On Mon, Oct 11, 2010 at 10:43 AM, jfh <j...@3kitty.org> wrote: > Aaah, right, Nyquist rate and all that... So, I guess that means that, > when you select a particular sample rate, there is an analog LPF of > appropriate cutoff switched into the analog signal path, before it gets > to the ADC. I hadn't realized that switching the digital sample rate > probably also changed the analog hardware configuration. >
In this case you will search in vain for that analog anti-aliasing filter. Here, decimation is your friend. The A:D converters used actually sample at a much higher sampling rate (internally) but use fewer bits of resolution. After sampling they filter (anti-aliasing filter) and then decimate (average groups of samples) to a lower sampling rate. The filter prior to decimation ensures that there is no power above the Nyquist frequency and then the decimation gets you more bits of resolution. In case you are thinking that you are somehow getting something for nothing, remember that Shannon sez that the bit-rate is the same, i.e. a 1L bag still holds 1L whether it is tall and skinny or short and fat. TANSTAAFL. One way to think about this is that a miracle occurs and there is no aliasing even if you change sample rates in the ADC. :-) (This to me is magik of the same order as impressing information on electromagnetic energy, blasting it out into the æther, intercepting it somewhere else, and recovering the information from it. Clarke got it right, "any sufficiently advanced technology is indistinguishable from magic.") -- Brian Lloyd, WB6RQN/J79BPL 3191 Western Dr. Cameron Park, CA 95682 br...@lloyd.com +1.767.617.1365 (Dominica) +1.931.492.6776 (USA) (+1.931.4.WB6RQN) _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/
