TC electronic SDN BHD LM5D Radar Detector User Manual

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Fig 11, Frequency weighting used with selected Leq measures.
Red curve: A weighting.
Green curve: RLB weighting.
Blue curve: R2LB weighting, AKA “K” weighting.
The frequency weighting employed in the loudness part of BS.1770 is shown in
fig 11. It means that you cannot perform a calibration tone sweep and expect the
reading to stay the same, see below.
Because of the frequency and channel weighting, and of the way channels sum, only
specific tones and input channels should be used for calibration.
The most transparent results are obtained using a 1 kHz sine tone for calibration.
Other frequencies or types of signal may be used (square wave, noise etc.), but don’t
expect similar results. The beauty of the system lies in its RMS foundation, so this is
a feature, not an error. The same feature enables the loudness measure to identify
overly hot CDs or commercials, and to take out of phase signals into account just as
much as signals that are in phase.
If we stick to standard methods for measuring peak audio level in a digital system,
where a sine wave (asynchronous of the sample rate) with digital peaks at 0 dBFS, is
regarded a 0 dBFS tone, BS.1770 and LM5 output these results:
One front channel fed with a –20 dBFS, 1 kHz sine tone => Reading of –23,0 LFS.
Two front channels fed with a –20 dBFS, 1 kHz sine tone => Reading of –20,0 LFS.
All 5.1 channels fed with a –20 dBFS, 1 kHz sine tone => Reading of –15,4 LFS.
See Fig 4 as examples of calibration tone Radar displays.
LM5 may use either the measurement unit of LU (Loudness Units) or LFS (Loudness
Full Scale). LU and LFS are measurements in dB, reflecting the estimated gain offset
to arrive at a certain Reference Loudness (LU) or Maximum Loudness (LFS) as defined
in BS.1770. Since a common reference point for LU has not been agreed on at the
time of writing (February, 2008), LFS (or “LKFS”, pointing specifically to the
Leq(R2LB) weighting of BS.1770), might be favored initially to avoid ambiguous use
of the term LU.
The effectiveness of any loudness meter depends on both the graphical appearance
and dynamic behavior of its display, as well as on its underlying measurement
algorithms. A short-term loudness meter also relies on the measurement algorithm’s
ability to output pertinent loudness information using different analysis windows, for
instance, 200-800 ms for running realtime updates. It should be noted how the
optimum size of this window varies from study to study, possibly because the objective
of a running display hasn’t been fully agreed upon.
Formal evaluation of a visualization system is challenging: First of all, one or more
metrics must be defined by which the display should be evaluated. The
correspondence between the sound heard and the picture seen is one aspect to be
evaluated. Another metric could characterize the speed of reading the meter reliably.
In LM5, short-term, mid-term and long-term of loudness measurements are tied
together coherently, and displayed in novel ways (angular reading and radar) that were
preferred in its development and test phases. However, we remain open to suggestions
for further improvement of the visualization of loudness.