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es 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 80 uac 2 0 introduced support for high speed usb in addition to full speed allowing greater bandwidth for multi channel interfaces higher sample rates 81 lower inherent latency 82 78 and 8 improvement in timing resolution in synchronous and adaptive modes 78 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 dsd audio effects channel clustering user controls and device descriptions 78 83 uac 1 0 devices are still common however due to their cross platform driverless compatibility 81 and also partly due to 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 84 85 83 uac 2 0 is also supported by macos ios and linux 78 however android only implements a subset of the uac 1 0 specification 86 usb provides three isochronous fixed bandwidth synchronization types 87 all of which are used by audio devices 88 asynchronous 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 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 mhz clock to a 1 khz sof signal a large frequency multiplication 89 90 adaptive the device s clock is synced to the amount of data sent per frame by the host 91 while the usb spec originally described asynchronous mode being used in low cost speakers and adaptive mode in high end digital speakers 92 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 93 94 86 in reality all types can be high quality or low quality depending on the quality of their engineering and the application 90 78 95 asynchronous has the benefit of being untied from the computer s clock but the disadvantage of requiring sample rate conversion when combining multiple sources connectors edit main article usb hardware connectors comparison of most legacy usb and current usb type c connectors the connectors the usb committee specifies support a number of usb s underlying goals and reflect lessons learned from the many connectors the computer industry has used the female connector mounted on the host or device is called the receptacle and the male connector attached to the cable is called the plug 44 2 5 2 6 the official usb specification documents also periodically define the term male to represent the plug and female to represent the receptacle 96 the design is intended to make it difficult to insert a usb plug into its receptacle incorrectly the usb specification requires that the cable plug and receptacle be marked so the user can recognize the proper orientation 44 the usb c plug however is reversible usb cables and small usb devices are held in place by the gripping force from the receptacle with no screws clips or thumb turns as some connectors use the distinction of a and b connectors was to enforce the directionality inherent in usb the single host has type a receptacles and each peripheral device has a single type b receptacle a hub provides multiple downstream facing type a receptacles and connects to the host through its single type b receptacle or a captive cable with a type a plug a hub may connect to the host either directly or through one or more additional hubs prior to type c usb on the go allowed a device such as a smartphone to take either the host or the peripheral device role with a single type ab receptacle micro ab superseded in 2014 or mini ab deprecated 2007 97 that accepted both type a and type b plugs usb connector types multiplied as the specification progressed the original usb specification detailed standard a and standard b plugs and receptacles these were originally referred to as simply type a and type b they were renamed standard out of necessity to distinguish from mini and later micro connectors the data contacts in the standard plugs are recessed compared to the power and ground contacts so that devices are safely electrically connected before the more delicate data communications circuitry is connected preventing damage some devices operate in different modes depending on whether the data connection is made simple power sources do not include data connections instead shorting the data contacts together but allow any capable usb device to charge or operate through a standard usb cable charging cables provide power connections but not data though the standard requires at least a usb 2 0 data connection capability in a non standard charge only cable the data wires are shorted at the device end otherwise the device may reject the charger as unsuitable cabling edit main article usb hardware cabling a variety of usb cables for sale in hong kong the usb 1 1 standard specifies that a standard cable can have a maximum length of 5 meters 16 ft 5 in with devices operating at full speed 12 mbit s and a maximum length of 3 meters 9 ft 10 in with devices operating at low speed 1 5 mbit s 98 99 100 usb 