Linear Acoustic LQ-1000, Руководство пользователя

Страница: 18 / 46

Linear Acoustic LQ-1000 User Guide Applications and Use
14
3.1 Measuring Loudness
Loudness measurement is decidedly different than simply measuring audio levels. This is
because loudness involves human perception and the measurement will therefore not be
objective, like voltage or current, but will instead be subjective. While the mechanics of
how humans hear are fundamentally the same for each person, what they perceive can vary
significantly.
As hearing research has evolved from over many decades, a truly universal method for
quantifying loudness in a repeatable manner was only formulated in the last few years. Un-
til that point, the only methods available combined objective meters like VU and PPM with
the “eyeball average” of an operator. Skilled operators got reasonable good results, but alas
the world is running in short supply of skilled operators.
3.1.1 ITU-R BS.1770
The International Telecommunications Union (ITU) formed a study group to investigate
a better and more universal answer to the question of how to objectively measure loudness.
The result was published as ITU-R BS.1770 and essentially specifies a system that filters,
weights, then power sums audio channels to create a repeatable numeric result. The basic
block diagram for the resulting system is shown below:
Figure 3-1
Block diagram of ITU-R BS.1770 multichannel loudness algorithm
It can be seen in the above drawing that applied multichannel audio is first filtered (by two
separate stages of filtering to be explained below), then are integrated (mean square) to al-
low power summing, which among other things removes the effects of inter-channel phase
difference causing cancellation. Note that the surround channels are boosted by approxi-
mately 1.5dB each before the sum and the LFE channel is not currently included in the
measurement.
The two stages of filtering are used to remove very low frequency information that does
not add to perceived loudness but will skew loudness results, and to apply a shelving high
frequency boost to make up for frequency response changes due to the shape of the hu-
man head. The two filters are shown below: