Editing USB (section)

=== Audio streaming ===
The USB Device Working Group has laid out specifications for audio streaming, and specific standards have been developed and implemented for audio class uses, such as microphones, speakers, headsets, telephones, musical instruments, etc. The working group has published four versions of audio device specifications:<ref>{{Cite press release |date=27 September 2016 |title=USB-IF Announces USB Audio Device Class 3.0 Specification |location=Houston, Texas & Beaverton, Oregon |url=https://www.businesswire.com/news/home/20160927006252/en/USB-IF-Announces-USB-Audio-Device-Class-3.0 |website=Business Wire |access-date=4 May 2018 |archive-date=4 May 2018 |archive-url=https://web.archive.org/web/20180504155618/https://www.businesswire.com/news/home/20160927006252/en/USB-IF-Announces-USB-Audio-Device-Class-3.0 |url-status=live }}</ref><ref>{{Cite web |url=http://www.usb.org/developers/docs/devclass_docs/ |title=USB Device Class Specifications |website=www.usb.org |access-date=4 May 2018 |archive-date=13 August 2014 |archive-url=https://web.archive.org/web/20140813051139/http://www.usb.org/developers/docs/devclass_docs/ |url-status=live }}</ref><ref>{{Cite web |title=USB Audio Devices Release 4.0 and Adopters Agreement {{!}} USB-IF |url=https://www.usb.org/document-library/usb-audio-devices-release-40-and-adopters-agreement |access-date=2025-05-20 |website=www.usb.org}}</ref> USB Audio&nbsp;1.0, 2.0, 3.0 and 4.0, referred to as "UAC"<ref name="xmos2015"/> or "ADC".<ref>{{Cite web |url=https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/usb-2-0-audio-drivers |title=USB Audio 2.0 Drivers |website=Microsoft Hardware Dev Center |access-date=4 May 2018 |quote=ADC-2 refers to the USB Device Class Definition for Audio Devices, Release 2.0. |archive-date=4 May 2018 |archive-url=https://web.archive.org/web/20180504155514/https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/usb-2-0-audio-drivers |url-status=live }}</ref>

UAC 3.0 primarily introduces improvements for portable devices, such as reduced power usage by bursting the data and staying in low power mode more often, and power domains for different components of the device, allowing them to be shut down when not in use.<ref>{{Cite web|url=https://www.synopsys.com/designware-ip/technical-bulletin/usb-audio-dwtb-q117.html|title=New USB Audio Class for USB Type-C Digital Headsets|website=Synopsys.com|access-date=7 May 2018|archive-date=7 May 2018|archive-url=https://web.archive.org/web/20180507221645/https://www.synopsys.com/designware-ip/technical-bulletin/usb-audio-dwtb-q117.html|url-status=live}}</ref>

UAC 2.0 introduced support for High Speed USB (in addition to Full Speed), allowing greater bandwidth for multi-channel interfaces, higher sample rates,<ref name=":2">{{Cite web |url=http://thewelltemperedcomputer.com/KB/USB.html |title=USB |website=The Well-Tempered Computer |last=Kars |first=Vincent |date=May 2011 |access-date=7 May 2018 |quote=All operating systems (Win, OSX, and Linux) support USB Audio Class 1 natively. This means you don't need to install drivers, it is plug&play. |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507153825/http://thewelltemperedcomputer.com/KB/USB.html |url-status=live }}</ref> lower inherent latency,<ref>{{Cite web |url=https://www.xmos.ai/file/fundamentals-of-usb-audio?version=latest |title=Fundamentals of USB Audio |publisher=XMOS Ltd. |website=www.xmos.com |format=PDF |date=2015 |access-date=10 December 2020 |quote=Note that Full Speed USB has a much higher intrinsic latency of 2ms}}</ref><ref name="xmos2015"/> and 8× improvement in timing resolution in synchronous and adaptive modes.<ref name="xmos2015"/> UAC2 also introduced the concept of clock domains, which provides information to the host about which input and output terminals derive their clocks from the same source, as well as improved support for audio encodings like [[Direct Stream Digital|DSD]], audio effects, channel clustering, user controls, and device descriptions.<ref name="xmos2015"/><ref name=":4"/>

