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Old 22nd January 2013, 11:42 AM   #11
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Quote:
Originally Posted by analog_sa View Post
This type of statement is making me scream. Honestly.

There is no simple answer without knowing what is on the receiving end of this cable. IME the difference can range between very substantial and almost imperceptible. In fact all USB implementations where the cable is audible are to some extent defective. And that's at least 99% of them.
Sorry if I made you blow a gasket.
Agreed on your comment that the implementation is very important. I try always try to pick the best equipment for my money. It will never be perfect but that's my starting point. So the question really is "Is it worth spending $150 on a USB for my audio gear?" Probably not. Getting a good quality cable at a decent price? Yes! But that's true for all cables in the chain.

I also tend to agree with the 99% implementations that are flawed, but just add a bad cable to the mix and you sure to reach 100% (flawed implementations). So if a small investment can make an audible difference, I will go for it
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Old 22nd January 2013, 12:09 PM   #12
marce is offline marce  United Kingdom
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Digital zero and one, two states no inbetweens, I would suggest the book "High Speed Digital Design" by Howard Johnson.
USB transmits digital data, digital data consist of two states hi and low, it is transmitted as differential digital square waves as the attached picture shows, and you employ digital transmission design techniques...
If cables sound different then somthing is wrong with the digital data transmission and the only way you will understand what is going on is to use a scope or network analyser.
99% are flawed how, there are billions of USB interfaces in the world, how come all the Audio ones are flawed!
As to $150 for a USB cable all I can say is "An Audiophile and his money isare soon parted"
One would hope with USB there is a transiever at each end (or reciever and transmitter at opposite ends)
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Old 22nd January 2013, 12:33 PM   #13
DF96 is offline DF96  England
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Simple question: do USB DACs buffer and reclock the data or do they rely on incoming data timing (but smoothed by a PLL) like most SPDIF DACs? If the latter then cable quality might make a tiny difference - probably too small to notice and not necessarily better with 'audiophile' cables. If the former then cables can make no difference.

For short cable runs correct termination is far more important than cable quality. To the extent that cable quality does matter, it is engineering issues such as characteristic impedance consistency which count rather than esoteric materials.
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Old 22nd January 2013, 01:48 PM   #14
marce is offline marce  United Kingdom
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Standard digital engineering practices... basicly.
here is an interesting article on USB cables...
http://www.gbs-elektronik.de/fileadm..._USB_Cable.pdf
Now I will concur that not all cables are equal, and getting a decent one that is up to the spec is paramountfor ANY signal transmission. But with digital you need to see the problem (ie use a scope or a network analyser) otherwise you are just guessing or imagining differences..why cos its digital (ie 0s and 1s)or it is in every other field of electronics, when it isn't 0s and 1s (ie the signal can vary, and be any voltage with infinate steps) its called analogue.
Rant over.
As to USB audio I believe there are 3 modes, from another web site:
Synchronous USB connections use a one way digital connection for music replay and are considered the worst type of connection for audio purposes
Adaptive mode is a little smarter. It asesses the amount of data in the frame and adjusts that dac's clock-timing to it.
Asynchronous mode is technically most advanced in that it has a feedback loop so that the amount of data in the frame can be controlled.
So if you use 1 you require the best transmission set up, 2 less so, and 3 all adverts for DACs I have looked at so far using mode 3 reclock.
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Old 22nd January 2013, 02:04 PM   #15
DF96 is offline DF96  England
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Quote:
Originally Posted by marce
all adverts for DACs I have looked at so far using mode 3 reclock.
So provided the cable quality reaches a minimum standard so that the bits are actually delivered undamaged, which almost any cable should achieve, there is precisely zero advantage to be had from a better cable?
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Old 22nd January 2013, 03:31 PM   #16
marce is offline marce  United Kingdom
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Certainly not a $150 dollar one
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Old 22nd January 2013, 04:10 PM   #17
GoatGuy is offline GoatGuy  United States
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The best quote so far, An audiophile and his money are soon parted

Its pretty clear that no one - not the "cables-make-a-difference" crowd, and not the "ones and zeros, get over it!" crowd ... seem to have even a tiny clue about WHAT travels over the digital USB cable.

OK, to cut the suspense: BLOCKS of bits, aligned into sophisticated packets, with error-correction codes, headers, routing information, device information, priority levels and more. The idea that the USB cable is somehow analogous to a digital audio fiber (or copper) cable couldn't be further from reality.

Know this: packetized data simply does not tolerate even single-bit errors. When they occur in an "unprotected" (no ECC=error correction coding) packet, the packet is tossed, and the receiver sends a packet to the transmitter saying, "hey, dude, resend that please". When ECC is part of the picture, even single-bit errors are not lethal to the packet. They're just corrected by the hardware, and the higher level receiver gets clean, perfect packets.

Further, because the USB interface is entirely packetized, both the transmitter and receiver are fully buffering the data stream. The delay - using an oscilloscope - can be as much as 3 or 4 packets, and as little as 1.5. The manufacturers of the respective transmitters and receivers know that USB contention will delay packets, so digital buffering is used to keep data from dropping entirely.

There simply can be NO discernible difference between inexpensive (functional) and ridiculous (functional) USB cables. IF there is, the defect(s) are so serious that there is entire packet loss, and buffer under run, which can sound at the worst like signal breakup and loud popping noise, to (if the manufacturer is preemptive and trying to make a "show must go on no matter what the error stream looks like" device) perceptible as strange nearly musical blurps and waveform inversions, repeats and in-fills.

No. If your cable is sturdy, untrampled, not made of dirt cheap worn filaments, but per-spec twisted, stranded wire, if it is shielded (more to prevent creating interference, than receiving it!) and again - mechanically sound, then there can be no difference in the delivered stream of digital data, that we call "music". Packetized communications simply won't allow for there to be a difference.

