250 likes | 352 Views
Fundamentals of. Fundamentals of Digital Audio. Analog vs. Digital. Analog vs. Digital. Analog. AMPLIFIER. MICROPHONE. SOUND PRESSUTRE WAVE. SOUND PRESSUTRE WAVE. VOLTAGE IN WIRE. Digital. DIGTAL TO ANALOG CONVERTER. ANALOG TO DIGITAL CONVERTER. COMUTER MEMORY. AMPLIFIER.
E N D
Fundamentals of ... • Fundamentals • of • Digital • Audio
Analog vs. Digital Analog vs. Digital Analog AMPLIFIER MICROPHONE SOUND PRESSUTRE WAVE SOUND PRESSUTRE WAVE VOLTAGE IN WIRE Digital DIGTAL TO ANALOG CONVERTER ANALOG TO DIGITAL CONVERTER COMUTER MEMORY AMPLIFIER
Idealized Model... Idealized Model Sampling / Reconstruction smoothing filter (low-pass) zero- order hold g(nT) g(t) f(t) f(nT) g(t) f*(t) quantizer computer sampler A/D converter D/A converter Model of the sampling and desampling process
Deconstruction of Sampling f(t) Deconstruction of Sampling f(t) analog (a) Graphic representation of types of signals: a) analog signal b) quantized (continuous time) signal c) sampled data signal d) digital signal quantized in amplitude (b) f*(t) quantized in time (c) f(nT) digital (d)
Fidelity Fidelity • depends on: • sampling rate • bit depth • (encoding scheme)
Sampling rate Sampling Rate Human hearing: 20-20,000 Hz Sampling Theorem: Must sample twice as high as highest component Typical rates: 8 K 11 K 22 K 44.1 K 48 K dark / dull bright / clean
Fletcher & Munson Intensity 10-2 10-4 10-6 10-8 10-10 10-12 2 x 10 2 2 x 10-1 2 x 10-2 2 x 10-3 2 x 10-4 10-5 Watt/m2 Newton/m4 Fletcher & Munson
Foldover in time domain Foldover in time domain Original signal Original signal Sampling pulse Sampling pulse Resultant Digitized Waveform Resultant Digitized Waveform
Positive and negative Frequency Positive and Negative Frequency |H(f)| analog f -f 0 Nyquist frequency
Digital Spectra Digital Spectra quantization in time |H(f)| digital 0 sampling rate SR Nyquist Frequency SR 2
folderover in freq... foldover in frequency domain foldover sampling rate foldover frequency = 1/2 sampling rate
quantization in amp. Quantization in Amplitude digital analog 4 3 2 1 0 -1 -2 -3 -4 error quantization levels
distribution of error Distribution of Error 0 -1 1 maximum error of ± 1/2
Computing S/N ideal Computing S/N “Ideal” 2N 1 dB = 20 log 2N-1 0.5 or 6 dB / bit
Bit depth Example S/N Bit Depth Example S/N bits 2 4 8 12 16 24 dB 12 24 48 72 96 144 voice CD proaudio
Dynamic range problem Dynamic Range Problem 16 bits 96 dB 24 dB 4 bits 4 bits 16 bits
low-level signal problem Low-level Signal Problem intended result +2 +1 0 -1 -2 a folded Folding Frequency
Dither Dither +2 +1 0 -1 -2 a f FF
Signal reconstruction Signal Reconstruction a Reconstruction Filter f SR/2 SR Ideal reconstruction filter hasn’t been found
Oversanpling Converters Oversampling Converters • Multibit oversampling • convert 44.1 K to • 176.4 K (4x) • 352.8 K (8x) • 1-bit oversampling • 1 bit at high rate • sigma-delta • MASH • 128 x 1-bit = • 8 * 16-bits
Multibit Oversampling Multibit Oversampling a t a f 3 SR 2 SR 2 SR a f
Actual Converters Actual Converters • Combine oversampling • first with 1-bit stream • second • Resulting noise lower • 4x 6dB • 8x 12dB • In computers ground isolation of • digital noise overwhelms everything else!