False Argument In Favor of Interlaced DTV Broadcasting, Part 2 — William F. Schreiber, MIT

by William F. Schreiber, Prof. Emeritus of Elec. Eng., MIT
Wednesday, April 29, 1998

Part 2
Other artifacts of interlace include image break-up when the camera is panned vertically. When the vertical motion is one line/field, then half the display lines disappear. Transcoding is also made more difficult (This is the reason why PAL<>NTSC transcoding is imperfect even after decades of trying.)

There is general agreement that P provides better images than I, so lip service is paid to an eventual migration from I to P. The I advocates, however, insist that it is too early to do so, for the various reasons mentioned above. This latest argument, here shown to be entirely without merit, is simply the most recent attempt to promote the use of existing interlaced production equipment at least for the initiation of digital broadcasting.

2 Million vs 1 Million

As shown above, an interlaced signal with 1080 lines per frame has an actual vertical resolution barely half that, while a progressive signal of 720 lines per frame has an actual vertical resolution of nearly 720. In the ATTC tests mentioned above, the objectively measured vertical resolution of 720P was *higher* than that of 1080I. As for the horizontal resolution, 1920 is indeed much higher than 1280, and if it had been achieved, one would expect that the perceived sharpness of the I image would have been higher than that of the P image. However, that was not the case. The subjective sharpness as measured by ATEL was about the same. (The subject matter was not specifically selected to illustrate interlace artifacts.) It is clear that the 1080I image did not resolve 1920 pixels horizontally. In all likelihood, this was caused by the camera itself or its filtering.

It should be noted that, with a 30 MHz bandwidth as used in the tests, the resolution is limited to about 1550 Horizontal pixels. Additional data on this issue has emerged in Japan and at the recent NAB show. In Japan, the Association of Radio Industries and Businesses (ARIB) has already changed 1080x1920 to 1080x1440 because the higher resolution causes coding artifacts (blocking) that can be reduced or eliminated, depending on the scene, by some reduction in horizontal resolution. There were also reports from NAB of blocking artifacts in 1080I coded material, no doubt from the same cause. Last December, Sony requested ATSC to change the 1080x1920 format to 1080x1440. On the other hand, there were no reports of compression artifacts with 720P at NAB.

In summary, the nominal resolution of 1080x1920 is not achieved in practice. The 1080I format does not have higher resolution than the 720P format, and it has all the well known interlace artifacts. *There is no quality advantage in using 1080I, and there are no valid reasons not to use progressive scan.*

Conclusion

The idea that 1080I has higher resolution than 720P has been shown to be false. The resolution actually achieved in the interlaced system is far below the nominal 1080x1920. The reduction in vertical resolution is due to the need to lessen the interline flicker that would otherwise be present. The reduction in horizontal resolution is partly a camera problem and partly a limitation of the MPEG compression system. These limitations are inherent; they cannot be removed within the given transmission data rate. There was a time when these matters were not fully understood, but that time is long past. There is now a mountain of evidence that shows that there is no advantage whatsoever to using interlace in digital TV broadcasting except to the manufacturers of interlaced production equipment. The fact that some interlace advocates are still pushing this obsolete technology shows that their viewpoint cannot be based on facts, but is almost surely due only to their last-ditch attempt to make the already developed 1125-line production equipment the appropriate equipment to use as HDTV broadcasting is initiated.

Glossary

ATTC Advanced Television Testing Center
ATEL Adavanced Television Evaluation Laboratory of the Canadian Dept. of Communications
NTSC National Television Systems Committee. The current analog TV system used in the US
PAL Phase alternation by line. The current analog TV system used in most
50-Hz countries
MUSE The analog compression system used for transmitting NHK signals by
satellite
SMPTE Society of Motion Picture and Television Engineers
ANSI American National Standards Institute
EBU European Broadcasting Union
NHK Japan Broadcasting System. Also the 1125-line interlaced system developed by NHK
ACTV Advanced Compatible Television. An analog system compatible with NTSC, developed at the Sarnoff Laboratories.
ACATS Advisory Committee on Advanced TV Systems.
ATSC Adcvanced Television System Committee
MPEG. Motion Picture Experts Group. Also the digital compression system developed by MPEG.
ARIB Association of Radio Industries and Businesses of Japan
NAB National Association of Broadcasters
RAI Italian Broadcasting System
EU European Union

N.B. Numbers such as 720x1280 refer to the structure of the visible television frame. Analog systems such as NTSC have a larger total number of lines (525) as compared with the 480 lines of the visible frame. The original NHK system had 1125 total lines, of which 1035 formed the visible image.

N.B. This note represents the personal opinion of the author, who has no financial interest in the outcome of the matters discussed herein.

William F. Schreiber, Prof. Emeritus of Elec. Eng., MIT [email protected]
25 April 1998