Hey, we have a music video which has been graded in color and features A LOT of crushed blacks falling below the legal limit. Due to time constraints we don't have time to completely re-grade all the shots to bring the blacks up to a legal level so we have a session booked with a video legalizer to output to digibeta.
I don't know quite how the legalizer handles the information, whether it re-compresses it or scrubs it off or shifts the whole gamut.. The question really is do we output from color and turn the broadcast safe on or leave it off and let the legalizer do all the work? Which is more likely to keep the blacks looking good without artifacting and not affect the rest of the image adversely?
They both have problems since they are not human eyes with human perceptions, they're each going to alter your overall look a bit wherever you were very far out of spec. And they do it in a more or less ham-fisted way. At which point, what was the point of all the noodling around in color correction, to just have it overridden by the machine anyway?
I would approach the question more from the other end, and say that no matter how cool it looks when you push the levels around, if it's not legal, it's not useable, your ultimate duty is to provide an aesthetically good as well as BROADCAST-LEGAL signal. Of the two, being legal has to override the aesthetic. This means not letting the shot pass on as "done" in color grading unless it is also already broadcast-safe. To me it's like a chef that arranges the plate for serving but neglects actually cooking the food first. Fine for gazpatcho and sushi, not so fine for video. or to put it another way, like spending hours sweetening audio, then having it go thru somebody's compressor and come out unrecognizable.
The overall clamping done by a "legalizer" should be considered a backup insurance for anything that was just a point high or low on the IRE scale, not to take the whole responsibility for grading away from the grader/editor. I don't mean to make this sound like scolding, just a strong point of view, but from a pure standpoint of efficiency, it saves time and money to do a job right the first time rather than doing it over or relying on a "quick fix" after the fact.
If I am going to use a properly configured hardware legalizer on my output (for whatever reason) then yes, I would leave the (software) broadcast safe off and let the hardware do what it does best.