Audio

This page will attempt to give examples of audio-based GStreamer pipelines using GStreamer-1.0, with 'enhanced' encoders and decoders from the gstreamer-imx set of plugins. Note that 'enhanced' just means that the software algorithms are especially tuned for the imx6 family.

To see GStreamer-0.10 (deprecated) examples, please refer to this older revision page.

For a description of Audio devices see:

• Ventana Audio devices

GStreamer alsasrc and alsasink

The GStreamer alsasrc and alsasink provide audio capture and playback to Advanced Linux Sound Architecture (ALSA) devices. ALSA is the modern device-driver API for the Linux kernel.

The Gstreamer alsasrc and alsasink can be passed a device property which can represent either the device name (reported from aplay -L for playback, or arecord -L for record) (ie sysdefault:CARD=sgtl5000audio) or the hw:x,y notation (ie hw:0,0 for first device, first subdevice)

If not specified via the device property, the device used for alsasrc and alsasink will depend on /etc/asound.conf and/or ~/.asoundrc.

Playback and Capture also depends on your ALSA mixer (amixer) settings. See mixer settings for more info.

Audio Output

Generally, a 'sink' plugin is one that will take an audio stream and send it to an audio device. Please refer to this audio page for more details on audio devices on the ventana platform.

A list of output sinks on the imx6:

• alsasink (recommended)
• autoaudiosink
• fakesink
• filesink

alsasink

This audio sink is our recommended audio sink. It can accept the following formats via audio/x-raw: S8, U8, S16LE, S16BE, U16LE, U16BE, S24_32LE, S24_32BE, U24_32LE, U24_32BE, S32LE, S32BE, U32LE, U32BE, S24LE, S24BE, U24LE, U24BE, S20LE, S20BE, U20LE, U20BE, S18LE, S18BE, U18LE, U18BE, F32LE, F32BE, F64LE, F64BE

fakesink

This is a very useful debugging sink. It takes whatever data is given to it and drops them. This might help debugging pipelines if problems ever arise.

filesink

This sink is very useful when a user wants to record and save audio to a file.

autoaudiosink

This GStreamer sink is not really an 'audio' sink in the traditional sense. Similar to playbin and decodebin, this element selects what it thinks is the best available audio sink and uses it. Generally not recommended.

Examples

The GStreamer alsasink element will playback a 1KHz tone to any Advanced Linux Sound Architecture (ALSA) device driver:

gst-launch-1.0 audiotestsrc ! alsasink device="hw:0,0"

• see here for details on specifying audio input and output devices

Audio Input

An input source is anything coming from a capture device on the SBC, e.g. HDMI audio in/analog audio in. Please refer to this audio page for more details on audio devices on the ventana platform.

A list of input sources on the imx6:

• alsasrc (recommended)
• autoaudiosrc
• audiotestsrc
• filesrc

alsasrc

This is the recommended audio capture source when capturing audio onto the SBC. This plugin can output the following source types in audio/x-raw: S8, U8, S16LE, S16BE, U16LE, U16BE, S24_32LE, S24_32BE, U24_32LE, U24_32BE, S32LE, S32BE, U32LE, U32BE, S24LE, S24BE, U24LE, U24BE, S20LE, S20BE, U20LE, U20BE, S18LE, S18BE, U18LE, U18BE, F32LE, F32BE, F64LE, F64BE

autoaudiosrc

This GStreamer source is not really an 'audio' source in the traditional sense. Similar to playbin and decodebin, this element selects what it thinks is the best available audio source and uses it. Generally not recommended.

audiotestsrc

This is a very useful plugin for testing. It can output 16-64bit audio/x-raw: S16LE, S32LE, F32LE, F64LE

This can output audio signals from sine to violet-noise.

filesrc

This source is very useful when a user wants to playback audio from a file.

Examples

The GStreamer alsasrc element will capture audio from any Advanced Linux Sound Architecture (ALSA) device driver:

gst-launch-1.0 alsasrc device="hw:0,0" ! "audio/x-raw,rate=32000,channels=2,depth=16" ! \
  audioconvert ! avimux ! filesink location=./audio.avi

• see here for details on specifying audio input and output devices

Encoding

Encoding is the term used to capture audio and encode it to a new format type.

