taint.org: Justin Mason’s Weblog

I’ve just spent a rather frustrating morning attempting to debug major performance problems with my home wireless network; one of my machines couldn’t associate with the AP at all anymore, and the laptop (which was upstairs in the home office, for a change) was getting horrific, sub-dialup speeds.

I did lots of moving of Linksys APs and tweaking of “txpower” settings, without much in the way of results. Cue tearing hair out etc.

Eventually, I logged into the OpenWRT AP over SSH, ran iftop to see what clients were using the wifi, and saw that right at the top, chewing up all the available bandwidth, was a multicast group called 224.0.0.56. The culprit! There was nothing wrong with the wifi setup after all — the problem was massive bandwidth consumption, crowding out all other traffic.

You see, “pulseaudio”, the new Linux sound server, has a very nifty feature — streaming of music to any number of listeners, over RTP. This is great. What’s not so great is that this seems to have magically turned itself on, and was broadcasting UDP traffic over multicast on my wifi network, which didn’t have enough bandwidth to host it.

Here’s how to turn this off without killing “pulseaudio”. Start “paman”, the PulseAudio Manager, and open the “Devices” tab:

(click on the image to view separately, if it’s partly obscured.)

Select the “RTP Monitor Stream” in the “Sources” list, and open “Properties”:

Hit the “Kill” button, and your network is back to normal. Phew.

Another (quicker) way to do this, is using the command-line “pacmd” tool:

echo kill-source-output 0 | pacmd

It’s a mystery where this is coming from, btw. Here’s what “paman” says it came from:

But I don’t seem to have an active ‘module-rtp-send’ line in my configuration:

: jm 98...; grep module-rtp-send /etc/pulse/* /home/jm/.pulse*
/etc/pulse/default.pa:#load-module module-rtp-send source=rtp.monitor

Curious. And irritating.

Tags: multicast, pulseaudio, rtp, wifi, wtf

For a while now, I’ve been using various hacks to play music from my Linux laptop, holding my main music collection, to client systems which drive the speakers.

Previously, I used this setup to play via my MythTV box. Nowadays, however, my TV isn’t in the room where I want to listen to music. Instead, I have my Nokia 770 hooked up to the speakers; this plays the BBC Radio 4 RealAudio streams nicely, and also the laptop’s MP3 collection using a uPnP AV MediaServer.

I specifically use TwonkyMedia right now, playing back via the N770’s Media Streamer app. (That works pretty well – uPnP AV is one of those standards plagued with incompatibilities, but TwonkyMedia and Media Streamer seem to be a reliable combination.)

However, TwonkyMedia sometimes fails to notice updates of the library, and nothing has quite as good a music-player user interface as JuK, the KDE music player and organiser app, so a way to play directly from the laptop instead of via uPnP would be nice…

A weekend’s hacking reveals that this is pretty easily done nowadays, thanks to some cool features in pulseaudio, the current standard sound server on Ubuntu gutsy, and the Esound server running on the N770.

Unfortunately, the N770 doesn’t (yet) support pulseaudio directly, otherwise we could use its seriously cool support for RTP multicast streams. Still, we can hack something up using the venerable “esd” protocol (again!) Here’s how to set it up…

On the N770:

You need to fix the N770’s “esd” sound server to allow public connections. Set up your wifi network’s DHCP server to give the N770 a static IP address. Log in over SSH, or fire up an xterm. Run the following:

mv /usr/bin/esd /usr/bin/esd.real

cat > /usr/bin/esd <<EOM
#!/bin/sh
exec /usr/bin/esd.real -tcp -public -promiscuous -port 5678 $*
EOM

chmod 755 /usr/bin/esd
/etc/init.d/esd restart

On the server:

Download this file, and save it as n770.pa. Edit it, and change server=n770:5678 on the fourth line to use the IP address or hostname of your Nokia 770 instead of n770. Then run:

cp n770.pa ~/.n770.pa

cat > ~/bin/sound_n770 <<EOM
#!/bin/sh
pulseaudio -k; pulseaudio -nF $HOME/.n770.pa &
EOM

cat > ~/bin/sound_here <<EOM
#!/bin/sh
pulseaudio -k; pulseaudio &
EOM

chmod 755 ~/bin/sound_here ~/bin/sound_n770

Now you just need to run ‘~/bin/sound_n770′ to redirect sound playback to the N770, and ‘~/bin/sound_here’ to reset back to laptop speaker output, for the entire desktop environment. Nifty!

Update: it appears that things may work more reliably if you add “rate=22050″ at the end of the “load-module module-esound-sink” line — this halves the bitrate of the network stream, which copes better with harsh wifi network conditions. The n770.pa file above now includes this.

Tags: esd, hacks, linux, n770, nokia, pulseaudio, sound