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Author Topic: Does NTSC have a "Non-Interlace" mode? Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-15-2004 08:44 PM                        Old school videogame systems like the Nintendo Entertainment System, Sega Master System, Sega Genesis, Super Nintendo, TurboGrafx-16/PC Engine, etc all ran at resolutions in the neighborhood of about 320x240. There was no visable interlacing at all when viewed on an NTSC CRT TV. Look at this picture: See the "empty scan lines" between the regular graphics? They are always there 100% of the time and do not flicker... ever. The colored lines also are 100% rock steady with zero flicker. it looks every bit as steady as 480p, just with much less resolution. I've only seen NTSC do this on video games and some menu screens in certain A/V components. My question is this: What is going on here? Does NTSC have some sort of built-in "240p" mode? I asked a very similar question about this on the AVS forum (a place where home theater losers hang out), and they simply could not comprehend what I was saying, even with the picture. It flies in the face of conventional wisdom. I got replies like: "It is interlaced. NTSC is NTSC" "It interlaces at 60 times per second specifically so you CAN'T see the flicker!" "It interlaces, but it uses only one field for the game graphics, but draws the other field completely black!" Some people actually seemed to get a bit angry that I would dare suggest NTSC is capable of not interlacing. Anyway, in answer to some of the quotes above, I do in fact notice interlace flicker. Interlacing is very annoying to me. If it were imperceptible as suggested, then there would be no need for progressive scan at all. Also, if only one screen were used for the game graphics and the other drawn completely black yet still interlaced, you'd notice a crazy amount of flicker and the game would be limited to drawing its graphics at 30 frames per second. That would make for some really choppy scrolling in games such as Super Mario Bros. But the scrolling is 60fps smooth. I can tell the difference between 30 and 60fps easily. Also, my analog to DV video converter does not support this "240p" mode. What it does when I feed it a signal like this is turn it into interlaced video, duplicating the scan lines and basically "filling in the blanks". It looks much different when it is interlaced, and it appears blockier. If one of the fields were actually a black screen, you'd be able to see that when the DV device did its thing. Here are pictures between the original composite input and the same input after being coverted to interlaced by my DV converter: Original NTSC image: Same image converted to interlaced mode: Both pictures were taken with the same camera using the same settings on the same TV maybe 15 seconds apart. There is no Photoshop going on here (other than resizing). So what is going on? Does NTSC really have some sort of half-assed 240p low resolution mode? Every NTSC TV I've played video games on that is a CRT seems to be able to do this. My theory is that the game uses one field and one field only. The second field is never drawn, ever. That way the game system can update its graphics 60 times per second and have smooth scrolling and whatnot, and it keeps it steady without the overhead of 640x480 graphics that these older systems could not handle. Since the graphics are all drawn in one pass 60 times a second across the same set of scan lines each time, technically it could be called progressive. Think about it before you respond with useless comments like "Sorry Joe, but NTSC is NTSC. It ALWAYS interlaces. End of story. PS - I am a moron". But if you have good info, questions, or whatever, please post with your thoughts!  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-15-2004 08:58 PM                     Yes it is very probable that the second field is being drawn on the screen but at a level near black level. That would all be done in the game itself and not in the set. This would be easy to determine with a scope or wavefrom monitor. The way the scanning circuits in an NTSC tv operate both set of lines HAS to be scanned because of the way the sync and scanning circuits are designed and have to operate. What is happenning is that the second set of lines is either at black level or is just blanked out. In other words the game is only putting out composite video with the even scanning lines and at black level or blanked on the odd scanning lines. The odd lines are just not visible or if at black level(7.5%) they would be just barely visible on a calibrated monitor. Mark  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-15-2004 10:03 PM                        But then why would they (the black or blanked lines) not show up when screen captured or converted to interlaced video via DV? Also, what about the 30 image per second limit that would happen in that case, since the games scroll at 60 images per second? Why no flicker at all? Also, why no black or blank lines when same game system is plugged into an HDTV (CRT)? You think it'd throw them in the picture if they were there. When I flip back and forth between interlace and my alleged "240p" mode, the screen changes modes and you can see it re-sync. I am not flipping through a switcher, but rather the game itself. Some games have full resolution interlaced title and menu screens, then drop back to the "240p" for the game itself. When it does this, there is a very obvious re-sync. I could try to videotape it with the camcorder pointing at the TV screen if it would help.