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Re: [Discuss-gnuradio] Re: FM stereo problem


From: Bernardo Gonçalves
Subject: Re: [Discuss-gnuradio] Re: FM stereo problem
Date: Tue, 25 Jan 2011 22:57:56 -0200

Hi Patrick,

First, thanks for the reply!

> But two other things I did notice:
> 1) The difference signal, which is shifted down from 38kHz is several
> 10dB lower than the sum signal. With different gains I got something
> more stereo-like.

I'll try to do that, I've seen through a FFT after the FM Demod block, but I didn't try to increase the gain of 38-53kHz!

> 2) The sum signal is faster than the difference signal. In the
> diff-signal path you have more blocks that are processed. Some delay
> (delay block) could maybe fix this problem, but I did not get the right
> value at a quick try.

Well, wouldn't that be also a problem during the modulation? Anyway I'll also try to look after the Delay block.

I'll post the results soon!

Thanks for the help!

Regards,
Bernardo

On Tue, Jan 25, 2011 at 10:25 PM, Patrick Strasser <address@hidden> wrote:
schrieb Patrick Strasser on 2011-01-25 16:55:
> schrieb Bernardo Gonalves am 2011-01-25 10:15:

>> Basically, the problem is that it works just as mono, not stereo...

> Second, I looked at your flowgraph. You shift the difference part from
> center frequency 38kHz to 0 with the Xlating FIR filter, and then use a
> complex-to-float block. This will not result in the positive
> frequencies. You'd rather have to filter out the negative frequencies
> with a Hilbert transformer, that is a special FIR filter keeping only
> positive frequencies, before converting to float again.

Sorry, I was wrong. You did the low-pass filtering, that was already right.

But two other things I did notice:
1) The difference signal, which is shifted down from 38kHz is several
10dB lower than the sum signal. With different gains I got something
more stereo-like.
2) The sum signal is faster than the difference signal. In the
diff-signal path you have more blocks that are processed. Some delay
(delay block) could maybe fix this problem, but I did not get the right
value at a quick try.

Patrick
--
Engineers motto: cheap, good, fast: choose any two
Patrick Strasser <patrick dot strasser at tugraz dot at>
Student of Telematik, Techn. University Graz, Austria


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