# Occasional choppiness/underruns (aUaU) with FCDPP-\>GRC-\>Pulse

**URL:** <https://www.ruby-forum.com/t/occasional-choppiness-underruns-auau-with-fcdpp-grc-pulse/240297>\
**Category:** GNU Radio\
**Created:** [April 7, 2014, 1:31pm UTC](https://www.ruby-forum.com/t/occasional-choppiness-underruns-auau-with-fcdpp-grc-pulse/240297 "2014-04-07T13:31:55Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [April 7, 2014, 1:31pm UTC](https://www.ruby-forum.com/t/occasional-choppiness-underruns-auau-with-fcdpp-grc-pulse/240297/1 "2014-04-07T13:31:55Z")

</div>

Chuck,

In most cases the problem comes from ALSA. If you’re able to change ALSA  
to  
something else, try to change it.

If you can’t change your default sound driver then you may try using  
“-Dplug:default” option for aplay. If you need to use this option with a  
Python script you may use it like " -c plug:default".

This was my only solution when I tried running rtl\_fm on Raspberry  
because  
default audio driver of Raspberry is ALSA and not possible to change  
(everything else like Jack, Pulseaudio etc. can’t be a solution because  
all  
of them run over ALSA, ALSA always remains at the bottom, so the problem  
always remain).

I hope this helps.

Regards,

Murat, TA1DB

From: Chuck R. [via GnuRadio]  
[[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)]

Sent: Sunday, April 06, 2014 4:10 AM  
To: Murat TA1DB  
Subject: Occasional choppiness/underruns (aUaU) with FCDPP-\>GRC-\>Pulse

Hello list, I need some help.

When running GRC 3.6.1 I am getting a problem with occasional choppiness  
in  
my audio output. Details:

- The choppiness sounds like only part of the buffer is filling, similar  
to  
the sound you get when a filmstrip’s playback gets unstable and you get  
that  
‘jitter’

- The issue usually does not immediately occur however occurs a few  
minutes  
into use.

- Sometimes computer activity such as un-minimizing a window will  
trigger  
the problem. Sometimes the same will fix it.

- Input is Funcube Dongle Pro+ 192khz on alsa hw:3

- Occurs regardless of using FCDPP+ Block or the Audio Input block.

- Output is pulse, originally set to 192khz (pulse resamples it),  
brought it  
down to native 48khz and used Rational Resampler block to bring the 192  
to

1. Did not resolve, however the issue occurred less often. (still  
intolerable)

- Turned on Realtime Scheduling and then adapted the JACK instructions  
for  
enabling system realtime scheduling permissions  
([http://jackaudio.org/linux\_rt\_config](http://jackaudio.org/linux_rt_config)). Afterward confirmed the python  
process to have a nice of -30 which is realtime. Issue did not resolve  
but  
again, it happened less often. (still intolerable)

- Occurs regardless of graph complexity. Simple in-\>out arrangement will  
reproduce issue.

- Sometimes you can hear it get worse as time goes by, and sometimes it  
will  
just as randomly fix itself, only to occur again later.

- I am using pulse as my audio output.

- When the issue occurs, the console output in GRC fills rapidly with  
“aUaU”

- When the issue occurs, looking into pavucontrol playback tab shows  
“ALSA  
plug-in [python 2.7]” entry flickering rapidly, as if it is resetting  
its  
connection over and over in an unreasonable manner.

- When the issue occurs, the realtime python process on core 2 shows no  
change in core usage, however the non-realtime gnuradio-companion  
process,  
also on core 2, spikes from its usual 0.9% usage to 93%. The only way I  
can  
see a high-priority process being pushed out by a low-priority process  
is if  
the high-priority process is waiting on the low-priority process for  
some  
operation to complete (this is a DSP no-no).

- When the issue occurs, pulseaudio’s verbose log fills rapidly.  
attached is  
a snippet of what repeatedly fills the log.

- There is chatter about similar underrun issues with mozilla and pulse.  
However the fixes appear to be occurring in mozilla only so there wasn’t  
much help from that.  
([779392 - Improve the ALSA PulseAudio bug workaround used to fix bug 761274](https://bugzilla.mozilla.org/show_bug.cgi?id=779392))

- When attempting to specify ‘pulse’ as signal input in GRC and then  
specify  
FCDPP as its input in pavucontrol, GRC gives silence and a stream of  
“aU”

- pulseaudio 4.0

- Xubuntu 13.10 AMD64.

There was a similar issue posted earlier however it was different in  
nature  
in the sense that the underruns occurred immediately instead of on  
occasion,  
implying a consistent sample rate mismatch.  
[https://lists.gnu.org/archive/html/discuss-gnuradio/2013-08/msg00516.html](https://lists.gnu.org/archive/html/discuss-gnuradio/2013-08/msg00516.html)

It is reasonable to rule out sample-rate mismatch in my case as the  
issue  
occurs only on occasion and sometimes fixes on its own, a mismatch would  
be  
expected to immediately fail. It would be mathematically impossible for  
a  
mismatched sample rate to cause such cascading underruns after minutes  
of  
proper functioning except for clock drift, which would be expected to be  
solved with a single buffer reset (a slight “blip”). This is not what is  
happening. It is thousands of consecutive blips after a while of proper  
functioning.

Thanks for reading. Log dump attached.

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