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101 lines
4.6 KiB
Markdown
101 lines
4.6 KiB
Markdown
# Playback tutorial 9: Digital audio pass-through
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## Goal
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This tutorial shows how GStreamer handles digital audio pass-through.
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## Introduction
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Besides the common analog format, high-end audio systems usually also
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accept data in digital form, either compressed or uncompressed. This is
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convenient because the audio signal then travels from the computer to
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the speakers in a form that is more resilient to interference and noise,
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resulting higher quality.
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The connection is typically made through an
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[S/PDIF](http://en.wikipedia.org/wiki/SPDIF) cable which can either be
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optical (with [TOSLINK](http://en.wikipedia.org/wiki/TOSLINK)
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connectors) or coaxial (with [RCA](http://en.wikipedia.org/wiki/RCA)
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connectors). S/PDIF is also known as IEC 60958 type II (IEC 958 before
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1998).
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In this scenario, GStreamer does not need to perform audio decoding; it
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can simply output the encoded data, acting in *pass-through* mode, and
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let the external audio system perform the decoding.
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## Inner workings of GStreamer audio sinks
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First off, digital audio output must be enabled at the system level. The
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method to achieve this depend on the operating system, but it generally
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involves going to the audio control panel and activating a checkbox
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reading “Digital Audio Output” or similar.
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The main GStreamer audio sinks for each platform, Pulse Audio
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(`pulsesink`) for Linux, `osxaudiosink` for OS X and Direct Sound
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(`directsoundsink`) for Windows, detect when digital audio output is
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available and change their input caps accordingly to accept encoded
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data. For example, these elements typically accept `audio/x-raw` data:
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when digital audio output is enabled in the system, they may also
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accept `audio/mpeg`, `audio/x-ac3`, `audio/x-eac3` or `audio/x-dts`.
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Then, when `playbin` builds the decoding pipeline, it realizes that the
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audio sink can be directly connected to the encoded data (typically
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coming out of a demuxer), so there is no need for a decoder. This
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process is automatic and does not need any action from the application.
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On Linux, there exist other audio sinks, like Alsa (`alsasink`) which
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work differently (a “digital device” needs to be manually selected
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through the `device` property of the sink). Pulse Audio, though, is the
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commonly preferred audio sink on Linux.
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## Precautions with digital formats
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When Digital Audio Output is enabled at the system level, the GStreamer
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audio sinks automatically expose all possible digital audio caps,
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regardless of whether the actual audio decoder at the end of the S/PDIF
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cable is able to decode all those formats. This is so because there is
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no mechanism to query an external audio decoder which formats are
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supported, and, in fact, the cable can even be disconnected during this
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process.
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For example, after enabling Digital Audio Output in the system’s Control
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Panel, `directsoundsink` will automatically expose `audio/x-ac3`,
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`audio/x-eac3` and `audio/x-dts` caps in addition to `audio/x-raw`.
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However, one particular external decoder might only understand raw
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integer streams and would try to play the compressed data as such (a
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painful experience for your ears, rest assured).
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Solving this issue requires user intervention, since only the user knows
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the formats supported by the external decoder.
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On some systems, the simplest solution is to inform the operating system
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of the formats that the external audio decoder can accept. In this way,
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the GStreamer audio sinks will only offer these formats. The acceptable
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audio formats are commonly selected from the operating system’s audio
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configuration panel, from the same place where Digital Audio Output is
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enabled, but, unfortunately, this option is not available in all audio
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drivers.
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Another solution involves, using a custom sinkbin (see
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[](tutorials/playback/custom-playbin-sinks.md)) which includes a
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`capsfilter` element (see [](tutorials/basic/handy-elements.md))
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and an audio sink. The caps that the external decoder supports are
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then set in the capsfiler so the wrong format is not output. This
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allows the application to enforce the appropriate format instead of
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relying on the user to have the system correctly configured. Still
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requires user intervention, but can be used regardless of the options
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the audio driver offers.
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Please do not use `autoaudiosink` as the audio sink, as it currently
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only supports raw audio, and will ignore any compressed format.
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## Conclusion
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This tutorial has shown a bit of how GStreamer deals with digital audio.
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In particular, it has shown that:
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- Applications using `playbin` do not need to do anything special to
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enable digital audio output: it is managed from the audio control
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panel of the operating system.
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It has been a pleasure having you here, and see you soon!
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