Vertical Interval Reference
The standard NTSC video image contains some lines (lines 1–21 of each field) that are not visible (this is known as the Vertical Blanking Interval, or VBI); all are beyond the edge of the viewable image, but only lines 1–9 are used for the vertical-sync and equalizing pulses. The remaining lines were deliberately blanked in the original NTSC specification to provide time for the electron beam in CRT-based screens to return to the top of the display.
VIR (or Vertical interval reference), widely adopted in the 1980s, attempts to correct some of the color problems with NTSC video by adding studio-inserted reference data for luminance and chrominance levels on line 19. Suitably equipped television sets could then employ these data in order to adjust the display to a closer match of the original studio image. The actual VIR signal contains three sections, the first having 70 percent luminance and the same chrominance as the color burst signal, and the other two having 50 percent and 7.5 percent luminance respectively.
A less-used successor to VIR, GCR, also added ghost (multipath interference) removal capabilities.
The remaining vertical blanking interval lines are typically used for datacasting or ancillary data such as video editing timestamps (vertical interval timecodes or SMPTE timecodes on lines 12–14), test data on lines 17–18, a network source code on line 20 and closed captioning, XDS, and V-chip data on line 21. Early teletext applications also used vertical blanking interval lines 14–18 and 20, but teletext over NTSC was never widely adopted by viewers.
Many stations transmit TV Guide On Screen (TVGOS) data for an electronic program guide on VBI lines. The primary station in a market will broadcast 4 lines of data, and backup stations will broadcast 1 line. In most markets the PBS station is the primary host. TVGOS data can occupy any line from 10-25, but in practice its limited to 11-18, 20 and line 22. Line 22 is only used for 2 broadcast, DirecTV and CFPL-TV.
TiVo data is also transmitted on some commercials and program advertisements so customers can autorecord the program being advertised, and is also used in weekly half-hour paid programs on Ion Television and the Discovery Channel which highlight TiVo promotions and advertisers.
Read more about this topic: NTSC
Famous quotes containing the words vertical, interval and/or reference:
“In bourgeois society, the French and the industrial revolution transformed the authorization of political space. The political revolution put an end to the formalized hierarchy of the ancien regimé.... Concurrently, the industrial revolution subverted the social hierarchy upon which the old political space was based. It transformed the experience of society from one of vertical hierarchy to one of horizontal class stratification.”
—Donald M. Lowe, U.S. historian, educator. History of Bourgeois Perception, ch. 4, University of Chicago Press (1982)
“I was interested to see how a pioneer lived on this side of the country. His life is in some respects more adventurous than that of his brother in the West; for he contends with winter as well as the wilderness, and there is a greater interval of time at least between him and the army which is to follow. Here immigration is a tide which may ebb when it has swept away the pines; there it is not a tide, but an inundation, and roads and other improvements come steadily rushing after.”
—Henry David Thoreau (1817–1862)
“If we define a sign as an exact reference, it must include symbol because a symbol is an exact reference too. The difference seems to be that a sign is an exact reference to something definite and a symbol an exact reference to something indefinite.”
—William York Tindall (1903–1981)