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USB.org回复
[quote name="TechAdmin [AT] usb.org"]to svyrdate Fri, Aug 5, 2011 at 5:46 AMsubject RE: usb audio (class 1 and 2) and cable affecting sound?Hello [svyr],USB transmits information digitally. Bits are either received correctly ornot received. What a bit looks like on the wire has no effect on quality ifthe bit is received correctly. If a bit is not receive correctly, errorchecking in USB protocols will flag the error in data transmission.Jitter is not a cable problem. Jitter is a transceiver (PHY) issue on thedevices.Can bits get scrambled within a cable assembly on occasion? Yes, primarilydue to EMI but this is highly unlikely -- more on that later. Is occasionaldata scrambling a problem for audio/video? Maybe. The answer depends on thehardware receiving/rendering the data.USB supports isochronous transport which is a timely delivery of data. Theisochronous transport has guaranteed bandwidth on USB. Isochronousprotocol, however, does not support error recovery. In other words, if datais flagged as an error by the receiver, there will be no attempt at dataretransmission. So if the receiver is using the isochronous protocol, thenthere can be errors in data. Most webcams use the isochronous transport.High-end audio/video equipment that does not mandate real-time delivery ofdata should not use the isochronous transport because accurate data deliveryis not guaranteed.USB also supports bulk transport. The Bulk transport shares bandwidth andtimely delivery is not guaranteed. Bulk protocol does have error recoveryand errors in data will be retried. If the receiver uses the bulk USBprotocol, then there will be no errors in the data. This is why USB massstorage devices always use the Bulk transport.Most USB audio/video devices use the bulk transport because real-timedelivery of the data is not necessary. Bulk audio/video devices will bufferdata before rendering it. I can think of only two situations where theaudio/video will be disturbed when rendered: 1) If the host is busyperforming IO to other USB devices, or 2) There are errors in datatransmission where continual retries cause buffer under-run to occur. Thesecond point could be cable related -- it could also be poor hardware designof the host or peripheral as well. The USB Bulk transport works very nicelyfor audio and video because data is accurately delivered. Now onto cable quality. A cheap USB cable will work perfectly fine in thevast majority of home/office environments. All USB certified cables usecertified connectors and are shielded, have minimal skew on the data lines,and meet criteria regarding impedance and voltage drop. If the environmentis extremely noisy with EMI, then a better shielded cable may be necessary.Usually relocating the cable or power strips will suffice to mitigate EMI.Personally, I would never recommend anyone buy an expensive USB cable unlessthey are experiencing problems not related to their hardware and thereexists definitive suspicions of environmental interference. I do alwaysrecommend that the cable purchased be USB certified which provides assurancethat the product is properly designed for USB. Using USB certifiedaudio/video equipment also assures that the USB signal quality and otherpacket parameters of the transceiver meets specifications.Of course, all of the above is premised upon properly designed andfunctioning hardware.Regards,Mark PaxsonUSB-IF Compliance AdministratorTechAdmin@usb.org(ReplyID 110804.105337)
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