Tecniche di registrazione e di riproduzione sonora
• Angelo Farina
Physical nature of sound
Origin 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
Transducers: microphones
Output signal:
voltage (Volts), current (Amperes) or charge (Coulombs) From acoustic pressure and particle velocity to physical (electrical) quantities
Pressure
Pressure
microphones (related with acoustic pressure)Velocity
Velocity
microphones (related with particle velocity)Hybrid
Hybrid
microphones (a proper combination of both quantities)From omnidirectional (100 p and 0 v) to figure of eight (0 p and 100 v)
Microphone directivity patterns
Omnidirectional (100,0) Subcardioid (75,25) Cardioid (50,50)
Hypercardioid (25,75) Figure-of-Eight (0, 100)
Microphones
Variable pattern microphone:
Neumann U89i variable-pattern microphone
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 opposite-
polarity signals
Balanced audio cables with XLR connectors (3 pins)
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
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).
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
DAT, DVD Video
48000 Hz
DVD audio HD rec.
96000 Hz
Special soundcards
192000 Hz
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, Nyquist freq. 24 kHz, difference = 11 kHz
After digital conversion will be 13 kHz (i.e. 24-11 kHz) Solution: low-pass antialiasing filters, oversampling
ADC (Analog to Digital Converter) 4
Also vertical axis (amplitude) is discretized.
Example: maximum voltage +5V
Discretization with 16 bit (32767 steps) means 90 dB Discretization with 20 bit (524272 steps) means 114 dB Typical resolutions:
16 bits; 20 bits; (24 bits)
High-end, 2-channels ADC unit (24 bits, 192 kHz, Firewire interface)
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.
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
Digital Signal Processing
Waveform editors
sampled waveform displayed as amplitude vs time (time domain)
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)
Metodo tradizionale
(registrazioni mono “panned”)
• Ciascuna traccia mono registrata rappresenta una sorgente in una diversa posizione, che viene posizionata mediante una
M I X
5 ch.
Surround panner
1 ch.
1 ch.
5 ch.
5 ch.
5 ch.
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
“Pairwise Panning” a potenza costante - il segnale viene inviato
a due altoparlanti per volta
Panning basato sulla teoria di Peter Craven - il segnale viene inviato sempre a tutti 5 gli altoparlanti, con
-0.2 0 0.2 0.4 0.6 0.8 1
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
C L R SL SR
-0.2 0 0.2 0.4 0.6 0.8 1
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
C L R SL SR
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 SR
C L R SL SR
ORTF Stereo
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
2 Microphones
60°
2 Loudspeakers
Binaural (Stereo Dipole)
Reproduction occurs over 2 loudspeakers angled at +/- 10°, being fed through a “cross-talk cancellation” digital filtering system
…
2 3
1
Original 2-channels recording of the signals coming from N sources
d1l d1r
d2l d2r
xr
xl
Cross-talk canceller
dNl dNr N
20°
Binaural (Stereo Dipole#2)
L R
hll hrr
hlr hrl Binaural
stereo signal yl yr
pl pr xl
xr
convolver
fll
flr
frl
frr
yl yr
Binaural (Stereo Dipole#3)
rl lr
rr ll
ll rr
rl rl
lr lr
rr ll
h h
h h
InvFilter InvDen
InvDen h
f
InvDen h
f
InvDen h
f
InvDen h
f ( ) ( ) ( ) ( ) ( )
rl lr
rr
ll FFT h FFT h FFT h
h FFT
C
C C
Conj
C InvDen Conj
hll
hlr
hrl
hrr
fll
flr
frl
frr
Binaural (Dual Stereo Dipole)
advantages advantages
: : 3D sound reproduction3D sound reproduction
Rotating of the headRotating of the head
The cross-talk filters The cross-talk filters could equalise also the could equalise also the
loudspeakers loudspeakers
disadvantages disadvantages
: : Low frequenciesLow frequencies
Coloration outside the Coloration outside the
“sweet spot
“sweet spot””
Scheme
Subwoofer
Binaural (Dual Stereo Dipole#2)
Frontal Rear
Quested 2108 monitors Quested F11P monitors
Array “discreti” di microfoni
• Sono stati proposti svariati standard di
posizionamento di 5 microfoni discreti con
opportuna caratteristica direttiva
Array discreti di microfoni
• In un sistema discreto, il segnale di ogni singolo microfono va ad alimentare un singolo altoparlante
Array “discreti” di microfoni
• Williams MMA
Schema del sistema
microfonico Williams MMA C : Cardioide, 0°
L, R : Cardioide, ± 40°
LS, RS : Cardioide, ± 120°
• INA-5
Schema del sistema microfonico INA-5 C : Cardioide, 0°
L, R : Cardioide, ± 90°
LS, RS : Cardioide, ± 150°
ITU 5.1 surround
OCT
73 cm
Schematic of the setup C : Cardioid, 0°
L, R : Super Cardioid, ± 90°
LS, RS : Cardioid, ± 180°
Array discreti di microfoni
Perchè hanno avuto ben poco successo?
