| 摘要 |
Processing of a received telephone-signal to limit sound-pressure from an earpiece 3 involves frequency-dependent limitation (7) after DTMF-tone suppression (5,6). The energies SyN of synthesised frequency-components 1-N of the sampled signal are computed within the limiter 7 from weighting coefficients cN1,cN2 derived iteratively by an adaptive LMS filter 10 for the sine and cosine pairs of the synthesised components. The computed energies SyN after weighting wN according to characteristics of the earpiece 3 are combined (17) to give an estimate ET of the energy-content of the signal-sample. Comparison (19) of the estimate ET with itself before and after application of a non-linear function (18) produces an attenuation factor that is applied (20) to limit the signal supplied to the earpiece 3. DTMF-tone detection (6) uses synthesis by LMS filter 25 of the sample-signal at DTMF-tone frequencies, and sorting (27) of their computed energies Sy1-8. The presence of DTMF tones in the sample-signal is detected if signals cS6-cS8 representing the three highest energies are found (31) to satisfy three criteria (28-30) attributable to the presence of DTMF-tone pairs.
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