Re: [Discuss-gnuradio] working AM radio - critique

[Eric Blossom]

On Sat, Aug 21, 2004 at 07:50:39AM -0400, cswiger wrote:
> Hi - With a big thanks to David Carr for the gr.complex_mag ()
> module I have an AM radio working. There was one last issue I
> had to get around with something probably not optimal - the
> audio was offset and very large. The fm demodulator distributed
> with gnuradio produces a working signal with amplitude about .2
> or so, centered about zero.  I simply put gr.complex_mag()
> in place of gr.quadrature_demod_cf() and got audio with
> magnitude about 300 centered about 350. My solution was to
> simply shift and scale and now get good audio, just like fm.
> 
> Q: Is there a better way ?   Is there a signal level standard
> we need to comply with?
> 

Glad you got it working.

Off the top of my head, I'd suggest that you follow the mag with a
high pass filter (probably IIR), with a cut off of about 300 Hz.  That
will remove the DC you've got.  Then I'd follow that with an AGC
block, and follow that with a volume knob (mult_const_ff).

The IIR and AGC are in 0.9, but need porting.  I'll handle the
IIR in the next day or so.

If anyone's looking for a project, we could use IIR filter design from
analog prototypes or pole/zero placement in Python (using Numarray if
you like).

Eric