Transcript Folie 1 - RAMSETE

University
of Parma
Sound Quality Assessment
IR measurements and AQT
Problems with traditional
measurements methods
 Traditional measurement are based on steady-state signals
- they provide a frequency response curve - this does not
provide significant match with listening experience in cars
 It was attempted to employ transient analysis (impulse
response) based on the same parameters employed for
concert halls (reverberation time, clarity, definition) - also
these do not correspond with the short time scale of time
transients inside cars
 In-car listening tests are useful, but difficult to standardize
 General lack of understanding of the relationship between
subjective perception and physical facts
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Solutions
Reliable in-car measurement technique
(MLS, sweep)
Transient analysis and equalization (AQT)
Virtual listening for comparative tests
Subjective (IPA) and objective (IPM)
evaluation indexes for audio quality
assessment
New metric based on all above for rating
with a sigle number the sound quality
(IQSB)
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Reliable in-car measurement technique
Methods:
⇨ Impulse Response (IR) as standard
measurement for characterization of the car
sound
⇨ Hardware: B&K type 4100 binaural head and
torso
⇨ Hardware: Notebook PC + Edirol FA-101
Firewire interface
⇨ Software: Adobe Audition and Aurora for IR
measure
⇨ AQT as advanced analysis (instead of FFT)
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Reliable in-car measurement technique
Standardized Position of the Dummy Head
tettuccio
30 cm
70 cm
verticale volante
ca. 25o
verticale a piombo
asse del dorso (sul piano dorsale)
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Reliable in-car measurement technique
Hardware: PC and audio interface
Edirol FA-101
Firewire sound
card:
10 in / 10 out
24 bit, 192 kHz
ASIO and WMA
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Reliable in-car measurement technique
Software
Aurora Plugins
Generate MLS
Deconvolve MLS
Generate Sweep
Deconvolve Sweep
Convolution
Kirkeby Inverse Filter
Speech Transm. Index
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Measurement process

The desidered result is the linear impulse response of
the acoustic propagation h(t). It can be recovered by
knowing the test signal x(t) and the measured system
output y(t). It is necessary to exclude the effect of the
not-linear part K and of the background noise n(t).
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Test signal: Log Sine Sweep
x(t) is a sine signal, which frequency is varied exponentially
with time, starting at f1 and ending at f2.


t  f2 




ln 


2    f1  T
T  f1 

x ( t )  sin
e
 1 


 f2  


 ln  

f
 1


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Deconvolution of Log Sine Sweep
The “time reversal mirror” technique is employed: the system’s
impulse response is obtained by convolving the measured signal
y(t) with the time-reversal of the test signal x(-t). As the log sine
sweep does not have a “white” spectrum, proper equalization is
required
Test Signal x(t)
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Inverse Filter z(t)
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Test Signal – x(t)
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Measured signal - y(t)
 The not-linear behaviour of the loudspeaker causes many harmonics to appear
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Inverse Filter – z(t)
The deconvolution of the IR is obtained convolving the measured signal y(t) with the inverse filter z(t) [equalized,
time-reversed x(t)]
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Result of the deconvolution
2°
5°
1°
3°
The last impulse response is the linear one, the preceding
are the harmonics distortion products of various orders
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Maximum Lenght Sequence vs. Sweep
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Post processing of impulse response
 Specific XFM modules have been developed for the computation of STI and
for performing standard analysis according to ISO-3382
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AQT Analysis
Goal:
⇨ Substitute classical analysis based on FFT with a new kind
of dynamic transient analysis
Reason:
⇨ FFT characterizes a only in steady state condition
BUT
⇨ Hearing is more sensible to transients
⇨ Musical signal is not a steady state signal
Solution:
⇨ AQT analysis (Acoustic Quality Test)
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AQT Analysis
Features:
⇨ Use transient stimulus (wavelet) instead of steady-state
(periodic sine)
⇨ Find a frequency response curve more related to perceived
curve than FFT response
⇨ Find resonances and tails that affect hearing
⇨ Useful during car development and for design of optimal
equalization
Output:
⇨ “Dynamic” frequency response curve of the sound
⇨ Articulation curve (transient capability at each frequency)
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AQT Analysis
Methods:
⇨ The test signal is a
sequence of sine bursts of
increasing frequency
⇨ Each burst is 200 ms long.
The gap between a burst
and the next is 33 ms
⇨ The instantaneous RMS
level is plotted versus time
(or frequency)
⇨ The same result can be
also obtained convolving
the test signal with the IR
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AQT Analysis
Details:
⇨ The envelope of the
maxima is the
dynamic frequency
response of the
⇨ The depth of the
valley between two
bursts shows the
dynamic transient
capability of the
Universiy of Parma
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AQT Analysis
Temporal masking according to Zwicker
The analysis of a single burst shows the level variation due to a
single burst, and the overshoot phenomenon at the beginning and
end of the burst.
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AQT Analysis
AQT Tool:
⇨ An automatic
tool was
developed to
perform a quick
processing
⇨ This tool can
also generate
the coefficients
for digital FIR
filters
The filters are designed for
producing a prescribed optimal
frequency response
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AQT Equalization results
Fiat STILO
Above: measured AQT
curve not equalized
vs. target curve (red)
Below: measured AQT
curve after equalization
vs. target curve (red)
Results (IPA listening test):
Not equalized
Traditional
FIR
Inverse AQT
7.18
7.20
7.50
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The new AQT plugin for Audition
 The new module is still under development and will allow for very fast
computation of the AQT curve from within Adobe Audition
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Conclusions
 The sine sweep method revealed to be systematically superior to
the MLS method for measuring electroacoustical impulse
responses
 In fact, it is now employed from top-grade measurement systemss,
includiong the Audio Precision (TM)
 Traditional FFT-based analysis of the measured impulse response
did noy show good correlation with subjective perception
 It was tehrefore necessary to derive new physical descriptors,
better correlating with the human listening experience inside a car.
 Three physical parametrs revealed to be more correlated with
subjective responses: IACC, STI and AQT
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