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Open Sound System OSS 4.0 User's Guide

softoss - Virtual Mixer audio driver.

Open Sound System provides an audio mixer device to mix multiple audio streams being played to the same audio device.

INTRODUCTION

Virtual Mixer is a feature of Open Sound System to overcome the problem of applications making exclusive write access to audio devices. The Virtual Mixer allows multiple concurrent audio applications to run and the output from each application is mixed and sample rate converted to the highest sampling rate of the soundcard hardware (typically 48Khz/stereo/16bit).

The normal Virtual Mixer mode is enabled by default and you don't need to do anything to enable it. It can be disabled by using the "Configuration options (options.cfg)" editor of soundconf command and by setting the value of softoss_disable to 1.

To use the pro mode you should first enable it by configuring the virtual mixer driver. This can be done by adding the "4Front Virtual Mixer Pro" device using the "Add new card/device" function of soundconf.

By default only 8 virtual devices will be created. If you need more you need to edit the softoss.conf (see below) and set the value of the softoss_devices option. Upto 32 devices can be configured.

After the virtual mixer driver is installed the additional audio devices will be shown as below by ossinfo -a.

Audio devices:

0: Creative AudioPCI (DUPLEX)
1: Creative AudioPCI (playback only)
2: SoftOSS v2.5 CH #0
3: SoftOSS v2.5 CH #1
4: SoftOSS v2.5 CH #2
5: SoftOSS v2.5 CH #3
6: SoftOSS v2.5 CH #4
7: SoftOSS v2.5 CH #5
8: SoftOSS v2.5 CH #6
9: SoftOSS v2.5 CH #7

In the above case the first virtual mixer device is /dev/dsp2 (/dev/audio2) and the last one is /dev/dsp9 (/dev/audio9). You can have up to 8 of these devices active at the same time. Note that the "real" audio device (usually /dev/dsp0) will not be available while any of the virtual mixer devices are open. Also the virtual mixer devices cannot be used while the actual hardware device (/dev/dsp0) is open.

IMPROVING SOUND QUALITY

The method used for sample rate conversion by the virtual mixer is optimized for CPU speed. The drawback is that sound quality is not perfect (should be enough for most users).

There is now an alternative sample rate conversion filter available. This converter provides quality that cannot be outperformed. It consumes more CPU power which means it's not suitable to be used with slower than Pentium III or AMD K7 machines. In fast machines the additional CPU overhead caused by the filter is about 1-3% (for each simultaneously active virtual device). In the current version of OSS this algorithm is enabled by default. To disable this feature you need to add a line containing softoss_use_src=0 to the options.cfg file.

FEATURES

1. Setting the number of Virtual Mixer devices You can now configure the number of Virtual Mixer devices by running soundconf, select Configuration Options and then select the option "softoss_devices" and type in the number of devices between 4 and 48. Save the configuration and run soundon.
2. Volume Control: You now have the ability to control the individual volumes for each of the Virtual Mixer channels. To do this you must use the ossxmix program located in the OSS installation directory. Find out the Virtual Mixer's mixer device number by typing ossinfo -a and then type ossxmix -d# (for example, if the Virtual Mixer's mixer device is device number 1 then type ossxmix -d1)
3. Equalizer: Virtual Mixer also provides you with a Professional Grade 4 band equalizer. By default this equalizer is disabled. To enable th equalizer, uncheck the "Bypass" button and now moving the equalizer sliders will enable you to change the sound output.
4. Loopback Recording: Virtual Mixer now enables you to record any audio that's played on the playback channel. The recording is completely sample rate controlled which means that even if you are playing audio at a given sample rate, bits per sample and stere/mono, you can record that same audio in a completely different speed/bits/channels. For example, you can play a mp3 (44.1Khz/16bit/Stereo) on SoftOSS i playback channel 0 and record that same mp3 at 22Khz 8bit mono. The loopback recording mode in the Virtual Mixer can be enabled by running soundconf->select Configuration Options->select softoss_loopdevs and type in a number between 1 and 48 in the box. Save the configuration and exit and now run soundon.
5. Setting the Sampling Rate: By default Virtual Mixer uses the maximum sampling rate of the soundcard. For instance, the Sound Blaster AWE64 only supports 44.1Khz while the Sound Blaster PCI128 supports 48Khz. Virtual Mixer will do sample rate conversion against the maximum sampling rate of the device. In some instances you may want to force a lower sampling rate like 44.1Khz on a sound card that does 48Khz (typically if you are processing CD audio in CDDA mode which will create PCM audio at 44.1Khz. In this case you wouldn't want the audio to be up sampled to 48Khz). To change the sampling rate of the Virtual Mixer, run soundconf, select Configuration Options menu, select "softoss_rate" and then enter the sampling rate desired. The legal values are 11025, 22050, 32000, 44100 and 48000.
6. Open redirect: If an application tries to open the "physical" device used by SoftOSS the open will be redirected to the virtual devices. This makes sharing the device between multiple applications very easy.
7. Effects: The SoftOSS virtual mixer provides the following audio effects that apply to any audio playing via the Virtual Mixer:

