From Wikipedia, the free encyclopedia
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 V
pp
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
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edit
]