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What is ideal length for a USB audio cable that runs from music server to DAC?


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Need to buy a new USB cable to go from my server to the DAC. Could make do with 1 meter but I vaguely remember hearing that 1.5 m is ideal (reduce reflections inside the cable etc.)

Anybody know more about this?

Thanks.

 

That 1.5M-2M optimal length refers to 75Ω coax digital cables. I can't speak to the validity of that belief, but, for USB, you should use the shortest length possible.

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Need to buy a new USB cable to go from my server to the DAC. Could make do with 1 meter but I vaguely remember hearing that 1.5 m is ideal (reduce reflections inside the cable etc.)

 

Anybody know more about this?

 

Thanks.

 

 

Robert Harley does:

 

[h=1]Why Does Bob Harley Recommend 1.5m Length for USB Cables?[/h] upvote.png 0 downvote.png

At the bottom of page 186 of the 5th edition of Robert Harley's book The Complete Guide to High-End Audio, he says this: "Finally, here's a hint regarding USB cable length. Unlike speaker cables and analog interconnects, where shorter is always better, the best-sounding USB cables are 1.5 meters long."

Now, why would that be so, I wonder? It hardly seems believable on the face of it, but perhaps there is a reason.

 

 

 

June 11, 2015 12:40 a.m.

 

 

 

 

[h=1]1 Answer[/h] Post an Answer

upvote.png 2 downvote.png

I have been told by several of the industry's brightest digital designers that 1.5m USB is preferable to 1m or to 2m. The reason involves signal reflections in the cable, which are a source of jitter. I don't understand the precise mechanism by which this occurs, but the 1.5m suggestion correlates with my listening experience.

 

Answered by RobertHarley

July 12, 2015 4:32 p.m.

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Since appeal to authority trumps basic facts here, this quotation clearly establishes the truth.

 

The reflection problem in coax cable appears in analogue TV transmission, where a short cable could cause ghosting. The claim is the digital version of this increases jitter, but unlike the analogue case, the digital transmission isn't high frequency. So it is a double stretch to say this applies to non-coaxial USB cables as well.

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Since appeal to authority trumps basic facts here, this quotation clearly establishes the truth.

 

The reflection problem in coax cable appears in analogue TV transmission, where a short cable could cause ghosting. The claim is the digital version of this increases jitter, but unlike the analogue case, the digital transmission isn't high frequency. So it is a double stretch to say this applies to non-coaxial USB cables as well.

 

And every decent USB DAC has a local clock anyway.

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Could make do with 1 meter but I vaguely remember hearing that 1.5 m is ideal (reduce reflections inside the cable etc.)

 

The original deliberation was that in a digital transmission you would not want the reflected signal to arrive at the receiver's end during the middle of a transition from low to high (or vice versa). Since receivers do not detect levels, but transitions, this might disrupt the timing of the reconstructed signal.

 

usb_20.jpg

 

With the assumption of signal speed c1/c0 = 0.75 and rise time around 25 ns you arrive at l = 2.8125 m. Since the (reflected) signal has to travel this distance twice (lambda/2), you have a minimum cable length of 1.4 m.

 

While 20-25 ns may be a realistic (but probably on the high side) value for S/PDIF transmissions [1], USB needs higher slew rates: 4-20 ns for full speed [2], 500 ps for high speed [3]. The latter reduces minimum cable length to approximately one inch.

 

[1] 93rd AES Convention, SF 1992, preprint 3360, p.5: We have measured 2 m interface links with a characteristic impedance of 75 ohms, correctly terminated both at the transmitter and receiver, with 10 - 90 % rise and fall times of less than 10 ns; this level of performance corresponds to a bandwidth of 35 MHz.

[2] Universal Serial Bus Specification Rev. 2.0 p.130: The rise and fall times must be between 4 ns and 20 ns and matched to within ± 10% to minimize RFI emissions and signal skew.

[3] Universal Serial Bus Specification Rev. 2.0 p.139: For a hub, or for a device with detachable cable, the 10% to 90% high-speed differential rise and fall times must be 500 ps or longer when measured at the A or B receptacles (respectively).

Primary ::= Nabla music server | Mutec MC-3+USB w/ Temex LPFRS-01 RB clock | WLM Gamma Reference DAC; Secondary ::= Nabla music server | WaveIO | PrismSound Lyra

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