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Re: [Discuss-gnuradio] usrp.source_s and usrp.source_c

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] usrp.source_s and usrp.source_c
Date: Fri, 3 Nov 2006 16:18:58 -0800
User-agent: Mutt/1.5.9i

On Fri, Nov 03, 2006 at 06:58:41PM -0500, Dawei Shen wrote:
> Hi, Guys
> I am suddenly a little bit confused about the difference between
> usrp.source_s and usrp.source_c. Basically I have two questions:
> 1. Since the ADC only has 12 bits, does it mean the short integers entering
> the computer have their last four bits as zeros? Or are their first four
> digits are zero (the first bit should be the sign)?

The short integers entering the computer have been through the DDC and
have been processed as 16-bit ints.  Without looking at the verilog
code I don't recall how they're adjusted before being clocked into the

> 2. Is anything interesting done when the short integers converted to float
> values? Or are they simply converted to a different type without changing
> the values?

At this point nothing besides the format conversion is done.

There have been some conversations about changing the _c format to be
normalized to +/- 1.0 but the details aren't worked out yet.   We'll
probably define a new interface so that we don't break a bunch of
code.  The normalization would also be a function of the kind of
daughterboard that's connected, and will probably factor in some kind
of two-tone IMD performance result.


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