| Frequency Domain Artificial
Reverberation using Spectral Magnitude Decay |
| Earl Vickers1, Jian-Lung (Larry) Wu2, Praveen Gobichettipalayam Krishnan3, and Ravirala Narayana Karthik Sadanandam3 | |
| 1 The Sound Guy, Inc., Seaside, CA 93955 USA | |
| 2 Stanford Center for Computer Research in Music and Acoustics, Stanford, CA 94305 USA | |
| 3 University of Missouri, Rolla, MO 65409 USA | |
| Presentation © 2006 Earl Vickers, The Sound Guy, Inc. | |
| sfx@sfxmachine.com |
| Overview |
| Introduction | |
| STFT & Phase Vocoder | |
| Time-Freeze, Time-Scaling, Looping & Reverberation | |
| Recursive Spectral Magnitude Decay | |
| Phase Generation | |
| Results | |
| Remaining Problems | |
| Conclusions | |
| Introduction |
| Frequency Domain Reverb | ||
| Not using convolution | ||
| Spectral Magnitude Decay | ||
| Late reverb only | ||
| Diffuse portion of the room response | ||
| Typically after 100 ms. | ||
| Best described statistically | ||
| Early reflections would be handled separately | ||
| Reverberation |
| Something about reverberation echoes, resonates & reverberates deep within the human psyche | |
| Echo & Narcissus | |
| ÒIn Babylonian mythology there are
hints of a specially constructed room in one of the ziggurats where whispers
stayed forever.Ó – Schafer |
|
| Existing Reverb Algorithms |
| Time domain reverbs, such as Feedback Delay Networks: | ||
| Low computational cost | ||
| Independent control over many perceptually relevant parameters | ||
| Convolution | ||
| Excellent simulation of the acoustics of a particular physical space | ||
| Inspirations for this approach |
| Frequency domain time-scaling often suffers from an unwanted ÒreverberantÓ effect, Òphasiness,Ó Òloss of presenceÓ | ||
| If weÕre trying to produce reverberation, this might prove to be an advantage, or at least not a liability | ||
| Ò...the responses in the finest concert halls sounded remarkably similar to white noise with an exponential amplitude envelope.Ó — James Moorer | ||
| Preview: Spectral Magnitude Decay |
| Uses STFT / Phase Vocoder | ||
| Attenuates & accumulates the spectral magnitudes | ||
| Combines them with a computed phase signal | ||
| Resulting impulse response: | ||
| Smooth, exponentially decaying envelope | ||
| Independent control over room energy
& decay time at each frequency |
||
| STFT & Phase Vocoder |
| Analyze signal into a time-frequency grid | |
| Change stuff | |
| Resynthesize in the time domain | |
| Time-Freezing (Infinite Time Scaling) |
| Ongoing Parallel Time-Freeze |
| Problems with the
Parallel Time-Freeze Reverb |
| Grows increasingly inefficient | ||
| Data storage & computation explosion | ||
| The impulse response consists of a decaying series of impulses, N samples apart | ||
| Optimizing the
Parallel Time-Freeze Reverb |
| Recursive Spectral Magnitude Decay |
| Frequency Domain Analogue of MoorerÕs Comb Filter |
| Frequency-Dependent Exponential Decay |
| Exponential Decay |
| Spectral Magnitude
Decay Impulse Response & EDC |
| The Phase Problem |
| Desired Phase Signal |
| Desired Phase Signal, cont. |
| Desired Phase Signal, cont. |
| The Auditory Sensation of Roughness |
| Out-of-tune piano: rapid beating results from nearby strings whose vibrations go in and out of phase | |||
| Rapid sequence of brief auditory events | |||
| Roughness most pronounced when sound includes spectrally coherent fluctuations | |||
| Can be minimized by randomizing the amplitudes and phases of its Fourier components | |||
| Randomization happens automatically in reverberant environments | |||
| Roughness reduction may be one of the benefits of adding reverb | |||
| Roughness & Beating
— Why CanÕt We All Just Get Along? |
| Amplitude fluctuations resulting from phase incoherence |
| Phase Generation
Methods 1 - Propagation of Instantaneous Frequency |
| Phase Generation
Methods 2 - Phase Randomization |
| Phase Generation
Methods 3 - Partial Phase Randomization |
| Final Spectral Magnitude Decay Algorithm |
| Initial Observations |
| Good impulse response | ||
| Smooth envelope | ||
| Independent control of Room Energy and Decay Time as functions of frequency | ||
| Echo density doesnÕt increase over time | ||
| Maybe OK, because late reverb is defined as the point where individual reflections are indistinguishable. | ||
| Gradual attack will help blend in late reverb | ||
| Decay Time Cross-Synthesis Effect |
| Advantages |
| Compared to time domain reverb: | |
| Less external memory needed | |
| Fewer external memory accesses | |
| Excellent control of room & decay spectra | |
Compared to convolution: |
|
| Parametric and flexible | |
| Computation cost independent of decay time | |
| Problems |
| Deterministic phases can produce echoes at period N | ||
| Less noticeable for shorter decay times | ||
| Phase randomization produces ÒwhisperizationÓ effect for short windows | ||
| Noticeable for N <= 1024 | ||
| Not bad for N = 2048 | ||
| Good for N >= 4192 | ||
| Can use partial phase randomization | ||
| Long windows cause long latency | ||
| Early reflections could fill in the gap, for reasonable Reflections & Reverb Delays | ||
| Conclusion |
| No delay lines needed | ||
| Could be used for cell phones and other systems with limited memory | ||
| Flexible, detailed control of room energy & decay time as functions of frequency | ||
| Could be especially useful if youÕre already in the frequency domain | ||
| Cheaper than convolution for longer decay times | ||
| Convolution could be used for the early reflections | ||
| Slide 34 |