[robomartin]

It's not "legal" as in "a matter of law". It's about the technical specifications of television and how television signals are processed and transmitted. In analog TV the transmission uses AM (Amplitude Modulation). You could create signals that over-modulate the transmitter if not careful. Hence the use of the terms "legal" to refer to signals that would pass through the transmitter without causing any trouble. Most stations would use boxes called "legalizers" to prevent bad signals from reaching the transmitter. However, broadcasters generally have specification documents that content providers must follow in order to submit material that will not be rejected by their QC department.

[commieneko]

No, it actually was "legal" as in FCC regulations; and you could get fines for broadcasting signal out of spec. (Though I've never known of it happening.)

I'm not sure what the situation is now with digital TV. My last years in the industry were before the mandatory switch, and the places I was working were simulcasting. So we were creating stuff that was NTSC safe. By then most of the software we were using took care of keeping things in spec, but some of the older equipment we were using, mostly character generators and few really cranky old switchers and effects boxes, still required that we be careful about our input levels.

[robomartin]

OK, you are probably right. It's been a while. I played in broadcast some time ago.

I think you jogged my memory further. I think the way it worked is that the transmitter over-modulation due to illegal levels would/could interfere with adjacent channels and that's why the FCC made the rules.