So I've decided to email http://www.usb.org/about - a non for profit forum founded by the developers of the usb specs, to find out some more authoritative answers to the questions of both USB cables and (as it turned out, I didn't ask, but the responder addressed) bulk vs async vs adaptive USB transfer modes. "If anyone would know - it's them" is my way of looking at it.
To my surprise they've replied to my initial email, and will hopefully reply to follow up questions.
You're welcome to ask any additional questions and I'll pass them along (probably more convenient for the people on the other end if it all comes from a single contact)
Of course the more selectively reading gifted of us will read in more magic and speculation, but I hope this will clear up some issues to people who really do want to know the hopefully objective facts behind it all, from people who don't have a financial interest in selling audio cables, gear or advertising in magazines. Or perhaps, while we're waiting for some objective measurements from people without vested interest/the right set of equipment and skills to conduct them.
Anyway, here are the org messages so far (waiting for a reply to the follow up atm) (my name and email removed, the email of the usb.org people separated by AT to prevent delicious spambots harversting)
Sent: Wednesday, August 03, 2011 4:27 AM
To: admin [AT] usb.org
Subject: usb audio (class 1 and 2) and cable affecting sound?
Major debates are raging about Class 1 or 2 USB audio devices (TE7022,
TAS1020B, etc receiver chips),
operating in adaptive or asynchronous isochronous modes being or not
being affected by
by 'quality' of the USB cable used.
I'm not going to bore you with much detail, but people seem to think
somehow improve sound quality by "reducing transmission errors and
'jitter'" over to spec/certified
USB cables.They also seem to think custom cables improve sound by
using better shielding/allegedly higher
Personally I think it's all expensive placebo and waffle and because
of the cost/margin on some of those
I was wondering if usb.org could comment on the issue, including any
influence they think
an after-market USB cable can have theoretically, and perhaps empirically.
To my surprise the reply also unexpectedly addressed the issue about adaptive/async transfer modes vs bulk mode usb audio devices. I've long thought bulk mode was superior because of error detection and retransmission and it seems usb.org people are of the same opinion. You can view the org deliberations about it in http://www.head-fi.org/forum/thread/546092/confirming-whether-your-dac-is-asynchronous-as-claimed-or-not I obviously don't advocate musiland products, in fact I'm vocally opposed to their dodgy QC, and bs marketing, that's probably quite clear from say, 3 threads from me catching them on their bs . Bolds are on my side for emphasis .
date Fri, Aug 5, 2011 at 5:46 AM
subject RE: usb audio (class 1 and 2) and cable affecting sound?
USB transmits information digitally. Bits are either received correctly or
not received. What a bit looks like on the wire has no effect on quality if
the bit is received correctly. If a bit is not receive correctly, error
checking in USB protocols will flag the error in data transmission.
Jitter is not a cable problem. Jitter is a transceiver (PHY) issue on the
Can bits get scrambled within a cable assembly on occasion? Yes, primarily
due to EMI but this is highly unlikely -- more on that later. Is occasional
data scrambling a problem for audio/video? Maybe. The answer depends on the
hardware receiving/rendering the data.
USB supports isochronous transport which is a timely delivery of data. The
isochronous transport has guaranteed bandwidth on USB. Isochronous
protocol, however, does not support error recovery. In other words, if data
is flagged as an error by the receiver, there will be no attempt at data
retransmission. So if the receiver is using the isochronous protocol, then
there can be errors in data. Most webcams use the isochronous transport.
High-end audio/video equipment that does not mandate real-time delivery of
data should not use the isochronous transport because accurate data delivery
is not guaranteed.
USB also supports bulk transport. The Bulk transport shares bandwidth and
timely delivery is not guaranteed. Bulk protocol does have error recovery
and errors in data will be retried. If the receiver uses the bulk USB
protocol, then there will be no errors in the data. This is why USB mass
storage devices always use the Bulk transport.
Most USB audio/video devices use the bulk transport because real-time
delivery of the data is not necessary. Bulk audio/video devices will buffer
data before rendering it. I can think of only two situations where the
audio/video will be disturbed when rendered: 1) If the host is busy
performing IO to other USB devices, or 2) There are errors in data
transmission where continual retries cause buffer under-run to occur. The
second point could be cable related -- it could also be poor hardware design
of the host or peripheral as well. The USB Bulk transport works very nicely
for audio and video because data is accurately delivered.
