Misura della Risposta all`Impulso

Tecniche di registrazione e di
riproduzione sonora
• Angelo Farina
9/12/2010
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Physical nature of sound
Origin of Sound:
Thermofluidodynamic phenomenon:
Particle velocity and variable density of medium (air)
Human body can detect sound (p and v) with:
ears, but also skin, chest, stomach
The trasducers should detect the same quantities
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Transducers: microphones
From acoustic pressure and particle velocity to physical
(electrical) quantities
Output signal:
voltage (Volts), current (Amperes) or charge (Coulombs)
Pressure microphones (related with acoustic pressure)
Velocity microphones (related with particle velocity)
Hybrid microphones (a proper combination of both
quantities)
From omnidirectional (100 p and 0 v) to figure of eight (0 p and 100 v)
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Microphone directivity patterns
Omnidirectional (100,0)
Subcardioid (75,25)
Hypercardioid (25,75)
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Cardioid (50,50)
Figure-of-Eight (0, 100)
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Microphones
Variable pattern microphone:
Neumann U89i variable-pattern microphone
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Cables
The weak microphone signal (few mV/Pa) has to be
amplified and transmitted by means of cables
Signal contamination can occur inside the cable, if not
properly shielded (balanced) with two oppositepolarity signals
Balanced audio cables with XLR connectors (3 pins)
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Preamplifiers
They should simply amplify the signals, but often they also
process the signals:
• linearly (band pass frequencies), for phantom power supply
to mics
• non-linearly (compression, harmonic distortion) it should be
avoided during room acoustics measurements.
2-channels tube microphone preamplifier
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ADC (Analog to Digital Converter)
Conceptually a black box connecting with two wires:
•
Analog input (sound signal)
•
Digital output (serial digital interface)
Two different types of ADCs:
1. PCM converters (Pulse Code Modulation – CD,
DAT, DVD)
2. Bitstream converters (DSD, Direct Stream
Digital, also called single-bit, employed in
SACD).
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ADC (Analog to Digital Converter) 2
PCM Converters
A master clock defines with high precision the
instants at which the analog signal has to be
“sampled” (Shannon theorem)
Applications
Resolutions
CDs
44100 Hz
48000 Hz
96000 Hz
192000 Hz
DAT, DVD Video
DVD audio HD rec.
Special soundcards
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ADC (Analog to Digital Converter) 3
Low-pass filtering must be applied before entering
the ADC, otherwise the signal will be aliased
Example: pure tone of 35 kHz
Sample rate 48 kHz,
kHz
Nyquist freq. 24 kHz, difference = 11
After digital conversion will be 13 kHz (i.e. 24-11 kHz)
Solution: low-pass antialiasing filters, oversampling
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ADC (Analog to Digital Converter) 4
Also vertical axis (amplitude) is discretized.
Typical resolutions:
16 bits; 20 bits; (24 bits)
Example: maximum voltage +5V
Discretization with 16 bit (32767 steps) means 90 dB
Discretization with 20 bit (524272 steps) means 114 dB
High-end, 2-channels ADC unit (24 bits, 192 kHz, Firewire interface)
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ADC (Analog to Digital Converter) 5
Bitstream Converters
• The idea arises from oversampling: increasing sample
frequency it would be possible to increase amplitude
resolution (bits)
• Sample rate: 2.88 MHz and 1 bit resolution: dividing to 2 for
6 time is equivalent to CD audio sample rate, but only with 7
bits!
• In order to enhance high freq. resolution, a proper noise
shaping of high order is required, suitable for static (non
transient) signals.
• Below 88 Hz the Bitstream converters outperform PCM conv.
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ADC (Analog to Digital Converter) 6
