Re: [Discuss-gnuradio] Basic question about filter

[Ian Buckley]

On Jun 18, 2012, at 6:54 PM, Marcus D. Leech wrote:

On 18/06/12 09:46 PM, signalswdm wrote:

Hello everyone:

   You see in theory there should be a low-pass filter after ADC for eliminating the image. Also after DDC there should also be a 

filter. But in USRP there are not such things. So do the cic filter and half band filter implement that function besides decimation?

Best regards

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Actually, in typical architectures using I/Q, there are low-pass filters *in front* of the ADC to eliminate aliasing prior
  to sampling.  So the CIC decimator "sees" a signal that has already been band-limited appropriate to the sample
  rate of the ADCs.

The CIC decimation process *is* filtering, so out-of-band products appearing at the output are usually suppressed
  by at least 80dB, and usually more.

Indeed, to re-enforce Marcus's point with precise English terminology:

Downsampling is a process that reduces the sample rate of a digital signal by simply discarding samples.

Decimation is the compound operation that incorporates downsampling and an appropriate low-pass filter.