How To Use a WAVEFORM and a VECTORSCOPE
and What Is An Illegal Level

I've had many requests to write a tutorial on how to use a WAVEFORM scope and a VECTORSCOPE, as well as a definition of what are illegal luma & chroma levels for NTSC broadcast, so here it is based on a response I made to someone on the VAedit list using the built-in software waveform & vectorscope in the DPS Reality/Velocity.

> Could someone please explain to me what the term "illegal color" implies in TV viewing reality? Power CG has a handy tool through which it is possible to check the legality of colors. To my dismay, I seem to be using these all over the place. I understand bleeding reds and yellows, but what about bright skies and a white, foggy mist covering the entire scene? > Paul

Hi Paul,

Strictly speaking, an illegal NTSC broadcast color is any pixel or area of the picture that has over 75% saturation of the chroma (color). For example, an RGB graphic (BMP or TGA) that had a yellow area of Red 191, Green 191, Blue 0, would have 75% saturated color & would be legal. 75% of a 0 - 255 range is 191, so as long as there isn't a "spread" of more than 191 between each of your RGB channels for any pixel, it's not illegal. An RGB graphic that had a yellow area of Red 255, Green 255, Blue 64, would also have 75% saturated color & would also be legal. But an RGB graphic that had a yellow area of Red 255, Green 255, Blue 0, would be 100% color saturated & would be illegal. If you look on your vectorscope (using 75% setting), any color that goes out beyond the six little target boxes is over 75% saturation & strictly speaking is illegal.

HOWEVER, here comes the part that muddies the definition of "illegal color" a bit. There are many combinations of colors over 75% color saturation that do not cause any kind of problem for NTSC broadcasters or receiving television sets, so some video producers take a chance & sneak up the color saturation beyond 75% for certain colors (we all want our video to look pretty). Specifically, if you look at your waveform monitor in the mode without the chroma stripped off (composite button on the waveform analyzer in Reality/Velocity), you can actually allow the chroma component to sneak as high as 121 IRE without concern that it will be illegal for broadcast. Many NTSC broadcasters use this criteria (rather than the vectorscope) to determine a more realistic (less stringent) illegality of colors measurement.

Using DPS NLE cards, only with YELLOW & CYAN range colors is it possible to go over the 121 IRE ceiling of legality for NTSC & as high as 131 IRE (at 100% saturation), which of course is very illegal for NTSC whether measuring via the waveform monitor or the vectorscope. Any color over 121 IRE in NTSC can spill over the allotted video bandwidth for a North American broadcast channel into the audio carrier (causing a buzzing audio noise), or even spill into the adjacent channel above causing picture interference, if the station isn't using hard clipping equipment prior to broadcast. Most modern broadcast stations automatically hard clip anything over 121 IRE, so any color saturation detail beyond that will be lost (clipped). NTSC broadcast colors are legally allowed to go as low as -16 IRE on the waveform monitor. Only RED & (Navy) BLUE range colors are able to go below -16 IRE if over 75% saturation.

With the current product line family of DPS NLE cards, it is not possible to produce illegal GREEN or MAGENTA, so you can crank those colors up to 100% color saturation if you wish. Of course it might just be your luck that some less than completely knowledgeable station engineer rejects your work because he thinks the colors are illegal (according to the vectorscope), but that's the chance you'll have to take.

You didn't ask about the luma (black & white) portion of the signal, but that portion is not something you have to be quite as concerned with if your NLE card is a PVR, PVR-RT, or Reality because these boards don't output higher luma than 100 IRE. The legal range of luma for most NTSC countries is 7.5 - 100 IRE, with PAL countries & Japan's NTSC being 0 - 100 IRE. The legality/illegality of the use of the 0 - 7.5 IRE black range for North American NTSC users is a whole other subject which is mostly an academic mute point these days. Anything over 100 IRE luma is illegal, but keep in mind that it is legal to have chroma go as high as 121 IRE & as low as -16 IRE in NTSC, so make sure you have your waveform monitor on the correct setting so you know whether you are scoping in luma only mode (luma button on the waveform analyzer in Reality/Velocity), or luma & chroma mode (composite button on the waveform analyzer in Reality/Velocity). PAL television transmissions in most countries can use 100% color saturation so no RGB color is illegal as long as your luma content doesn't go over 100 IRE & your chroma content doesn't go over 131 IRE. This is because most PAL countries use a wider channel bandwidth for broadcast than NTSC countries (7 or 8 MHz rather than 6 MHz) & because PAL transmitters don't invert the signal before transmission.

I haven't used Power CG so I'm not familiar with the illegal color level indicator tool in it, but I can tell you that from a technically correct point of view the illegal color level indicator in Inscriber is completely wrong. I have discussed this extensively with Inscriber & took 2 - 3 days of my time to research & prove it to them with references, but they refuse to do anything about it, so I finally gave up on them.

Assuming you proc amped all your NTSC video levels correctly & used legal level graphics, you may still have someone wrongly criticize or reject your work for illegal chroma or luma levels because they played it back at the wrong levels from their proc amp controls. Make sure your master tape has SMPTE color bars on it as a reference so the television station playing back your work can calibrate their proc amps to the correct levels. Even if you & the television station get all the levels correct & legal, the signal is still often messed up by the cable or satellite distributors who sometimes over boost a signal.

You asked about "bright skies and a white, foggy mist covering the entire scene". Generally speaking, foggy mist is low in color saturation, but bright skies can be very high in color saturation. Make sure you don't clip the luma levels during capture, or when proc amping the clip from playback on the timeline. Remember to check the IRE levels of the cyan color of the sky to make sure they aren't over 121 IRE for NTSC (use the composite button on the waveform analyzer), or use the vectorscope to make sure the Cyan sky isn't more than 75% saturated because the Cyan range is one of the colors that can go beyond 121 IRE (the yellowish range is the other one). If you capture illegal Cyan sky colors, your DPS NLE card will capture them, but the NTSC television station will clip anything over 121 IRE (some stations set their clippers even lower).

I hope this explanation has been helpful in understanding legal/illegal chroma & luma levels for broadcast. I'd sure like to hear from others (especially any station engineers) who have other information to contribute to legal/illegal levels.

By Doug Hembruff.