2 0 provides for a maximum cable length of 5 meters 16 ft 5 in for devices running at high speed 480 mbit s 100 the usb 3 0 standard does not directly specify a maximum cable length requiring only that all cables meet an electrical specification for copper cabling with awg 26 wires the maximum practical length is 3 meters 9 ft 10 in 101 usb bridge cables edit two computers hosts can easily be connected through a usb c cable but before type c hosts could not be connected to each other with common usb cables usb bridge cables or data transfer cables can be found within the market offering direct pc to pc connections a bridge cable is actually an electronic device that appears as a usb peripheral device to each of the connected hosts allowing peer to peer communication between the computers such usb bridge cables are used to transfer files between two computers via their usb ports popularized by microsoft as windows easy transfer the microsoft utility used a special usb bridge cable to transfer personal files and settings from a computer running an earlier version of windows to a computer running a newer version in the context of the use of windows easy transfer software the bridge cable can sometimes be referenced as easy transfer cable many usb bridge data transfer cables are still usb 2 0 but there are also a number of usb 3 0 transfer cables despite usb 3 0 being ten times as fast as usb 2 0 usb 3 0 transfer cables are only two to three times as fast given their design clarification needed the usb 3 0 specification introduced an a to a cross over cable without power for connecting two pcs these are not meant for data transfer but are aimed at diagnostic uses dual role usb connections edit usb bridge cables have become less important with usb dual role device capabilities introduced with the usb 3 1 specification under the most recent specifications usb supports most scenarios connecting systems directly with a type c cable for the capability to work however connected systems must support role switching dual role capability requires there be two controllers within the system as well as a role controller while this can be expected in a mobile platform such as a tablet or a phone desktop pcs and laptops often do not support dual roles 102 power edit main article usb hardware power usb host and hub recepticles supply power at a nominal 5 v dc via the v_bus pin to upstreaming usb devices low power and high power devices edit this section describes the power distribution model of usb that existed before usb power delivery usb pd on devices that do not use bc or pd usb provides up to 4 5 w through type a and type b connectors and up to 15 w through usb c all pre pd usb power is provided at 5 v for a host providing power to devices usb has a concept of the unit load any device may draw power of one unit and devices may request more power in these discrete steps it is not required that the host provide requested power and a device may not draw more power than negotiated low power devices can draw no more than one unit all devices must act as low power devices when starting out as unconfigured for usb devices up to usb 2 0 a unit load is 100 ma or 500 mw while usb 3 0 defines a unit load as 150 ma 750 mw full featured usb c can support low power devices with a unit load of 250 ma or 1250 mw high power devices e g typical 2 5 inch hard disk drives can draw more than one unit usb up to 2 0 allows a host or hub to provide up to 2 5 w to each device in five discrete steps of 100 ma and superspeed devices usb 3 x allows a host or a hub to provide up to 4 5 w in six steps of 150 ma usb c allows for dual lane operation of usb 3 x with larger unit load 250 ma up to 7 5 w 103 usb c also allows for type c current as a replacement for usb bc signaling power availability in a simple way without needing any data connection 104 usb power standards specification max current voltage max power low power device up to usb 2 0 100 ma 5 v a 0 50 w low power superspeed usb 3 x device 150 ma 5 v a 0 75 w high power device up to usb 2 0 500 ma b 5 v 2 5 w high power superspeed usb 3 x single lane device 900 ma c 5 v 4 5 w high power superspeed usb 3 x dual lane device d 1 5 a e 5 v 7 5 w battery charging bc 1 5 a 5 v 7 5 w type c 1 5 a 3 a 5 v 7 5 w 15 w power delivery spr d 5 a f up to 20 v 100 w power delivery epr d 5 a f up to 48 v g 240 w 1 2 the v bus supply from a low powered hub port may drop to 4 40 v up to five unit loads with non superspeed devices one unit load is 100 ma up to six unit loads with superspeed devices one unit load is 150 ma 1 2 3 for usb c only up to six unit loads with multi lane devices one unit load is 250 ma 1 2 3 a 60 w operation requires an electronically marked cable rated at 5 a 20 v 100 w operation requires an electronically