Noise / harmonic rejection

Frequency-response analysis [1] is the most widely used technique for impedance testing. Similar to the lock-in technique, it can extract a small signal from a very high background of noise, automatically rejecting DC and harmonic responses. The difference is that a frequency-response analyzer (FRA) correlates the input signal with the reference sine waves. To achieve faster measurements, FRAs are usually equipped with separate analyzers for each input chaimel. 

Refer to section 3.1.2.7 for more information about the single sine correlation process. Even measurement of one cycle of the waveform to be analyzed provides rejection of all harmonic frequencies, as can be seen from the upper trace in Figure 3.2.4. Additional cycles can be averaged to provide rejection of noise and subharmonics from the waveform, as demonstrated by the other two traces which show the improved rejection of spurious signals for 10 and 100 cycles of integration. 

Figure 3.2.4. Single sine correlation (rejection of noise and harmonics).

Comb filtering capitalizes on the periodicity of voiced speech signals. It uses pitch information to design a comb filter that passes the harmonics of the signal while rejecting noise between these harmonics. This and other related techniques are discussed in Lim and Oppenheim (1979). The main drawback with this approach is that accurate estimates of the voiced speech pitch are required and such estimates are difficult to obtain from noisy speech. 

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