Re: Filter design.


<tanmay.zargar@xxxxxxxxx> wrote in message
news:1133479883.975943.33090@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>I have to design a simple low pass filter (for audio) to reject
> frequencies beyond 3.6kHz. The design of the filter is quite simple.
> However, I want to know what affect the sampling frequency of the audio
> will have on such a filter design.
>
> In other words, I can sample at 16kHz, filter the data and downsample
> to 8kHz (which is finally the sampling rate I want) OR I can sample at
> 8kHz and filter the data. (This will save me precious MIPS, which I am
> in great need of). I want to know what I am losing (if anything at all)
> if I sample at 8kHz instead of 16kHz. Bear in mind that this is audio
> (voice) data I am talking about, so I am not interested in frequencies
> above say 3.8kHz.
>

Since you have to sample then it's necessary for the data to have bandwidth
effectively limited to below fs/2 - perhaps 0.4 fs before sampling.

Sampling at 16kHz may be one way without prefiltering the input - if the
bandwidth is effectively limited to less than 7kHz or so. Why do I doubt
that??

Sampling at 8kHz would require that the bandwidth is effectively limited and
certainly to less than 4kHz already. So, if the data is bandlimited in that
range to begin with, then you may not need a digital filter at all....

Fred


.

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