[pulseaudio-discuss] Pulseaudio-equalizer module crackling with libao

[Sean Greenslade]

Hi, folks. I've been trying to track down the source of a rather odd
problem I'm having. I've been using the pulseaudio-equalizer module, but
it seems to have some issues when using it via libao.

My hardware: 
- Asus eeePC 1015PEM
- Intel Corporation NM10/ICH7 Family High Definition Audio Controller
(on motherboard)
- FiiO E10 USB DAC

My software:
- Linux wheatley 3.14.1-1-ARCH #1 SMP PREEMPT Mon Apr 14 20:40:47 CEST
2014 x86_64 GNU/Linux <ArchLinux>
- pulseaudio 5.0
- libao 1.2.0
- alsa-lib 1.0.27.2

To enable the EQ, I've added the following to my default.pa:
load-module module-dbus-protocol
load-module module-equalizer-sink

The issue: When running [libao_sound_source] -> equalizer -> Audio card,
the audio becomes sparse crackles. When running the same
[libao_sound_source] -> Audio card, the audio comes through unmolested.

But the equalizer is not broken. [S]Mplayer can feed its audio through
the equalizer no problem, as can mpg123. 

Here is where things get a bit...bizarre, though. After playing with
libao's settings, I discovered that if I set buffer_time to any value
between 49 and 97 (inclusive), the audio will play through correctly.
Any other value causes the sparse static. But(!) there is another issue.
When playing pianobar (which uses libao), the audio buffers in a
"caterpiller style." It will immediately dump ~20-30 seconds of audio
into the equalizer sink and block, wait until the buffer underruns, then
dump another ~20-30 seconds of audio. 

I realize that this may be an issue with libao (or something else
entirely), but I'm not knowledgeable enough to dig into the
codebases myself. If it's not a pulse issue, please point me in the
right direction.

Thanks,

--Sean