Flutterind Sound

[didi...@googlemail.com]

Hi
I am trying to do my first steps in SDR reception with a Logilinc USB 2.0 DVB-T (RTL 2832U).
For first tests I tried to receive FM radio. I succeeded to find our local station in the spectrum,
could tune to it and could hear something, but it is a useless fluttering sound coming out the PC speaker.
I recorded some of the audio as evidence but was surprised that the recorded audio was perfect.
I am running GQRX under MINT 17 Cinnamon and installed the standard GQRX version offered
in the Synaptics package manager. The PPA found in the internet doesn't work.

Any idea what is wrong with my setup?

Claus

[Alexandru Csete]

The only thing I can think of is too high CPU load.
I would be interested to know why the PPA doesn't work. Perhaps it is
a matter of selecting the right distribution?

Alex

[WA7RCQ]

Morning Alexandru,

By build, I meant a new installation, unbuntu 14.04 LTS, on an Apple iMac Intel Core i5-2500 CPU @ 2.70 GHz x2 64bit OS type.

Using Realtek 2838UHIDIR.

Interesting, so look at it this morning - pulseaudio never seems to use more than 27% and gqrx uses 100% of both cores by the graphics display.  Thanks for the tip!  I had not noticed that bit.

Pressed the [ Record ] button, twice.  [ On / Off ]  The moment the 'Audio recorder stopped' message appeared in Terminal, audio was clear for over a minute.  Then the 'UaUaUa's' resumed filling Terminal and the audio is stuttered.  CPU for gqrx was still at 100% during clear audio.

Question: What is the compiz process?  30% during clear audio.  100% during stutter.

Sigh!  So, what is the right distribution?  Missing dependencies / updated [ over / newer ] resource(s) ????

Thanks!

[Alexandru Csete]

I think compiz is the window manager, though I'm not sure. I abandoned
standard Ubuntu desktop when they introduced the unity desktop.
Instead I am using Xubuntu which is Ubuntu with XFCE desktop - a
lightweight desktop with traditional look & feel.

If you have the complete gnuradio package installed there should be a
command line tool called volk_profile installed. Running it might
improve performance of gqrx.

Alex

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[WA7RCQ]

Okay, I accept that challenge!

Hmmmm, interesting!  Over on the the much more appealing Xfce, [ much nicer GUI ], everything is different.  Gqrx now shows no more than 35% CPU.  Pulseaudio is never more than 27%.  The Task Manager shows no more than 73% total CPU usage.

Now, ( on Xfce ), activating the [ Record ] does not clear up the audio.  Terminal still populating with 'UaUaUa's.  Interesting!  Recordings play back clear as before!

Sigh!  Still stuttering audio.

Keeping Xfce.  Thanks very much!  { May just choose to remove the unbuntu unity and all dependencies altogether. }

I'll look at the volk profile tool.

Question: What are the 'yellow' highlighted processes in Task Manager.  Still looking for documentation ...

Thanks,
--
Michael

[James Lemley]

WA7RCQ de KG5CCG.  I'm experiencing the same choppy audio.  So far, the best combination I've found is Fedora 20, LXDE desktop environment, compiled gqrx from Bookmarks branch, and commented out the pulse audio backend in the gqrx project file. Audio still ends up going through Pulse via gr-audio, but the CPU use stays sane and the audio sounds pretty good for long periods (though not perfect).  

The audio problems are why I am compiling from source.  I'm profiling the program with gprof, though it really looks quite good so far - I can't figure out what is eating all the CPU. I hope to be able to offer some constructive improvements soon. Maybe tweaking the Pulse interface latency code would help.

More later I hope. 

James

[wonkoderverstaendige]

I'm having the same issue. Linux Mint 17, gqrx installed from the Trusty repositories. Audio works fine for a while, but sometimes randomly, but usually when changing the filter or mode, the audio becomes choppy and gqrx is spitting out "Ua". There is a temporary jump in CPU usage (from ~45% on both cores to ~ 70% on both cores), but this quickly settles again, while audio is messed up and can only be recovered by stopping/starting DSP processing. Recorded audio is fine.

[some one]

env PULSE_LATENCY_MSEC=30 gqrx

that command might help 

[steven jones]

same deal here, i had run into this before using dsp to decode motorbo signals in my local area, cant recall  but i think the dsp build was found on radio reference forums. which brings me to this, can new forms of demodulation be added easily?

[steven jones]

Problem isnt your distribution, nor is it compiz. works great for a minute then craps out with the ua thing.

On Sunday, June 22, 2014 at 8:16:32 AM UTC-4, didi...@googlemail.com wrote:

[jmly...@gmail.com]

Hi:

Is there any update on this problem?

I have a recent install from Ubuntu 14.04LTS package manager.

Runs clear for one or more minutes then starts fluttering. Have to kill and restart to clear the flutter.

