Skype for Linux

This is the page to share my findings regarding to Skype for Linux.

Skype 1.3
Skype 1.2

Skype 1.3

The BETA version of Skype 1.3 has been released. Has ALSA support.

Skype for Linux 1.3 BETA (ChangeLog)

Skype for Linux 1.3.0.30 BETA — first impressions

The most anticipated feature — ALSA support is finally in. Unfortunately it is not yet fully functional. If you have single sound card - there should be no major problems (except loosing sound...). Default configuration should just work.

In more advanced config with more than one sound device, I found out that choosing from dropdown list any other sound device then first one makes Skype deaf and mute.

Moreover Skype does not support selection of virtual sound devices. Selection box features only basic hardware devices, so you cannot easily benefit from ALSA flexibility, where you can define own device, that consist of speakers of one sound card and microphone of another (eg. USB webcam).

Fortunately this is up to some degree workaroundable. Current implementation seems to always choose default ALSA configuration (at least when first device is selected in dropdown list). And you can redefine what default device is. The downside is that other applications will use that device as a default as well. However many of them (eg. mplayer, xmms, xine) gives the way to specify what device should be used.

See the ALSA configuration hints for Skype for some examples. If you configure that right skype_dsp_hijacker shouldn't be necessary anymore. But if you wish to continue using OSS drivers, you can continue using it with no harm. Moreover hijacker has now a workaround so you may leave default ALSA device unchanged and use custom one (eg. named 'skype) with Skype.

Problems in Skype 1.3 BETA

Skype for Linux 1.3.0.30

Not fully functional ALSA configuration (device selection does not work, no virtual device selection) →skype_dsp_hijacker v0.8
Still no support for selecting device for ringing
Loosing sound problem (no sound in headphones after few seconds of connection) →forum
Do you have problems with DTMF driven services (banking, passwords, etc.)? After a year of troubles (lost digits, double digits etc), and different tries, I found a solution that more-or-less worked for me recently. If others have these problem too, I'm willing to describe it here.

Skype 1.2

Sound problems

Skype is using obsolete OSS sound system and this fact has several implications. One is that with most sound cards and configurations allows only one application to access sound device in a time. Another is not directly OSS fault, but rather how the Skype is implemented. Skype setup doesn't give much flexibility in configuring sound. In fact you can setup only one OSS device (/dev/dsp*) that will be used for all purposes: speakers, microphone, ringing. It sounds reasonable to allow user to select different devices, eg. earphones for listening, USB (camera) microphone for recording, and loudspeakers for ringing. This is doable with OSS, just there is no such option in Skype.

Separate problem is a bug introduced in Skype for Linux 1.2.0.11 and not fixed in 1.2.0.17, 1.2.0.18 or 1.2.0.21, that makes Skype unusable after first call. Sometimes a few calls can be made, before it happens, but later only solution is to restart Skype. Technically it seems that Skype "forgets" to close sound device, before trying to open it again and locks up. Messages given on the screen & on the console aren't very helpful: Problem with sound device., or /dev/dsp1: Device or resource busy.

Solution

Real solution would be to fix the bug, implement ALSA support, and add more detailed sound configuration to Skype. Unfortunately due a closed source development model, only Skype employees are able to do the work.

Workarounds

OSS support only
Lack of support modern sound architecture (ALSA) can be (up to some extend) replaced with ALSA OSS emulation.
This is common configuration and in most cases there is no need to do anything. In fact there are two kinds of ALSA OSS emulations. One is done in Linux kernel, and compatibility /dev/dsp device is provided for each ALSA sound card, another is done via special emulation library (libaoss), and to use it you need to prepend application call with aoss wrapper (eg. call aoss skype instead of just skype). In the later case you need to properly configure the wrapper (/etc/asound.conf).
Multiple applications to use sound card
Some sound cards can do hardware mixing, and the device is not opened in exclusive mode.
Also using libaoss ALSA OSS emulation with software mixing (dmix) configured will help. Unfortunately Skype crashes whenever I try to use it via aoss wrapper.
Another device for speakers, another for microphone
This can be done with proper aoss (ALSA OSS emulation via libaoss library) configuration. You can reroute recording to a different ALSA device.
Last resort is using dirty hack to reroute sound streams do different OSS devices by hijacking them. This method can work with ALSA OSS kernel emulation, and can be done with use of skype_dsp_hijacker.
Sound device blocked after first call bug workaround #1
Find Skype's sound directory and delete or rename hangup.wav file. On Fedora it would be:

