This is an analog TV question not digital TV question but I couldn't find an analog TV section on this forum.
My question is what exactly is meant by the 75% and 100% color bars test patterns? It's a percent within what range? There are several possible ranges I can think of that could be used.
blanking level to white level (IRE 0 to 100)
black level to white level (IRE 7.5 to 100)
blanking level to maximum IRE (IRE 0 to 130)
black level to maximum IRE (IRE 7.5 to 130)
full range including sync levels (IRE -40 to 130)
Also when talking about the percentage that the signal has, does this just refer to the Y (luma) signal for the bar that's at that level, or is it referring to the peak voltage of the composite signal (Y and chroma carrier added together) for the bar that's at that level?
I'm trying to make my own color test pattern algorithmically in some computer software I'm writing (both the 100% bars and 75% bars versions), and I need to make sure I'm using the exact correct values. If possible, could you point me to the official document that provides the specifications for generating the color bars test patterns?
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Last edited by Videogamer555; 21st Jan 2021 at 02:19.
75% = 75 percent of intensity. The Luma for "white" would be at 75IRE, with Chroma excursions going above & below (it is modulated, remember).
100% = 100 percent intensity. The Luma for "white" would be at 100IRE, with Chroma excursions again going above & below.
See the Wikipedia pages for SMPTE bars and for IRE.
Why are you doing this? For all intents & purposes, analog TV retired in the US almost 12 years ago.
75 percent RGB primaries. For example, the yellow bar will have red at 75 percent, green at 75 percent, blue at 0 percent.
Not quite true, at least in the US. While 100% of full luma for white being 100 IRE is correct, it's not the same for anything below 100% of full luma. Remember, 0% intensity (black) is NOT at 0 IRE, but rather at 7.5 IRE (at least in the US). Blanking level (0 IRE) is actually considered DARKER than black, and BOTH colors (blanking and black) should be completely black (emitting no light at all) on a properly adjusted TV display.
This is the reason I even asked about the color bars, because I know that 0 IRE is not the same as the color black, in the US. So in the US, is a 75% white color bar at 75% between blanking and full white? Or is it 75% between black and white?
But the 7.5 setup is only for analog video. Digital video doesn't use setup -- setup is applied by the hardware that converts digital to analog (or removed by the hardware that converts analog to digital). So if you're making your own digital video color bars setup is of no importance to you.
The "75% colorbars" spec refers, not to the luma values, but to the RGB components before being converted to YIQ/YUV and after being converted back to RGB. Remember that the RGB primaries contribute different amounts to the luma. So full blue (R=0, G=0, B=255) has a limited range, 8 bit, rec.601, Y value of only 41, not 255. 100 percent red has a Y value of 81, and 100 green 146.
Last edited by jagabo; 21st Jan 2021 at 08:41.
Last edited by Sharc; 21st Jan 2021 at 10:06.
For RGB=(0,0,255) I get Y=29 and an illegal Cb=258, or for RGB=(255,0,0) I get Y=76 and an illegal Cr=258. I understand RGB outside the [16....235] range leaves some headroom for occasional excursions to prevent clipping, but 'full red' is represented by RGB=(235,16,16) following this standard. Maybe I misread something.
Last edited by Sharc; 21st Jan 2021 at 10:56.
Because you're using the limited range RGB/YUV equations. Computer monitors are set up with black at RGB=0, white at RGB=255.
On-screen RGB is almost always 0-255. Almost all commercial YUV formats are Y=16-235, UV=16-240 (DVD, Blu-ray, youtube, Netflix, etc.). The limited range YUV to/from full range RGB equations take care of the range difference. You want to use the YCbCr equations:
Y' = 0.257*R' + 0.504*G' + 0.098*B' + 16 Cb' = -0.148*R' - 0.291*G' + 0.439*B' + 128 Cr' = 0.439*R' - 0.368*G' - 0.071*B' + 128 R' = 1.164*(Y'-16) + 1.596*(Cr'-128) G' = 1.164*(Y'-16) - 0.813*(Cr'-128) - 0.392*(Cb'-128) B' = 1.164*(Y'-16) + 2.017*(Cb'-128)
Last edited by jagabo; 21st Jan 2021 at 13:27.
Yes, understood. Thanks.
Apparently ITU-R BT.601-7 addresses the limited range equations only. I did not find the full range RGB to limited range Y'CbCr conversion in that BT.601 specification (or I did not read it carefully enougth or just didn't get it). I see no obvious reason why they did not include it. I have read about leaving headroom for filter overshoot, ringing, inaccuracies etc. Well, maybe ......