Reading Vectorscopes

Vectorscopes can seem mysterious and even perhaps a little intimidating. However, you don't need a phD in engineering to understand how to read one. ***

-seems like some crazy engineering stuff; Don't be scared!

***You don't have to be a geek to read one. Don't need thick glasses to read one.

 

Composite video signals consist of two elements: luma and chroma. In simple terms, luma contains the "black and white" information in a television signal, while chroma contains the "color" information in a signal. Vectorscopes provide a visual representation of the chroma signal.

Totally achromatic (colorless) values will appear at the very center of the vectorscope. The stronger the chroma value, the farther away from the center of the vectorscope. The angle around the center of the vectorscope indicates the hue of colors.

An example reading, using color bars:

color bars vectorscope reading

Sony Vegas' vectorscope conveniently labels the colors Red, Blue, Green, Magenta (blue+red), Cyan (blue+green), and Yellow (red+green). Roll-over the image to see how the trace (i.e. the white dots) corresponds to particular colors.

Vectorscopes display chroma, not saturation!

Chroma values are proportional to their associated luma values- higher luma have higher chroma values, assuming that saturation stays the same. By extension, higher/brighter exposure means higher chroma values.

high exposure
aggressive exposure
low exposure
conservative exposure

Note that the trace on the left extends much further than the one on the right. Remember: vectorscopes do not display saturation. When matching scene colors, a vectorscope cannot be used to match saturation reliably, unless the luma values are the same.

If you need to match scene colors, the fastest approach would be to match colors numerically. Unfortunately, not all NLEs/editing applications provide the tools to do so. The most accurate approach (perceptually) is to use your eye. Numerical approaches do not account for the quirks of human vision such as surround effects. In any case, a vectorscope is unnecessary in matching colors.

Hardware vectorscopes versus NLE-based vectorscopes

***Now, I've just told you that software***

Hardware vectorscopes have a different usage and can be useful in different ways than the vectorscope in your NLE/editing application. In general, NLE-based vectorscopes are somewhat useful as an aid in color correction (see discussion above). They are not very useful when dealing with analog signals (i.e. making a betaSP master) or for field/studio-recording use. They do not replace hardware vectorscopes for those uses.

The manufacturer of analog vectorscopes (i.e. Tektronix, Leader, etc.) will have documentation on how to install and use/read their products. I won't bother duplicating their information. Rather, I will point out the differences between analog vectorscopes and those found in NLEs.

Note: Hardware vectorscopes can measure both analog signals (i.e. composite, component) and digital signals (SDI, HD-SDI). For brevity's sake, I will mainly talk about hardware vectorscopes that measure analog signals.

White balance

If setting white balance on a camera, a vectorscope can be used to check that the white balance is accurate. If the white balance is off, the majority of the trace be slightly off the center of the vectorscope. Analog vectorscopes offer the ability to magnify the display to zoom in. This makes the center of the display easier to see.

In a NLE, there are better way to do things. Simply use the NLE's color correction filter and eyedropper/sample a neutral color. The color corrector should provide some sort of visual indication about the change in white balance. If you are still unsure about white balance, increase the chroma in the signal (this setting is sometimes marked as "saturation"). **

Video levels

A vectorscope can be useful for lining up the color bars on a tape. Most analog vectorscopes will have targets that show where the trace from color bars should end up. This aids in setting video levels when printing to analog formats. The targets in NLE-based vectorscopes tend to be way too large and imprecise. Also, NLE-based vectorscopes can only measure digital levels, not analog ones.

Analog vectorscopes will also have markings for 75% and 100% color bars. This allows the vectorscope to work in conjunction with both types of color bars. 100% color bars are generally unnecessary for video editing. They are more useful for particular engineering uses.

Camera matching

Camera alignment charts from companies like DSC Labs aid in matching cameras through their menu settings (i.e. color matrix). Most NLE-based vectorscopes are not designed for this purpose (i.e. no gain control, no marking for 75%/100% color bars, no live monitoring).

Unexpected Behaviours

NLE-based vectorscopes tend to have some unexpected behaviours compared to hardware vectorscopes. They may even have settings that affect the accuracy of the readings.

*Please note that these behaviours are only correct for the time of writing. Software upgrades may change the behaviour of vectorscopes in the NLEs mentioned.

**I have not mentioned other NLEs since I am not very familiar with them.

Rec. 601 versus Rec. 709

Rec. 709 defines a different set of luma co-efficients than Rec. 601, resulting in a different color space. This changes the locations of the color bar targets. Some NLE-based vectorscopes do not account for this difference.

Typically, SD formats use Rec. 601 color space and HD formats use Rec. 709 color space.