Minimum length for 75Ω coax cable (Revisited)

[Speedskater]

The above 'SoNic67' post is just a bunch of random facts and assumptions, taken way out of context and then mis-applied to SPDIF.

 

At the end of the day, SPDIF is a very robust, very low frequency system. For the short length cables that most of us have, any cable in your house with the correct connectors will work just fine. If however you need cables about 10 meters (33 feet) or longer, then it should be a 75 Ohm co-ax.

[Speedskater]

.......................

My dealer loaned me a Rogue Sphinx hybrid integrated amp to try out. This unit has a 2x 12AU7 tube-powered preamp, a class D power section, and includes a phono stage. With the source knob set to "phono" and the volume up full, I could hear my line-source music "bleeding through" the circuit, and faintly coming from the speakers (along with a lot of hiss from the not-so-great phono stage in this amp).

When I did the same thing using the longer cable, the "ghost" music disappeared! If there is "data reflection" in the short cable that is corrupting something down the line of the signal path, this might be evidence of it.

....................

 

Well, you can't get analog cross-talk or bleed through into a digital input, so I would look for a different cause than cable length.

[Speedskater]

And you have... what qualifications for that assessment? Electrical Engineering college?

 

Over 50 years of broadcast engineering, recording engineering and industrial R & D engineering experience.

[Speedskater]

So, in all those years you didn't hear about signal reflections on transmission lines? Let me doubt that you are really EE.

Refresher: http://www.ddpp.com/DDPP4student/Supplementary_sections/Zo.pdf

 

Good transmission lines don't reflect! That could be a description of a good transmission line. From the sender end of a good transmission line, it's impossible to tell how lone the line is. It could be infinitely long or just 1 meter long.

[Speedskater]

From your linked paper:

 

However, suppose that we have a 5-foot-long pair of conductors, and we place a resistance equal to

Z0 across the far end. If we instantaneously place a voltage source across the pair

as shown in (b), the same current flows forever as in the infinite-length case.

Thus, when a line is terminated by its characteristic impedance, we needn’t

consider transmission-line effects any further.

[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.

[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.