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Alexa Auto SDK System Audio Extension

The Alexa Auto SDK System Audio extension provides the default audio capturing and playback functionality for various platforms.

Table of Contents

What's Included

The System Audio extension contains the platform implementations of:

  • aace::audio::AudioOutput and aace::audio::AudioOutputProvider for audio playback capability
  • aace::audio::AudioInput and aace::audio::AudioInputProvider for audio capturing capability

Supported Audio Backends

Currently the System Audio extension supports:

  • GStreamer tested with:

    • Generic Linux (Ubuntu 18.04)

      Note: You'll need extra GStreamer plugins to play content from Amazon Music (which uses HLS) and other Music Service Providers. Installing plugins such as gst-plugins-good1.0, gst-plugins-bad1.0, and gst-plugins-ugly1.0 is recommended.

      Note: GStreamer on Poky Linux for iMX8 does not support Audible or Amazon Music playback.

    • Automotive Grade Linux with 4A framework support (FF or GG)

  • OpenMAX AL (Encoded audio playback only) tested with:
    • QNX Multimedia Suite
  • QNX Sound Architecture (QSA) (Raw audio only) tested with:

    • QNX 7.0.0

      Note: You'll need a QNX Multimedia Suite license to use OpenMAX AL, and both OpenMAX AL and QSA are required in order to enable full funcitonality on QNX.

Getting Started

Prerequisites

You'll need GStreamer development packages such as libgstreamer1.0-dev and libgstreamer-plugins-base1.0-dev.

Building the Alexa Auto SDK with the System Audio Extension

The Alexa Auto SDK Builder automatically includes the System Audio extension when you build the Auto SDK. For example, assuming AAC_SDK_HOME is the location where you've installed the Alexa Auto SDK, use this command to build the Auto SDK for generic Linux:

$ cd ${AAC_SDK_HOME}
$ builder/build.sh linux -t native [options]

Running the C++ Sample App with the Audio Configuration

When you run the C++ Sample App, a config-system-audio.json file with default audio settings for Linux platforms is installed under /opt/AAC/etc.

Changing the Default Audio Settings for Linux (required for Poky 32)

If you need to modify the configuration defined in the config-system-audio.json file for your Linux platform, follow these steps:

  1. Open the config.json.linux file (located in the aac-sdk/extensions/system-audio/modules/system-audio/assets directory).
  2. Edit the config.json.linux file as necessary.

Note: For Poky 32 boards, you must set "shared" to "true" in the "default" node.

  1. Save the config.json.linux file as config-system-audio.json under /opt/AAC/etc.
  2. Include a --config /opt/AAC/etc/config-system-audio.json \ line when you run the C++ Sample App.

Changing the Default Audio Settings for QNX (required)

The default audio settings will not work for QNX. To modify the configuration defined in the config-system-audio.json file for your QNX platform, follow these steps:

  1. Open the config.json.qnx file (located in the aac-sdk/extensions/system-audio/modules/system-audio/assets directory).
  2. Edit the config.json.qnx file as necessary. See the default QNX configuration for guidance.
  3. Save the config.json.qnx file as config-system-audio.json under /opt/AAC/etc.
  4. Include a --config /opt/AAC/etc/config-system-audio.json \ line when you run the C++ Sample App.

Note: You may need to set the AAL_CAPATH system environment value to specify which path should OpenMAX AL used for CURLOPT_CAPATH internally. If you don't set the AAL_CAPATH system environment variable, /etc/ssl/certs will be used by default.

Advanced Usage

For complex audio setup, you may need to write your own config file. To use this file, save it as config-system-audio.json under /opt/AAC/etc and include a --config /opt/AAC/etc/config-system-audio.json \ line when you run the C++ Sample App.

