mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-18 14:26:43 +00:00
101 lines
4.6 KiB
Markdown
101 lines
4.6 KiB
Markdown
# Playback tutorial 9: Digital audio pass-through
|
||
|
||
## Goal
|
||
|
||
This tutorial shows how GStreamer handles digital audio pass-through.
|
||
|
||
## Introduction
|
||
|
||
Besides the common analog format, high-end audio systems usually also
|
||
accept data in digital form, either compressed or uncompressed. This is
|
||
convenient because the audio signal then travels from the computer to
|
||
the speakers in a form that is more resilient to interference and noise,
|
||
resulting higher quality.
|
||
|
||
The connection is typically made through an
|
||
[S/PDIF](http://en.wikipedia.org/wiki/SPDIF) cable which can either be
|
||
optical (with [TOSLINK](http://en.wikipedia.org/wiki/TOSLINK)
|
||
connectors) or coaxial (with [RCA](http://en.wikipedia.org/wiki/RCA)
|
||
connectors). S/PDIF is also known as IEC 60958 type II (IEC 958 before
|
||
1998).
|
||
|
||
In this scenario, GStreamer does not need to perform audio decoding; it
|
||
can simply output the encoded data, acting in *pass-through* mode, and
|
||
let the external audio system perform the decoding.
|
||
|
||
## Inner workings of GStreamer audio sinks
|
||
|
||
First off, digital audio output must be enabled at the system level. The
|
||
method to achieve this depend on the operating system, but it generally
|
||
involves going to the audio control panel and activating a checkbox
|
||
reading “Digital Audio Output” or similar.
|
||
|
||
The main GStreamer audio sinks for each platform, Pulse Audio
|
||
(`pulsesink`) for Linux, `osxaudiosink` for OS X and Direct Sound
|
||
(`directsoundsink`) for Windows, detect when digital audio output is
|
||
available and change their input caps accordingly to accept encoded
|
||
data. For example, these elements typically accept `audio/x-raw` data:
|
||
when digital audio output is enabled in the system, they may also
|
||
accept `audio/mpeg`, `audio/x-ac3`, `audio/x-eac3` or `audio/x-dts`.
|
||
|
||
Then, when `playbin` builds the decoding pipeline, it realizes that the
|
||
audio sink can be directly connected to the encoded data (typically
|
||
coming out of a demuxer), so there is no need for a decoder. This
|
||
process is automatic and does not need any action from the application.
|
||
|
||
On Linux, there exist other audio sinks, like Alsa (`alsasink`) which
|
||
work differently (a “digital device” needs to be manually selected
|
||
through the `device` property of the sink). Pulse Audio, though, is the
|
||
commonly preferred audio sink on Linux.
|
||
|
||
## Precautions with digital formats
|
||
|
||
When Digital Audio Output is enabled at the system level, the GStreamer
|
||
audio sinks automatically expose all possible digital audio caps,
|
||
regardless of whether the actual audio decoder at the end of the S/PDIF
|
||
cable is able to decode all those formats. This is so because there is
|
||
no mechanism to query an external audio decoder which formats are
|
||
supported, and, in fact, the cable can even be disconnected during this
|
||
process.
|
||
|
||
For example, after enabling Digital Audio Output in the system’s Control
|
||
Panel, `directsoundsink` will automatically expose `audio/x-ac3`,
|
||
`audio/x-eac3` and `audio/x-dts` caps in addition to `audio/x-raw`.
|
||
However, one particular external decoder might only understand raw
|
||
integer streams and would try to play the compressed data as such (a
|
||
painful experience for your ears, rest assured).
|
||
|
||
Solving this issue requires user intervention, since only the user knows
|
||
the formats supported by the external decoder.
|
||
|
||
On some systems, the simplest solution is to inform the operating system
|
||
of the formats that the external audio decoder can accept. In this way,
|
||
the GStreamer audio sinks will only offer these formats. The acceptable
|
||
audio formats are commonly selected from the operating system’s audio
|
||
configuration panel, from the same place where Digital Audio Output is
|
||
enabled, but, unfortunately, this option is not available in all audio
|
||
drivers.
|
||
|
||
Another solution involves, using a custom sinkbin (see
|
||
[](tutorials/playback/custom-playbin-sinks.md)) which includes a
|
||
`capsfilter` element (see [](tutorials/basic/handy-elements.md))
|
||
and an audio sink. The caps that the external decoder supports are
|
||
then set in the capsfiler so the wrong format is not output. This
|
||
allows the application to enforce the appropriate format instead of
|
||
relying on the user to have the system correctly configured. Still
|
||
requires user intervention, but can be used regardless of the options
|
||
the audio driver offers.
|
||
|
||
Please do not use `autoaudiosink` as the audio sink, as it currently
|
||
only supports raw audio, and will ignore any compressed format.
|
||
|
||
## Conclusion
|
||
|
||
This tutorial has shown a bit of how GStreamer deals with digital audio.
|
||
In particular, it has shown that:
|
||
|
||
- Applications using `playbin` do not need to do anything special to
|
||
enable digital audio output: it is managed from the audio control
|
||
panel of the operating system.
|
||
|
||
It has been a pleasure having you here, and see you soon!
|