Second Guessing the TV Makers
Too often the TV set digests the incoming video signal seemingly unnecessarily so it occupies a different number of scan lines. This scaling process usually causes noticeable loss of picture quality. Recently a few line doubler, de-interlacer, and scaler makers have added their own scaling so the video signal bypasses the scaling in the TV set. Because both de-interlacing and scaling require that the video be digitized, doing both in the same unit avoids one of the analog to digital conversions that also detract from picture quality.
Updated 4/25/02
Return to Video Topics
1. A well known problem seen from time to time in TV sets is being stuck in widescreen (16:9) mode when progressive scan or HDTV video is being viewed. if the program material is 4:3, it is stretched unnaturally as viewed. So some DVD player makers have had to resort to reformatting the picture to occupy a 4:3 shaped space even though the scan lines on the picture tube are spanning a 16:9 shaped space.
Here are some other situations that have been found:
2. Wide screen "letterbox" material not enhanced for 16:9 occupies typically 360 or fewer (for NTSC) of the scan lines with the first several and last several scan lines all black as recorded on the DVD. On a 16:9 TV using the zoom feature, the inner 360 scan lines are are spaced out a bit more. If the inner 360 scan lines could be upscaled to 480, the picture will look much smoother although the resolution would stay the same. On a 16:9 TV set stuck in widescreen mode, the picture cannot be enlarged, or zoomed. At best we have the side squeeze from the player or de-interlacer (#1 above) but the finished picture has black on all four sides.
3. Many HDTV sets display everything in 1080i. DVD and other non-HDTV video is scaled from 480 scan lines (NTSC) to 540. How this is accomplished varies widely in quality.
The fewer times we rescale the video, the better. So we want the de-interlacer to have at the very least the following vertical scalings, or electronic zoom:
a. None, for 4:3, 5:3, and anamorphic material on 480p or 960i* displays,b. Inner 360 scan lines to become 480 scan lines for non-anamorphic wide screen material on 480p or 960i* displays,
c. 480 scan lines to become 540 scan lines for 4:3, 5:3, and anamorphic material on 1080i displays.
d. Inner 360 scan lines to become 540 scan lines for non-anamorphic wide screen material on 1080i displays,
And if we add PAL to NTSC conversions there are several more useful scalings, such as 576 down to 540 for full frame 4:3 material and 432 to 540 for non-anamorphic material.
*The distinction between 480p and 960i has to do with the positioning of the electron beam on the picture tube, does not detract from picture quality, and either will work with a progressive scan DVD player or line doubler. If a viewer selection is offered, the slight difference in picture appearance is a viewer preference only.
From the standpoint of inexpensive line doublers, it is fortunately not necessary to resort to continuously variable scaling. Under the current DVD standard, the same fixed scalings suggested above work for all movie aspect ratios from 1.78:1 on up to 2.65:1.
When interlaced video is converted to progressive scan video, the best results are had if the device that does the de-interlacing also does the scaling. Both de-interlacing and scaling require working with digital video. If the video is converted back to analog in between these activities, which always happens if the scaling is performed in a different place, some quality is lost. So it is advantageous to do the scaling in a progressive scan DVD player although few players so far have the feature.
For best results the de-interlacing should be completed before the vertical scaling is done, and the vertical scaling completed before the horizontal scaling is done. It may be noted that scaling interlaced video first and then doing de-interlacing will result in added or dropped scan lines occurring in pairs relative to the finished progressive scan frames, causing artifacts to appear larger. Also, by de-interlacing before scaling, it is not necessary, in this writer's opinion, to do vertical filtering (blending of adjacent scan line content).
Actually, horizontal scaling, as needed to address #1 above, does not need any specific formula. Just the necessary final step of converting to analog will perform it. For 16:9 to 4:3 conversion using side to side squeezing, the right way is to time the output of the pixels during the digital to analog conversion so the pixels all land on the inner 75% of each scan line. Each of the 720 pixels now occupies 1/960'th of the scan line.
If stand alone de-interlacers implemented features (c) and (d) described above, the progressive scan DVD player would lose its advantage when the TV displays everything in 1080i. The progressive DVD player has a direct digital path between its MPEG decoder and its de-interlacer, but there is an analog to digital conversion needed when the TV upscales the incoming 480p progressive scan video to 1080i. Meanwhile the stand-alone de-interlacer needs an analog to digital conversion for the incoming interlaced video from the DVD player but no additional analog to digital conversion is needed because the 1080i output from the de-interlacer skips the scaler in the TV.
Scaling and Downconversion Test
All parts (c) copyright 2002, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.
P.O. Box 762, Nashua, NH 03061
603-889-1111 -- ajaynejr @ aol.com
If you would like to contribute an idea for our web page, please send
us an e-mail. Sorry, but due to the volume of e-mail we cannot reply personally
to all inquiries.