Color
A Viable Transition To DTV
For Everyone--
Not HDTV For Everyone
by
Craig Birkmaier
PCube.com
The following was developed by the Japanese public broadcaster NHK in the 1970s when they conducted their basic research into the requirements for HDTV display.
The HDTV viewing experience requires a display that:
A. delivers what is perceived as a sharp image at the designed viewing distance--the ability to resolve horizontal, vertical and diagonal spatial details with a resolution equal to or greater than 22 cycles per degree;
B. delivers static and moving images that are free of artifacts that interfere with the perception of the original source material.
C. covers at least 30 degrees of the field of view
NHK found that a viewing distance in the range of 3.3 picture heights and an aspect ratio of 3:2 would produce what they called "the induction effect." This is the point at which the human visual system responds to the images as if we were seeing a real-world event, and it requires a big screen that covers a large portion of our field of view. The induction effect also seems to be sensitive to viewing distance; although it is possible to meet all of the criteria above with a direct view display, it doesn't work when the eyes must focus on an image plane that is very close, as is the case with a computer display. One European study, where vieweres were allowed to position themselves at any distance from the display, found that on average, people would not sit 3 picture heights from a display until it had a screen diagonal above 100 inches, at a viewing distance of approximately 17 feet.
NHK also reported that picture impairments from interlace and other forms of aliasing are the biggest barrier to the perception of a high definition image. The removal of picture impariments makes the largest contribution to the perception of a high quality image. This finding appears to be an important factor in why consumers rate the MPEG-2 encoded images from DirecTV and the Dish Network so highly, despite the fact that they contain no more resolution than a good NTSC image--these digital images are free from the impairments of NTSC encoding...but not interlace artifacts.
NHK found that progressive scanning of the display was highly desirable, as it reduced picture impairments, allowing the source image samples to be seen properly. These studies suggest that a progressive scan image with about 700 lines is adequate to deliver the HDTV viewing experience.
So why did they then choose to develop an interlaced HDTV system with more than 1000 lines?
Bandwidth!
The NHK 1125/60 system was designed for an analog world where bandwidth is still a precious commodity. Interlacing is a very effective form of compression for analog systems--it allowed NHK to meet their design goals, whereas an approximately 700 line progressive scan system would have required greater transmission bandwidth and significantly higher CRT scanning frequencies. 1080i requires a horizontal scanning frequency of 33.75 kHz, while 720P requires a horizontal scan rate of approximately 48 kHz.
Thus the notion that an HDTV image requires more than 1000 scan lines was born, and still lives today in the CEMA definition of HDTV. CEMA defines HDTV as any display which scans all of the original 720P or 1080i source in a "window" with a 16:9 aspect ratio. Thus any computer monitor that supports a horizontal scan rate of 33.75 MHz or greater is capable of delivering HDTV, even if it has a screen diagonal of only 15 inches.
It is impossible to deliver the HDTV viewing experience on a 36 inch diagonal display, no matter how many pixels it can resolve. These 36" 4:3 displays have a picture width of about 28.8 inches and height of 21.6 inches. The 16:9 window of these displays has a picture height of 16.2 inches, which is equivalent to a 33 inch diagonal 16:9 display. This means that I would need to sit about 4 feet from the display to enjoy the HDTV viewing experience.
Not only does this NOT stimulate the induction effect, it does not work in most family rooms.
So the issue does become one of how to deliver the HDTV viewing experience at an affordable price, or how to deliver an enhanced viewing experience at an affordable price. The premise that is set forth by a recent New York Times Article by Joel Brinkley is that you can see the difference between 1080i and 480P source at normal viewing distances between 7 and 12 feet. He is mistaken due to his testing criteria.
The premise of the computer industry is that 480P resolution is adequate on affordable mass market displays, even the "relatively expensive" models that were used in the NYT Brinkley test. Keep in mind that the average display sold today is 27" diagonal and costs about $350. Less than 15% of all displays sold have a screen diagonal of 36" or greater.
The laws of physics and economics tell us that HDTV is not a mass market product...it is a niche of a niche market...home theater systems. We are in the process of asking every consumer to upgrade their television viewing infrastructure for DTV. Some will be satisfied with a set-top box for their existing television receivers...witness the hugh success of DBS and the new terrestrial DTV service in Great Britain. Some will opt for an enhanced television experience such as the 36" multimedia displays evaluated by Mr. Brinkley in the NYTimes article. And a small percentage will opt for the more expensive HDTV displays.
The reality is that television is finally breaking out of the "one-size-fits-all" mold. HDTV is not for everyone, nor is it necessary for every application. There are many good reasons for having multiple levels of resolution available to the producers of television content, just as there are many good reasons to offer DTV receivers at various price points with different Quality Of Service levels.
There is, however, a major problem looming on the horizon. HDTV and SDTV are largely incompatible. It is virtually impossible to deliver content to a wide range of receivers with different screen sizes, aspect ratios and levels of resolution, without compromising that content for the lowest common denominator display.
Take for example the ABC Monday Night Football broadcasts that will begin in the fall. On a large screen (>50" diagonal), these broadcasts will create an entirely new way to cover professional football, an a new experience for those who can afford an HDTV display. But how satisfying will these broadcasts be on a 33 inch diagonal display, or the 16:9 window of my new 36 inch multimedia monitor. Something tells me that most people who buy one of these new sets will tune to the NTSC broadcast, which is optimized for the 4:3 aspect ratio, with shots composed for the lower resolution of SDTV.
THE ISSUE is a viable transition to DTV for everyone, not HDTV for everyone.
