Preventing Illegal Broadcast Levels

Broadcast facilities have limits on the maximum values of luma and chroma that are allowable for broadcast. If a video exceeds these limits, distortion can appear in the form of colors bleeding into one another, the whites and blacks of your program washing out, or the picture signal bleeding into the audio signal and causing audible distortion. In all these cases, exceeding standard signal levels can result in unacceptable transmission quality.

For this reason, as you are performing color correction on clips in your edited sequence, you need to make sure that the luma and chroma levels of your video stay within the parameters referred to as broadcast-legal, or acceptable for broadcast. It is easy to inadvertently push the levels of clips in your sequence too high, so it’s important to use Final Cut Pro scopes and range-checking options to make sure that the luma and chroma levels you set stay legal.

Legal Broadcast Colors

A mistake beginners often make when creating graphics for video is to use colors that are vivid on the computer display, but have chroma and luma levels that are outside of the “legal” range of color that can be broadcast. Broadcasters must adhere to these specifications or risk fines from the FCC. Therefore, if you submit a tape with signals out of specification, it may not be accepted.

Many graphics and compositing applications have a “broadcast safe” filter that you can use to limit the color range of a graphic so that it will be broadcast properly without distortion. However, you should use these filters only as a last resort. It’s best to be aware of the specifications of the video signal and keep your chroma and luma values within proper limits.

If your program has been accepted for broadcast, you can usually get a set of guidelines specifying the broadcaster’s criteria for a legal video signal. The Corporation for Public Broadcasting has a frequently cited set of guidelines for defining what levels of luma and chroma are acceptable for broadcast. You will probably be in the clear with most broadcasters if your program adheres to these guidelines, since they are fairly conservative. Other broadcast companies publish their own guidelines.