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Re: [Discuss-gnuradio] working AM radio - critique
From: |
Eric Blossom |
Subject: |
Re: [Discuss-gnuradio] working AM radio - critique |
Date: |
Sat, 21 Aug 2004 17:54:32 -0700 |
User-agent: |
Mutt/1.4.1i |
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