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Re: [Discuss-gnuradio] Stereo FM receiver

From: Al Davis
Subject: Re: [Discuss-gnuradio] Stereo FM receiver
Date: Mon, 24 Jan 2005 22:16:15 -0500
User-agent: KMail/1.7.2

On Monday 24 January 2005 09:34 pm, Matt Ettus wrote:
> Perhaps there is an FM guru out there that can clear this up.
>  I was under the impression that the output of the FM demod
> should be run through the deemphasis, and then the stereo and
> subcarriers can be picked off.  You have it the other way
> around -- you do deemphasis on the left and right separately
> after they have been filtered out from the main signal.  I
> think this causes the L-R to have incorrect deemphasis.

The implementation is correct.  The subcarrier is processed 
before deemphasis, then deemphasis is applied to both channels 

In the transmitter, preemphasis is applied to both channels 
separately.  Then L+R and L-R are formed.  L-R modulates the 
subcarrier.  The sum of L+R + the subcarrier + pilot modulates 
the main carrier.

A common variant on this is to simply sample L and R at 38 kHz, 
using alternate samples.  If the sampling waveform is 
sinusoidal, this produces the same result as above.

Some receivers work this way too.  The pilot is doubled.  The 
resulting 38 kHz operates a switch to alternately select left 
and right.  Then it is low-pass filtered, then deemphasis is 

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