IRE (unit)

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An IRE is a unit used in the measurement of composite video signals. Its name is derived from the initials of the Institute of Radio Engineers.[1]

A value of 100 IRE was originally defined to be the range from blanking level to peak white in a video signal. A value of 0 IRE corresponds to the voltage value of the signal during the blanking period. The sync pulse is normally 40 IRE below this 0 IRE value, so the total range covered from peak to trough of an all white signal would be 140 IRE. [2]

Note that video signals use the "IRE" unit instead of DC voltages to describe levels and amplitudes. Based on a standard 1 Vpp NTSC composite-video signal that swings from -286 mV (sync tip) to +714 mV (peak video), a 140 IRE peak-to-peak convention has been established. One NTSC IRE unit is 7.14mV, where -40 IRE is equivalent to -286 mV, and +100 IRE is equivalent to +714 mV. Note that 0 IRE is equivalent to 0 V. The Black Level is equivalent to 53.57 mV (7.5 IRE)[3]

The PAL video signal is slightly different in that it swings from -300 mV to +700 mV, instead. Thus, one PAL IRE unit is 7mV, where -43 IRE is equivalent to -300 mV at the sync tip, and +100 IRE is equivalent to +700 mV at the peak video level. Black level is the same as the blanking level 0 mv (0 IRE) [4][5]

The reason IRE is a relative measurement (percent) is because a video signal may be any amplitude. This unit is used in the ITU-R BT.470 which defines PAL, NTSC and SECAM.[6]

References[edit]

  1. ^ Robin & Poulin; Michael Robin; Michel Poulin (2000). Digital television fundamentals: design and installation of video and audio systems. McGraw-Hill Professional. p. 17. ISBN 0-07-135581-2. 
  2. ^ "World Analogue Television Standards and Waveforms". Retrieved 31 May 2018. 
  3. ^ "Composite Video Signals (CVBS)". National Instruments. Retrieved 31 May 2018. 
  4. ^ Cliff Win, Jr. "Measuring composite-video signal performance requires understanding differential gain and phase, Part 1 of 2". National Semiconductor Corporation. Retrieved 31 May 2018. 
  5. ^ "Vertical Blanking Interval of 625-Line Standard (PAL Colour)" (PDF). Archived from the original (PDF) on 28 April 2016. Retrieved 3 September 2015. 
  6. ^ "BT.470 : Conventional analogue television systems". International Telecommunications Union. Retrieved 28 August 2012.