• Perchè consentono poco controllo in fase di registrazione
• Forniscono un angolo di ripresa fissato, a meno che siano realizzati con una struttura snodata, peraltro difficile da regolare sul campo
• Non consentono facilmente di mixare i segnali provenienti da tracce mono registrate a breve distanza dalla sorgente (ad
esempio voci dei cantanti, strumenti singoli, etc.)
• I pattern direttivi degli usuali microfoni per riprese stereo (cardioidi, ipercardioidi) non corrispondono all’esigenza di
avere una legge di “panning” omogenea rispetto alla geometria non regolare del layout degli altoparlanti di un sistema
surround 5.1
Ambisonics 3D 1
storder
Reproduction occurs over an array of 8-24 loudspeakers, through an Ambisonics decoder
Original Room Sound Source
SoundField Microphone
B-format 4- channels signal
(WXYZ) Ambisonics decoder
Speaker array in the reproduction room
3D extension of the pressure-velocity measurements
• The Soundfield microphone allows for simultaneous measurements of the omnidirectional pressure and of the three cartesian components of particle velocity (figure-of-8 patterns)
Directivity of transducers
Soundfield ST-250 microphone
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 180
210 240 270
300 330
125 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 180
210 240 270
300 330
250 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 180
210 240 270
300 330
500 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 180
210 240 270
300 330
1000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 210
240 270
300 330
2000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 210
240 270
300 330
4000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 210
240 270
300 330
8000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150 210
240 270
300 330
16000 Hz
A-format microphone arrays
• Today several alternatives to Soundfield microphones do exists. All of them are providing “raw” signals from the 4 capsules, and the conversion from these signals (A-format) to the standard Ambisonic signals (B-format) is performed digitally by means of software running on the computer
Ambisonics encoding
Oppure dalla sintesi dei 4 canali (WXYZ) a partire dalla loro definizione matematica (armoniche sferiche di ordine 0 ed 1)
Il segnale B-format deriva da un microfono Soundfield…
Algoritmo per decoding Ambisonics
• Il segnale di ciascun altoparlante è semplicemente un “mix” dei 4
canali di trasmissione (WXYZ)
• I guadagni dipendono solo dalla posizione di ciascun altoparlante
• Viene aggiunto un piccolo filtro FIR specifico per ogni altoparlante, per equalizzarne la risposta
W X Y Z
G1 G2 G2 G2
cos 1
cos 1
cos 1
InvFilt1
cos 2
cos 2
cos 2
InvFilt2 r
y z
x
Ambisonics decoding
vantaggi vantaggi: :
TridimensionaleTridimensionale
Buona percezione dei Buona percezione dei suoni laterali
suoni laterali
Buona risposta ai bassiBuona risposta ai bassi
““sweet spot” largo, sweet spot” largo, nessuna colorazione nessuna colorazione
uscendo da esso uscendo da esso
svantaggi svantaggi: :
Non isotropicoNon isotropico
Richiede un decoding Richiede un decoding avanzato (Y trattato avanzato (Y trattato
differentemente da X,Z) differentemente da X,Z)
Bi-square Ambisonics array
Bi-square Ambisonics array
8 Turbosound Impact 50 loudspeakers 8 Turbosound Impact 50 loudspeakers: :
Light, easily fixed and orientedLight, easily fixed and oriented
Good frequency responseGood frequency response
Very little distortionVery little distortion
Front-Right
Down-Right Up-Right
BSS Soundweb 9088-II (8 ins, 8 outs) BSS Soundweb 9088-II (8 ins, 8 outs)
Sistema DSP per decoding Ambisonics
SID Futureclient fanless PC (Pentium-III 1 GHz)
SID Futureclient fanless PC (Pentium-III 1 GHz)
Programmazione del decoder
sul processore digitale SoundWeb
Effetto dell’equalizzazione FIR di ciascun altoparlante
Funzione di tarsferimento Filtro inverso a fase Effetto equalizzante del
Decoder Ambisonics via software
Audiomulch VST host
Gerzonic bPlayer
Gerzonic Emigrator
Sale Ambisonics 3D
University of Ferrara ASK – Reggio Emilia
Sistema di gestione audio completo
Philips 15” Brilliance LCD display Logitech wireless keyboard & mouse Echo Layla Soundboard (8 ins, 10 outs)
BSS Soundweb digital processor 2 Crown K1 amplifiers
QSC CX168 8-channels amplifier Signum Data Futureclient fanless PC
Metodi ibridi (Ambiophonics)
• Sistema Ambiophonics 3D (10 altoparlanti):
Ambiophonics
In sostanza, vengono impiegati simultaneamente il sistema Ambisonics e gli Stereo-Dipoles