| melchman's guide to: | HDTV |
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| Prerequisites: | None |
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| Sources: | www.atsc.org |
| Scientific Atlanta | |
| M. S. Telecommunications |
HDTV
High Definition Television is the topic, so let's get some terms straight:
- NTSC - National Television Standard Committee - Established the US television standards for broadcasters and manufacturers to ensure compatibility for the consumer. The (simplified) standard was analog, 4:3 aspect, 525 lines.
- SDTV - a Digital Version of NTSC
- EDTV - a wider (704 pixel) Digital that can stretch to 16:9 or squish to 4:3
- HDTV - the new TV standard: Wide aspect (16:9) 1080 line
- ATSC - Advanced Television Systems Committee - Developing the new standard
ATSC
The ATSC was formed to set a new US standard for television. The old NTSC standard was about 50 years old and we invented some new stuff in the last few decades. The goal is the same as the old NTSC: set the standard for the producers, broadcasters and equipment manufacturers so that it all works together for the consumer.
Digital capture and digital production began to drive the desire to deliver the improved image and audio quality to consumers through new equipment. The creative possibilities of digital thrilled the artists. The new perceived need to buy new digital equipment pleased the manufacturers. A better signal over broadcast thrilled the old networks who were "losing" to cable. Enhanced digital services (music channels, shopping channels and Video on Demand) thrilled the cable providers.
Formats
This august body (ATSC) agreed on no less than 18 standards for the new television standard. Pretty much confusing and confounding every consumer they were trying to protect by enforcing a standard. This made adoption slow.
The table below lists the formats. The columns need some explanation.
First, a few abbreviations are used: i means interlaced, p is progressive. Interlaced frames are really only half frames. Each frame is either the odd or even lines of the picture. Two frames (or fields) are needed to make a complete picture. The human eye (and mind) will automatically blend the two fields into one complete picture with some blurring, especially in fast motion situations. Progressive scan or non interlaced is a complete frame each time. Progressive scan is compressed by sending only the changes from the last frame to create the next.
Second, SD is Standard Definition while HD is High Definition. ED is Enhanced definition. ED causes more discussion and grief than it is worth, but the manufacturers needed to describe their "better than before, but not all the way there" sets in some catchy marketable way.
Resolution is measured in lines and pixels. The old NTSC standard was measured in lines only. The analog signal did not lend itself to easy quantification as pixels. Given this legacy we still use the term lines to measure the height of the picture. We added pixels to measure the width of the picture. The new digital media is easy to quantify so we can be accurate in this dimension.
Most people struggle with the idea of overscan. Analog transmission required the image to be larger than the minimum resolution. This would guard against loss and slight variations in manufacture and design of the receiver. In recent years we have watched TV tubes move from round bubbles to flat glass to LCD grids and plasma layers. The transmitter could not be sure of the final shape of the viewable area, so they overcompensated with overscan. The NTSC signal used to send 525.5 lines with an expected viewable area of 480. The overscan column lists the transmitted lines including the overscan.
| Def. | Scan Lines | Pixels | Frame Rate | Aspect | Overscan | Comments |
|---|---|---|---|---|---|---|
| SD | 480 | 640 | 24p | 4:3 | 525 | Film speed |
| 480 | 640 | 60i | 4:3 | 525 | NTSC | |
| 480 | 640 | 30p | 4:3 | 525 | like NTSC, but Progressive | |
| 480 | 640 | 60p | 4:3 | 525 | Super NTSC? | |
| ED | 480 | 704 | 24p | 4:3 | 525 | The 704 allows us to squash into 4:3 aspect and stretch to 16:9 without too much distortion. |
| 480 | 704 | 60i | 4:3 | 525 | ||
| 480 | 704 | 30p | 4:3 | 525 | ||
| 480 | 704 | 60p | 4:3 | 525 | ||
| 480 | 704 | 24p | 16:9 | 525 | ||
| 480 | 704 | 60i | 16:9 | 525 | ||
| 480 | 704 | 30p | 16:9 | 525 | ||
| 480 | 704 | 60p | 16:9 | 525 | ||
| HD | 720 | 1280 | 24p | 16:9 | 750 | Film speed, near film aspect |
| 720 | 1280 | 30p | 16:9 | 750 | Wide aspect with 30 fps | |
| 720 | 1280 | 60p | 16:9 | 750 | Wide aspect and better motion | |
| HD | 1080 | 1920 | 24p | 16:9 | 1125 | The DVD dream: wide, crisp and 24fps |
| 1080 | 1920 | 30p | 16:9 | 1125 | 2:3 pulldown gives us the NTSC 30 fps | |
| 1080 | 1920 | 60i | 16:9 | 1125 | Interlaced like NTSC but twice as tall and 3 x wider |
Sources
Broadcast
Some in the NAB were hoping that DTV was going to get some antennae back on the rooftops! When the standards were being discussed the broadcast networks were losing revenue to the cable operators. DTV was hoped to be broadcast's saviour because they were given bandwidth for this new standard while cable operators would need to dump channels from their line ups to make space for the fatter HD channels.
Who is broadcasting what? I enjoy CSI (on CBS) off the air. The pictures are beautiful! Each network has adopted a standard which all their affliates should use. Affiliates sometimes break with the network standard and sometimes only break the standard for home games or special events. This is the best list I have found:
| Signal | Network |
|---|---|
| 1080i | CBS |
| 1080i | NBC |
| 1080i | PBS |
| 1080i | HBO |
| 1080i | Showtime |
| 720p | ABC |
| 480p | FOX |
| 480p | WB |
Satellite
DirecTV offers HDTV. LAst I looked they offer 8 HD channels Plus Premium (subscription) channels like HBO. They are also the only way I know to record HD on TiVo.
Cable
As the broadcasters believed, cable providers have been slow to offer HDTV. Many now offer digital service which transmits the same old NTSC programs in compressed format so they can squeeze more channels on to the existing wiring.
It also makes smaller channels possible. My service offers 45 channels of music. It is CD Quality, but the video is mostly static. I get a picture of the album or band and some text with the track and album name with the occasional trivia fact. I love it because I am a music geek, too. This audio and "light video" probably compresses 45 channels into the bandwidth of 3 regular NTSC channels. More channels in less bandwidth makes a cable provider rich!
What about HD? I had one of the best cable providers in the nation according to J. D. Powers. We had 3 HD channels: CBS, PBS and a pay per view! This service has only become available recently (October 2003). The broadcast CBS looks better to me, but the cable version is more stable. Cable just isn't there yet!
My prospective cable company in Arizona offers 5 HD channels, plus 3 subscription and 4 PPV.
DVD
DVDs are my favorite HD source! I buy Widescreen editions and enjoy the quality. A special CSS encoded MPEG2 stream is encoded on the DVD.
DVD players send a 480p signal across the component (YPrPb) connection. If you are using the composite or S-Video connection you are getting a decoded analog signal probably similar to the 525 NTSC signal.
MPEG Stream
If the MPEG2 stream from a DVD can be played by an HDTV receiver then any old MPEG2 should be playable across the same wiring. This opens the doors for internet and other cable streaming sources.