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PostPosted: Fri Mar 31, 2017 12:45 am 
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Location: Australia, the land of British 50hz and obscure sports games.
I've seen a couple of instances of people complaining about the use of 0D color palette entry as it messes with their tube televisions by creating some weird warping effect. I have no idea why this happens on some TVs but not others. I also have no clue on how video signals are transmitted and i want to know how it interferes with certain televisions.


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PostPosted: Fri Mar 31, 2017 1:11 am 
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This has been asked before, though not precisely.
whicker wrote:
I'm hoping everyone understands why "blacker than black" is a problem...
It's because it looks like a sync pulse-- The pulse that tells the electron beam to go back to the left side of the screen or to the top-left of the screen.


Successful CRT televisions in general had to be very tolerant of noisy, out of spec signals because of those dastardly battery powered VHS camcorders that were pervasive during the 1980's-90's. There's so much that needs to go on and not enough room to do it exactly right based on the technology of the time. Plus we're talking a mechanical tape feed that is subject to wobble and actual stretching of the recording material. So the played back signal is going into unsafe ranges constantly. The family that bought their shiny new top-of-the-line (ultimately piece of crap, heavy NiCad battery powered, belt driven, DC brush motor, power sucking, overheating) camcorder is not going to blame the brand-new camcorder, but complain to the TV manufacturer that it doesn't display correctly. The picture is stable in the viewfinder, it must be the TV, they'd (correctly) say.

I've seen the many different responses from TVs and capture devices I've tested with an $0D colour signal:

1. Wobbly areas of the screen in horizontal strips. (loss of horizontal blank integrity?)
2. Wobbly/rolling screen in general. (loss of vertical blank integrity?)
3. Drops the signal entirely. (probably same as 2 but for a TV that's converting to digital)
4. $0D rendered at same colour as regular black.
5. $0D rendered slightly darker than regular black.
6. Introduction of $0D causes the black level to renormalize after several seconds, causing a slow transition from 4 to 5 as it becomes the "new" black.

tepples wrote:
NTSC, but not PAL, has something called setup. This is a difference between the blanking level (not the sync level) and the black level. Sync is -40 IRE, blanking is 0 IRE, and black is 7.5 IRE.

Someone should convert the voltage levels on NTSC video to IRE units so that analog video geeks can make better sense of it.

I feel like there's a better explanation somewhere whose indexing terms I'm missing.


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PostPosted: Fri Mar 31, 2017 1:15 am 
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This would be better explained with a graphic, but I'll try to explain. Basically video signal is transmitted by sending the analog luminosity value as a varying voltage. When the end of a line is reached, a short negative voltage pulse is sent to indicate HBlank. When the end of the frame is reached, a long negative voltage pulse is sent to indicate VBlank. PAL and NTSC have different standards when it comes to the timing of those pulses, and also have different colour encodings. Colour encoding was added later on the top of existing monochrome video standards and was designed to be fully backwards & forward compatible with monochrome displays - however I'm getting out of topic here.

The value $0d sends a negative voltage pulse, which can be mistaken for a HBlank pulse. The timing is not right so it'll confuse some TVs. I unfortunately cannot tell more details, as I am ignorant to how TVs actually decode the signal and in what way exactly they are "confused" by fake blanking pulses.


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PostPosted: Fri Mar 31, 2017 2:34 am 
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Location: Australia, the land of British 50hz and obscure sports games.
Bregalad, is there a reason why some modern televisions (obviously the ones that support PAL analogue encoding) also have problems with this minor oversight? My modern flatscreen television makes the screen extremely bright and washes out any other color onscreen when at least half the screen has 0D on it. This makes playing some Codemasters games very difficult to look at, let alone play.

I'm getting a new CRT soon and i'd also like to find out if theres a easy way to test if a TV has this problem without carrying my NES and a copy of Bignose Freaks Out! around with me. Thanks for the quotes but i couldn't find the source topic. I'd like to learn more about this intriguing issue.


EDIT: I realised I forgot about another question i had in mind. Bregalad you said that some Televisions have this problem and others don't. Why is that? wouldn't TVs need strict specifications in order to receive a translatable signal at all?


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PostPosted: Fri Mar 31, 2017 3:07 am 
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I have absolutely no idea about either of your questions. This relates to how individual TVs decode an invalid signal - and this depends on how each TV is made inside - and as I already mentioned I have zero knowledge in that domain.


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PostPosted: Fri Mar 31, 2017 6:04 am 
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The "strict specifications" aren't quite so strict. First, they say what to do when receiving a conforming signal. The NES signal is anything but conforming: 227.333 cycles per line instead of 227.5, 262 lines per field instead of 262.5, $0D lower than black setup level, $20/$30 possibly lighter than expected, etc. Second, there's no hard boundary between the luma and chroma frequency bands at least in NTSC, meaning there's no well-defined way to tease apart a composite signal in a way that produces 0 dot crawl and 0 color fringing.


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PostPosted: Fri Mar 31, 2017 7:59 am 
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Meanwhile, I have an old TV where color 0x0D + all emphasis bits set just looks like a nice deep black color. Definitely not true on other TVs.

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PostPosted: Fri Mar 31, 2017 8:56 am 
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I don't think there's any way to know how a TV will react to color $0D besides trying an actual NES on it. I personally wouldn't worry about this too much, considering that games aren't supposed to use this color in the first place. The few games that do are "doing it wrong" (TM). For development purposes, it's actually better that your TV freaks out with color $0D, so that you can easily notice it in case you use it by accident.


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PostPosted: Fri Mar 31, 2017 9:09 am 
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TR3KT wrote:
Bregalad, is there a reason why some modern televisions (obviously the ones that support PAL analogue encoding) also have problems with this minor oversight? My modern flatscreen television makes the screen extremely bright and washes out any other color onscreen when at least half the screen has 0D on it. This makes playing some Codemasters games very difficult to look at, let alone play.


Maybe you're tv adjusts itself to the darkest black onscreen? I remember the TV I used to have did that a lot and it drove me nuts. Whenever there was a lot of black onscreen my tv would automatically whiten every color onscreen including black.


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PostPosted: Fri Mar 31, 2017 10:13 am 
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Here's a few links to things that might help you feel that you have a better understanding of how analog CVBS video works, including how non-compliant video works:
http://martin.hinner.info/vga/pal.html
http://www.danalee.ca/ttt/analog_video.htm
http://www.ronaldsnoeck.com/vcr.htm


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PostPosted: Mon Apr 03, 2017 2:00 am 
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Location: Australia, the land of British 50hz and obscure sports games.
Thank you for your responses so much.

it's a bit of a shame that there's no easy way to test TV color sensitivity. i'll have to use a NES to test out if the tv will accept certain games


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