Topic: Digital Cable Minimum length?

[Roving Sign]

I remember some high-end audio type of argument about making digital coax cables too short?

I was going to make some shorties for a stealth bag...any reason not to make some as short as 6 inches...?

[cshepherd]

Shorter is generally preferred when it comes to cable lengths, with the exception of a/c mains cables that employ filtering properties.  I've never heard of minimum length requirements for spdif cables.

Chris

[cshepherd]

OK, it seems I may be wrong on this one   .  UHF in Canada (http://www.UHFmag.com) aparently took the digital cable length issue to task.  Apparently, there is a reflection that gets sent down the wire when the first bit is transmitted from the source.  When the cable length reaches 1.5m, the first bit reaches its destination before the reflection...and this is good.  It was a big enough difference that UHF is going to phase out digital cables at lengths shorter than 1.5m.  Our Atlas sales rep told us about this today, who is also UHF's Atlas rep (they have a retail outlet and online store as well).  He took some Harmonix spdif cables in varying lengths home to check this out for himself.  These cables cost about $1,000 each.  He said he was completely shocked at how much better the 1.5m cable sounded over the 1m cable.  So much that May Audio (our distributor) is returning all of their spdif cables under 1.5m to the manufacturers.

Just thought I would pass along the info.

Chris

[JasonR]

It sounds like this is an argument against using optical cable shorter than 1.5m.  Is that the case?  I wouldn't imagine there's any such 'reflection' in the world of coaxial digital, unless you believe in the magic goblins that eat jitter and such.   

- Jason

[Roving Sign]

Thanks - that sounds very much like the info I was remembering...

1.5 M - thats pretty long! - like about 5ft right?

I wonder if this is as critical recording a signal as it seems to be feeding a DAC?

[cshepherd]

It sounds like this is an argument against using optical cable shorter than 1.5m.  Is that the case?  I wouldn't imagine there's any such 'reflection' in the world of coaxial digital, unless you believe in the magic goblins that eat jitter and such.   

- Jason

This was definitely about a spdif connection.  Feel free to keep using your imagination though.   

Chris

[rodeen]

This was definitely about a spdif connection.

So what you are saying is any SP/DIF connection whether it is optical or coax?
I guess it's time to get longer cables.  I should go read the article, but did it say
what the issues were?  You talked about reflections but I'm not sure what that means.
Do they cause jitter or is it an issue like standing waves in electromagnetic signal that
causes data loss? 

Interesting topic!

[JasonR]

This was definitely about a spdif connection.  Feel free to keep using your imagination though.   

Chris

SPDIF can be coaxial or optical.  My point was that if the article talked about "reflections" being a problem, that sounds like an optical issue.

- Jason

[MattD]

Reflections can occur in coaxial cable too. From my experience, it's caused by an impedance mismatch, but apparently there are other mechanisms.

[cshepherd]

The topic came up over Atlas cables, and they don't make optical cables.  UHF does a lot of business in Atlas so it was news when they asked to return all their 1m digital cables.

Chris

[cshepherd]

Here's the review.  The whole issue is in one pdf file, the review is on pg. 24.  UHF used two Atlas digital interconnects, the Compass ($93) and the Opus All Cu ($278), in both 1m and 1.5m lengths in a blind comparison.  The differences were more dramatic in the cheaper interconnect, but the Opus All Cu also benefitted from the extra length.  They weren't talking about splitting hairs either.  It's an interesting read.  It does a pretty good job of explaining how 1's and 0's can actually have trouble with signal integrity.

The theory was published some time ago in Positive Feedback, written by Steve Nugent of Empirical Audio.  The readers digest version is that the longer cable gives the DAC more time to sample the data ('transmission" as it's referred to in the article) before its reflection hits the DAC.  The reflection travels from the DAC to the transport and then bounces back to the DAC.

http://www.uhfmag.com/Issue74/UHF74.pdf

Chris

[SparkE!]

When cables are connected to a device whose input impedance is the same as the intrinsic impedance of the cable itself, there will be NO reflections from that end of the cable.  S/PDIF is supposed to be 75 ohms, so if you are using 50 ohm cable, you're going to get reflections.  If you are careful to use 75 ohm cable and 75 ohm connectors and your S/PDIF equipment is properly designed with 75 ohm port impedance, it doesn't matter how long the cable is, except that the cable will attenuate the signal.  Accepted design practice is to limit S/PDIF cables to less than 10 meters in length.  Misinformation like Nugent is spreading is usually based in some tiny morsel of truth, but the fact is  that if the bits get through the cable unaltered, a properly designed playback system will not be affected by the cable length of the S/PDIF cable that feeds it.  The only way that Nugent's argument has real life merit is if you are using cables of the wrong impedance.  In that case, length does matter.