UAC 1.0 devices are still common, however, due to their cross-platform driverless compatibility,<ref name=":2"/> and also partly due to [[Microsoft Windows|Microsoft]]'s failure to implement UAC 2.0 for over a decade after its publication, having finally added support to [[Windows 10]] through the Creators Update on 20 March 2017.<ref>{{Cite web |url=https://blogs.windows.com/windowsexperience/2016/09/21/announcing-windows-10-insider-preview-build-14931-for-pc/ |title=Announcing Windows 10 Insider Preview Build 14931 for PC |website=Windows Experience Blog |date=21 September 2016 |access-date=7 May 2018 |quote=We now have native support for USB Audio 2.0 devices with an inbox class driver! This is an early version of the driver that does not have all features enabled |archive-date=23 September 2016 |archive-url=https://web.archive.org/web/20160923032703/https://blogs.windows.com/windowsexperience/2016/09/21/announcing-windows-10-insider-preview-build-14931-for-pc/ |url-status=live }}</ref><ref>{{Cite web |url=http://amplioaudio.blogspot.com/2017/09/usb-audio-class-20-support-in-windows.html |title=Ampliozone: USB Audio Class 2.0 Support in Windows 10, FINALLY!!!! |last=Plummer |first=Gregg |date=20 September 2017 |website=Ampliozone |access-date=7 May 2018 |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507154036/http://amplioaudio.blogspot.com/2017/09/usb-audio-class-20-support-in-windows.html |url-status=live }}</ref><ref name=":4">{{Cite web|url=https://www.computeraudiophile.com/ca/bits-and-bytes/this-just-in-microsoft-launches-native-class-2-usb-audio-support-wait-what-r647/|title=This Just In: Microsoft Launches Native Class 2 USB Audio Support. Wait, What?|website=Computer Audiophile|date=2 May 2017 |access-date=7 May 2018|quote=Class 2 support enables much higher sample rates such as PCM 24 bit / 384 kHz and DSD (DoP) up through DSD256.|archive-date=2 September 2018|archive-url=https://web.archive.org/web/20180902023557/https://www.computeraudiophile.com/ca/bits-and-bytes/this-just-in-microsoft-launches-native-class-2-usb-audio-support-wait-what-r647/|url-status=live}}</ref> UAC 2.0 is also supported by [[macOS]], [[iOS]], and [[Linux]],<ref name="xmos2015"/> however [[Android (operating system)|Android]] only implements a subset of the UAC 1.0 specification.<ref name=":5">{{Cite web |url=https://source.android.com/docs/core/audio/usb#hostAudio |title=USB Digital Audio |website=Android Open Source Project |access-date=16 February 2023 |quote=Synchronous sub-mode is not commonly used with audio because both host and peripheral are at the mercy of the USB clock. }}</ref>

USB provides three isochronous (fixed-bandwidth) synchronization types,<ref>{{cite web|url=http://www.atmel.com/Images/doc32139.pdf|title=32-bit Atmel Microcontroller Application Note|date=2011|publisher=Atmel Corporation|access-date=13 April 2016|url-status=live|archive-url=https://web.archive.org/web/20160506204128/http://www.atmel.com/Images/doc32139.pdf|archive-date=6 May 2016}}</ref> all of which are used by audio devices:<ref>{{Cite web|url=http://www.ti.com/lit/ds/symlink/pcm2906c.pdf|title=PCM2906C datasheet|date=November 2011|website=Texas Instruments|quote=The PCM2906C employs SpAct architecture, TI's unique system that recovers the audio clock from USB packet data.|access-date=4 May 2018|archive-date=4 May 2018|archive-url=https://web.archive.org/web/20180504225235/http://www.ti.com/lit/ds/symlink/pcm2906c.pdf|url-status=live}}</ref>

