2022-10-12 14:06:26 +00:00
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# Audio control
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2022-10-12 19:58:29 +00:00
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- [`[synit]/protocols/schemas/audio.prs`](https://git.syndicate-lang.org/synit/synit/src/branch/main/protocols/schemas/audio.prs)
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The audio control protocol configures sound sources and sinks, for instance for call audio
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control on a mobile phone. This protocol is still *experimental*, as configuration of audio
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stacks is complex and I have only a few simple examples to work from thus far. As Synit is
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ported to new systems (e.g. new mobile handsets, to desktop machines, and so on), this protocol
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will be refined.
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**Implementation.** The protocol is (as of October 2022) implemented entirely within the
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SqueakPhone Smalltalk image, in class `AudioProfileDaemon`. The audio profile daemon has three
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tasks: first, to track Linux input `HEADPHONE_INSERT` and `MICROPHONE_INSERT` events and state,
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producing `HeadsetSpeakerPresent` and `HeadsetMicrophonePresent` assertions in response;
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second, to track application state such as whether a call is in progress or not; and third, to
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respond to changes in device and application state, along with changes in sink/source mapping
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configuration, by using the external `amixer` program to enable and disable audio sinks and
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sources. The schema definition file contains a few
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[comments](https://git.syndicate-lang.org/synit/synit/src/commit/a2babb63d55743ce942562e7ccb9106fdb239cf4/protocols/schemas/audio.prs#L3-L17)
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that give some of the flavour of the logic involved.
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## Source/Sink mappings
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Each different kind of device asserts `Mapping` records to connect abstract `Endpoint` names to
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concrete ALSA device names.
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```
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Mapping = <alsa-mapping @abstract Endpoint @concrete string> .
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Endpoint = <source @value Source> / <sink @value Sink> .
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Sink = =speaker / =headset / =earpiece .
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Source = =speakerphone / =headset / =handset .
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```
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**Example.** When the Smalltalk code detects that it is running on a PinePhone, it asserts
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- `<alsa-mapping <sink speaker> "Line Out">`
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- `<alsa-mapping <sink headset> "Headphone">`
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- `<alsa-mapping <sink earpiece> "Earpiece">`
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- `<alsa-mapping <source speakerphone> "Mic1">`
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- `<alsa-mapping <source handset> "Mic1">`
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## Audio device state
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The current daemon interprets low-level Linux input subsystem events, asserting
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`HeadsetSpeakerPresent` and `HeadsetMicrophonePresent` when the relevant hardware is physically
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attached to the system. These assertions are in turn used when selecting an audio profile to
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enable.
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```
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HeadsetSpeakerPresent = <headset-speaker-present> .
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HeadsetMicrophonePresent = <headset-microphone-present> .
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```
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## Relevant application state
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The current daemon reacts to changes in
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[`AlertSoundPlaying`](./soundEffects.md#AlertSoundPlaying),
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[`PhoneRinging`](./telephony.md#PhoneRinging),
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[`CallInProgress`](./telephony.md#CallInProgress), and
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[`Speakerphone`](./telephony.md#Speakerphone) when selecting an audio profile to enable.
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