detections. The situation worsens when several tags are simultaneously within range of a single reader. Although many current tagging systems are unable to operate properly with multiple tags present, the anticollision schemes implemented in most multi-tag systems introduce even more delay. In addition, commercial RFID products seldom provide an indication of detected signal strength, corresponding to tag position and orientation (a very useful interface parameter). Likewise, because of the

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... nse of fabricating the tag's chip and bonding it to a potentially complicated antenna structure, today's RFID systems are generally more expensive than simpler technologies, such as resonant analog tags, which were adopted in this study. We have developed a passive tag reader optimized for applications in human-computer interaction. It sweeps through a 50-300 kHz read frequency, flagging any magnetically-coupled resonators in that range. It is a minimally-complicated circuit, and is able to provide the center frequency, resonance width, and amplitude for each detected tag over a serial line at 30 Hz continuous updates. The tags are easily fashioned, consisting only of an inductor and capacitor or magnetostrictor tag cut to appropriate length. We have written an engaging musical application to demonstrate this system, tagging over 11 different objects and tracking their proximity and state, launching or modifying musical sounds in accordance.