* Asynchronous&nbsp;— The ADC or DAC are not synced to the host computer's clock at all, operating off a free-running clock local to the device.
* Synchronous&nbsp;— The device's clock is synced to the USB start-of-frame (SOF) or Bus Interval signals. For instance, this can require syncing an 11.2896&nbsp;MHz clock to a 1&nbsp;kHz SOF signal, a large frequency multiplication.<ref>{{Cite web|url=http://www.cypress.com/file/102921/download|title=Designing Modern USB Audio Systems|last=Castor-Perry|first=Kendall|date=October 2010|website=Cypress Semiconductor|access-date=4 May 2018|archive-date=5 May 2018|archive-url=https://web.archive.org/web/20180505172950/http://www.cypress.com/file/102921/download|url-status=live}}</ref><ref name=":1">{{Cite web|url=http://www.cypress.com/file/122521/download|title=Programmable Clock Generation and Synchronization for USB Audio Systems|last=Castor-Perry|first=Kendall|date=2011|website=Cypress Semiconductor|quote=Early USB replay interfaces used synchronous mode but acquired a reputation for poor quality of the recovered clock (and resultant poor replay quality). This was primarily due to deficiencies of clocking implementation rather than inherent shortcomings of the approach.|access-date=4 May 2018|archive-date=4 May 2018|archive-url=https://web.archive.org/web/20180504181023/http://www.cypress.com/file/122521/download|url-status=live}}</ref>
* Adaptive&nbsp;— The device's clock is synced to the amount of data sent per frame by the host<ref>{{Cite web|url=http://www.thewelltemperedcomputer.com/Lib/Hitoshi%20Kondoh%20story.pdf|title=The D/A diaries: A personal memoir of engineering heartache and triumph|last=Kondoh|first=Hitoshi|date=20 February 2002|quote=The fact that there is no clock line within the USB cable leads to a thinner cable, which is an advantage. But, no matter how good the crystal oscillators are at the send and receive ends, there will always be some difference between the two...|access-date=4 May 2018|archive-date=12 December 2019|archive-url=https://web.archive.org/web/20191212230749/http://www.thewelltemperedcomputer.com/Lib/Hitoshi%20Kondoh%20story.pdf|url-status=live}}</ref>

While the USB spec originally described asynchronous mode being used in "low cost speakers" and adaptive mode in "high-end digital speakers",<ref>{{Cite web |url=http://www.usb.org/developers/docs/usb20_docs/ |title=USB 2.0 Documents |website=www.usb.org |access-date=7 May 2018 |archive-date=3 December 2017 |archive-url=https://web.archive.org/web/20171203144114/http://www.usb.org/developers/docs/usb20_docs/ |url-status=live }}</ref> the opposite perception exists in the [[hi-fi]] world, where asynchronous mode is advertised as a feature, and adaptive/synchronous modes have a bad reputation.<ref>{{Cite web |url=https://www.cambridgeaudio.com/usa/en/blog/our-guide-usb-audio-why-should-i-use-it |title=Our Guide to USB Audio – Why Should I Use it? |website=Cambridge Audio |date=9 May 2016 |access-date=7 May 2018 |quote=Synchronous USB DAC is the lowest quality of the three ... Adaptive ... means that there is no continuous, accurate master clock in the DAC, which causes jitter in the audio stream. ... Asynchronous – this is the most complex to implement but it is a huge improvement on the other types. |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507153701/https://www.cambridgeaudio.com/usa/en/blog/our-guide-usb-audio-why-should-i-use-it |url-status=live }}</ref><ref>{{Cite web |url=http://thewelltemperedcomputer.com/Intro/SQ/USB_USB.htm |title=USB versus USB |website=The Well-Tempered Computer |last=Kars |first=Vincent |date=July 2012 |access-date=7 May 2018 |quote=Synchronous is not used in a quality DAC as it is very jittery. ... asynchronous is the better of these modes. |archive-date=22 April 2018 |archive-url=https://web.archive.org/web/20180422204100/http://thewelltemperedcomputer.com/Intro/SQ/USB_USB.htm |url-status=live }}</ref><ref name=":5"/> In reality, all types can be high-quality or low-quality, depending on the quality of their engineering and the application.<ref name=":1"/><ref name="xmos2015"/><ref>{{Cite web |url=https://www.head-fi.org/threads/low-jitter-usb-dan-lavry-michael-goodman-adaptive-asynchronous.493152/#post-6661517 |title=Low-Jitter USB: Dan Lavry, Michael Goodman, Adaptive, Asynchronous |website=Head-Fi |access-date=7 May 2018 |quote=Some manufacturers may lead you to believe that Asynchronous USB transfers are superior to Adaptive USB transfers and that therefore you must believe in the asynchronous solution. This no more true than saying that you "must" hold the fork in your left hand. In fact, if you know what you are doing, you will feed yourself with either hand. The issue is really about good engineering practices. |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507153738/https://www.head-fi.org/threads/low-jitter-usb-dan-lavry-michael-goodman-adaptive-asynchronous.493152/#post-6661517 |url-status=live }}</ref> Asynchronous has the benefit of being untied from the computer's clock, but the disadvantage of requiring [[sample rate conversion]] when combining multiple sources.