Bitstream Converters
• The Bitstream converters are widely employed with SACD
system (Super Audio CD), co-developed by Sony and Philips.
• However they are much more expensive that 24 bit 96 kHz
PCM
a low-cost multichannel USB-2 soundcard, equipped with 2 microphone
preamps
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Digital Signal Processing
Waveform editors
sampled waveform displayed as amplitude vs time (time
domain)
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Recording/playback methods
•
•
•
•
•
•
•
Mono followed by amplitude panning (stereo or surround)
Stereo (ORTF on 2 standard loudspeakers at +/- 30°)
Discrete ITU 5.1 (from different 5-mikes layouts)
Full 3D Ambisonics 1st order (decoding the B-format
signal)
2D Ambisonics 3rd order (from Mark Poletti’s circular
array microphone)
Wave Field Synthesis (from the circular array of
Soundfield microphones)
Hybrid methods (Ambiophonics)
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Metodo tradizionale
(registrazioni mono “panned”)
5 ch.
5 ch.
1 ch.
1 ch.
5 ch.
M
I
X
5 ch.
Surround panner
• Ciascuna traccia mono registrata rappresenta una sorgente in
una diversa posizione, che viene posizionata mediante una
appropriata legge di “panning” multicanale
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Leggi di “panning”
• Si è visto sin dagli albori che non conviene posizionare le tracce mono sui
singoli canali del surround in modo discreto, ma che conviene utilizzare
appropriate leggi di “panning” in modo da alimentare sempre piu’ di un
altoparlante per volta
1
1
0.8
0.8
0.6
0.6
C
C
L
R
0.4
L
R
0.4
SL
SL
SR
SR
0.2
0.2
0
0
-0.2
-180
-150
-120
-90
-60
-30
-0.2
0
30
60
90
120
150
180
“Pairwise Panning” a potenza
costante - il segnale viene inviato
a due altoparlanti per volta
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-180
-150
-120
-90
-60
-30
0
30
60
90
120
150
180
Panning basato sulla teoria di Peter
Craven - il segnale viene inviato
sempre a tutti 5 gli altoparlanti, con
opportuni guadagni 17
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Leggi di “panning”
• Un modo alternativo per visualizzare le leggi di panning consiste
nell’ipotizzare l’esistenza di 5 microfoni virtuali con opportuni “pattern” di
direttività
C
L
R
SL
C
L
R
SL
SR
“Pairwise Panning”
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SR
Panning di Peter Craven
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ORTF Stereo
60°
2 Microphones
2 Loudspeakers
Playback occurs over a pair of loudspeakers, in the
standard configuration at angles of +/- 30°, each
being fed by the signal of the corresponding
microphone
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Binaural (Stereo Dipole)
Original 2-channels recording of the signals coming from N sources
dNr
N
…
3
dNl
xr
Cross-talk
canceller
d2r
2
20°
xl
d2l
1
d1r
d1l
Reproduction occurs over 2 loudspeakers angled at +/- 10°,
being fed through a “cross-talk cancellation” digital filtering
system
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Binaural (Stereo Dipole#2)
convolver
fll
flr
xl
Binaural
stereo signal
frl yl
frr
xr
yr
yl
R
L
hlr
hrl
hll
hrr
pl
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yr
pr
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Binaural (Stereo Dipole#3)
 f ll  hrr   InvDen

 f lr   hlr   InvDen

 f rl   hrl   InvDen
 f  h   InvDen
ll
 rr
 InvDen  InvFilterhll  hrr  hlr  hrl 
C ( )  FFT (hll )  FFT (hrr )  FFT (hlr )  FFT (hrl )
InvDen  
hll
hrl
fll
frl
hlr
hrr
flr
frr
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ConjC  
ConjC    C      
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Binaural (Dual Stereo Dipole)
advantages:
Scheme



3D sound reproduction
Rotating of the head
The cross-talk filters
could equalise also the
loudspeakers
disadvantages:

Subwoofer
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
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Low frequencies
Coloration outside the
“sweet spot”
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Binaural (Dual Stereo Dipole#2)
Frontal
Rear
Quested 2108 monitors
Quested F11P monitors
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