marked extended power range epr cable to recognize battery charging mode a dedicated charging port places a resistance not exceeding 200 ω across the d and d terminals shorted or near shorted data lanes with less than 200 ω of resistance across the d and d terminals signify a dedicated charging port dcp with indefinite charging rates 105 106 in addition to standard usb there is a proprietary high powered system known as poweredusb developed in the 1990s and mainly used in point of sale terminals such as cash registers signaling edit main article usb communications signaling usb phy usb signals are transmitted using differential signaling on twisted pair data wires with 90 ω 15 characteristic impedance 107 usb 2 0 and earlier specifications define a single pair in half duplex hdx usb 3 0 and later specifications define one dedicated pair for usb 2 0 compatibility and two or four pairs for data transfer two data wire pairs realising full duplex fdx for single lane 1 variants require at least superspeed ss connectors four pairs realising full duplex for two lane 2 variants require usb c connectors usb4 gen 4 requires the use of all four pairs but allow for asymmetrical pairs configuration 108 in this case one data wire pair is used for the upstream data and the other three for the downstream data or vice versa usb4 gen 4 use pulse amplitude modulation on three levels providing a trit of information every baud transmitted the transmission frequency of 12 8 ghz translate to a transmission rate of 25 6 gbd 109 and the 11 bit to 7 trit translation provides a theoretical maximum transmission speed just over 40 2 gbit s 110 usb data operation modes operation mode name introduced in lanes encoding data wires nominal signaling rate original label usb if current 53 current old marketing name logo low speed n a usb 1 0 1 hdx nrzi 2 1 5 mbit s half duplex low speed usb ls basic speed usb full speed 12 mbit s half duplex full speed usb fs high speed usb 2 0 480 mbit s half duplex hi speed usb hs usb 3 2 gen 1 1 usb 3 0 usb 3 1 gen 1 usb 3 0 1 fdx 1 hdx a 8b 10b 6 5 gbit s symmetric superspeed usb ss usb 5gbps usb 3 2 gen 2 1 usb 3 1 gen 2 usb 3 1 128b 132b 10 gbit s symmetric superspeed ss usb 10gbps usb 3 2 gen 1 2 n a usb 3 2 2 fdx 1 hdx a 8b 10b 10 10 gbit s symmetric n a usb 3 2 gen 2 2 128b 132b 20 gbit s symmetric superspeed usb 20gbps usb 20gbps usb4 gen 2 1 usb4 1 fdx 1 hdx a 64b 66b b 6 used of 10 10 gbit s symmetric usb 10gbps usb4 gen 2 2 2 fdx 1 hdx a 10 20 gbit s symmetric usb 20gbps usb4 gen 3 1 1 fdx 1 hdx a 128b 132b b 6 used of 10 20 gbit s symmetric usb4 gen 3 2 2 fdx 1 hdx a 10 40 gbit s symmetric usb 40gbps usb4 gen 4 2 usb4 2 0 2 fdx 1 hdx a pam 3 11b 7 t 10 80 gbit s symmetric usb 80gbps asymmetric 1 hdx a 40 gbit s up 120 gbit s down n a 120 gbit s up 40 gbit s down 1 2 3 4 5 6 7 8 usb 2 0 implementation 1 2 usb4 can use optional reed solomon forward error correction rs fec in this mode 12 16 b 128 bit symbols are assembled together with 2 b 12 bit 4 bit reserved synchronization bits indicating the respective symbol types and 4 b of rs fec to allow to correct up to 1 b of errors anywhere in the total 198 b block low speed ls and full speed fs modes use a single data wire pair labeled d and d in half duplex transmitted signal levels are 0 0 0 3 v for logical low and 2 8 3 6 v for logical high level the signal lines are not terminated high speed hs uses the same wire pair but with different electrical conventions lower signal voltages of 10 to 10 mv for low and 360 to 440 mv for logical high level and termination of 45 ω to ground or 90 ω differential to match the data cable impedance superspeed ss adds two additional pairs of shielded twisted data wires and new mostly compatible expanded connectors besides another grounding wire these are dedicated to full duplex superspeed operation the superspeed link operates independently from the usb 2 0 channel and takes precedence on connection link configuration is performed using lfps low frequency periodic signaling approximately at 20 mhz frequency and electrical features include voltage de emphasis at the transmitter side and adaptive linear equalization on the receiver side to combat electrical losses in transmission lines and thus the link introduces the concept of link training superspeed ss uses a new coding scheme with an increased signaling rate gen 2 1 mode and or the additional lane of usb c gen 1 2 and gen 2 2 modes a usb connection is always between an a end a downstream facing port dfp of either a host or a hub and a b end the upstream facing port ufp of either a peripheral device or a hub historically this was made clear by the fact that hosts had only type a and peripheral devices had only type...
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