Now onto cable quality. A cheap USB cable will work perfectly fine in the
vast majority of home/office environments. All USB certified cables use
certified connectors and are shielded, have minimal skew on the data lines,
and meet criteria regarding impedance and voltage drop. If the environment
is 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 unless
they are experiencing problems not related to their hardware and there
exists definitive suspicions of environmental interference. I do always
recommend that the cable purchased be USB certified which provides assurance
that the product is properly designed for USB. Using USB certified
audio/video equipment also assures that the USB signal quality and other
packet parameters of the transceiver meets specifications.
Of course, all of the above is premised upon properly designed and
USB-IF Compliance Administrator
Please visit http://compliance.usb.org for the latest updates to the USB-IF
again, I trust that's about as clear as it could be about cable jitter, and transfer modes. More info about usb transfer modes and adaptive vs bulk vs async and the associated lack or presence of error detection and correction is in. http://www.head-fi.org/forum/thread/546092/confirming-whether-your-dac-is-asynchronous-as-claimed-or-not I obviously don't advocate Musiland, in fact I'm vocally opposed to their dodgy QC, and bs marketing, that's probably quite clear from say, 3 threads from me catching them on their bs
Anyway, I thought I'd ask a few more questions:
follow up message
TechAdmin [AT] usb.org
date Fri, Aug 5, 2011 at 10:08 AM
subjec tRe: usb audio (class 1 and 2) and cable affecting sound?
Thank you very much for a detailed reply on the topic of USB audio!
It's good to see that my interpretation of the USB spec regarding Bulk
vs adaptive and async usb
transfer modes was correct. (including the bit about error correction
in bulk mode and it's absence for guaranteeing delivery latency in the
other isochronous transfer modes). I would assume the only time bulk
mode might not be so good, is if you for some reason need a guaranteed
latency both ways and there are transmission errors... Or maybe
something to do with the reserved bandwidth for modes other than bulk
(I presume that may impede its use in applications where you need
bidirectional streaming like pro audio recording and simultaneously
monitoring, but I'm not too sure about that either. I think I just
have to get out of the habit of assuming transmission errors are
common and out to get us).
I think it's somewhat unfortunate that nearly all the consumer grade
USB audio devices I looked at were using
adaptive and in best case async modes.
It's I've recently purchased a musiland md series card that
uses bulk mode - custom drivers to view the usb receiver on its end
as a separate bus on the usb bus (bulk mode PC to the receiver), and
then attaches the audio device on that bus.
Unfortunately, as far as I know this is the only bulk mode USB
transfer mode consumer grad card out there (along with other musiland
series cards). If you know more consumer or pro grade ones, could you
please name one or two as examples?
It's good to know my interpretation of the error correction (bulk mode
only, and I presume error checking on receipt and retransmission on
and the circumstances where it stops helping (although adaptive and
async wouldn't be of use there either it seems) are also correct
(buffer under-run on either the receiver or sender side, either
because the host is busy or in the unlikely scenario the transmission
is bad because of a very high error rate and retransmission doesn't
rectify it (or maybe cannibalizes too much from the useful bandwidth)
at that stage you probably would have more problems than not being
able to guarantee latency even softly and you probably won't hear much
if it was an adaptive or async device at that point anyway). Not that
latency seems to be much of a problem for playback applications.
> If the environment is extremely noisy with EMI, then a better shielded cable may be necessary.
Usually relocating the cable or power strips will suffice to mitigate EMI.
Hmm, interesting, thank you. Ummm, I don't know how feasible is it,
but could you maybe name some common household sources you'd generally
find in a home office type environment, that could have high enough
EMI to warrant that.
Once again, thank you very much for the interesting and extended reply !
So I'm currently waiting for a reply to those...
I hope this sheds some objective light on the usb cable issue, and will make more USB audio makers get off their lazy backsides and make DACs with proper bulk transfer mode USB implementations (I can't be angry enough with musiland being the only one from the audiophile market to date that do it)...Writing drivers is difficult, especially for multiple OSes, but is it a good enough reason to continue making adaptive/async usb transfer mode DACs instead of bulk mode ones.
Edited by svyr - 8/5/11 at 9:52am