JVC NAB-2007 - page 35

JVC Professional APRIL 2007 NAB-2007 The ProHD Report

The HD delivery method is ATSC OTA or QAM CATV, both being relatively equal

in full bandwidth quality at their higher bitrates. (I.e. the use of sufficient bits in the

19.4 ATSC transmission for the primary HD channel, and not compromise the

encoding quality due to squeezing multiple channels through the ATSC pipe.) Thus

we assume 1920x1080i60 and 1280x720p60 at 4:2:0 consumer delivery is the pipe

limitation, thus the benchmark in practical terms is a live HD studio camera (full

HD bandwidth 4:2:2 at 10-bit depth, HD-SDI out) at the TV station shooting a

well lit (news) set, with real-time delivery over ATSC OTA to the home HDTV.

Interlaced HD on a Progressive HDTV?

Viewing in HD is by definition viewing on a progressive HDTV, as all HDTVs sold

today are of the progressive kind, with a refresh rate of 60 frames per second. (We

forget about the very few CRT-based HDTVs still being sold.) The ATSC 720p60

progressive standard is of course no problem, as it maps frame by frame, and, if a

native 1280x720p60 HDTV, pixel by pixel.

But, when receiving the ATSC 1080i60 OTA, the interlaced HD video must be de-

interlaced, as the progressive display must draw the entire screen 60 times per

second. All current displays except for CRT screens require to de-interlace 1080i. In

theory, there is no reason why LCD, DLP or Plasma displays could not display two

fields sequentially, but the requirement that half of the pixels remain black half of the

time would result in less (half?) perceived brightness. Remember that interlaced

CRTs are made with phosphorous material which illuminates when hit by the

scanning electron beam through a mask, and the phosphor has an intended

“illumination decay time” while a pixel in a new non-CRT display is either on (with

the appropriate intensity) or off, without any intended (longer) decay time.

De-interlacing is an imperfect process and how much of the delivered ATSC OTA

1080i resolution is lost in the de-interlacing process? And, remember, in the home

HDTV, de-interlacing must be a real-time process with low latency. We assume that

the de-interlacing/conversion process from 1080i to progressive produces a loss in

perceived resolution more or less equal to the Kell/Interlace factor of 0.7.

The pixels/sec benchmark (ATSC 4:2:0) presented to the viewer is then:

1280x720p60 x 1.5 = 83 million pixels/sec (no Kell/I reduction, progressive)

Luminance 1280x720p60 = 55 million

Chrominance = 28 million

1920x1080i60 x 1.5 (x 70%) = 65 million pixels/sec (Kell/I reduced to 70%)

Luminance 1920x1080i60 = 44 million

Chrominance = 21 million