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-16-2004 08:10 PM                        So is there anyone who can offer a logical explanation for my questions in my last post above? Anyone?  |  IP: Logged Elena N. Solovyova Film Handler Posts: 44 From: Voronezh, Russia Registered: Feb 2003 posted 09-17-2004 03:53 AM                     It was an unpleasent surprise to me when I realized that my 100Hz TV set will not show my vintage Sinclair ZX Spectrum output! As a consultant explained, there is indeed a progressive mode in TV sets. It is done by tweaking the frame sync signal frequency. The drawback is that you get half of vertical resolution. It seems that Sony Playstation still uses it today. My TV set was enabled to show such output after in-house servicing (it was done by tweaking the parameters from "hidden menu" envoked by special remote controller brought by serviceman).  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 09-19-2004 08:38 PM                     Joe actually figured it out himself.... A function of NTSC sets (at least from the late '60s) is how they interpret the sync signals. A video game will indeed put out an effective 240p signal. It does this by omitting one of the sync pulses that would have advanced the scan lines and instead redraws the same line twice in the same place (still 60 refreshes) so no flicker but the absent line is visible. I do not know if this "feature" was the intent of the NTSC system but it was certainly capitalized upon by every video game manufacturer in the '70s and onward. Fortunately, I have a brother that was a designer of one of the better video display chips of the '70s (TI's TMS-9918) to get the scoop on this. It found its way into the TI 99/4 home computer, Colecovision and other video game machines Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-19-2004 09:41 PM                     Thats interesting Steve!! I wonder how this would behave on older tubed TV sets that don't have very selective sync seperator circuits and whose circiutry requires that both sets of scan lines be completed in order to start the next frame. Any idea? Mark  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-19-2004 10:18 PM                        Thanks, Steve! It's good to know.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-21-2004 09:15 PM                        Just some random information: NTSC can be operated in noninterlaced mode by setting register bit MODE1 high. When in noninterlaced master mode, the Bt866 always displays Field 1, meaning that the falling edges of HSYNC and VSYNC will be output coincidentally. NSYNC will never be output as hopefully they have been muted forever. A 30 Hz offset will be added to the color subcarrier frequency while in NTSC mode so that the color subcarrier phase will be inverted from field to field. Subcarrier phase is reset to 0 upon rising RESET and every four fields for NTSC or eight fields for PAL. In master mode, transition from interlaced to noninterlaced occurs during Field 1 to prevent synchronization disturbance. In slave mode, transition occurs after a subsequent falling edge of VSYNC. NOTE: Consumer VCRs can record noninterlaced video with minor noise artifacts, but special effects (e.g., scan > 2x) may not function properly. Yeah!  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 09-21-2004 09:30 PM                     The difference in image quality after going through your DV (damaged video) converter may be that DV cuts color resolution in half. This produces a "smeared" look.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-22-2004 12:09 AM                        No, the DV device actually duplicates field 1 of the non-interlaced image into field 2, thus creating a true interlaced image. The DV codec is incapable of handling "240p" in its native form. DV compression really isn't that bad, except for the handling of the color red. Images that are interlaced BEFORE going into the convertor come out looking nearly identical.  |  IP: Logged Evans A Criswell Phenomenal Film Handler Posts: 1579 From: Huntsville, AL, USA Registered: Mar 2000 posted 09-22-2004 04:08 PM                        It's interesting that with the several technical books I have on video and television engineering, none of them mention this type of output from video games. Every time I've read this thread, I've been at home, but today, I read it while I was in the office and was going to see if any of my books mentioned video games outputting signals like this, but they don't seem to.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-22-2004 06:16 PM                        Your books suck.  |  IP: Logged Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 09-22-2004 09:50 PM                        Nah, it's video games that suck. You don't see things like this in the real world of broadcast NTSC engineering or operations.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 09-23-2004 12:22 AM                        Videogames are cool. The book sucks because it doesn't even try to talk about the full functionality of NTSC. Also, there would be absolutely no need to broadcast in 240p, so that's why you don't see it in the "real world" of broadcast engineering or operations. Last I checked, videogames are in the real world and are a multi-billion dollar-a-year industry. The consoles of old did not have the horsepower to produce a full 640x480 picture. So instead of letting the games look like ass with field duplication in interlaced mode, they went for the progressive scan in their native resolution which allowed for a rock steady picture. Damn, videogames really do kick ass! So what's NOT in the "real world" then? Anime. Only geeks like anime.  |  IP: Logged

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