OVER 100,000 ILLEGAL COLORS!

With the latest advancements in electronic memory and computer technology, sophisticated video graphic systems and character generators are becoming more powerful. Recently, many video graphic systems and character generators have been touting the availability of more than 16.7 million colors. Mathematically this is correct for systems that can generate colors from 256 levels, each of Red, Green and Blue. (256 X 256 X 256 = 16,777,216). But the limits of the availability of these colors is sometimes overlooked.

Many products such as television character generators and paint systems have numerous limits that restrict the use of available colors on a given object, scan line or the screen. Many times these products use memory saving techniques such as palettes to allow more colors from less memory. These are generally hardware limitations that vary from one piece of equipment to the next and should be addressed by the manufacturers specification sheets. The intention of this article is to address the true availability of colors that fit into the NTSC composite specifications. Just how many of these 16.7 Million colors are valid or legal colors?

First the limits must be established for (composite) NTSC. For the purposes of this article, I will call a "VALID" color any color that when properly encoded to NTSC will create a signal which falls within the limits of 100 IRE on the positive excursion and -20 IRE on the negative excursion, while not exceeding the 100% saturation limit (S.M.P.T.E. color bars meet this criteria). By this definition, 100% saturated color bars would be invalid, as would a "Modulated Ramp" test signal because both contain excursions that go above 100 IRE on a waveform monitor. The excursions are measured as the instantaneous sum of the luminance information and the chrominance information.

While it is true that signals with chroma peaks just over 100 IRE can be legally transmitted, the reason this level was chosen as the limit of "VALID" colors is many other pieces of equipment in the video path may not be able to handle excursions beyond this. VCR's and Time Base Correctors for example, typically have severe distortion (D.P., D.G., clipping, compression, fold-over, blooming, etc.) problems with this type of chroma excursion. Chroma excursions that go below -20 IRE can also cause a disruption in the sync or burst separation circuits.

For the definition of a "Legal" color, the composite peak excursions can not go beyond 120 IRE in the positive direction. The modulation level of a NTSC transmitter is established so that 100 IRE corresponds to 12.5% modulation (sync tip is at 100%). This means a signal with 120 IRE peaks develops 0% modulation (effectively turning off the carrier). Needless to say, signals that go beyond this level cannot be transmitted and are illegal.
One common method that has been used to determine which colors, from an electronic video generation device, are valid is to use a waveform monitor to observe violations. This method would only work if it were practical to have an engineer looking over the shoulder of the artist, as the artist created new colors on his palette. If you allow the artist to create freely, then try to review his work after completion, he will be quite upset that he must rework his art. It becomes quite frustrating to say some colors can't be used, while no one can explain which colors may be used. There is no simple rule that can be used to describe valid colors.

A better way to prevent illegal and invalid colors is to simulate the effects of an ideal encoder mathematically and test for violations. This could have the effect of having a simulated engineer looking over the artist's shoulder, warning if a chosen color might be a problem. Some character generators and video graphics products have taken the first step in this process by using a color space more suited to television such as H.S.V. (Hue Saturation and Value) or H.L.S. (Hue, Lightness and Saturation) instead of the traditional RGB color space. These color models are often more intuitive to the artist, but the only trouble with these color models is that they are not based on the levels created by an NTSC encoder. Even after correction factors have been taken into account, one significant piece of information about the signal is still missing, the instantaneous peak levels represented by the sum of the luminance and the chrominance signals.

A computer model may be constructed to simulate the ideal encoder, once we further understand the mathematical relationships between RGB and Encoded NTSC. We begin by deriving the relative luminance portion of the NTSC from the RGB components:

E_Y = (0.299*E_R + 0.587*E_G + 0.114*E_B)

Whereas:

E_Y = The relative voltage potential for the luminance component after matrixing of the RGB signals, expressed as a percentage of 1, where 1 is the level produced when each of the RGB components are at maximum, and 0 is set-up level.

E_R,G,B = The relative voltage potential for each of the individual color channels expressed as a percentage of 1, where 1 is the maximum level that may be generated, and 0 is the minimum level, or set-up. It is assumed that a gamma correction of 2.2 has been added to each of the RGB signals.

It is important to note, that these levels are a relative percentage of 1. and are proportional to, but do not represent the actual peak-to-peak voltage levels. This formula also assumes that the gamma correction and the set-up (pedestal) have already been added to the R,G & B components. This relative level is handy because any resolution of Digital to Analog converter (D/A) may be incorporated into the formula by using the chosen level divided by the number of levels available.

Next the absolute luminance level in IRE units (140 IRE = 1.00 V.) will be determined, based on the absolute levels of the RGB components.

NTSC ENCODER FORMULAS:

           660*E_Y + 53.55
Y_IRE = -----------------
                   7.14

E_I = 0.596*E_R - 0.274*E_G - 0.322*E_B

E_Q = 0.211*E_R - 0.522*E_G + 0.311*E_B


E_SAT = Ö(E_I2 + E_Q2)

SAT_IRE = E_SAT * 185.232

PHASE⁰ = ARCTAN (E_I/E_Q)+33 (IF E_Q < 0, THEN ADD 180)

IF E_I = 0, AND E_Q = 0, THEN PHASE = N/A.

IF SAT_IRE > 100,

OR SAT_IRE / 2 + Y_IRE > 100,
OR IF Y_IRE - SAT_IRE / 2 < -20


THEN COLOR IS INVALID

OR IF Y_IRE - SAT_IRE / 2 < -40

THEN COLOR IS ILLEGAL

TRANSCODER FORMULAS:

E_R-Y = (0.701*E_R - 0.587*E_G - 0.114*E_B) = E_SAT * COS(PHASE⁰)

E_B-Y = (0.886*E_B - 0.299*E_R - 0.587*E_G) = E_SAT * SIN(PHASE⁰)

Y_BETA = Y_IRE * 0.00714

R-Y_BETA = 0.6662 * (E_R-Y)

B-Y_BETA = 0.5271 * (E_B-Y)

Y_MII = E_Y * 0.7

R-Y_MII = 0.4618 * (E_R-Y)

B-Y_MII = 0.3654 * (E_B-Y)



Y_MII = The Y signal used by MII which has no set-up, and is a 7:3 video to sync ratio.

R-Y_MII = The R-Y color component used by the MII system. When 100% color bars are used this component will have a 0.6475 Volt P-P level. No set-up is used on the RGB components.

B-Y_MII = The B-Y color component used by the MII system. When 100% color bars are used this component will have a 0.6475 Volt P-P level. No set-up is used on the RGB components.

Y_BETA = The Y signal used by the BETACAM system. This Y signal is essentially the same as Y_IRE, but is expressed in volts and has sync added at 0.286 Vp-p. Set-up is used and the video to sync ratio is 10:4.

R-Y_BETA = The R-Y color component used by the BETACAM system. When 75% color bars are used this component will have a 0.700 Volt P-P level. Set-up is used on the RGB components. BETACAM-SP will allow for a P-P level of 0.934 Volts, which is what is created by 100% saturated bars with this gain factor.

B-Y_BETA = The B-Y color component used by the BETACAM system. When 75% color bars are used this component will have a 0.700 Volt P-P level. Set-up is used on the RGB components. BETACAM-SP will allow for a P-P level of 0.934 Volts, which is what is created by 100% saturated bars with this gain factor.

Y_SMPTE = Y_MII

P_b = The SMPTE B-Y color component. The level is 0.700 V p-p, (no set-up) when 100% color bars are applied.

P_r = The SMPTE R-Y color component. The level is 0.700 V p-p, (no set-up) when 100% color bars are applied.