DESCRIPTION

The loopback audio driver makes it possible to create special purpose virtual audio devices based on user land server processes.

Loopback devices are driven by a timer interrupt and no real audio hardware is required.

INTRODUCTION

Audio loopback devices are like named pipes or pseudo terminals. They are grouped in client and server device pairs. The server side device must be open before the client side device can be opened.

Loopback devices are typically used to implement server based special purpose audio devices. This kind of server can for example transfer the audio data played by the client application to some remote system using some VoIP protocol. However the server application doesn't need to be any dedicated server. Practically any audio application can be used as the server.

SERVER SIDE DEVICE

The server side applications sets up the native sampling rate and sample format (number of bits and channels). The server side device can be opened for input (O_RDONLY) pr output (O_WRONLY). Opening for simultaneous input and output (O_RDWR) is not permitted.

The server application will automatically be paused at the moment it tries to read or write audio data for the first time. It will be kept in sleep until the client side application starts writing or reading data. This sleep period may last forever and in some cases the first write/read call never returns. For this reason it's not recommended to use GUI based audio applications as the server. Note that this wait will occur even in the non-blocking (O_NONBLOCK) mode (this is intentional feature and not a bug).

CLIENT SIDE DEVICE

The client side device is typically used by any ordinary audio application. There is nothing special in loopback devices.

Since the loop is unidirectional the client side will be forced to be write only if the server side device is open for recording and vice versa.

The loop will use the sample rate and sample format (number of bits and channels) set by the server side application. If the client uses different settings then OSS will perform the required sampling rate and format conversions automatically.

COMPATIBILITY ISSUES

Audio loopback devices differ from "normal" audio devices because an application is needed at the both ends of the loop. The loop device will return a "Connection reset by peer" error (ECONNRESET) error. Applications designed to be used as loopback based server applications can/should use this error (returned by read or write) as an end-of-stream indication.

OPTIONS

o audioloop_instances:

Specifies how many loopback client/server audio device pairs to be created.

Values: 1-16

Default: 1

KNOWN PROBLEMS

• There is no mixer (volume control) related with loopback audio devices. This may prevent poorly designed audio applications (that expect/require a mixer) from working. There is no workaround available.
• The server side application will wait until the client side application starts using it. This wait may last forever which in turn may cause unrecoverable (network) problems with some applications.
• Loopback devices may return "Connection reset by peer" error when the reote side of the loop disconnects the device. Some recording applications may fail to save the recorded data properly because of this. Use some other application (such as ossrecord) if this happens.

FILES

/etc/oss4/conf/oss_audioloop.conf Device configuration file

AUTHOR

4Front Technologies