USB cable length

I wonder how many people had the opportunity to compare the same aftermarket cable in different lengths., posted on September 11, 2014 at 08:20:30
carcass93 Audiophile Posts: 7181 Location: NJ Joined: September 20, 2006	I suspect only our manufacturer inmates - if you're lucky. I suspect the sound quality should degrage somewhat, but don't have any first-hand experience, using 0.5m Oyaide 5S myself at the moment (2-legged Acoustic Revive is in the plans).

RE: USB cable length, posted on September 11, 2014 at 11:17:31
audioengr Manufacturer Posts: 6017 Location: Oregon Joined: April 12, 2001	I recommend to my customers 1.25-1.5m minimum on any digital cable. This moves the reflections on the cable so they are outside the time that the receiver detects the signal transition. See: http://www.positive-feedback.com/Issue14/spdif.htm

RE: USB cable length, posted on September 11, 2014 at 11:35:13

Tony Lauck
Audiophile

Posts: 13629
Location: Vermont
Joined: November 12, 2007

Your recommendation is based on SPDIF. It has no relationship to USB.

Signaling rate on USB 2.0 is much higher, so these arguments don't apply. Also, USB is a packet bus which further complicates the issue. There is no relationship between the data clock rate on USB and the audio clock rate. There is a relationship between the packet clock rate on USB and the audio clock rate, but the packet clock rate is 1000 Hz, so the wavelength is measured in hundreds of kilometers.

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

This thread might be informative, posted on September 11, 2014 at 11:49:11

G Squared
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Posts: 8491
Location: Washington, DC Metro Area
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Contributor
Since: May 23, 2023

nt
Gsquared

http://db.audioasylum.com/mhtml/m.html?forum=pcaudio&n=13737

I did the comparison, posted on September 11, 2014 at 12:07:57
Recorder Audiophile Posts: 311 Location: Washington DC Joined: November 1, 2007	I had a 1 m stereovox hdxv SPDIF cable and then read Steve Nugent's Positive Feedback article. I then bought a 1.5 M version of the same model and noticed an improvement. It wasn't huge, but it clearly made a difference, so I sold the 1 m cable. I can't give you a detailed account of the differences, since this was several years ago. Cheers, Bill

RE: USB cable length, posted on September 11, 2014 at 13:03:57
Sprezza Tura Audiophile Posts: 4585 Location: New York City Joined: August 24, 2012	Lots of computer audio mythology out there. And sadly, being propagated by manufacturers. No wonder folks are sticking with CDs and vinyl.

This is valuable, and confirms audioengr's findings. However, that's not USB cable., posted on September 11, 2014 at 13:07:06
carcass93 Audiophile Posts: 7181 Location: NJ Joined: September 20, 2006	The challenges there are different (see Tony's post above), and it's highly possible that the same rules do not apply. My gut tells me that it's "the shorter, the better" in case with USB cables.

RE: USB cable length-Audioengr, posted on September 11, 2014 at 15:13:03

fmak
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Posts: 13158
Location: Kent
Joined: June 1, 2002

was not discussing data rate. He was talking about how the signal was being reflected from the termination end and how this affects the signal.

Now I personally prefer short usb cables in my systems. having made up several sets with the same 5m or 7m lengths of Wireworld usb cables. The way in which the shield is grounded makes as much difference as anything else but this does not mean that Steve is wrong.

RE: This is valuable, and confirms audioengr's findings. However, that's not USB cable., posted on September 11, 2014 at 15:19:41
fmak Audiophile Posts: 13158 Location: Kent Joined: June 1, 2002	See my post above. Steve's remarks has nothing to do with spdif, it is to do with digital signal transmission. I do find long usb cable difficult to dress in my setups and prefers short one, from which I get v good sound. However, the sound does change with cable length using cut lengths from the same cable but this has not prevented me from using 0.3-0.5m lengths..

RE: USB cable length-Audioengr, posted on September 11, 2014 at 20:25:37

Tony Lauck
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Posts: 13629
Location: Vermont
Joined: November 12, 2007

"Audioengr was not discussing data rate. He was talking about how the signal was being reflected from the termination end and how this affects the signal."

I question if you have any technical understanding whatsoever about how digital signals travel down wires. You should really take more care in your posting if you actually know what you are talking about, lest you come across as a blustering idiot.

The impact of reflections depends on where the reflections fall in the digital waveform. When a reflection hits a point where the waveform is transitioning from a 0 to a 1 or conversely then it will affect the time the receiver detects the transition ("jitter"). This effect depends on the length of the cable from the point of reflection compared to the wavelength of the bits on the wire. With a much higher clock rate (USB 2.0) the wavelength will be much shorter.

Carcass93's intuitive understanding of the situation "shorter is better" makes perfect sense as far as the USB itself is concerned, but there may be other reasons dictating the use of longer cables, e.g. some equipment may best be kept physically separated from other equipment.