ADVANCED TELEVISION PUBLISHING
1999
A Viable Transition To DTV
For Everyone--
Not HDTV For Everyone
by
Craig Birkmaier
PCube.com
The following was developed by the Japanese public broadcaster NHK in the 1970s when they conducted their basic research into the requirements for HDTV display.
The HDTV viewing experience requires a display that:
A. delivers what is perceived as a sharp image at the designed viewing distance--the ability to resolve horizontal, vertical and diagonal spatial details with a resolution equal to or greater than 22 cycles per degree;
B. delivers static and moving images that are free of artifacts that interfere with the perception of the original source material.
C. covers at least 30 degrees of the field of view
NHK found that a viewing distance in the range of 3.3 picture heights and an aspect ratio of 3:2 would produce what they called "the induction effect." This is the point at which the human visual system responds to the images as if we were seeing a real-world event, and it requires a big screen that covers a large portion of our field of view. The induction effect also seems to be sensitive to viewing distance; although it is possible to meet all of the criteria above with a direct view display, it doesn't work when the eyes must focus on an image plane that is very close, as is the case with a computer display. One European study, where vieweres were allowed to position themselves at any distance from the display, found that on average, people would not sit 3 picture heights from a display until it had a screen diagonal above 100 inches, at a viewing distance of approximately 17 feet.
NHK also reported that picture impairments from interlace and other forms of aliasing are the biggest barrier to the perception of a high definition image. The removal of picture impariments makes the largest contribution to the perception of a high quality image. This finding appears to be an important factor in why consumers rate the MPEG-2 encoded images from DirecTV and the Dish Network so highly, despite the fact that they contain no more resolution than a good NTSC image--these digital images are free from the impairments of NTSC encoding...but not interlace artifacts.
NHK found that progressive scanning of the display was highly desirable, as it reduced picture impairments, allowing the source image samples to be seen properly. These studies suggest that a progressive scan image with about 700 lines is adequate to deliver the HDTV viewing experience.
So why did they then choose to develop an interlaced HDTV system with more than 1000 lines?
Bandwidth!
The NHK 1125/60 system was designed for an analog world where bandwidth is still a precious commodity. Interlacing is a very effective form of compression for analog systems--it allowed NHK to meet their design goals, whereas an approximately 700 line progressive scan system would have required greater transmission bandwidth and significantly higher CRT scanning frequencies. 1080i requires a horizontal scanning frequency of 33.75 kHz, while 720P requires a horizontal scan rate of approximately 48 kHz.
Thus the notion that an HDTV image requires more than 1000 scan lines was born, and still lives today in the CEMA definition of HDTV. CEMA defines HDTV as any display which scans all of the original 720P or 1080i source in a "window" with a 16:9 aspect ratio. Thus any computer monitor that supports a horizontal scan rate of 33.75 MHz or greater is capable of delivering HDTV, even if it has a screen diagonal of only 15 inches.
It is impossible to deliver the HDTV viewing experience on a 36 inch diagonal display, no matter how many pixels it can resolve. These 36" 4:3 displays have a picture width of about 28.8 inches and height of 21.6 inches. The 16:9 window of these displays has a picture height of 16.2 inches, which is equivalent to a 33 inch diagonal 16:9 display. This means that I would need to sit about 4 feet from the display to enjoy the HDTV viewing experience.
Not only does this NOT stimulate the induction effect, it does not work in most family rooms.
So the issue does become one of how to deliver the HDTV viewing experience at an affordable price, or how to deliver an enhanced viewing experience at an affordable price. The premise that is set forth by a recent New York Times Article by Joel Brinkley is that you can see the difference between 1080i and 480P source at normal viewing distances between 7 and 12 feet. He is mistaken due to his testing criteria.
The premise of the computer industry is that 480P resolution is adequate on affordable mass market displays, even the "relatively expensive" models that were used in the NYT Brinkley test. Keep in mind that the average display sold today is 27" diagonal and costs about $350. Less than 15% of all displays sold have a screen diagonal of 36" or greater.
The laws of physics and economics tell us that HDTV is not a mass market product...it is a niche of a niche market...home theater systems. We are in the process of asking every consumer to upgrade their television viewing infrastructure for DTV. Some will be satisfied with a set-top box for their existing television receivers...witness the hugh success of DBS and the new terrestrial DTV service in Great Britain. Some will opt for an enhanced television experience such as the 36" multimedia displays evaluated by Mr. Brinkley in the NYTimes article. And a small percentage will opt for the more expensive HDTV displays.
The reality is that television is finally breaking out of the "one-size-fits-all" mold. HDTV is not for everyone, nor is it necessary for every application. There are many good reasons for having multiple levels of resolution available to the producers of television content, just as there are many good reasons to offer DTV receivers at various price points with different Quality Of Service levels.
There is, however, a major problem looming on the horizon. HDTV and SDTV are largely incompatible. It is virtually impossible to deliver content to a wide range of receivers with different screen sizes, aspect ratios and levels of resolution, without compromising that content for the lowest common denominator display.
Take for example the ABC Monday Night Football broadcasts that will begin in the fall. On a large screen (>50" diagonal), these broadcasts will create an entirely new way to cover professional football, an a new experience for those who can afford an HDTV display. But how satisfying will these broadcasts be on a 33 inch diagonal display, or the 16:9 window of my new 36 inch multimedia monitor. Something tells me that most people who buy one of these new sets will tune to the NTSC broadcast, which is optimized for the 4:3 aspect ratio, with shots composed for the lower resolution of SDTV.
THE ISSUE is a viable transition to DTV for everyone, not HDTV for everyone.
1999