1. We saw that the impulse response of the cascade of two LTI systems is the convolution of the impulse responses of the individual systems and that the overall impulse response does NOT depend on the ORDER in which the systems are cascaded.
2. Since the impulse response of the cascade of two LTI systems is the convolution of the individual impulse responses, the convolution property then implies that the overall frequency response of the cascade of two systems is simply the PRODUCT of the individual frequency responses.
3. while the ideal lowpass filter does have perfect frequency selectivity, its impulse response has some characteristics that may not be desirable.
4. h(t) is NOT zero 0 for t<0. Therefore, the ideal lowpass filter is NOT causal, and have oscillatory behaviour ( 游移不定).
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