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Tecniche di registrazione e di riproduzione sonora

Tecniche di registrazione e di riproduzione sonora. Angelo Farina. Physical nature of sound. Origin of Sound:. T hermofluidodynamic phenomen on: Particle velocity and variable density of medium (air). Human body can detect sound (p and v) with: ears, but also skin, chest, stomach.

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Tecniche di registrazione e di riproduzione sonora

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  1. Tecniche di registrazione e di riproduzione sonora • Angelo Farina Registrazione e Riproduzione

  2. 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 Registrazione e Riproduzione

  3. 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) Registrazione e Riproduzione

  4. Microphone directivity patterns Omnidirectional (100,0)Subcardioid(75,25)Cardioid(50,50) Hypercardioid(25,75)Figure-of-Eight(0, 100) Registrazione e Riproduzione

  5. Microphones Variable pattern microphone: Neumann U89i variable-pattern microphone Registrazione e Riproduzione

  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) Registrazione e Riproduzione

  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 Registrazione e Riproduzione

  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: • PCM converters (Pulse Code Modulation – CD, DAT, DVD) • 2. Bitstream converters (DSD, Direct Stream Digital, also called single-bit, employed in SACD). Registrazione e Riproduzione

  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) Registrazione e Riproduzione

  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. 24-11 kHz) Solution: low-pass antialiasing filters, oversampling Registrazione e Riproduzione

  11. 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) Registrazione e Riproduzione

  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. Registrazione e Riproduzione

  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 Registrazione e Riproduzione

  14. Digital Signal Processing Waveform editors sampled waveform displayed as amplitude vs time (time domain) Registrazione e Riproduzione

  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 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) Registrazione e Riproduzione

  16. Metodo tradizionale(registrazioni mono “panned”) • Ciascuna traccia mono registrata rappresenta una sorgente in una diversa posizione, che viene posizionata mediante una appropriata legge di “panning” multicanale MIX 5 ch. 5 ch. 5 ch. 1 ch. 1 ch. 5 ch. Surround panner Registrazione e Riproduzione

  17. 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 Panning basato sulla teoria di Peter Craven - il segnale viene inviato sempre a tutti 5 gli altoparlanti, con opportuni guadagni “Pairwise Panning” a potenza costante - il segnale viene inviato a due altoparlanti per volta Registrazione e Riproduzione

  18. 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 “Pairwise Panning” Panning di Peter Craven Registrazione e Riproduzione

  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 60° 2 Microphones 2 Loudspeakers Registrazione e Riproduzione

  20. Binaural (Stereo Dipole) xr Cross-talk canceller xl Original 2-channels recording of the signals coming from Nsources Reproduction occurs over 2 loudspeakers angled at +/- 10°, being fed through a “cross-talk cancellation” digital filtering system dNr N … dNl 3 d2r 20° 2 d2l d1r 1 d1l Registrazione e Riproduzione

  21. Binaural (Stereo Dipole#2) Registrazione e Riproduzione

  22. Binaural (Stereo Dipole#3) frl fll flr frr hrl hll hlr hrr Registrazione e Riproduzione

  23. Binaural (Dual Stereo Dipole) Subwoofer Scheme advantages: • 3D sound reproduction • Rotating of the head • The cross-talk filters could equalise also the loudspeakers disadvantages: • Low frequencies • Coloration outside the “sweet spot” Registrazione e Riproduzione

  24. Binaural (Dual Stereo Dipole#2) Frontal Rear Quested 2108 monitors Quested F11P monitors Registrazione e Riproduzione

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