Here is the config file template:

{
  "aace.systemAudio": {
    "<provider>": {
      "enabled": <boolean>,
      "devices": {
        "<device-name>": {
          "module": "<module-name>",
          "card": "<id>",
          "rate": "<sample-rate>",
          "shared": <boolean>
        }
      },
      "types": {
        "<type>": "<device-name>"
      }
    }
  }
}
  • aace.systemAudio.<provider>: Set to AudioInputProvider or AudioOutputProvider. You can write a configuration for each <provider>, depending on the direction (input or output).
  • aace.systemAudio.<provider>.enabled: Set to true or false. Setting this parameter to false disables the registration of the <provider> platform implementation. The default setting is true.
  • aace.systemAudio.<provider>.devices.<device-name>: Set to any "<device-name>" or to default. If you set the "<device-name>" to "default" audio will be routed by default if there is no explicit "<device-name>" available for the configured "<type>". You can configure multiple devices, depending on your platform.
    • "module": Specify a "<module-name>" to explicitly define which audio backend to use. By default, "module" is set to an empty string, which configures the system audio extension to use whatever backend is available.
    • "card": Specify the card id for the specific audio backend you defined with the "<module-name>" parameter. By default, "card" is set to an empty string since by default "<module-name>" is not defined.
    • "rate": Specify the sample rate of audio input. By default the "rate" is set to 0.
    • "shared" (AudioInputProvider only): Set to true or false. Set "shared: to true for Poky 32 boards or in cases where the device should be shared within the Auto SDK Engine; otherwise, the System Audio extension will try to open the device for every audio input type. The "shared" option is useful when the underlying backend doesn't support the input splitter. By default "shared" is set to false.
  • aace.systemAudio.<provider>.types.<type>: Use the "type" option to specify which device should be used for various types of audio. If you do not explicitly specify a device, the default type is used. See aace::audio::AudioInputProvider::AudioInputType and aace::audio::AudioOutputProvider::AudioOutputType for the possible "<type>" values.

Default QNX Configuration

Here is the default configuration for QNX platforms:

{
  "aace.systemAudio": {
    "AudioInputProvider": {
      "devices": {
        "default": {
          "module": "QSA",
          "shared": true
        }
      }
    },
    "AudioOutputProvider": {
      "devices": {
        "default": {
          "module": "OpenMAX AL"
        },
        "raw": {
          "module": "QSA"
        }
      },
      "types": {
        "COMMUNICATION": "raw"
      }
    }
  }
}

If you use this configuration:

  • The audio capturing for all types will use QSA, but it will be shared. This means that only a single PCM channel will be opened by the Engine.
  • The audio playback for all types except COMMUNICATION will use OpenMAX AL. COMMUNICATION audio will use QSA instead. Note that the multiple PCM channels will be opened for each types.

Linux Configuration Example

Here is a configuration example for Linux platforms:

{
  "aace.systemAudio": {
    "AudioInputProvider": {
      "devices": {
        "default": {
          "module": "GStreamer"
        },
        "loopback": {
          "module": "GStreamer",
          "card": "hw:Loopback,1,0",
          "shared": true
        }
      },
      "types": {
        "LOOPBACK": "loopback"
      }
    },
    "AudioOutputProvider": {
      "devices": {
        "default": {
          "module": "GStreamer"
        }
      }
    }
  }
}

The above example shows how you could provide "Speaker reference" into the engine. Note that the card parameter corresponds to the ALSA device name.

Playlist URL Support

The System Audio extension supports playback of playlist URL from media streaming services (such as TuneIn) based on PlaylistParser provided by AVS Device SDK. The current supported formats include M3U and PLS. Note that only the first playable entry will be played in the current implementation. Choosing a variant based on stream information or continuing playback of the second or later entry is not supported right now.

After the user asks Alexa to play on TuneIn, if Alexa acknowledges the request but says TuneIn is not available, the parser displays the following error:

2021-03-02 05:04:25.745 [AVS] E PlaylistParser:doDepthFirstSearch:url=http\://www.podtrac.com/pts/redirect.mp3/chtbl.com/track/5899E/traffic.megaphone.fm/HSW1953246087.mp3:getHeaderFailed

To avoid this error, provide a valid cacert.pem to CURLOPT_CAINFO in the Auto SDK configuration. Download the cacert.pem file from here.

      "libcurlUtils": {
        "CURLOPT_CAPATH": "/path/to/certs",
        "CURLOPT_CAINFO": "/path/to/cacert.pem"
      }