Minimum length for 75Ω coax cable (Revisited)

[Speedskater]

How can you say there are absolutely no impedance mismatches from Tx chip to Rx chip though the cable-connector interface, to through the circuit board and through to the silicon? And to say this with enough confidence to neglect the small and simple accommodation of just using a 1.5m wire? Surely, not all SPDIF sources and receivers are built to a perfect ideal.

 

Belden had a short note in a newsgroup about 10 years ago.

 

Turns out that with an cable impedance mis-match of about a 50 Ohm cable in our 75 Ohm system. Once the cable is maybe 40 feet long you could have problems.

 

Thinks about RCA/BNC connector impedance's at the very low SPDIF frequencies is just silly.

[junker]

Moreover the description of the XS Adapter on the manufacturers site only describes it as an RCA adapter. It would give you a pretty short cable! I've heard that you can go very very short with SPDIF cables too (<0.15m), so this would probably work as a minimal cable.

 

ViaBlue™ XS Adapter Cinchverlängerung | vergoldet

 

Product description:

 

ViaBlue™ XS adapters RCA jack to RCA jack are used for extending cables. High-end internal contacts of the ViaBlue™ XS adapter series guarantee absolutely low resistance transmissions of audio and video signals. The contact surfaces are 24 karat gold-plated.

 

Even though I think it is good practice to not go <1.5 with SPDIF cables, I was primarily referring to BNC even though I suppose it applies equally well to RCA.

 

3) I've used this:

[ATTACH=CONFIG]7333[/ATTACH]

 

My "2 cents" opinion? I've used as S/PDIF cable one of this:

[ATTACH=CONFIG]7381[/ATTACH]

 

It works. Very well

 

No, that's just RCA connectors. Adding probably a bunch of reflection and jitter. The original page doesn't even state nothing about 'quality', just that it will let you extend cables. Use Google translate:

Audio Hifi Shop - Viablue XS Adapter - Cinch-Stecker auf Cinch-Stecker Angebot

[SoNic67]

Thinks about RCA/BNC connector impedance's at the very low SPDIF frequencies is just silly.

It's not about frequency (that would imply attenuations like on analog signals), but transitions, since SPDIF is digital and all about transition timing and eye openings. This is why REFLECTIONS are important.

[drez]

I would just stick to minimum length as a safest policy, or if you are really curious buy different lengths of the same cable and compare. I never found any of the longer lengths to sound more focussed or precise than the 45cm cable I am using. Bnc attenuators similarly don't really improve things, just makes things more smooth.

 

So in summary I wouldn't worry about signal reflections, especially if you are using expensive cables. If you are using a cheap cable then sure try for yourself with different lengths.

[SoNic67]

Reflections are not generated by the cable itself, but by your devices (transport - DAC). Cable length just helps masking imperfections in devices.

Some devices are better build, and they have proper impedance terminations, therefore they are less sensitive to cable length of RCA/BNC connectors.

The minimum 1.5m is a general rule and exceptions to that rule are just that... exceptions.

[mecanica]

On 8/24/2013 at 8:11 PM, junker said:

Actually, his post uses some nice back-of-the-envelope math to estimate the reflection delay.

 

If you don't buy into the digital reflection thing, then don't and run short cables if you want. That's your business. Personally, I'd stick with 1.5 - 2.0M

 

More info on this is presented by Steve Nugent of Empirical Audio here:

 

spdif

 

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 meters, 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.

 

 

But I do agree that what he is describing sounds like something completely different. Phono, maxed-out, tube, bleeding-through...

I am waking up to this topic. If we use a source which has a little jitter as same as allo digione (average jitter 65ps) can we use too short digital cable without any reflection issue? One of my friend offer that use 0.10m bnc cable for best quality of connection. Has anybody experienced with as this too short cable? 

[marce]

Yes, shorter the better.

[Speedskater]

For a S/PDIF cable, any length from zero to several meters should work.  If it doesn't, then one of digital components has a serious problem!  And yes, over the decades some highly rated audiophile digital products have had serious design problems.

[esldude]

Suggestions in the industry are don't use Coax SPDIF for more than 10 meters.  Keep it to 5 meters to be safe I suppose.  

As for minimum length.  There isn't one other than the shorter the better. 

[buonassi]

On 8/26/2013 at 9:57 PM, drez said:

Bnc attenuators similarly don't really improve things, just makes things more smooth.

what do you mean by "smooth"?  

[mecanica]

I have 90 cm Gepco 75 ohm digital cable with  canare bnc connectors.  I will try one of the 10cm cable of this Gepco with the same connectors.  Average jitter of allo digione 65ps. This value is nearly perfect.  So i think that too short cable doesn't a problem for reflection.