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SND_UAUDIO(4) Kernel Interfaces Manual SND_UAUDIO(4) NAME snd_uaudio -- USB audio and MIDI device driver SYNOPSIS device sound device usb device snd_uaudio In rc.conf(5): kld_list="snd_uaudio" In sysctl.conf(5): hw.usb.uaudio.buffer_ms hw.usb.uaudio.default_bits hw.usb.uaudio.default_channels hw.usb.uaudio.default_rate hw.usb.uaudio.handle_hid hw.usb.uaudio.debug DESCRIPTION A USB audio device consists of a number of components: input terminals (e.g. USB digital input), output terminals (e.g. speakers), and a num- ber of units in between (e.g. volume control). If the device supports multiple configurations, and there have been no user-supplied values specified through the sysctl(8) interface, the driver will select the best matching configuration supported by the de- vice during attach. "Best" means the configuration with the most chan- nels and highest quality in sample rate and sample size. Refer to the `USB Audio Class Specification' for more information. HARDWARE The snd_uaudio driver provides support for USB audio class devices and USB MIDI class devices. SYSCTL VARIABLES The following settings can be entered at the loader(8) prompt or in loader.conf(5) and can also be changed at runtime with the sysctl(8) command. For a change to take effect during runtime, the device has to be re-attached. hw.usb.uaudio.buffer_ms Period of audio data processed at once, in milliseconds, from 1 to 8 (default is 4). Lower values mean less latency, but this can result in audible gaps due to frequent CPU wakeups. hw.usb.uaudio.default_bits Preferred sample size in bits, from 0 to 32 (default is 0). A value of 0 sets the sample size to the maximum supported sample size. Set this to select a smaller sample size if the device supports multiple sample sizes. hw.usb.uaudio.default_channels Preferred number of sample channels, from 0 to 64 (default is 0). USB 1.1 devices are limited to 4 channels due to bandwidth constraints, unless a higher value is explicitly requested. A value of 0 sets the sample channels to the maximum supported channel number. Set this to select a smaller channel number if the device sup- ports multiple channel configurations. hw.usb.uaudio.default_rate Preferred sample rate in Hz (default is 0). If set to 0, the device's highest supported sample rate will be used. Note that if VCHANs are enabled, the sample rate will be over- ridden by dev.pcm.%d.[play|rec].vchanrate (see sound(4)), which can also be used to adjust the sample rate during runtime. If hw.usb.uaudio.default_rate is non-zero, dev.pcm.%d.[play|rec].vchanrate will use it as its maximum al- lowed value. hw.usb.uaudio.handle_hid Let snd_uaudio handle HID volume keys, if any (default is 1). 0 Disabled. 1 Enabled. If usb(4) has been compiled with USB_DEBUG on, the following setting is also available: hw.usb.uaudio.debug Debug output level (default is 0). SEE ALSO sound(4), usb(4), loader.conf(5), loader(8), sysctl(8) USB Audio Class Specifications, http://www.usb.org/developers/docs/devclass_docs/. HISTORY The snd_uaudio driver first appeared in FreeBSD 4.7. AUTHORS This manual page was adopted from NetBSD 1.6 and modified for FreeBSD by Hiten Pandya <hmp@FreeBSD.org>. BUGS The PCM framework in FreeBSD currently does not support the full set of USB audio mixer controls. Some mixer controls are only available as dev.pcm.%d.mixer sysctls. FreeBSD 15.0 July 17, 2025 SND_UAUDIO(4)
NAME | SYNOPSIS | DESCRIPTION | HARDWARE | SYSCTL VARIABLES | SEE ALSO | HISTORY | AUTHORS | BUGS
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