• Reverb: This feature adds simple reverb effects to audio.
• Fidelity Enhance: This feature adds "fidelity" or "brightness" to audio like MP3 and Ogg. This feature is also known as "exciters" in audio terminology.
• 3D Audio: This feature spreads the stereo field giving the listner the impression that the audio is coming from speakers in front and side of the listners ears.

CPU USAGE

The virtual mixer driver consumes some additional CPU time when compared to a "real" hardware device. This extra load is barely measurable on fast systems.

However the load may be very significant on older (486) systems. When the load on a fast 1.5GHz PII/P4/K7 system is just few percents it can be 50% or more on slow (486 and oldest Pentium) CPUs.

On a PIII/686 system the "iddle" load when none of the virtual devices is playing audio is about 5% of CPU time. The load gets increased by 1.5% for each active (playing) virtual device. This measurement was made when the master device was running at 48 kHz and each of the virtual devices were outputting 44.1 kHz stream. Low quality SRC (Sample Rate Conversion) algorithm was used (softoss_use_src option was set to 0).

When the sampling rate of the virtual stream equals to the sampling rate of the master device the load increase caused by an active virtual device was just 0.3%.

Using the professional quality SRC algorithm (GRC3) the load increase caused by each active virtual channel is just slightly larger than with the low quality SRC method. The measured difference (PII/866) was about 0.3%.

MINIMIZING CPU USAGE

Minimizing the CPU usage of the virtual mixer driver may be necessary when using slower CPUs (such as most processors designed for embedded systems). There are several ways to do this:

1. Decrease the sampling rate used by the by the master device by setting the softoss_rate option in options.cfg. The decrease in the CPU usage is proportional with the decrease in the sampling rate. In ideal case use the same sampling rate as majority of the sounds to be played.
2. Whenever possible use the same sampling rate in the virtual devices as the one used by the master device (softoss_rate option). When the sampling rates match the time consuming sample rate conversion step can be skipped. This decreases the CPU load associated with an active virtual device to 1/5th of the case where the sampling rates are different.
3. In some situations using the lower quality SRC algorithm may help. This can be done by setting the softoss_use_src option to 0 in options.cfg. However in some cases using this method may actually make things worse since the low auality SRC code is executed in the interrupt context.

Notice that the virtual mixer driver will use some CPU time always when any of the virtual devices are open even none of them is actually outputting anything. This "base load" equals to the CPU time taken by 3 active virtual channels (when SRC is used).

OPTIONS

• softoss_rate: Specifies the sampling rate for Virtual Mixer - normally its set to the highest frequency of the
  soundcard eg 48000. However if you want Virtual Mixer
  to use 44.1Khz then specify 44100.
  Values: 5000-96000 Default: 48000
  

• softoss_masterdev: This tells Virtual Mixer which physical device to use. Type cat ossinfo -a and locate the physical devices
  from the list and then set softoss_masterdev to one.
  Normally this parameter defaults to the first playback
  only or Duplex device (if no playback only device is
  available)
  Values: 1-number of physical devices, Default: 0
  

• softoss_channels: Specifies how many interleaved channels to use. Values: 1,2,4,6 Default: 2 (stereo)
  

• softoss_devices: Specifies how many /dev/dsps to configure. Values: 1-32 Default: 8
  

• softoss_loopdevs: Specifies how many loopback recording devices to configure in Virtual Mixer PRO.
  Values: 1-4 Default: 0
  

• softoss_redirect: Specifies if you want OSS to automatically set /dev/dsp to open a Virtual Mixer channel.
  Values: 1 or 0 Default: 1
  

• softoss_disable: Specifies if you want to disable Virtual Mixer and OSS will not load the virtual mixer devices.
  Values: 1 or 0 Default 0
  

• softoss_use_src: Turns on the 4Front GRC Professional sample rate convertors. If disabled, Virtual Mixer defaults to a
  linear interpolated SRC.
  Values: 1 or 0 Default: 1
  

• softoss_fragment: Specifies the DSP_SET_FRAGMENT setting. Some apps require small fragments for better performance.
  Values: 1-15 Default: 7
  

FILES

softoss.conf Device configuration file

AUTHOR

4Front Technologies