Hence ... why trillions of digital packets are accurately transmitted between billions of internet devices, every day. Oh, there are a lot of packet-transmission errors ... but the use of packetized data, low-level ECC and auto-retransmission-on-error and other techniques guarantee that YOUR data gets to the destination, all in all.

GoatGuy
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Old 22nd January 2013, 04:19 PM   #18
DF96 is offline DF96  England
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Quote:
Originally Posted by GoatGuy
Its pretty clear that no one - not the "cables-make-a-difference" crowd, and not the "ones and zeros, get over it!" crowd ... seem to have even a tiny clue about WHAT travels over the digital USB cable.
Which is precisely why I asked a simple question, and received a helpful answer from marce. I don't carry round in my head details of every interface, although I suspected that USB might be packet based.

So my conclusion that correct bit delivery is all that is needed is therefore incorrect? USB can cope with less than this, provided it is not much less than this so most bits are delivered safely. High BER will cause stutters. Sufficiently low BER will deliver perfection.
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Old 22nd January 2013, 05:16 PM   #19
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Quote:
Originally Posted by GoatGuy View Post
Know this: packetized data simply does not tolerate even single-bit errors. When they occur in an "unprotected" (no ECC=error correction coding) packet, the packet is tossed, and the receiver sends a packet to the transmitter saying, "hey, dude, resend that please".
This is unfortunately not entirely correct. Most USB audio devices of generations up to the current use isochronous transfer, where there is no provision for retransmission in the case of errors. Isochronous clocking depends on the host's clock, the data is transmitted willy-nilly, and the DAC must adapt its clock to accommodate the transmitted data rate, usually using a PLL. This mode can be more accurately described as adaptive isochronous. Drivers for this mode are provided in later versions of Windoze and Linux.

Adaptive asynchronous is similar to adaptive isochronous in that the data is sent willy-nilly, but a local clock is employed, and any difference in speed is accommodated using ASRC (asynchronous sample rate conversion).

'Bulk' transfers do permit retransmission, but are not generally employed for audio, because timely transmission is not guaranteed. Some designers have employed bulk transfer for audio with a local clock, but they are very much in a minority, and the system requires a custom driver to be written.

With the arrival of USB audio class 2, asynchronous devices have appeared, where the host sends data on demand and a totally independent local clock can be employed.
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Old 22nd January 2013, 05:18 PM   #20
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Quote:
Originally Posted by GoatGuy View Post
The best quote so far, An audiophile and his money are soon parted

Its pretty clear that no one - not the "cables-make-a-difference" crowd, and not the "ones and zeros, get over it!" crowd ... seem to have even a tiny clue about WHAT travels over the digital USB cable.

OK, to cut the suspense: BLOCKS of bits, aligned into sophisticated packets, with error-correction codes, headers, routing information, device information, priority levels and more. The idea that the USB cable is somehow analogous to a digital audio fiber (or copper) cable couldn't be further from reality.

Know this: packetized data simply does not tolerate even single-bit errors. When they occur in an "unprotected" (no ECC=error correction coding) packet, the packet is tossed, and the receiver sends a packet to the transmitter saying, "hey, dude, resend that please". When ECC is part of the picture, even single-bit errors are not lethal to the packet. They're just corrected by the hardware, and the higher level receiver gets clean, perfect packets.

Further, because the USB interface is entirely packetized, both the transmitter and receiver are fully buffering the data stream. The delay - using an oscilloscope - can be as much as 3 or 4 packets, and as little as 1.5. The manufacturers of the respective transmitters and receivers know that USB contention will delay packets, so digital buffering is used to keep data from dropping entirely.

There simply can be NO discernible difference between inexpensive (functional) and ridiculous (functional) USB cables. IF there is, the defect(s) are so serious that there is entire packet loss, and buffer under run, which can sound at the worst like signal breakup and loud popping noise, to (if the manufacturer is preemptive and trying to make a "show must go on no matter what the error stream looks like" device) perceptible as strange nearly musical blurps and waveform inversions, repeats and in-fills.

No. If your cable is sturdy, untrampled, not made of dirt cheap worn filaments, but per-spec twisted, stranded wire, if it is shielded (more to prevent creating interference, than receiving it!) and again - mechanically sound, then there can be no difference in the delivered stream of digital data, that we call "music". Packetized communications simply won't allow for there to be a difference.

Hence ... why trillions of digital packets are accurately transmitted between billions of internet devices, every day. Oh, there are a lot of packet-transmission errors ... but the use of packetized data, low-level ECC and auto-retransmission-on-error and other techniques guarantee that YOUR data gets to the destination, all in all.

GoatGuy
Thanks for the great info. I did not know (or was too lazy to dig up the info) that USB data was packetized. Sorry for my ignorance or lack of curiosity. I sometimes take things for granted .

Just as you stated:
http://www.ftdichip.com/Support/Docu...0Structure.pdf

USB mmade simple
USB Made Simple

They also have analysers
Packet-Master - Buy or Rent

and lots of detailed info here:
USB in a NutShell - Chapter 1 - Introduction

The packets vary in size, but have a max size of about 8K and with 2 CRC fields.

All we need now are WAN accelerators at both ends of the USB cable to compensate for latency on long runs

I'll stick to the good quality cable and that's it. I will spend my money on better sources or DACs (and that's another interesting discussion). I'm just happy I asked the question after all.. At least, I will take an informed decision There is still so much to learn about the reproduction of analog music in the digital age... sources, codecs, drivers, DACs ...

Now I have more money for whisky too but it's usally not much cheaper than a SUPER USB cable
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Last edited by lowtherdream; 22nd January 2013 at 05:24 PM.
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