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