14/12/2012sound recording and playback 1 angelo farina
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14/12/2012 Sound recording and playback 1
Sound recording and playback
• Angelo Farina
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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
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Transducers: microphones
Output signal:
voltage (Volts), current (Amperes) or charge (Coulombs)
From acoustic pressure and particle velocity to physical (electrical) quantities
PressurePressure microphones (related with acoustic pressure)VelocityVelocity microphones (related with particle velocity)HybridHybrid 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) Cardioid (50,50)
Hypercardioid (25,75) 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 cablesSignal contamination can occur inside the cable, if not properly shielded (balanced) with two opposite-polarity 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 ConvertersA master clock defines with high precision the instants at which the analog signal has to be “sampled” (Shannon theorem)
Applications Resolutions
CDs 44100 HzDAT, DVD Video 48000 Hz
DVD audio HD rec. 96000 HzSpecial soundcards 192000 Hz
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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 aliasedExample: 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
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ADC (Analog to Digital Converter) 4Also 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)
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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)• HOA (high order Ambisonics, Eigenmike)• SPS (Spatial PCM Sampling)
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Traditional “surround” – amplitude panning
• Each mono track is positioned artificially, by means of “pairwise” or “advanced” amplitide panning
MIX
5 ch.
Surround panner
1 ch.
1 ch.
5 ch.
5 ch.
5 ch.
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Panning laws• Instead of just feeding a single loudspeaker with each mono
track, it is advantageous to “pan” each signal ver two or more loudspeakers
“Pairwise Panning” with constant power- each signal is always sent
to just 2 loudspeakers
Peter Craven’s Ambisonics panning at 5th order – all the loudspeakers are
always fed
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Panning laws• An alternative display of teh spanning laws is by charting the polar patterns
of a number of “virtual microphones”, each feeding the corresponding loudspeaker
“Pairwise Panning” Peter Craven’s panning
C
L
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SR
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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
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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°
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Binaural (Stereo Dipole#2)
L R
hll hrr
hlr hrl
Binaural stereo signal yl yr
pl pr
x l
x r
convolver
f ll
f lr
f rl
f rr
y l y r
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Binaural (Stereo Dipole#3)
rllrrrll
llrr
rlrl
lrlr
rrll
hhhhInvFilterInvDen
InvDenhf
InvDenhf
InvDenhf
InvDenhf )()()()()( rllrrrll hFFThFFThFFThFFTC
CCConj
CConjInvDen
hll
hlr
hrl
hrr
fll
flr
frl
frr
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Binaural (Dual Stereo Dipole)
advantagesadvantages: :
3D sound 3D sound reproductionreproduction
Rotating of the headRotating of the head The cross-talk filters The cross-talk filters
could equalise also could equalise also the loudspeakersthe loudspeakers
disadvantagesdisadvantages: :
Low frequenciesLow frequencies Coloration outside Coloration outside
the “sweet spotthe “sweet spot””
Scheme
Subwoofer
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Binaural (Dual Stereo Dipole#2)
Frontal Rear
Quested 2108 monitors Quested F11P monitors
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“Discrete” microphone arrays
• Many different geometrical layouts were proposed – each microphone feeds the corresponding loudspeaker
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“Discrete” microphone arrays• In a discrete system, each microphone feeds just the
corresponding loudspeaker:
Microfoni Altoparlanti
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“Discrete” microphone arrays• Williams MMA
Schema del sistema microfonico Williams MMAC : Cardioide, 0°L, R : Cardioide, ± 40° LS, RS : Cardioide, ± 120°
• INA-5Schema del sistema microfonico INA-5C : Cardioide, 0°L, R : Cardioide, ± 90° LS, RS : Cardioide, ± 150°
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ITU 5.1 surround
OCT73 cm
Schematic of the setupC : Cardioid, 0°L, R : Super Cardioid, ± 90° LS, RS : Cardioid, ± 180°
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“Discrete” microphone arraysWhy such a limited success?• Little control during recording and during post-processing• Fixed angle of covering, which cannot be adjusted after the
recording is done• It makes it difficult to add to the recording the sound track of
separately-recorded voices or instruments• Currently available directivity patterns of first-order
microphones do not correspond with the ideal, asymmetrical patterns implemented by optimal panning laws.
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Ambisonics 3D (1st order)
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
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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)
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Directivity of transducersSoundfield ST-250 microphone
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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
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Ambisonics decoding
• Each speaker fed is just obtained as a weighted sum of the 4 B-format signals (WXYZ)
• The weighting factors only depend on the position of each loudspeaker
• It is possible to add a small FIR filter for matching perfectly the gain and phase of all loudspeakers
W X Y Z
G1 G2 G2 G2
cos 1cos 1cos 1
InvFilt1
cos 2cos 2cos 2
InvFilt2
r
y
z
x
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Ambisonics decoding
advantagesadvantages: : TridimensionalTridimensional Good later sound Good later sound
perceptionperception Good bass responseGood bass response Wide “sweet spot”, no Wide “sweet spot”, no
colouring outside itcolouring outside it
disadvantagesdisadvantages: : Not isotropicNot isotropic Advanced decoding Advanced decoding
required (Y gets more required (Y gets more weight than X and Z, weight than X and Z, and this must be and this must be compensated for)compensated for)
Bi-square Ambisonics array
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Bi-square Ambisonics array
8 Turbosound Impact 50 loudspeakers8 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
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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 SID Futureclient fanless PC (Pentium-III 1 GHz)GHz)
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Programmazione del decodersul processore digitale SoundWeb
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Effetto dell’equalizzazione FIR di ciascun altoparlante
Funzione di tarsferimento
misurata dell’altoparlante
Front-left
Filtro inverso a fase minima (100 coefficienti)
Effetto equalizzante del filtro
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Decoder Ambisonics via software
Audiomulch VST host
Gerzonic bPlayer
Gerzonic Emigrator
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Sale Ambisonics 3D
University of Bologna
University of Ferrara
ASK – Reggio Emilia
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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
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Metodi ibridi (Ambiophonics)• Sistema Ambiophonics 3D (10 altoparlanti):
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Ambiophonics
In sostanza, vengono impiegati simultaneamente il sistema Ambisonics e gli Stereo-Dipoles