Digital Cable

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KingStyles

Digital Cable
« on: 13 May 2011, 06:32 am »
I have seen your white paper that a digital cables optimum length is 1.5m. I know that depends on a few factors. Do you feel that with the offramp 4 you need a 1.5m cable for best results or will a 1m work as good.

Elizabeth

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Re: Digital Cable
« Reply #1 on: 13 May 2011, 06:44 am »
A different look is that the rise time of the average signal in a digital transfer at the receiving end is 25 nano seconds. The signal travels down the wire about (roughly) 2 nano seconds per foot.
(now it could be done faster, but that creates problems, so the usual setup runs around 25 nanoseconds for the signal rise time)
So then you have the inevitable reflection, if it is an RCA, they just are not 75 ohms. (and the stuff in the receiving end mostly is not really a perfect 75 ohms, so you are going to have reflections travelling back up the wire, then bouncing back at the origin)
So the signal takes 25 nano to peak from zero, well if the wire is short, the bounce and back is going to mess with the system getting the correct reading of the initial pulse. and thus a problem with the signal getting messed up is added.
So if your wire is two meters, or six feet, then the bounce and back is 6 ft plus six ft  times two nono per ft or 24 nonoseconds and the bounce back hits and the wave bump arrives back just at the moment you want.
Now this is a pretty lame condensation of various things I have read. But it is a good enough reason to buy a longer wire.
Right now i have a 26 ft wire because it has just by chance some so called 75 ohm RCA crimp on connectors, and I thought I would try it. It has less high frequency hash than a shorter cable.
I plan on buying a 13ft cable.
But  i would go with a minimum of two meters if you want it shorter. but not under that.
Longer is not nearly as much of a problem as too short.
If the DAC has a faster rise time, the the wire could be shorter, but like I mentioned, too long does not seem to be nearly as much of a problem as too short.

KingStyles

Re: Digital Cable
« Reply #2 on: 13 May 2011, 05:22 pm »
is the rise time controlled by the dac or by the cdp/offramp? I guess I was asking because if it is controlled by the offramp, it might have a better rise time allowing for shorter cables. Or I may be way off.

audioengr

Re: Digital Cable
« Reply #3 on: 13 May 2011, 08:41 pm »
I have seen your white paper that a digital cables optimum length is 1.5m. I know that depends on a few factors. Do you feel that with the offramp 4 you need a 1.5m cable for best results or will a 1m work as good.

The white paper does essentially the analysis that Liz does above.  It however is based on other companies products, which have slower risetimes than the Off-Ramp 4.  The Off-Ramp 4 is typically in the 3nsec range, so a 1m cable is usually good enough.  I actually use a 1.5m length.  That way I can use it with more devices.  This is what I recommend.

The problem with really long cables is losses, which result in more jitter at the receiver, as well as the opportunity for second and third reflections to cause jitter.

Steve N.

JayNYC

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Re: Digital Cable
« Reply #4 on: 9 May 2018, 08:29 pm »
Hi.  Reviving a discussion from 7 years ago....  is there a way for an end consumer to measure the rise time of the SPDIF/AES outputs on his/her devices?   Asked differently, what product or set of products/software would a consumer buy to be able to measure the rise time and then *know* for sure what his/her options are for cable length?


Speedskater

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Re: Digital Cable
« Reply #5 on: 9 May 2018, 08:37 pm »
Because SPDIF/AES signal is low-pass filtered per FCC rules, rise time is not a factor.

Speedskater

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Re: Digital Cable
« Reply #6 on: 9 May 2018, 08:43 pm »
Only if the digital output stage or digital input stage are incorrectly designed, can cable length matter. 1 inch or 100 feet, no difference in a well designed digital interconnect system.

JayNYC

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Re: Digital Cable
« Reply #7 on: 9 May 2018, 08:51 pm »
Only if the digital output stage or digital input stage are incorrectly designed, can cable length matter. 1 inch or 100 feet, no difference in a well designed digital interconnect system.

@speedskater perhaps I am looking at an old article from 2004 which is no longer current?
https://positive-feedback.com/Issue14/spdif.htm


audioengr

Re: Digital Cable
« Reply #8 on: 11 May 2018, 08:36 pm »
Hi.  Reviving a discussion from 7 years ago....  is there a way for an end consumer to measure the rise time of the SPDIF/AES outputs on his/her devices?   Asked differently, what product or set of products/software would a consumer buy to be able to measure the rise time and then *know* for sure what his/her options are for cable length?

If you have a fast oscilloscope and know how to use it. You would need at least 100MHz bandwidth and even that is marginal.  I use 7GHz scope to measure risetimes.

The safe thing is to get 1.25-1.5m length.

Steve N.

audioengr

Re: Digital Cable
« Reply #9 on: 11 May 2018, 08:45 pm »
Because SPDIF/AES signal is low-pass filtered per FCC rules, rise time is not a factor.

Misinformation. S/PDIF signal if made as fast as possible reduces jitter significantly.

The 60958-3 spec says:
"The time difference between the 10% and 90% points of any transition shall be less than 0.4UI."

UI is half the period.

This means that one can make the risetime as fast as you want.

I have had my Synchro-Mesh reclocker measured for emissions and it has risetime of less than 700psec.  Passed emissions testing.  If other companies cannot pass emission testing without low-pass filtering their outputs, then that is their problem.

Steve N.

audioengr

Re: Digital Cable
« Reply #10 on: 11 May 2018, 08:48 pm »
Only if the digital output stage or digital input stage are incorrectly designed, can cable length matter. 1 inch or 100 feet, no difference in a well designed digital interconnect system.

Also misinformation.

As the length increases significantly above 10 feet in even the lowest loss cable, the losses and dielectric absorption cause waveform distortion and slower risetimes, which delivers more jitter to the destination receiver.  This will be audible, unless there is reclocking circuitry that is immune to jitter in the DAC.  Most DACs don't qualify.

Steve N.
Empirical Audio