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[docs] guide: document current Looking Glass features
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Geoffrey McRae
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doc/audio.rst
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doc/audio.rst
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.. _client_audio:
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Audio and microphone
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####################
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The client selects audio independently from video and input. The current LGMP
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transport does not provide audio, so SPICE normally supplies it in an IDD
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setup. Selecting SPICE as the primary video transport also uses its audio
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provider.
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.. warning::
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Do not use Scream for a Looking Glass setup. Use the built-in classic SPICE
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or USB audio path. Scream over IVSHMEM can also conflict with the shared
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device used for Looking Glass frames.
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Looking Glass offers two SPICE audio paths:
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USB audio (recommended)
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Presents an emulated Looking Glass USB Audio Class 2 device to Windows over
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a SPICE USB channel. It supports stereo, quadraphonic, 5.1 and 7.1 playback
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using 24-bit PCM at up to 192 kHz. It also provides asynchronous playback
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feedback and a lower-latency clock relationship with supported Linux audio
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backends. This path is enabled by default when it is available.
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Classic SPICE audio
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Uses a virtual guest sound card and SPICE playback and record channels.
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QEMU limits it to stereo, 16-bit samples at 48 kHz. Use it as a compatibility
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fallback when the emulated USB audio device is not available.
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Set ``spice:audio=no`` to disable both paths.
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.. _client_usb_audio:
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USB audio setup
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---------------
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USB audio requires:
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* a client built with ``libusbredirparser-0.5`` version 0.7.1 or newer;
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* at least one unused SPICE USB redirection channel in the VM; and
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* a working client playback backend.
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For libvirt, add a USB redirection device:
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.. code:: xml
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<redirdev bus='usb' type='spicevmc'/>
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In virt-manager, this is a **USB Redirector** device using the SPICE channel.
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It is dedicated to the virtual Looking Glass audio device while USB audio is
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active.
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The emulated USB audio device is enabled by default. Its settings are:
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.. code-block:: ini
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[spice]
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audio=yes
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usbAudio=yes
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Playback supports stereo, quadraphonic, 5.1 and 7.1 speaker layouts using
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24-bit PCM at sample rates up to 192 kHz. Recording supports stereo at up to
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192 kHz.
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After the device connects, select the Looking Glass USB Audio speakers as the
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Windows output device.
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If USB audio could not be created at startup, the client falls back to classic
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SPICE audio and logs the reason. A working USB audio device also requires the
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SPICE USB redirection channel to connect; if the channel is absent, add it to
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the VM rather than reinstalling the Windows endpoint.
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Classic SPICE setup
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-------------------
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Add an Intel HDA sound device and a SPICE audio backend to the VM as shown in
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:ref:`libvirt_spice_server`, then select the classic path:
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.. code-block:: ini
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[spice]
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audio=yes
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usbAudio=no
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QEMU's classic SPICE audio implementation exposes a fixed stereo format: two
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channels, 16-bit samples at 48 kHz. Use the emulated USB audio device when
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other sample rates, sample formats or channel layouts are required.
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Microphone access
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-----------------
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PipeWire supports playback and recording. The current PulseAudio backend does
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not provide microphone recording. Press :kbd:`ScrLk` + :kbd:`E` to toggle
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recording when it is available.
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When a guest application opens the microphone, ``audio:micDefault`` controls
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the response:
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``prompt``
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Ask before sending microphone audio. This is the default.
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``allow``
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Allow requests automatically.
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``deny``
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Refuse requests automatically.
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Press :kbd:`ScrLk` + :kbd:`C` to cycle this policy. Keep
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``audio:micShowIndicator=yes`` to show when recording is active.
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Latency settings
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----------------
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``audio:periodSize`` requests the audio backend period in samples. The default
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is 512; 0 requests a 10 ms period. Smaller values may reduce latency but also
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make underruns more likely.
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``audio:latencyOffset`` applies only to classic SPICE audio. It adds safety
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margin to the calculated minimum buffer; the default is 6 ms. Reduce it only
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after confirming that classic SPICE playback remains free of dropouts under
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load.
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``audio:resampler`` also applies only to classic SPICE audio. The default
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``auto`` setting chooses the appropriate path; the other choices are
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``libsamplerate`` and ``backend``. The emulated USB audio device instead uses
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feedback to adjust the Windows packet rate.
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Set ``audio:debug=yes`` to log ring-buffer level, backend delay, clock feedback
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and underrun or overrun counts. Disable it after diagnosis to keep the normal
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log concise.
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