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Echo Generation and Simulated Reverberation R.C. Maher ECEN4002/5002 DSP Laboratory Spring 2003
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher2 Recirculating Delay Line Recall that a simple delay FIFO (first-in, first-out) queue can be implemented using a modulo buffer Now, consider a recirculating buffer: + Delay Line (z -N ) k output can also be taken here
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher3 Recirculating Delay (cont.) + Delay Line (z -N ) k x[n]y[n] N poles equally spaced around circle (radius k 1/N )
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher4 Recirculating Delay (cont.) Example: unit sample response (N=8, k=0.8)
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher5 Recirculating Delay (cont.) Frequency response of system A “comb” filter
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher6 Echo Generator If length N is short, comb filter effect (audio coloration) is noticeable If length N corresponds to a time delay of 200ms or more, output perceived as an echo of the input Scaling of input is typically needed to prevent overflow
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher7 Acoustics Sound propagates through air as a longitudinal wave Speed of sound: –343 m/s @ 20°C (331 m/s @ 0°C) –About 1125 ft/s –About 1 ft/ms (light travels about 1 ft/ns) –About 5 seconds per mile Audible wavelengths: 1.7cm to 17m
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher8 Human Hearing Characteristics Frequency sensitivity roughly 20 Hz – 20kHz –3 decades, or 10 octaves –Compares to less than one octave for human visual system (wavelengths: 3800 – 7600 angstroms) Amplitude sensitivity roughly 20 Pa – 20Pa –Atmospheric pressure: 10 5 Pa –Minimum audible sound corresponds to motion a fraction of diameter of hydrogen atom! –Any better, and we would “hear” thermal noise Difference judgments good, absolute judgments poor
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher9 Acoustical Transfer Function Linear Time-Invariant (LTI) systems can be fully described by an impulse response Fourier Transform of the impulse response is the transfer function Acoustical path from a sound source to the listener’s ear is LTI, so we can describe the acoustical system with a transfer function
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher10 Reflections and Reverberation Direct sound arrival is followed by reflections from room surfaces Overlapping reflections are heard as reverberation Direct-to-Reverberant ratio gives cues to size of room, type of room surfaces, and distance from source
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher11 Direct Sound and Reflections Sound Source
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher12 Reverb Simulation Method 1: obtain impulse response of real room, then convolve with input signal –Results can be good, but computation and storage requirements are high: impulse response sequence for 3-5 seconds is ~200k Method 2: use recirculating delay lines to create an artificial impulse response –Less computation and storage, but tricky to get satisfactory sound quality
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher13 Reverb Simulation (cont.) For good reverb: –echo density (echoes per second) must increase as time goes on –response should be quasi-random (no spectral coloration) –high frequencies typically decay faster than low frequencies
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher14 Reverb Simulation (cont.) Plan: use parallel recirculating delay lines with incommensurate delays + Delay Line (z -N 1 ) k1k1 + + Delay Line (z -N 2 ) k2k2
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher15 Other Improvements Cascade of all pass recirculators + Delay Line (z -N ) k + -k N poles and zeroes equally spaced around circle (pole radius k 1/N, zero radius k -1/N )
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher16 Other Improvements (cont.) Add lowpass filter to feedback path: shortens high frequency reverb time: more like natural reverberation + Delay Line (z -N ) k LPF
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ECEN4002 Spring 2003Echo and Reverb R. C. Maher17 EVM Note: External Memory To use external memory on EVM, need to program the bus control register and the address attribute register 0 (see 56300 Family Manual) movep #$040821,x:M_AAR0;Compare 8 most significant bits ;Look for a match with address ;Y:0000 0100 xxxx xxxx xxxx xxxx ;No pack, no mux, Y enabled ;P and X disabled ;AAR0 pin active low movep #$012421,x:M_BCR ;One ext. wait state Access to external memory is slower than internal memory: wait state stalls processor
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