How Ads Limit Dynamic Range

At the close of episode 3 of the Korean zombie series All of Us Are Dead, the beam of a helicopter searchlight gradually grows larger as the chopper approaches the high school where undead are feasting on classmates until the entire screen is engulfed by blinding white light, and we were eager to see what transition would follow and how long it would take our eyes to recover, but then the credits began to roll…

The following excerpt is from a paper prepared for the Society of Cable Telecommunications Engineers.

“The PQ transfer function defined in SMPTE standard ST 2084 is defined as an “EOTF”, an electro optical transfer function, meaning that it defines the characteristic function in a PQ reference display that maps electronic digital code values in a PQ signal to absolute luminance values on that reference display in order to match the image seen on the reference display used for production or color grading/mastering. This emphasis on EOTF is why PQ is referred to as a “display referred” video signal – unlike for all prior television standards (BT.601, BT.709 and BT.2020) and for HLG (per BT.2100), which are all “scene referred.”

“The PQ transfer function was designed to allow encoding that, as a practical matter, can represent video using a chosen subset [steady-state dynamic range] (at best, about 14-stops in range) from within its potential 28 stops of screen luminance coding range, and to do so in a 10-bit (or 12-bit) displayed signal without visible contouring (“banding”) caused by quantization of HDR signal levels. In television use, assuming black level display is typically limited to about .01 nits, some of both the lower end of the PQ encoding function is not usable, so the PQ range is then closer to 21 stops of dynamic range; but even then, all of this cannot in fact be used in television service.”

“The reason is that the dynamic range of the human visual system in any state of visual adaptation is about 14 stops. Today’s best video cameras also have a similar 14-stop range. So, a real-world PQ signal cannot, in practice, deliver a visible contrast range exceeding this same 14-stop limit. In a motion picture theater, or with uninterrupted Blu-ray movie viewing, careful scene brightness planning (over time as the story proceeds) can take advantage of viewers’ ability to visually adapt to allow more than a 14-stop range to be exploited, but this is not possible for television service, because television content is frequently interrupted by ads and other interstitial materials which would disrupt any attempt to take advantage of slow audience changes in visual adaptation.”