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

Understanding?, posted on September 11, 2014 at 23:58:25

fmak
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Posts: 13158
Location: Kent
Joined: June 1, 2002

''The impact of reflections depends on where the reflections fall in the digital waveform. When a reflection hits a point where the waveform is transitioning from a 0 to a 1 or conversely then it will affect the time the receiver detects the transition ("jitter"). This effect depends on the length of the cable from the point of reflection compared to the wavelength of the bits on the wire. With a much higher clock rate (USB 2.0) the wavelength will be much shorter.''

So what? You state the obvious which does not contradict what audioengr said. How and where the reflections affect transitions depend on a given system, impedance matching, detector performance, cable return loss etc.

I would advise you to stop making simplistic, 1D theory and irrelevant observations so very frequently.

RE: USB cable length, posted on September 12, 2014 at 11:54:24
audioengr Manufacturer Posts: 6017 Location: Oregon Joined: April 12, 2001	Has less to do with the signaling rate and more the risetime, which is similar on my S/PDIF and USB. Steve N.

RE: USB cable length, posted on September 12, 2014 at 15:18:41
fmak Audiophile Posts: 13158 Location: Kent Joined: June 1, 2002	Forget it, the guy doesn't know what he's on about. But it's not only risetime but also discontinuities in the up and down ramps.

RE: USB cable length, posted on September 13, 2014 at 11:28:27

Tony Lauck
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Posts: 13629
Location: Vermont
Joined: November 12, 2007

The assumption on which your Positive Feedback article was based what that the time of signal transition mattered. This is correct with SPDIF, because this is how the audio clock is encoded. Unless some kind of reclocking is used jitter in this clock signal will make it directly to the clock used to do the digital to analog conversion. With regard to USB there is no relationship between the bit clock used to transmit individual bits in a USB packet. Jitter on this clock is only relevant if it becomes large enough to close the eye pattern at the receiver and cause data errors.

Going into a little more detail on how SPDIF works, it is useful to contrast it to the simpler I2S. Clock transitions on SPDIF depend on reflections from previous signals on the wire. These signals are constantly changing, because of changing audio data bits. This means that the time of transition at the receiver will be constantly varying due to cable reflections. Now contrast this with I2S. With I2S there is a separate clock wire. Reflections on this clock wire will not cause jitter, because the clock signal is periodic. The most any reflections will do is to create a constant phase shift in the transition time of the clock signal at the receiver. This won't add any jitter to the received clock.

USB is vastly more complex than SPDIF and I2S, being a packet based system. USB transmits audio data ischronously, with one frame sent every 1 msec, with the number of samples in each frame depending on the audio sample rate. The frame rate must be kept synchronized with the DAC master clock.

In the original USB audio, the frame rate was controlled by the source and the DAC forced to derive its master clock from the frame rate. Note that the frame rate has nothing to do with the data rate used to transmit bits on the USB, which is a separate clock altogether. The USB data clock does not have to be synchronized to the USB frame rate. For this reason, jitter on the USB data is irrelevant to the audio clock, except as to determination of the start of frame. This is handled by the USB protocol with each frame starting with a fixed header. At the time a frame is transmitted, the USB cable is completely idle so there will be no reflections. As the header is transmitted there will be reflections, but as the same header is transmitted with each frame the impact of these reflections will be constant. Hence cable reflections will not be a source of jitter. (There is a much worse source of jitter when running at 44.1 kHz, namely the fact that the audio clock is not an exact multiple of the frame rate, causing serious audio quality issues with early USB receiver chips.)

With the newer asynchronous USB audio, the connection between the frame clock and the DAC master clock is broken. The DAC master clock runs at its own rate. The computer's frame clock runs at its own rate, and the size of data in each packet is adjusted to keep buffers from underflowing or overflowing. With this mode there is no connection between the timing of signals on the USB cable and the DAC master clock. If transport or cable changes affect the sound it is not due to jitter, its must be due to other factors, such as noise coupling between portions of the DAC circuitry.

As to rise times. The data rate on USB is much higher than SPDIF (except the older USB 1.1 which is not suitable for hi-res audio). This requires fast rise times, otherwise there won't be an open eye pattern. With the slower SPDIF it is debatable whether fast risetimes are desirable. They reduce the jitter impact of noise at the receiver, but at the expense of creating high frequency noise that may be hard to keep away from critical DAC circuitry. Better to have a system architecture that avoids this dilemma. Both I2S and USB meet this criteria.

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

http://www.positive-feedback.com/Issue14/spdif.htm

RE: USB cable length, posted on September 13, 2014 at 20:04:40
internethandle Audiophile Posts: 42 Location: CA Joined: November 30, 2011	I've long since stopped reading this forum except for skimming due to the relative lack of insight vs. other CA forums, but somehow fmak has become one of the more insightful 'inmates' and "Tony Lauck" seems to have taken his place. Just endless contrarian prolix babble. "tl;dr" one might say in 2014.

RE: USB cable length, posted on September 14, 2014 at 11:42:19
info@skolis.com Audiophile Posts: 48 Joined: September 14, 2014	Up to 5 meters - ok, beyond that, degrades quickly. but do get a audiophile USB ( not a computer cable) Look at audioquest - they offer various lengths up to 5 meter. nothing beyond that. and in listening tests, you can't hear any difference in shorter lengths.

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USB cable length

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