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]
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]
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 or 7+1/7 mV),[3] where -40 IRE is equivalent to -285.7 mV, and +100 IRE is equivalent to +714.3 mV. 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 recommendations BT.470 and BT.1700 which define PAL, NTSC and SECAM:[7][8][9]
| Sync level | Blanking level | Reference black | Reference white | Peak 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 edit
- ^ 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.
- ^ "World Analogue Television Standards and Waveforms". Archived from the original on 11 July 2018. Retrieved 31 May 2018.
- ^ Lee, Paul. "Introduction to Analog Video" (PDF).
- ^ "Composite Video Signals (CVBS)". National Instruments. Retrieved 31 May 2018.
- ^ 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.
- ^ "Vertical Blanking Interval of 625-Line Standard (PAL Colour)" (PDF). Archived from the original (PDF) on 28 April 2016. Retrieved 3 September 2015.
- ^ "BT.470 : Conventional analogue television systems". International Telecommunication Union. Retrieved 13 November 2023.
- ^ "BT.1700 : Characteristics of composite video signals for conventional analogue television systems". International Telecommunication Union. Retrieved 13 November 2023.
- ^ "Video Levels - NI Signal Generators Help (NI-FGEN) - National Instruments". National Instruments. Retrieved 13 November 2023.