The following are quotes -- I will paste site at bottom:
"North American NTSC standards historically used a level for the analog output of black that was 53.6mV (aka 7.5 IRE) above the blanking voltage of 0mV (0 IRE), and we still do. Other standards like HD, Japanese NTSC, etc use 0mV to output black, and lack the 53.6mV (7.5 IRE) ‘setup pedestal’ that North America uses."
"Both HD material and DVDs use the same digital levels. It is the ANALOG output standards which can vary. The encoded levels represent black just as black. The dynamic range is the same. Video encodes black at digital 16. Period. That’s black, and whether it is output at 0 IRE or 7.5 IRE the resultant image should be equally black. If there is any difference in the visible levels of black, your video chain is miscalibrated. If you connect sources that output at various voltages, you have to calibrate to EACH of these sources. Users will often connect a DVD player that outputs black at 7.5 IRE, then connect an HD source that outputs black at 0 IRE, discover that the HD source is darker, and mistakenly conclude that HD has improved black capabilities. This discovery of changed blacks is merely that you have different sources sending different signals. You must calibrate your display to each individually."
"Unfortunately, in reality DVD players are often designed poorly and may clip BTB data. Some will clip BTB in both settings, others will clip it in one or the other. A correctly designed DVD player will maintain BTB data at analog output regardless of whether it is set to output black at 0 IRE or 7.5 IRE."
“Why does data below black even exist? That makes no sense! What can be blacker than black!?”
"In video, headroom and footroom is important for a number of reasons. The most basic is that mastering can be less than perfect, so some “fudge-room” has always existed. However, even with ‘perfect’ mastering, data regularly extends outside reference black/reference white. Peak white data allows for highlight details to be maintained, which is common in clouds and other bright objects. BTB data helps prevent image anomalies from hard clipping of the analog waveform at black when converted to analog. BTB can also sometimes become visible as the actual black level on a CRT display floats up and down with image content because black level retention on CRTs is not perfect. The mastering engineer is viewing on a CRT display and actively changing the encoded levels so that they are imaged correctly on the display. This reverse-float compensation in black is allowed with BTB footroom. This reverse float compensation should be the only times data encoded below black is visible in the final image. If you are using a PLUGE pattern with BTB bars to calibrate, you should calibrate so that the BTB data is not visible. BTB data also helps define dithering duty patterns on DLP projectors. Lastly, BTB and peak white data is quite useful for any image processing/scaling etc applied to an image. For all these potential reasons, video engineers advocate preserving full BTB and peak white data whenever possible. Lastly, this data is helpful for any image processing that is applied on an image, such as sharpening, scaling, etc."
Refererance setting = 16-235
Peak setting = 0-255 (What most of us refer to as BTB and WTW)
This information was pasted from this site:
http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=494606
@party :clap :clap 
