14/12/2012sound recording and playback 1 angelo farina

14/12/2012 Sound recording and playback 1

Sound recording and playback

• 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

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)


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 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)


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 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



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


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)



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.



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)• HOA (high order Ambisonics, Eigenmike)• SPS (Spatial PCM Sampling)


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.


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

-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


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

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

x l

x r

convolver

f ll

f lr

f rl

f rr

y l y r


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


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


Binaural (Dual Stereo Dipole#2)

Frontal Rear

Quested 2108 monitors Quested F11P monitors


“Discrete” microphone arrays

• Many different geometrical layouts were proposed – each microphone feeds the corresponding loudspeaker


“Discrete” microphone arrays• In a discrete system, each microphone feeds just the

corresponding loudspeaker:

Microfoni Altoparlanti


“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°


ITU 5.1 surround

OCT73 cm

Schematic of the setupC : Cardioid, 0°L, R : Super Cardioid, ± 90° LS, RS : Cardioid, ± 180°


“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.


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


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 transducersSoundfield 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

180

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

180

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

180

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

180

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 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


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


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


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)


Programmazione del decodersul processore digitale SoundWeb


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


Decoder Ambisonics via software

Audiomulch VST host

Gerzonic bPlayer

Gerzonic Emigrator


Sale Ambisonics 3D

University of Bologna

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

14/12/2012sound recording and playback 1 angelo farina

Documents

sound p

hz slide

oversampling slide

digital converter

analog signal

adc analog

firewire interface slide

angelo farina slide