On Thu, 11 Oct 2001, Billy Biggs wrote: > Hey, > > I have a bttv-compatible card (BT878) and I was noticing that my > images are too 'bright'. I quickly discovered this is because the BT878 > samples the luma channel in the range 16-253 instead of the normal > 16-235 specified by Rec. 601. I don't think you're correct about this. The bt848 DOES use the CCIR 601 colorspace with a luma excursion of 16-235. If you take the yuv output of the bt848 and convert it to RGB with the same equations you would use for rec 601 input, you will get the correct output. The bt848 does NOT use a full range yuv colorspace (0-255), as is used by the JFIF standard for the colorspace used in most JPEG files. Check the archives of this list from around April-2001 for posts from Benedict Bridgwater and myself that go into somewhat more detail. The chip will output chroma from 2-253 and luma from 16-253 (you can turn on "full range luma" and get 0-255 by setting some bit in a control register). I think that output outside of the 16-235 range represent invalid color or luma information in the original video signal. In fact, I think the reason that rec 601 specified a range less than 0-255 in the first place was to allow headroom for invalid signals. So, you do not need to scale the bt848 output to produce 601 format data. You should, however, clamp the output to the 601 specified range if your software wasn't designed to handle that.
