theaters.
ATSC Digital Television Standard
New television capabilities require new television broadcast standards. The United States is in the process of phasing out NTSC analog television. The Advanced Television Systems Committee (ATSC) has developed new television broadcast standards for Digital Television (DTV). One of the great things about DTV is that it is not limited to just one type of signal. DTV allows for the display of programming in 18 different formats. These formats offer several resolutions, scanning methods, and aspect ratios. In NTSC there is only one format used, one resolution, one scanning method, and one aspect ratio.
Since the DTV signal is digital, data transfer rates determine the amount of picture and sound information that will be received by a television set. Data reception for DTV requires bit rates measured in megabits per second (Mbps). The speed of data transfer in DTV makes dial-up modem bit rates seem pitiful. More picture and sound data means better video and sound for your home television.
ATSC vs. DVB
The ATSC standard is only being used in the United States. The digital television standard that most of the world is using is DVB (Digital Video Broadcast). Despite the United States adoption of the ATSC standards, there is a huge amount of DVB programming that is available to the United States via satellite. Satellite TV receivers that are DVB compatible are not very expensive and enable the consumer to receive a huge array of free satellite TV.
ATSC MPEG-2 Compression and bit rates
DTV can squeeze more picture information into a given broadcast channel than is possible with the old NTSC television signal. ATSC incorporates MPEG-2 (Motion Picture Experts Group standard 2) compression, which is a world standard for the transmission of compressed digital video and sound. MPEG-2 reduces a digital signal's size dramatically. The analog 6 MHz bandwidth used in the U.S. for off-air and cable broadcasting can carry digital information at 19.4 Mbps. There are 18 approved video formats for DTV broadcasts. The bit rates required for a DTV video format are dependent upon the amount of picture information transmitted by that format. When talking about Digital Television, information about bit rates makes more sense than talking about bandwidths, but it is worth noting that the variable bit rates for DTV make it possible for broadcasters to put multiple programs within a 6 MHz bandwidth.
Frame Rates and ATSC scanning
A video frame is the complete image displayed when all the lines have been completely scanned. A complete video frame is like the individual film frames displayed in movie theaters at 24 frames per second. Each film frame is a complete image.
In interlaced NTSC scanning it takes 2 passes of the electron beam to produce one frame. In NTSC each pass of the electron beam produces half on the video frame, so each pass of the electron beam produces a field. NTSC frames are made up of two fields.
Many of the ATSC formats use progressive scanning. In progressive scanning all of the lines are scanned with one pass of the electron beam. Progressive scanning produces a picture with better perceived resolution. Progressive scanning produces a better picture than NTSC interleaved scanning.
The faster the frame rate for video, the better it will look on the screen. ATSC provides variable frame rates. The 24 frames per second is especially useful for showing films because films use 24 frames a second.
HDTV and SDTV
High Definition Television (HDTV) is the flag ship of the new ATSC digital television standard. All DTV is not HDTV. HDTV video formats are those that have a 16:9 aspect ratio and display at least 720 visible scan lines. The ATSC standard also includes video formats that have a 4:3 aspect ratio and have lower resolution than the HDTV formats. These non HDTV signals are called SDTV (Standard Definition TV).
ATSC Video Format Chart
The chart below shows the 18 approved ATSC video formats. You can see from this chart that when more picture information is transmitted (Total pixels) or faster (Frame Rates), then the required (Bit Rate) is higher. In the frame rate column an "i" indicates interleaved scanning and a "p" indicates progressive scanning. In order to compare DTV to the old NTSC standard. I have put the NTSC standard information at the bottom of the chart.
| Signal Type | Aspect Ratio | Pixels per Line - Horizontal Resolution | Visible Scan Lines - Vertical Resolution | Total Pixels | Frame Rates | Bit Rate |
|---|---|---|---|---|---|---|
| HDTV | 16:9 | 1920 | 1080 | 2,073,600 | 24p 30p 30i |
18 Mbps 18 Mbps 18 Mbps |
| HDTV | 16:9 | 1280 | 720 | 921,600 | 24p 30p 60p |
8 Mbps 10 Mbps 18 Mbps |
| SDTV | 16:9 | 720 | 483 | 347,760 | 24p 30p 30i 60p |
3 Mbps 4 Mbps 4 Mbps 8 Mbps |
| SDTV | 4:3 | 720 | 486 | 349,920 | 24p 30p 30i 60p |
3 Mbps 4 Mbps 4 Mbps 7 Mbps |
| SDTV | 4:3 | 640 | 480 | 307,200 | 24p 30p 30i 60p |
3 Mbps 3 Mbps 3 Mbps 7 Mbps |
| NTSC analog |
4:3 | ~ 440 | ~ 480 | 211,200 | 30i | Not Applicable |
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