Dante Inovonics 552, Installation & User Manual, Page: 29 / 60

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ITU-R BS.1770
Loudness
Measurement
The 552 incorporates a program loudness measurement utility
based on (human) ‘perceived loudness’ per the ITU-R BS.1770
Standard. Meter readings under this standard are expressed in
“LUs” (Loudness Units), one LU equivalent to 1dB.
‘Zero-LU’ does not conveniently correlate to a specific modula-
tion percentage because of the Standard’s frequency weighting.
However, a 1kHz constant tone at the nominal 100% modula-
tion point brings the meter to about +22LU.
For our monitoring purposes here, we should probably consider
552 LU measurements “locally-relative”; that is, most useful in
comparing loudness between on-site audio programs, market
companions, music cuts, processing presets, talent voices, etc.
‘LU’ loudness is displayed as a bargraph, featuring three col-
ored
LU
indicator bar-segments labeled
M
,
S
and
I
.
M The yellow segment shows a ‘momentary’ value of loud-
ness, actually the loudness of a 400ms block of audio
level refreshed every 100ms. This gives a ‘right now’
loudness reading. The yellow ‘wash’ behind the display
represents the level range considered in this calculation,
and provides a quick visual of program dynamic range.
S The green segment is a short-term-integrated value de-
rived from a 3-second block of audio refreshed every
second. This yields a readout primarily indicating the
loudest sections of program audio.
I The blue segment is a longer-term integrated measure-
ment and is ‘gated’ to ignore speech pauses and other
periods of silence. This is probably the most useful
overall, average-program-loudness reading.
This metering method reflects the ‘broadcast’ implementation
of the BS.1770 Standard, and is shared in intimate detail in EBU
Loudness Standard R-128.
Synchronous
and AM Noise
Sync , or ‘synchronous’ FM noise, generally refers to amplitude
modulation of the FM carrier by the FM baseband signal, pri-
marily by the program audio source. If the passband of the
transmitter is not frequency-flat, then as the carrier deviates
toward the channel frequency extremes the carrier level under-
goes amplitude rolloff at the band edges.
AM , or asynchronous noise, is carrier amplitude modulation
from any source other than the modulating signal, such as
power supply ripple. This AM noise measurement should be
taken without modulation.