Thanks,
James

On Sunday, June 22, 2014 at 8:16:32 AM UTC-4, didi...@googlemail.com wrote:

[jmly...@gmail.com]

Update:

Found I can toggle DSP off/on a few times and audio will clear up for awhile.

Using 1,200,000 sample rate at this time.

PC is an HP 15 AMD64 Quadcore showing 60% utilization on each core.

James

[Simon Kennedy]

I found that simply cleaning the build up of dust from my CPU heatsink solved this problem. Processors run at a lower clock rate when the power is not needed and the speed is increased on demand. My CPU would not increase the speed because it was over-heating. This meant that the CPU was not able to keep up with the sample rate it was presented with.

However, if you are using a laptop it might not be so easy to clean the dust out. I use a desktop machine so access is easy.

Simon.

[Buford T. Justice]

Make sure you are not accidentally on AM when you want Narrow FM or vice versa.

[Buford T. Justice]

Also make sure to go (for Ubuntu for example) /etc/modprobe.d as Administrator.  Create a new file called no-rtl.conf and add these 4 lines to it with a text editor...

blacklist dvb_usb_rtl28xxu
blacklist e4000
blacklist rtl2830
blacklist rtl2832

Save the no-rtl.conf file then reboot your computer.


[Andrew Douglas]

env PULSE_LATENCY_MSEC=30 gqrx

Worked for me!!!!! Using OS X host with Ubuntu 14.03 LTS guest through Virtualbox

On Sunday, June 22, 2014 at 8:16:32 AM UTC-4, didi...@googlemail.com wrote:

[Ian Board]

I have the same issue - I can receive fm broadcast stations, but the sound stutters a bit (comes and goes).  My cpu loading (total)  is about 60%, but I notice this with less demanding processing, like just selecting AM demod.  One other interesting thing I notice is the behavior of the spectrum display and the waterfall. After a few minutes, the spectrum display updates in a slightly jerky, slower way, the waterfall remains smooth and scrolling. 

The stuttering audio seems like a buffer underrun.

 Question: how is the mismatch in receive adc to audio sample rates resolved?  The adc rate in the dongle and the hardware rate in the audio dac are both fixed but they are in different clock domains and one can't be an exact multiple of the other - is resampling being used somewhere?  I have a signal processing background, but don't know much about what goes on inside gqrx or pulseaudio (yet). I'm using the rtl-sdr dongle, sample rate set to 2 Msps.

Another question: is it possible to specify a buffer size for the audio? Assuming samples aren't being lost, a FIFO somewhere would smooth out the stuttering. Is there a config option for this?

One more question: it may very well be a cpu loading question, but I'm not sure what constitutes a 'heavy' utilization. It is running about 60% for wbfm on my thinkpad.

I am running this on ubuntu 14.04, gnuradio built using the build-gnuradio script and gqrx-sdr (2.4) built from source. I ran the volk-profile to try to leverage the vector processing as much as possible.  It seems very stable and all features work, except for the sometimes stuttering audio. I was just using the normal analog headphone out on my laptop.

As a side note: it really didn't seem that hard to build - some minor editing of the build-gnuradio script, bringing libusb up to date (downloading the latest and building) but it was one of the longer builds I have done - wow.

 I just started with gqrx/gnuradio a few days ago so I'm not panicking yet. 

- Ian

[Alexandru Csete]

On Thu, Dec 17, 2015 at 8:06 AM, Ian Board <ian....@gmail.com> wrote:
> I have the same issue - I can receive fm broadcast stations, but the sound
> stutters a bit (comes and goes). My cpu loading (total) is about 60%, but
> I notice this with less demanding processing, like just selecting AM demod.
> One other interesting thing I notice is the behavior of the spectrum display
> and the waterfall. After a few minutes, the spectrum display updates in a
> slightly jerky, slower way, the waterfall remains smooth and scrolling.

The jerky FFT is a different issue, I think. It is observed when using
rtl-sdr dongles at 250 kHz sample rate, so I am surprised that you see
it at higher rates.

The reason for this behavior is that the driver uses large transfer
buffers, which at low sample rates will mean longer time between
updates. Since the FFT runs using its own clock without using any
dedicated buffering to workaround such issues, it will often not have
any new data and will just replot the previous data.

You can add buflen=16384 to the device string to reduce the transfer
sizes and this will give smooth FFT at 250 ksps sample rate. Other
values may also work but buffer length must be multiple of 512 and
preferably multiple of 16384 (URB size):
http://sdr.osmocom.org/trac/wiki/GrOsmoSDR#RTL-SDRSource

But I am surprised that you should have this issue at 2.4 Msps sample rate.

Alex

[Ian Board]

Thanks, what you say makes sense. I'll play around with it. BTW - I really like the program.

Thanks,

Ian

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