[hpsdr] Bandwith and decimation questions

[Joe Martin K5SO]

Hi Maximo,

On Mar 22, 2011, at 12:22 PM, Maximo EA1DDO_HK1DX wrote:
>
> What I don´t understand good is "There are three fixed sample rates  
> that will come out of Ozy or Metis: 48000, 96000 and 192000".
> I thought that 48000, 96000 and 192000 are radio spectrum bandwith,  
> but I did not know that radio bandwith is same as transmission  
> bandwith. I mean, Mercury´s sample rate is always same, 130 Mb/s.
>
> I need some more light on that...
>

Yes, Mercury's ADC continuously runs at 122.88 M conversions per  
second producing 16-bit values at that rate.  Those data are decimated  
to produce one of three lower, user selectable, data rates (192,000,  
96,0000, 48,000) and by virtue of "processing gain" produces 24 bit  
integers from those initial 16-bit values.  To understand exactly what  
is going on in Mercury I highly recommend that you obtain a copy of  
the DVD recording of Phil's presentation concerning Mercury given at  
the 2008 Digital Communications Conference in Chicago.  I think the  
DVD is still available at

http://www.arvideonews.com/dcc2008/index.html#Nick_Luther_K9NL

and it is an excellent tutorial on Mercury and it's inner workings.   
Phil explains the design philosophy of Mercury and how each of the  
most important trade offs were handled in the design for Mercury.  It  
will educate you as to how DSP works generally and, more to the point  
of your questions,  how Mercury works in particular.

Of course, many of the answers to your questions can be found in any  
DSP text but the DVD I referenced is focused on Mercury in particular.

73,  Joe K5SO
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