Digital Cable Length

[fmak]

A lot of high cost digital cables (spdif and aes/ebu) are being offered in 0.5 m lengths to reduce cost. Does this pose theoretical and real sonics problems? What is the consensus?

[Fred Dieckmann]

Many of the 1/2 meter digital cables I have heard sound much worse than the 1 meter version. The reflection time for that length of cable can put the refected signal back on the rising edge on the digital signal. This depends on the rise of the cable driver circuit and the velocity factor of the coax. I have found 1.5 meters to sound better than one meter cable and that was the standard length of the cables I designed for Audient Technologies.

[UrSv]

Originally posted by Fred Dieckmann
Many of the 1/2 meter digital cables I have heard sound much worse than the 1 meter version.

Any explanation as to why?

[Fred Dieckmann]

Originally posted by UrSv


Any explanation as to why?

yes

[tiroth]

cable resistance damps reflections

[peranders]

I shouldn't interfere here but is reflection really something to care about? How much do you really get?

This is a constant defect and do you really get distorted signals, after the reciever? 1 meter is 5 ns in one direction.

[Fred Dieckmann]

"I shouldn't interfere here but is reflection really something to care about?"

Only if you actually care what it sounds like. It is pretty audible in my experience. I believe Jock did some experimants with reducing the cable length incrementally and listening to the results. He said it was possible to find a length that sounded really bad.

How much do you really get?

Enough to be easily measureable on a 100MHz scope. Obviously it is a function of the impedance discontinuity, signal rise time, cable velocity factor, length, and lossiness. he likes loooong cables, like 10 feet and more. Lossy cables also often work well.

This is a constant defect and do you really get distorted signals, after the reciever?

Reflections cause noise. If timing is such that the reflection returns at the part rising edge where the the receiver is looking for a logic transistion, jitter can occur. The right reflection time can also result in undershoot or increased ringing. They can also contribute to EMI and the effects of that on sonics can be pretty bad. Don't foget that these reflections bounce back from the source end to the reciever. The presence of EMI, logic power supply noise, and receiver hysteresis compond this problem.

1 meter is 5 ns in one direction

That depends on the velocity factor of the cable and is a function of the dielectric constant of the cable insulator. Interestingly digital cable are often directional. Differences in termination and the fact that the cables center conductors distance from the sheild result in changes in the characteristic impedance down the cable length. Cable direction change the time delays of the reflections caused by these impedance continuities. One of the audio magazines reported different jitter measurements from changing the cable direction. Anybody still doubt digital cables can sound different.

For a good Signal integrity book:

http://www.sigcon.com/books.htm

I went to one of his seminars also. He invented the field of study of signal integrity which is crucial in high speed logic design.

[UrSv]

Originally posted by Fred Dieckmann
"I shouldn't interfere here but is reflection really something to care about?"

Only if you actually care what it sounds like. It is pretty audible in my experience. I believe Jock did some experimants with reducing the cable length incrementally and listening to the results. He said it was possible to find a length that sounded really bad.

How much do you really get?

Enough to be easily measureable on a 100MHz scope. Obviously it is a function of the impedance discontinuity, signal rise time, cable velocity factor, length, and lossiness. he likes loooong cables, like 10 feet and more. Lossy cables also often work well.

This is a constant defect and do you really get distorted signals, after the reciever?

Reflections cause noise. If timing is such that the reflection returns at the part rising edge where the the receiver is looking for a logic transistion, jitter can occur. The right reflection time can also result in undershoot or increased ringing. They can also contribute to EMI and the effects of that on sonics can be pretty bad. Don't foget that these reflections bounce back from the source end to the reciever. The presence of EMI, logic power supply noise, and receiver hysteresis compond this problem.

1 meter is 5 ns in one direction

That depends on the velocity factor of the cable and is a function of the dielectric constant of the cable insulator. Interestingly digital cable are often directional. Differences in termination and the fact that the cables center conductors distance from the sheild result in changes in the characteristic impedance down the cable length. Cable direction change the time delays of the reflections caused by these impedance continuities. One of the audio magazines reported different jitter measurements from changing the cable direction. Anybody still doubt digital cables can sound different.

For a good Signal integrity book:

http://www.sigcon.com/books.htm

I went to one of his seminars also. He invented the field of study of signal integrity which is crucial in high speed logic design.

Thanks, kind of what I thought. I know all to well what reflections can do after working with networking for a fair amount of time and learning quickly the difference between proper and bad termination.

As a reflection (!), did you, or somebody else that you know of, ever test to use a much longer cable than needed and put connectors at reasonable distance, say less than 1 m, but extending the line some extra meters from both end points and terminating there. I just thought that maybe one could get the reflection attenuated slightly and moving out in time to where it could be less harmful. Stupid or interesting?

[tiroth]

If you have reflections at the reciever, they propagate from the reciever, not the end of the cable.

You might /also/ get reflections from the cable end though with this termination. Not to mention that driving a 25-ohm line is rather hard. ^_^

[Cobra2]

UrSv, this was nearly how we used to tune our PR-radio antennas! (with SWR-meter).

Arne K