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This question already has an answer here:

I am trying to implement a logic to downsample data in non-integer factor.Lets say,converting 48Ksps to 1344 samples per second.Have two step implementation of upsample by K ,and downsample by L is in my mind, such that K/L gives me the required ratio.

The thing that troubles me is the filter(if any) required in between the upsampling and downsampling. Can anyone help me understand the necessity of this filter? Also is there any better methods to do so?

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marked as duplicate by Marcus Müller, MBaz, lennon310, hotpaw2, jojek Apr 18 '17 at 10:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ hotpaw2's answer to the question above would be a good starting point to learn about rational resamplers (which is the term I'd look for in a DSP book or wikipedia or just google) $\endgroup$ – Marcus Müller Apr 17 '17 at 9:09
  • $\begingroup$ Unlike the other question,I really would like to know about the filter required between the two sampling stages. $\endgroup$ – Binary_World Apr 17 '17 at 9:40
  • $\begingroup$ Well, I don't really know why I should write an answer that just says "I've opened the second link google gives me when searching for rational resampler filter and copy/pasted the explanation there". I do think you either need to do more research on your own, or explain more clearly what you've already understood and what your precise question is :) $\endgroup$ – Marcus Müller Apr 17 '17 at 9:48
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If the data is already bandlimited to below half the new sample rate, then a low pass filter may not be required. If not, low pass filtering is required to prevent aliasing, which can corrupt your result. Low pass filtering can be combined with Sinc kernel interpolation of all the new points needed at the new sample rate, e.g. only one step, no up/down pair of sample computations required.

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