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 Post subject: Re: Emulating NTSC video
PostPosted: Wed Oct 26, 2016 8:15 pm 
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Here, just to follow up, a video by "My Life in Gaming" about CRTs for gaming. Maybe a bit heavy handed, but they do have a lot of good video examples and I think they explain it well.
https://www.youtube.com/watch?v=RAi8AVj9GV8


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 Post subject: Re: Emulating NTSC video
PostPosted: Thu Oct 27, 2016 1:56 pm 
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Here's a list of things I wish HDTVs would stop doing.

-Deinterlacing everything into 30fps
-Gaussian blurring the entire screen
-Input lag
-Sampling everything at 640 pixels across, and upscaling it (with the dumb gaussian blur filter)

Also, is it just me, or did CRTs usually have better Y/C separation than HDTVs do.


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 Post subject: Re: Emulating NTSC video
PostPosted: Thu Oct 27, 2016 2:06 pm 
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Location: NE Indiana, USA (NTSC)
Let me guess the excuses that TV makers will use:

TVs don't attempt to deinterlace video that meets a progressive video standard. This means 480p, 720p, 768p (RGB), or 1080p. The double-struck 240p mode used by classic video game consoles was never published as a formal standard.

It's not Gaussian blur. In many cases, it appears to be either linear or cubic interpolation. This is visually preferable to nearest-neighbor for live action, photorealistic CGI, and cel animation, just not for the nonstandard output of the original PlayStation and earlier video game consoles.

Input lag is not a problem for noninteractive video from broadcast, cable, or satellite television or Internet streaming. Better TVs tend to include a "game mode" that reduces lag from a standard progressive source at the expense of somewhat reduced picture quality.

The only composite picture source I'm aware of that requires a sampling rate greater than 13.5 MHz is an Apple II in 80-column text, double hi-res, or IIGS double super hi-res mode.


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 Post subject: Re: Emulating NTSC video
PostPosted: Fri Oct 28, 2016 8:00 pm 
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For NTSC filtering there is also the possibility of steep notch filters, like the one below. If the dot-crawl is too noticeable, it probably could use a Gaussian filter on top of it.

http://www.dspguide.com/ch19/3.htm


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 Post subject: Re: Emulating NTSC video
PostPosted: Sun Oct 30, 2016 7:32 pm 
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This is totally not how analog TVs work, but I just thought of a neat way to separate luma and chroma. Demodulate chroma. Limit the slope of the I and Q signals. Remodulate the chroma signal and subtract it from the composite signal to form the luma signal.


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 Post subject: Re: Emulating NTSC video
PostPosted: Sun Oct 30, 2016 7:49 pm 
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Location: NE Indiana, USA (NTSC)
I'm pretty sure some analog TVs do subtract the remodulated IQ or UV from the composite signal.


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 Post subject: Re: Emulating NTSC video
PostPosted: Sun Oct 30, 2016 7:59 pm 
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Do any of them fake filtering by limiting the rate the I and Q signals change from high to low?


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 Post subject: Re: Emulating NTSC video
PostPosted: Sun Oct 30, 2016 8:03 pm 
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It'll only work if done in digital domain, I cannot see it work with any decent performance when done entirely in analog domain mostly because time delaying original signal is required, demodulation isn't instant and because you got to perfectly line up the original and processed signal, any desync and linearity error results in exaggerated artifacts.

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 Post subject: Re: Emulating NTSC video
PostPosted: Mon Oct 31, 2016 4:53 am 
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"Slope limiting" sounds mostly like another way of describing "lowpass filter"...


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 Post subject: Re: Emulating NTSC video
PostPosted: Mon Oct 31, 2016 5:10 am 
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"Slope limiting" or "maximum slew rate" has a low-pass characteristic, but unlike the common IIR filter designs, it's not linear.


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 Post subject: Re: Emulating NTSC video
PostPosted: Tue Nov 01, 2016 3:20 pm 
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I've been experimenting with different filters using a calculator and a painting program, and I think a 49 point Hamming Sinc filter with the picture scaled to 512x448 looks best. If the original poster wants to use this type of filter the equation is this:

y=sin(7xpi/12)/(xpi)*(25+21cos(xpi/24)/46*256

... and the convolution matrix itself:

{0,0,0,0,-1,0,1,0,-1,1,1,-3,0,4,-3,-5,7,2,-12,4,17,-19,-20,78,153,78,-20,-19,17,4,-12,2,7,-5,-3,4,0,-3,1,1,-1,0,1,0,-1,0,0,0,0}

edit:
Fudged center point to from 149 to 153 so that every 3rd sample adds up to 85, and the whole thing adds up to 255.


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