Re: trying to play two radio stations simultaneously

[address@hidden]

First of all, thank you for the feedback and letting me know that someone has 
watched
the video :)

I believe the issue with your flowgraph is that the second channel is too far 
from the
center frequency: with 800 kHz offset with respect to the center frequency and 
a span
of 1.536 MHz (spectrum spanning from -768 kHz to +768 kHz) you are looking at 
the alias
at -736 kHz (768-(800-768)). If you don't have two stations closer than 800 
kHz, then
I would select a center frequency (OsmoSDR source) in between the two stations, 
and shift one
station by +400 kHz and the other by -400 kHz. Both stations will fit nicely 
within
the available bandwidth of 1.536 MHz with no risk of aliasing with one station 
being
at the very limit of the spectrum with its boundary at the Nyquist frequency 
(half the sampling
rate).

As I had received another request this morning, I am trying to upload my 
flowgraphs (GRC 3.8)
at http://jmfriedt.free.fr/, hitting on the left menu "Licence 3 EEA" and 
scrolling down to the
video links, but I only saved some of the more complex flowgraphs (RDS 
decoding, phase synchronization
simulation and POCSAG decoding at the moment). I must re-generate the other 
flowgraphs by watching
my own video and remembering what I said ! This is excellent feedback though to 
improve teaching
material, thank you.

JM

--
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, 
France

January 11, 2021 6:13 PM, "Chris Gorman" <chrisjohgorman@gmail.com> wrote:

> Hello All,
> 
> I'm trying out jean-michel.friedt's course on digital communications
> and have a problem with getting two radio stations to play
> simultaneously. I have tried to follow jean-michel's example, but am
> getting caught somewhere. I get one radio station and hiss on the
> other.
> 
> I've tried a few combinations, but haven't got a logical successful
> solution. I am choosing two local radio stations and can tune them
> independently. The first radio station is the osmocom source ch0
> frequency. This station I can tune without trouble. The second
> station is determined by s frequency translating filter and I make the
> center frequency a negative difference between the ch0 frequency and
> it's nearest neighbour. In this case I'm choosing ch0 frequency
> 90.7e6 and center frequency -800e3. (For a radio station at 91.5
> MHz.) Somewhere here I am getting into trouble. This station can be
> heard but is mostly hiss.
> 
> I'm including my grc file and a screenshot of my flow graph with the
> hope that someone can point me to my error. Thanks in advance.
> 
> Chris