psycopathicteen wrote:
I keep looking at schematic diagrams and technical documents of RGB to NTSC converter chips and I don't see anything that limits the composite signals to 120 IRE, nor anything that desaturates the chrominance signal to fit under this 120 IRE broadcast "limit." It just adds the luminance signal to the chrominance signal assuming no valid RGB value can possibly go over the limit.
...and it's not like video game developers pay attention to avoid certain oversaturated colors. Nintendo seems to use a lot of "illegal" RGB triplets in their games and my TV doesn't go berserk everytime I play my Nintendo Wii.
After reading up awhile back on this a bunch, it isn't well defined. What I did read, however, said that you are supposed to saturate on decoding and clip on encoding.
i.e. if your chroma+luma exceeds the thresholds (too high or too low) you clip.
on decoding, if R, G, or B exceeds the maximum range, you clip. It is possible I think to have a negative value for R, G, or B. In which case you clip to 0. This clipping can cause a colour shift if say, red saturates and green hasn't yet. This could cause it to shift from red to orange as red pegs and more green is added.
It's been about 3-4 months since I poked around with this stuff but that's what I remember. Incidentally, finding information on how to actually encode/decode NTSC and PAL video was extremely difficult to do. I ended up finding a book about it titled "Video demystified: a handbook for the digital engineer". This book is extremely good and has everything you'd ever want to know about video processing in the digital domain.
I used the info in it to make an FPGA RGB to NTSC/PAL video encoder.