IRE (unit)

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

Diagram of composite video field with IRE measurements

A value of 100 IRE is defined to be +714 mV in an analog NTSC video signal. A value of 0 IRE corresponds to the voltage value of 0 mV, the signal value 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 is established. Thus, one NTSC IRE unit is 7.143 mV (1/140 V),[3] where -40 IRE is equivalent to -285.7 mV, and +100 IRE is equivalent to +714.3 mV. Note that 0 IRE is equivalent to 0 V. The black level is equivalent to 53.57 mV (7.5 IRE).[4]

The PAL video signal is slightly different in that it swings from -300 mV to +700 mV, instead. Thus, one PAL IRE unit is 7 mV, 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).[5][6]

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:[7][8]

Sync level Blanking level Reference black Reference white Pick level Burst Amplitude
M-NTSC -40 IRE, -285.7 mV 0 IRE 7.5 IRE, 53.57 mV +100 IRE, 714.3 mV 120 IRE 20.0 IRE
B/G-PAL -43 IRE, -300 mV 0 IRE 0 IRE +100 IRE, 700 mV 133 IRE 21.5 IRE
SECAM -43 IRE 0 IRE 0 IRE +100 IRE 130 IRE N/A

References
  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. Lee, Paul. "Introduction to Analog Video" (PDF).
  4. "Composite Video Signals (CVBS)". National Instruments. Retrieved 31 May 2018.
  5. 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.
  6. "Vertical Blanking Interval of 625-Line Standard (PAL Colour)" (PDF). Archived from the original (PDF) on 28 April 2016. Retrieved 3 September 2015.
  7. "BT.470 : Conventional analogue television systems". International Telecommunications Union. Retrieved 28 August 2012.
  8. "Video Levels - NI Signal Generators Help (NI-FGEN 18.1) - National Instruments". zone.ni.com. Retrieved 2020-11-26.
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