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14/12/2012Sound recording and playback 1 Angelo Farina.

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1 14/12/2012Sound recording and playback 1 Angelo Farina

2 14/12/2012Sound recording and playback 2 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

3 14/12/2012Sound recording and playback 3 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)

4 14/12/2012Sound recording and playback 4 Microphone directivity patterns Omnidirectional ( 100,0) Subcardioid ( 75,25) Cardioid ( 50,50) Hypercardioid ( 25,75) Figure-of-Eight (0, 100)

5 14/12/2012Sound recording and playback 5 Microphones Variable pattern microphone: Neumann U89i variable-pattern microphone

6 14/12/2012Sound recording and playback 6 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)

7 14/12/2012Sound recording and playback 7 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

8 14/12/2012Sound recording and playback 8 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).

9 14/12/2012Sound recording and playback 9 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) ApplicationsResolutions CDs Hz DAT, DVD Video Hz DVD audio HD rec Hz Special soundcards Hz

10 14/12/2012Sound recording and playback 10 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 kHz) Solution: low-pass antialiasing filters, oversampling

11 14/12/2012Sound recording and playback 11 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 ( steps) means 114 dB Typical resolutions: 16 bits; 20 bits; (24 bits) High-end, 2-channels ADC unit (24 bits, 192 kHz, Firewire interface)

12 14/12/2012Sound recording and playback 12 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.

13 14/12/2012Sound recording and playback 13 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

14 14/12/2012Sound recording and playback 14 Digital Signal Processing Waveform editors sampled waveform displayed as amplitude vs time (time domain)

15 14/12/2012Sound recording and playback 15 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 1 st order (decoding the B-format signal) 2D Ambisonics 3 rd order (from Mark Polettis 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)

16 14/12/2012Sound recording and playback 16 Traditional surround – amplitude panning Each mono track is positioned artificially, by means of pairwise or advanced amplitide panning MIXMIX 5 ch. Surround panner 1 ch. 5 ch.

17 14/12/2012Sound recording and playback 17 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 Cravens Ambisonics panning at 5 th order – all the loudspeakers are always fed

18 14/12/2012Sound recording and playback 18 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 Cravens panning

19 14/12/2012Sound recording and playback 19 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

20 14/12/2012Sound recording and playback 20 Binaural (Stereo Dipole) Reproduction occurs over 2 loudspeakers angled at +/- 10°, being fed through a cross-talk cancellation digital filtering system … Original 2-channels recording of the signals coming from N sources d 1l d 1r d 2l d 2r xrxr xlxl Cross-talk canceller d Nl d Nr N 20°

21 14/12/2012Sound recording and playback 21 Binaural (Stereo Dipole#2)

22 14/12/2012Sound recording and playback 22 Binaural (Stereo Dipole#3) h ll h lr h rl h rr f ll f lr f rl f rr

23 14/12/2012Sound recording and playback 23 Binaural (Dual Stereo Dipole) advantages : 3D sound reproduction 3D sound reproduction Rotating of the head Rotating of the head The cross-talk filters could equalise also the loudspeakers The cross-talk filters could equalise also the loudspeakers disadvantages : Low frequencies Low frequencies Coloration outside the sweet spot Coloration outside the sweet spot Scheme Subwoofer

24 14/12/2012Sound recording and playback 24 Binaural (Dual Stereo Dipole#2) FrontalRear Quested 2108 monitorsQuested F11P monitors

25 14/12/2012Sound recording and playback 25 Discrete microphone arrays Many different geometrical layouts were proposed – each microphone feeds the corresponding loudspeaker

26 14/12/2012Sound recording and playback 26 Discrete microphone arrays In a discrete system, each microphone feeds just the corresponding loudspeaker: MicrofoniAltoparlanti

27 14/12/2012Sound recording and playback 27 Discrete microphone arrays 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°

28 14/12/2012Sound recording and playback 28 ITU 5.1 surround OCT Schematic of the setup C : Cardioid, 0° L, R : Super Cardioid, ± 90° LS, RS : Cardioid, ± 180 °

29 14/12/2012Sound recording and playback 29 Discrete microphone arrays Why 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.

30 14/12/2012Sound recording and playback 30 Ambisonics 3D (1 st order) Reproduction occurs over an array of 8-24 loudspeakers, through an Ambisonics decoder

31 14/12/2012Sound recording and playback 31 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)

32 14/12/2012Sound recording and playback 32 Directivity of transducers Soundfield ST-250 microphone

33 14/12/2012Sound recording and playback 33 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

34 14/12/2012Sound recording and playback 34 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

35 14/12/2012Sound recording and playback 35 Ambisonics decoding advantages : Tridimensional Tridimensional Good later sound perception Good later sound perception Good bass response Good bass response Wide sweet spot, no colouring outside it Wide sweet spot, no colouring outside it disadvantages : Not isotropic Not isotropic Advanced decoding required (Y gets more weight than X and Z, and this must be compensated for) Advanced decoding required (Y gets more weight than X and Z, and this must be compensated for) Bi-square Ambisonics array

36 14/12/2012Sound recording and playback 36 Bi-square Ambisonics array 8 Turbosound Impact 50 loudspeakers: Light, easily fixed and oriented Light, easily fixed and oriented Good frequency response Good frequency response Very little distortion Very little distortion Front-Right Down-Right Up-Right

37 14/12/2012Sound recording and playback 37 BSS Soundweb 9088-II (8 ins, 8 outs) Sistema DSP per decoding Ambisonics SID Futureclient fanless PC (Pentium-III 1 GHz)

38 14/12/2012Sound recording and playback 38 Programmazione del decoder sul processore digitale SoundWeb

39 14/12/2012Sound recording and playback 39 Effetto dellequalizzazione FIR di ciascun altoparlante Funzione di tarsferimento misurata dellaltoparlante Front-left Filtro inverso a fase minima (100 coefficienti) Effetto equalizzante del filtro

40 14/12/2012Sound recording and playback 40 Decoder Ambisonics via software Audiomulch VST host Gerzonic bPlayer Gerzonic Emigrator

41 14/12/2012Sound recording and playback 41 Sale Ambisonics 3D University of Bologna University of Ferrara ASK – Reggio Emilia

42 14/12/2012Sound recording and playback 42 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

43 14/12/2012Sound recording and playback 43 Metodi ibridi (Ambiophonics) Sistema Ambiophonics 3D (10 altoparlanti):

44 14/12/2012Sound recording and playback 44 Ambiophonics In sostanza, vengono impiegati simultaneamente il sistema Ambisonics e gli Stereo-Dipoles

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