Re: [Discuss-gnuradio] GNU Radio Companion - ALSA

[Marcus Müller]

Hi Robin,

first of all: Welcome to the GNU Radio community!


On 02/18/2017 05:29 PM, Robin A. Jensen wrote:
> Hello all of you.
>
> I've just recieved my RTL-SDR dongle and is all new to this sdr-stuff,
> so please bear over with me, if i'm at the wrong place.
> I'm using GNU Radio Companion on a RPi 3 and no mather what i'll do, i
> can't get the sound to work.
> If a'im using rtl_fm and aplay, i'll get sound but it won't set on the
> radiostation.
aha, so that's good, the sound system as it does work.
You'll probably want to use "aplay -L" to find the possible ALSA device
names that you can use in the GNU Radio Audio sink.
> I'll take on that later.
> I've createt a small FM Reciever in GNU Radio companion and everytime
> i'll execute the script i'll get an error:
>
> RuntimeError.audio.alsa.sink
Hm, I've never seen a GNU Radio error being printed like this; but that
might just be me. However, I can't reproduce this error printing shape
as hard as I try.
>
> I've been all over the internet to find a solution but with no luck.
> So now i'm have a hope that this mailling list can help me?
>
My suspicion is that your audio device doesn't like the sampling rate
your trying to use, or you need to specify a device name (or both). Can
you make things work on the PC you use to design these flow graphs?

I'd start with a signal source (sampling rate == the sampling rate that
you set in your Audio sink), configured to produce a "float" output sine
of 1 kHz, directly connected to an Audio sink. If that works, move on.

--------------------------------------------------------------------------------

What I say about the flow graph in the following has, as far as I can
tell, nothing to do with the error you're getting. Still, there's
mistakes in the flow graph that would make it impossible to successfully
run it, and thus I'd like to avoid frustration later on by pointing them
out know:


So, the main issue with your flow graph is that the sampling rate at the
audio sink must be what you configured your audio sink for (48 kHz).
But: that rate is the result of your SDR's sampling rate (2MS/s),
multiplied with all interpolations, divided by all decimations in the
path between.

> My setup is:
> RTL-SDR Source: samplerate:  2M, frequency 96.5 MHz ->
> Rationel Sampler: Interpolation 4, Decimation: 1 ->
Uh, that means that you have now 8MS/s. That seems unnecessary, since to
capture a <100 kHz wide FM channel, you wouldn't even need the 2MS/s you get
> Low Pass Filter: Sample rate: 2M, Cutoff freq: 100K, Transition Width:
> 100k ->
Which contradicts the 2MS/s used here, so you're actually getting 400kHz
passband width, 400kHz transition width. Also, this feels like a prime
candidate for including decimation in the filter (because the resulting
bandwidth is 200 kHz (if you overlap the two transition widths), and for
that you'd only need 200 kS/s of complex digital signal).
> WBFM Recive: Quadrature: 500K, Audio Decimation: 1 ->
This is now off by a factor of 16; are you sure you should be using
"interpolation=4,decimation=1" instead of the inverse?
> Rationel Sampler: Interpolation: 500
Certainly not :) 8 MS/s · 500 = 4 GS/s
> Decimation: 48 
Aside from that not even having greatest common denominator of 1 with
500 (you can't know that this is important, not blaming you), this would
give you an output sampling rate of 4GS/s/48 = 1 GS/s / 12 = 83.333
MS/s. Which isn't even a multiple of 48 kHz, which you use in the audio
sink:
> ->
> Audio Sink: Sample Rate: 48 KHz, Device Name: HW:0,0
>
> I've found that recipie on a Hack5 video and there it's working
I think there's some error in the way you configured these resamplers. I
don't know the Video you're referring to, but the amount of rational
resamplers used here alone, paired with the fact that you don't decimate
in the WBFM receiver makes me kind of suspicious this might not have
been the optimal video to take reference designs from!

--------------------------------------------------------------------------------

Best regards,
Marcus
>
> With best regards
> Robin.
>
>
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