| 摘要 |
A system for reducing periodic noise, which includes a plurality of harmonically related noise signals, comprises an actua- tor (1) for producing a cancelling acoustic signal, a sensor (3) for detecti ng a residual noise signal, synchronizing signal generat- ing means (7) and processing circuitry (4-1, ... 4-n, 5, 6). The processing circuitry comprises a plurality of tunable harmonically related band pass filters (4-1, ... 4-n), a tuning signal generator (6) and a summer (5) which sums the outputs of the filters (4-1, ... 4-n). The tuning signal generator (b) receives the synchronizing signal from the synchronizing signal generating means (7) and outputs the tuning signals to the band pass filters (4-1, ... 4-n). As the frequency of the synchronizing signal changes, the tuning signal generating means (6) causes the tunable filters (4-1, ... 4-n) to tra ck harmonics of the noise to be cancelled. After summing by the summer (5) and suitable amplification, the outputs from the filters ( 4- 1, ... 4-n) are used to drive the actuator (1) so as to reduce the residual noise detected by the sensor (3). In one embodiment the filters (4-1, ..., 4-n) are implemented by means of switched capacitor filters.
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