Re: [Discuss-gnuradio] A question on wfm_rcv_gui.py

[Eric Blossom]

On Sun, Mar 20, 2005 at 04:47:11PM -0800, Sachi wrote:
> I still can't figure it out. Can anyone of you guys
> help me with it?
> 
> > Hi guys
> > 
> > I got a bit confused by the code for FM demodulator.
> > In function parseargs(), why we return freq1 - 128e6
> > instead of freq1 itself? What does 128M stand for?
> > 
> > Thanks
> > 
> > Sachi

Let me take a shot at it.  Matt wrote it, but I think he's off-line
for the weekend.

wfm_rcv_gui was written assuming that there was no RF front end doing
any downconversion.

The A/D sample rate is 64M.

The Nyquist zones are therefore:

[0, 32M]    normal
[32M, 64M]  inverted
[64M, 96M]  normal
[96M, 128M] inverted

etc.

The problem is that 96M, one of the folding points, occurs right in
the middle of the broadcast band.

This means that 95.0 and 97.0 both alias down to the same frequency
and can't be distinguished from each other.

freq1 - 128M will give a valid frequency if freq1 is >= 96M.

Bottom line: it's a kludge that sort of works.

Eric