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How does an equalizer prevent a sample and hold effect?

Does a reconstruction filter only smooth out the output of an DAC (+S&H) or does it have another function?

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Equalizers do not prevent the sample and hold effect, though if you are talking about compensation filters, see the edit below.

There are two types of reconstruction filters (also known as low-pass filters): analog and digital. An analog low-pass filter can smooth out the output of a DAC. A digital low-pass filter cannot, since it comes before the DAC. What it can do is reduce the effect of the sample and hold by increasing the sample rate (assuming it is an interpolation filter) and then getting rid of the aliases via the low-pass filtering. The effect of the sample and hold is reduced due to the increased sample rate.

EDIT: As Robert points out, the frequency droop that happens as a result of the zero-order hold can be partially compensated for with digital filters whose frequency domain gain is the inverse of the zero-order hold effect. While this will not eliminate the sample and hold effect, it will mitigate it.

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  • $\begingroup$ Jim, i cannot agree with you. if the zero-order-hold effect of the DAC is known in advance (like, say, it's a "conventional" DAC, not a sigma-delta DAC), you can, if you're willing to put in a little delay, compensate in the frequency domain for the droop (which is -3.92 dB at Nyquist) of the ZOH. at least to some extent. but that compensation would be bad if the same samples went out to a DAC that operated at a high internal sampling frequency (like a $\Sigma\Delta$ DAC). $\endgroup$ – robert bristow-johnson Jan 8 '14 at 17:22
  • $\begingroup$ @robertbristow-johnson Good point. Edited the answer. $\endgroup$ – Jim Clay Jan 8 '14 at 19:16
  • $\begingroup$ cool. i wish i could edit my comment now (to make it compatible with the current answer). one other issue (not a disagreement, but to sorta be clear): some folks (even profs, like who i had for "Digital Control Systems" at NU) incorrectly ascribe to the S/H before the ADC input, this Zero-Order Hold effect. it doesn't matter how the ADC gets the numbers, just that it gets the correct numbers. the ZOH effect is due to the DAC holding the voltage constant (instead of outputting a dirac-like pulse) between sample instants of time. $\endgroup$ – robert bristow-johnson Jan 8 '14 at 19:52

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