Nelson Lee

Lazy inference on object identities in wireless sensor networks

Maintaining the identities of moving objects is an important aspect of most multi-object tracking applications. Uncertainty in sensor data, coupled with the intrinsic combinatorial difficulty of the data association problem, suggests probabilistic formulations over the set of possible identities. While an explicit representation of a distribution over all associations may require exponential storage and computation, in practice the information provided by this distribution is accessed only in certain stylized ways, as when asking for the identity of a given track, or the track with a given identity. Exploiting this observation, we proposed a practical solution to this problem based on maintaining marginal probabilities and demonstrated its effectiveness in the context of tracking within a wireless sensor network. That method, unfortunately, requires extensive communication in the network whenever new identity observations are made, in order for normalization operations to keep the marginals consistent. In this paper, we have proposed a very different solution based on accumulated log-likelihoods that can postpone all normalization computations until actual identity queries are made. In this manner the continuous communication and computational expense of repeated normalizations is avoided and that effort is expended only when actual queries are made of the network. We compare the two methods in terms of their computational complexities, inference accuracies, and distributed implementations. Simulation and experimental results from a RFID system are also presented.

Analysis and Synthesis of Coupled Vibrating Strings Using a Hybrid Modal-Waveguide Synthesis Model

The linear coupling of two strings or of a single string vibrating in two orthogonal polarizations leads to two observable phenomena: two-stage decay and beating. In this paper, we present methods for accurately measuring and modeling the lower partials of a recorded guitar tone, where coupling effects are most audible. These estimated parameters are then used for accurate resynthesis in a hybrid modal/waveguide model. We make use of the fact that two-stage decay occurs in analyzed tones to allow direct measurement of sinusoidal decay rates. A traditional iterative optimization algorithm is explored and found to be most effective in the special case when only beating occurs. Sound examples are provided on the Web.

Practical implementation of low‐latency DSP for feedback control of sound in research contexts

Feedback control of sound requires low‐latency signal processing. In addition, because the human range of hearing extends roughly from 20 Hz to 20 kHz, controller hardware must process signals with relatively large bandwidths in comparison with common control applications. Over the past decade, the appropriate embedded hardware has become a niche product, so its cost has actually increased. The open source community has developed an excellent alternative: a general‐purpose computer runs Linux with the real‐ time application interface (RTAI). Open source drivers (see www.comedi.org) enable software to communicate efficiently with data acquisition cards. For the first time, we describe in detail from start to finish how to configure such a system. In particular, we explain how to run control code from user space while still disabling interrupts. We further explain how to reserve a processor for running only feedback control code. This configuration achieves less than one sample of total system delay at samp...

Vibrating‐String Coupling Estimation from Recorded Tones

Coupling of vibrational polarizations in a single string, for an instrument such as the acoustic guitar, produces psychoacoustically significant effects such as beating and two‐stage decay (Weinreich, JASA v62 n6). Previous considerations of string coupling phenomena appear not to have addressed the practical problem of calibrating computational models based on recorded tones. In this work, we take a data‐driven approach using measured data from a vibrating string from an acoustic guitar, the motion of the string in two orthogonal planes, and formulate a regularized least‐squares problem for computing the coupling between the measurements. Such a formulation ensures that the resulting coupling is physically admissible, in that the resulting coupling factors do not generate energy, and are easily found as the problem is convex. Well‐studied algorithms for solving convex problems, such as interior‐point and gradient descent methods can be used and are widely available in the form of open‐source libraries.

Use of the energy decay relief (EDR) to estimate partial‐overtone decay times in a freely vibrating string

The energy decay relief (EDR) was proposed by J. M. Jot [IEEE, ICASSP (1992)] for displaying the impulse response of artificial reverberation systems. The EDR is a frequency‐dependent generalization of Manfred Schroeder’s energy decay curve (EDC), defined at time n as the sum of squared impulse‐response samples from time n until decay is complete. The EDR is similarly defined for each band in a uniform filter bank, typically implemented using the short‐time Fourier transform (STFT). In this work, we apply the EDR to the problem of estimating decay times for the partials of a freely vibrating string. Previously, such decay times have been measured based on STFT magnitude data. We show that the EDR has certain advantages over the STFT, such as being less sensitive to ‘‘beating’’ in the amplitude envelopes of the partial overtones. Results in the context of virtual acoustic guitar modeling will be presented.

Virtual String Synthesis

In this chapter, we discuss methods for real-time synthesis of stringed instruments. Interest in this topic is wide and varying, as both studio and performance uses for realistic virtual stringed instruments are becoming increasingly possible with gains in computing power.

Combining physical REALity with SIMulations in pedagogical laboratory experiments

The RealSimPLE project provides teachers with a modular collection of online musical acoustics laboratory experiments, including experiments on elementary wave motion, vibrating strings, pipes, sensors, digital waveguides, psychoacoustics, PID control, sound cards, and more. Hands‐on laboratory sessions are complemented by pedagogical computer‐based simulations or animations of the same systems. This dichotomous approach illustrates both the behavior of the real system together with corresponding idealized theoretical simulation. Moreover, the simulations can illustrate hard‐to‐measure details of the real system, or highlight details which easily escape the eye or ear. In this way, the traditional lab bench is enhanced rather than replaced. Detailed instructions help teachers and/or students easily construct the minimal amount of low‐cost laboratory equipment needed. The RealSimPLE website (http://ccrma.stanford.edu/realsimple) also includes rollover ‘‘pop‐up’’ definitions for many technical terms. These ...

An online virtual acoustic guitar laboratory

We present a ‘‘virtual acoustic guitar laboratory’’ in which students can bring in their own acoustic guitars, record several notes, and follow a step‐by‐step procedure to create their own real‐time computational model, suitable for use in a synth plugin in an electronic music studio. The guitar synthesis model is based on the commuted waveguide synthesis technique (http://ccrma.stanford.edu/∼jos/cs.html). Online laboratory materials are presently available at (http://ccrma.stanford.edu/∼jos), the project website (http://ccrma.stanford.edu/realsimple/), and are also being ported to Connexions (http://www.cnx.org/), an online repository of educational materials freely distributable under a Creative Commons license. Connexion’s ‘‘modules’’ and ‘‘courses’’ (which are simply module sequences) are readily and freely accessible and modifiable to allow instructors to adapt the materials as appropriate for their own classroom needs. The online lab materials contain Web pointers to relevant portions of online text...