Understanding Video Levels

The video signal is made up color information (chrominance) superimposed on top of black-and-white information (luminance). Adjusting this balance gives you the values of brightness, contrast, color intensity (saturation) and hue (tint). When you look at a waveform monitor in the IRE mode, you can see the values of the grayscale that represent the black-and-white portion of the picture. A vectorscope displays the distribution of the color portion around a circular scale, representing saturation and the various component colors.

The Red, Green and Blue components of light form the basis for video. All images that end up as video originally started as some sort of RGB representation of the world - either a video camera or telecine using CCDs for red, blue and green - or a computer graphics file created on an RGB computer system. This RGB version of the world is converted into a luma+chroma format by the circuitry of the camera or your editing workstation. Originally an analog encoding process, this conversion is now nearly always digital, conforming to ITUR-601 or DV specs. This is commonly referred to as YCrCb - where Y = luminance and CrCb = two difference signals used to generate color information. You may also see this written as YUV, Y/R-Y/B-Y or other forms. In the conversion from RGB to YCrCb, luminance detail is given more weight, because research has shown that the eye responds better to brightness and contrast than pure color information. Therefore in 601, YCrCb is expressed as the ratio of 4:2:2, so by definition, chroma has half the resolution of the black-and-white values of the image. In DV, the ratio is 4:1:1 (NTSC) - even less color information.

Although most post-production systems keep YCrCb signal components separate, they are nevertheless encoded into a composite signal of some sort before the viewer sees the final product, even if only by the very last display technology in the chain. This means that your pristine image capture in RGB has undergone a truncation of information during the post and end use of the video. This truncation gives us the concept of "legal colors" and "broadcast safe" levels, because there are color values and brightness levels in RGB and even YCrCb that simply will not appear as intended by the time the viewer sees it broadcast or on a VHS dub or DVD. That's why it's important to monitor, adjust and restrict levels to get the best possible final results.

Interpreting the Waveform

The NTSC video signal is one volt of energy from the absence of video (black) until the maximum video brightness level (white). This signal is divided into the sync portion (below zero on a waveform monitor) and the video portion (above zero). The video portion of the scale is divided into 100 IRE units, with black set at 7.5 IRE (in the US) and peak whites at 100 IRE. The luminance information should not dip lower than 7.5 or higher than 100. The color information, which you see superimposed over the luminance information when a waveform monitor is set to FLAT, can exceed these 7.5 and 100 IRE limits. In fact, color can legally dip as low as -20 and as high as 120. On top of this, many cameras are actually adjusted to limit (clip) their peak whites at higher than 100 IRE - usually at 105 or even 110. This is done so that you have a bit of artistic margin between bright parts of the image - that you would like to keep very white - and specular highlights, like reflections on metal - which are supposed to be clipped. Therefore, if you set up a camera tape to the recorded bars, you will often find that the peak levels on the tape are actually higher than 100 IRE. On the other end of the scale, you may also find chroma levels, such as highly saturated blues and reds that dip below the -20 mark. In order to correct these issues, you must do one of the following: a) lower the levels during capture into the NLE, b) adjust the image with color-correction, c) add a software filter effect to limit the video within the range, or d) add a hardware proc amp to limit the outgoing video signal.

These values are based on an analog signal displayed in the composite mode, but things get a bit confusing once digital monitoring is introduced. A digital waveform displaying a 601 serial digital video signal will frequently use a different scale. Instead of 100 IRE as the topmost limit, it will be shown as .7 volts (actually 714 millivolts) - the electrical energy at this level. Digital video also has no "set up" to the signal. This is the NTSC component that sets black at 7.5 IRE instead of 0. In the digital world, black is 0, not 7.5. There is nothing missing, simply a difference in scales, so the analog range of 7.5 to 100 IRE equals the digital range of 0 to 714 millivolts. Sometimes digital scopes may label their scale as 0 to 100, with black at 0 and the peak white level at 100. This tends to confuse the issue, because it is still a digital and not an analog signal, so operators are often not sure what the proper value for black should be.

256 Levels

Digital video is created using an 8-bit (or sometimes 10-bit) quantization of the incoming analog image. With 8-bit digital video, the 7.5 to 100 IRE analog range is divided into 256 steps between total black (0) and total white (255). In RGB values, 0 to 255 is the full range, but according to the 601 specifications, the range for YCrCb digital video was reduced so that black = 16 and white = 235. This was done to accommodate "real world" video signals that tend to exceed both ends of the scale and to accommodate the set-up level of NTSC.

Unfortunately, not all NLEs work this way. Some take video in and digitize it based on the 0-255 range, while others use the 16-235 range. Since no video can exist below digital zero, any analog video, which is lower than 7.5 IRE or higher than 100 IRE, will be clipped in a system that scales according to the 0-255 range, since there is no headroom on such an NLE. Some NLEs that work in this RGB mode will correct for the headroom issue as part of the conversion done with the incoming and outgoing video signals.

DV and Set-up

It was all pretty easy in the days of analog tape formats or even professional digital formats, like Digital Betacam, which correctly converted between analog and digital scales. Then came DV. DV is a consumer format, which has been aggressively adopted by the professional world. Like other digital recordings, DV does not use a set-up signal. If you capture video from a DV VTR into an NLE, using the DV signal over FireWire (iLink, 1394), then the lack of set-up isn't an issue because the signal path has always been digital.

Many DV decks also have analog outputs and the analog signals coming out of these frequently have not been corrected with an added set-up value. This results in DV video - with blacks at digital 0 - being sent out via the analog spigots with the black level at analog 0, not 7.5 IRE. The problem really becomes compounded if you capture this analog signal into an NLE, which is expecting an analog signal to have a 7.5 IRE limit for black. It will scale this 7.5-point to equal the digital black value. If you use an NLE that scales digital video according to the 16-235 range, then you are safe, because the darker-than-expected portion of the signal is still within a useable range. On the other hand, if your NLE scales according to the 0-255 range, then your darkest video will be clipped and cannot be recovered because nothing can be darker than digital 0. There are four solutions to this issue: a) use a DV deck with SDI I/O - no set-up required, b) use the 1394 I/O only - no set-up required, c) use a DV deck that adds set-up to the analog signals, or d) place a converter or proc amp in line between the deck and the NLE to adjust the levels as needed.

Color

I've spent a lot of time discussing the black-and-white portion of the image, but color saturation is also quite important. There are two devices that best show color-related problems: the vectorscope and the waveform monitor's diamond display. Vectorscopes show chroma saturation (color intensity) in a circular display. The farther out from the center that you see a signal, the more intense the color. Legal color levels can go to the outer ring of the scale, but cannot go past it.

The diamond display on a waveform monitor shows the combination of brightness with color intensity. This display shows two diamond patterns - one over the other. The video signal must fall inside the diamonds in order to be legal. Certain colors, like yellow, can be a problem because of their brightness. Yellow can exceed the upper limits of the legal range due to either chroma saturation or video level (brightness). An excessive yellow signal - a "hot" yellow - would easily fall outside the edges of these diamond patterns. Either reducing the brightness or the saturation can correct the level, because it is the combination of the two that pushes it into the illegal range.

Proper levels are easy to achieve with a little care and attention to details. To understand more about the video signal, I recommend a book formerly published and distributed by Snell and Wilcox. Check out Video Standards - Signals, Formats and Interfaces by Victor Steinberg (ISBN 1900739 07 0).


© 2004 Oliver Peters

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