When the cable length reaches 1.5m, the first bit reaches its destination before the reflection...and this is good.

Let's do a little math...  48kHz times 16 bits times 2 channels is 1.536 Mb/s.  Each bit, then, is about .65 us long. In .65 us, the pulse could travel about 195 meters at the speed of light.  The speed of propagation on a cable is roughly 2/3 the speed of light, so we're talking a cable that is about 130 meters long if the cable is as long enough to hold a whole bit at once.  The cable would have to be half that long for the bit to be reflected back to the input at the time the next bit starts to be transmitted.  So, from your description of the cable, we'd need a cable that was longer than 65 meters!

Remember, when someone is trying to justify selling $1000 S/PDIF cables, they're going to have to resort to fairy tales in order to sell any of them.

The bottom line is if you use the correct cable impedance, it does not matter how long the cable is, except that the cable cannot be so long as to unacceptably attenuate the signal it carries.

[cshepherd]

Nobody was selling $1000 digital cables.  The cables UHF used to confirm validity of Nugent's claim awere Atlas' entry-level and top-end digital ables ($93 and $278 for 1m).  Our rep didn't believe UHF either so he grabbed the most expensive cables they carried to do a similar comparison, which were from Harmonix.  Actually, the 1.5m cable he used sells for $1700.  Different cables, different ears, same results.

Nugent goes through some different math so I'm just going to copy the text from the article.

Chris

"There is a short but finite time between the initial rise of the signal voltage and its arrival at full voltage. The receiver circuit, at the front of the converter, must interpret the signal, and “decide” when it will consider that a “1” has arrived. That interpretation presents a potential for an error in timing, unless it is absolutely consistent from one data block to the next.  In fact, it may not be.  Because neither the circuits nor the connectors nor the wire are perfectly matched for impedence, some of the energy arriving at the converter will actually bounce back toward the transport, from which it will bounce back again. Thus the converter will receive the imperfect square wave, and its echo. Now we have potential for confusion. Here’s Nugent’s take:

When a transition is launched into the
transmission line, it takes a period of time
to propagate or transit to the other end.
This propagation time is somewhat slower
than the speed of light, usually around
2 nanoseconds per foot, but can be longer…
When the transition reaches the end of the
transmission line (in the DAC), a reflection
can occur that propagates back to the driver
in the transport. Small reflections can occur
in even well matched systems. When the
reflection reaches the driver, it can again
be reflected back towards the DAC. This
ping-pong effect can sustain itself for several
bounces depending on the losses in the cable. It
is not unusual to see 3 to 5 of these reflections
before they finally decay away.
So, how does this affect the jitter? When
the first reflection comes back to the DAC,
if the transition already in process at the
receiver has not completed, the reflection
voltage will superimpose itself on the transition
voltage, causing the transition to shift in
time. The DAC will sample the transition
in this time-shifted state and there you have
jitter.
If the rise-time is 25 nanoseconds and the
cable length is 3 feet, then the propagation
time is about 6 nanoseconds. Once the transition
has arrived at the receiver, the reflection
propagates back to the driver (6 nanoseconds)
and then the driver reflects this back to the
receiver (6 nanoseconds) = 12 nanoseconds).
So, as seen at the receiver, 12 nanoseconds
after the 25 nanosecond transition started,
we have a reflection superimposing on the
transition. This is right about the time that
the receiver will try to sample the transition,
right around 0 volts DC. Not good. Now if
the cable had been 1.5 metres, the reflection
would have arrived 18 nanoseconds after
the 25 nanosecond transition started at
the receiver. This is much better because
the receiver has likely already sampled the
transition by this time.

[Brian Skalinder]

Okay, so it seems this is an explanation of one potential cause of jitter.  It seems a simple buffer would solve the jitter problem.  Do not quality DACs have a buffer in which to store the 0s and 1s as they arrive, re-clocking them to the proper timing upon playback?  Seems (to me) like a simple solution, regardless of the cable used.

[Roving Sign]

It seems like they are counting on this:

"He said he was completely shocked at how much better the 1.5m cable sounded over the 1m cable.  So much that May Audio (our distributor) is returning all of their spdif cables under 1.5m to the manufacturers."

...to bolster their argument and make the sell - as much as any real eveidence.

I'm always super skeptical about the "sound" of digital cables...as longs as it locks - its working.

On the other hand - I think DATs can sound REALLY wierd - especially on these old Sony D7/D8s...its like the signal is working but sounds unfocused. When you get them transfered on a full size machine - they seem to sound much better...