Bala, I think your question about sub-Nyquist sampling was in reply to my posts that Jeff kindly referenced. Your question used kilohertz (kHz) as units for frequency while my post used only hertz (Hz), so if you thought I was saying that you can successfully sample a 15-30 kHz signal with only 30 samples per second (sps) then you're wrong; it would require 30 kilosamples per second (ksps). The sample rate must be twice the band-WIDTH, i.e. Fs > 2*(Fmax-Fmin), AND the resulting aliased spectrum must map every input frequency to a unique aliased frequency. So in the first case, 15-30 Hz, sampling at 30 sps gives a reverse-mapped spectrum from 15 to 0 Hz, i.e. 30 Hz input will map to 0 Hz, and 15 Hz input maps to 15 Hz. Since the sampled signal has a periodic spectrum, the signal can be reconstructed by converting back to continuous-time (i.e with a D-A converter) and analog filtering with a band-pass filter with brick-wall cut-off frequencies at 15 and 30 Hz. You probably need to draw the periodic spectrum of the sampled signal to visualize this. Now, in the second case, 30-45 Hz, sampling at 30 sps causes the 30 Hz input to map to 0 Hz (DC), and the 45 Hz input becomes 15 Hz. This alias-mapping is non-reversed, but as my second post noted, this does not really matter because the same reconstruction approach still works: just change the brick-walls of the analog band-pass filter to 30 Hz and 45 Hz. Does that make sense? Regards, Mark > -----Original Message----- > From: Jeff Brower [mailto:] > Sent: Tue, August 06, 2002 11:29 AM > To: Daniar Hussain > Cc: Bala Sekhar; > Subject: Re: [matlab] sampling rate ???? > Daniar, Bala- > > Hey guys, Mark Egler's post reply to Prijit Debnath (copy > below) from a few days ago > is fantastic on this subject. I suggest you read it before > you try to re-cover the > ground he has already explained in detail. > > Jeff Brower > DSP sw/hw engineer > Signalogic > > > In order to understand the Nyquist rate, it is easiest to > do so in the > > FREQUENCY domain rather than in the time domain, which you > are trying to > > do. > > > > In the Freq. domain, the signal must be BAND-limited -- ie: > zero beyond > > some max. frequency. You must sample at TWICE this max. > frequency in > > order to capture all information. This is because in DT, > the frequency > > domain is periodic, and in order for frequencies not to > overlap, you need > > to sample at twice the max. frequency. > > > > I hope this makes sense. > > > > Daniar > > > > On Mon, 5 Aug 2002, bala sekhar wrote: > > > > > Hi all, > > > > > > can you please explain me some thing about sub-nyquist > > > sampling. > > > > > > I cannot understand the concept of sub-nyquit > > > sampling. > > > > > > suppose,the bandwidth is 15 Khz,sampling rate is 30 > > > sps > > > > > > if you need to sample in the range 15-30khz,it could > > > be done without > > > downsampling the signal to 0-15 khz.I am aware that > > > the the frequency > > > gets mapped properly and could be retrived back. > > > > > > In the case of 30-45Khz ,i read there is NO MAPPING of > > > signals... > > > > > > questions: > > > Could some one explain in detail how is this done? > > > what is the exatly is happening in > > > 1.15-30khz > > > 2.30-45khz > > > > > > thank you > > > > > > regards, > > > Bala. > |
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