cd /usr/share/skype/sound
mv hangup.wav hangup.wav-disabled
Sound device blocked after first call bug workaround #2
skype_dsp_hijacker from version 0.5 implements workaround to catch situation that leads to lockup, and workaround it. See below.
Sound device blocked after first call bug workaround #3
Mandriva users can use soundwrapper skype command. (based on information from forum.mandriva.pl; not tested by me)

skype_dsp_hijacker

skype_dsp_hijacker is GPL licensed wrapper wrote by snow-x, Jan Albin & others.
Functions:

use of separate devices for sound input and output
workaround device open bug in Skype 1.2.0.11, Skype 1.2.0.17, Skype 1.2.0.18 & Skype 1.2.0.21

How does hijacker it work

skype_dsp_hijacker is loaded before Skype itself, and override open/close/read/write operations. Depending on configuration it can reroute reads and/or writes that comes from Skype to other device than Skype intended.

For bug introduced in Skype 1.2.0.11, wrapper detects the situation when Skype tries to open the device it didn't close before, and close it.

Technically skype_dsp_hijacker is shared library that is preloaded (LD_PRELOAD) before Skype is executed, and the script that wraps it with some command line interface. skype_dsp_hijacker is distributed as a C source code and needs to be compiled.

Skype 1.3 with ALSA update: Since new Skype 1.3 BETA has ALSA support, using OSS is not recommended anymore. Also bug introduced in 1.2.0.11 is not longer a problem. The new problem is that Skype always use default ALSA device. skype_dsp_hijacker v0.8 can workaround that too. Instead of 'default' will use ALSA device named 'skype' (see configuration hints) or any other you configure it to use.
Technically it would be possible to use separate ALSA device for input & output, but there is no need to, since this can be achieved through ALSA configuration as well.

To use skype_dsp_hijacker with ALSA support, compile it with make CFLAGS=-DHAVE_ALSA. Make sure that you have ALSA device named 'skype' configured.

Downloads

ChangeLog
skype_dsp_hijacker-0.8.tar.gz (ALSA device swapping)
skype_dsp_hijacker-0.7.tar.gz (check if sound device is truly busy, debug output to stderr so URLs opens, compilation/installation updates)
skype_dsp_hijacker-0.6.tar.gz (fix for sound quality loss after device open workaround)
skype_dsp_hijacker-0.5.tar.gz (source updates, updated run script and device open bug workaround)

Hints

AMD64 compilation

Resources

Home page of skype_dsp_hijacker
initial version: http://suedpol.dyndns.org/skype/ seems to be off-line
Skype for Linux forums, Sound device blocked after first call topic
ALSA OSS emulation(s)

Usage

Skype 1.2.0.11 bug workaround only
$ ./skype_dsp_hijacker
or
$ MIXERDEV=/dev/mixer MICDEV=/dev/dsp SPEAKERDEV=/dev/dsp LD_PRELOAD=./libskype_dsp_hijacker.so:/lib/libdl.so.2 skype
Use second sound card
$ ./skype_dsp_hijacker --2nd
or
$ MIXERDEV=/dev/mixer1 MICDEV=/dev/dsp1 SPEAKERDEV=/dev/dsp1 LD_PRELOAD=./libskype_dsp_hijacker.so:/lib/libdl.so.2 skype
Use microphone from the first sound card and speakers connected to the second (eg. USB ones)
$ ./skype_dsp_hijacker --speakers2nd
or
$ MIXERDEV=/dev/mixer1 MICDEV=/dev/dsp SPEAKERDEV=/dev/dsp1 LD_PRELOAD=./libskype_dsp_hijacker.so:/lib/libdl.so.2 skype
See also:
$ ./skype_dsp_hijacker --help
and edit skype_dsp_hijacker to customize it even more

Debian dependency problem

Q: how to fix the problem with the libqt3c102-mt lib?
A: Download the fedora rpm and "alien" it:

sudo alien skype-versionblabla.rpm
sudo dpkg -i skype_version_blabla.deb

Credits to Rattlesnake at Skype Forum for this workaround.

Slow startup

In some configurations Skype takes way too long to start, and slows down, or even freeze other applications by using 100% of CPU. This happens even on systems that are much more powerful then the system requirements.

Known workaround is to start Skype with --disable-dbus option.

skype --disable-dbus

→ Go back to Linux resources page

2006.07.04, Jan Słupski, email: jslupski at juljas dot net