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71 lines
3.6 KiB
Markdown
71 lines
3.6 KiB
Markdown
---
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layout: doc
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title: Audio Virtualization
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permalink: /doc/audio-virtualization/
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---
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Audio Virtualization
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====================
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VMs on Qubes OS have access to virtualized audio through the PulseAudio module.
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It consists of two parts:
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- `pacat-simple-vchan` running in a dom0/Audio VM (standalone application, one per VM, connected to the PulseAudio daemon)
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- `module-vchan-sink` running in a VM (loaded into the PulseAudio process)
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Protocol
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--------
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The protocol between these two parts is designed to be as simple as possible.
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Specifically, there is no audio format negotiation, no compression, etc.
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All the audio data is transferred as raw 44.1kHz samples in S16LE format, 2 channels.
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These two parts are connected with two vchan links:
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1. Connection on vchan port 4713 used to transfer audio from VM to dom0/Audio VM.
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There is no negotiation, no headers, etc.
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Only raw samples are sent over this channel.
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2. Connection on vchan port 4714 used to transfer audio from dom0/Audio VM to VM.
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Like the previous one, audio data is sent as raw samples.
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Additionally, the second vchan connection (on port 4714) is used in the opposite direction (VM to dom0) to send simple notifications/commands about the audio state.
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Each such notification is a 4-byte number in little-endian format.
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List of defined codes:
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- `0x00010001` -- VM wants to receive audio input (some process is listening); prior to this message, `pacat-simple-vchan` will not send any audio samples to the VM.
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- `0x00010000` -- VM does not want to receive audio input (no process is listening anymore); after this message, `pacat-simple-vchan` will not send any audio samples to the VM.
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- `0x00020000` -- VM does not want to send audio output; informational for dom0, to avoid buffer under runs (may affect PulseAudio calculated delays).
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- `0x00020001` -- VM does want to send audio output.
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pacat-simple-vchan
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------------------
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This is the dom0 (or Audio VM) part of the audio virtualization.
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It is responsible for transferring audio samples between the PulseAudio daemon in dom0/Audio VM (which has access to the actual audio hardware) and a VM playing/recording sounds.
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A separate `pacat-simple-vchan` process runs in dom0 for each VM with audio.
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Each of them opens separate input and output streams to their local PulseAudio, which allows for controlling the volume of each VM separately (using the `pavucontrol` tool, for example).
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Audio input to the VM is not sent by default.
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It needs to be both requested by the VM part and explicitly enabled in `pacat-simple-vchan`.
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The mechanism to do this differs between Qubes versions.
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In Qubes before R4.1, `pacat-simple-vchan` is controlled over system D-Bus:
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- destination: `org.qubesos.Audio.VMNAME` (where `VMNAME` is the VM's name)
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- object path: `/org/qubesos/audio`
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- interface: `org.qubesos.Audio`
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- property: `RecAllowed` (which can be set using the `org.freedesktop.DBus.Properties` interface)
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In Qubes R4.1 and later, `pacat-simple-vchan` is controlled over a UNIX socket at `/var/run/qubes/audio-control.VMNAME` (where `VMNAME` is the VM's name).
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Supported commands:
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- `audio-input 1\n` - enable audio input
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- `audio-input 0\n` - disable audio input
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These commands can be sent using the `qubes.AudioInputEnable+VMNAME` and `qubes.AudioInputDisable+VMNAME` qrexec services, respectively.
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The current status is written into QubesDB at `/audio-input/VMNAME` (where `VMNAME` is the VM's name) with either `1` or `0` values.
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The lack of a key means that the `pacat-simple-vchan` for a given VM is not running.
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In either version, it is exposed to the user as device of class `mic`, which can be attached to a VM (for example, using the `qvm-device mic` command).
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