A list of audio encoders on the imx6 platform:

• gstreamer-imx specific
  • imxmp3audioenc
• Other GStreamer encoders
  • vorbisenc
  • mulawenc
  • wavenc
  • alawenc
  • flacenc
  • lamemp3enc

There are many more. You can search for your specified one by running a similar search: gst-inspect-1.0 | grep enc.

imxmp3audioenc

This encoder can encode audio/x-raw S16LE to audio/mpeg.

vorbisenc

This encoder can encode audio/x-raw F32LE to audio/x-vorbis.

mulawenc

This encoder can encode audio/x-raw S16LE to audio/x-mulaw.

wavenc

This encoder can encode audio/x-raw S32LE, S24LE, S16LE, U8, F32LE, F64LE, audio/x-alaw, and audio/x-mulaw to audio/x-wav.

alawenc

This encoder can encode audio/x-raw S16LE to audio/x-alaw.

flacenc

This encoder can encode audio/x-raw S24LE, S24_32LE, S16LE, S8 to audio/x-flac.

lamemp3enc

This encoder can encode audio/x-raw S16LE to audio/mpeg.

Examples

This will take in an audio source from the mic and save it as an mp3.

gst-launch-1.0 alsasrc device="hw:0,0" ! audioconvert ! imxmp3audioenc ! filesink location=/tmp/mic.mp3

• Note the audioconvert element is used to convert audio samplerate / bitwidth / format (via software algorithms) as needed between two elements.

Decoding

Decoding is the term used to decode an encoded audio stream to a raw audio stream.

A list of audio encoders on the imx6 platform:

• gstreamer-imx specific
  • imxuniaudiodec
• Other GStreamer encoders
  • vorbisdec
  • ivorbisdec
  • a52dec
  • mulawdec
  • alawdec
  • flacdec

imxuniaudiodec

This decoder can decode audio/mpeg to audio/x-raw S32LE, S24LE, S16LE, S8.

vorbisdec

This decoder can decode audio/x-vorbis to audio/x-raw F32LE.

ivorbisdec

This decoder can decode audio/x-vorbis to audio/x-raw S16LE.

a52dec

This decoder can decode audio/x-ac3, audio/ac3, and audio/x-private1-ac3 to audio/x-raw F32LE.

mulawdec

This decoder can decode audio/x-mulaw to audio/x-raw S16LE.

alawdec

This decoder can decode audio/x-alaw to audio/x-raw S16LE.

flacdec

This decoder can decode audio/x-flac to audio/x-raw S8, S16LE, S24_32LE, S32LE.

Examples

We can decode the MP3 audio saved previously and playback to the default audio device with the following:

gst-launch-1.0 filesrc location=/tmp/mic.mp3 ! mpegaudioparse ! imxuniaudiodec ! alsasink

Audio Format Conversion

In specify the details of the audio format, you use a GStreamer capsfilter and to convert it from one format to another you use the GStreamer audioconvert element.

gst-launch-1.0 alsasrc device="hw:0,0" ! "audio/x-raw,rate=32000,channels=2,depth=16" ! \
  audioconvert ! avimux ! filesink location=./audio.avi

Audio loopback (Useful for testing audio input and output)

A simple aduio loopback test can take audio input from an input device and output it to an output device.

loopback audio from the first audio card:

gst-launch-1.0 alsasrc device="hw:0,0" ! alsasink device="hw:0,0"

To send audio from the sgtl5000 (analog input) to the imxhdmisoc (HDMI output):

• using device/function numbers (ie from aplay -l):

  gst-launch-1.0 alsasrc device="hw:0,0" ! alsasink device="hw:2,0"
  

• or, using names (ie from aplay -L):

  gst-launch-1.0 alsasrc device="sysdefault:CARD=sgtl5000audio" ! \
   alsasink device="sysdefault:CARD=imxhdmisoc"
  

To send audio from the tda1997x HDMI receiver (digital input) to the imxhdmisoc (digital output):

gst-launch-1.0 alsasrc device="sysdefault:CARD=tda1997xaudio" ! \
  "audio/x-raw,rate=44100" ! alsasink device="sysdefault:CARD=imxhdmisoc"

• Note here we need to specify the audio sample-rate as it can vary per input stream (and gstreamer does not validate the rate). This rate must match the source stream samplrate which can be found via sysfs /sys/bus/i2c/drivers/tda1997x/2-0048/audmode. If your output device requires a different sample-rate than the source input device, you need to perform a sample-rate conversion.