Minimum length for 75Ω coax cable (Revisited)

[SoNic67]

Shortest cable is good for analog domain because you worry about interference/noise pick up. For digital, you need to worry more about reflections than anything else and a longer cable is almost mandatory - or at least helps mitigating the reflections (the matching of impedances is never perfect).

 

This is because the SPDIF receivers have 'gated' inputs. When they 'lock' on signal they ignore any other signal that arrives past the small window of time when the input 'gate' is open. The 'gate' is opened periodically to re-sample the incoming signal.

If the reflected signal comes back quick enough following the real signal (short cable), it will find the 'gate' open, it will over-impose over the original signal and affect the decoding process.

The average receiver has a 8ns 'window'. Light travels 8x0.3m=2.4m in vacuum in that time. Over a 75 ohm cable, that has slower speed (0.8 of light in vacuum), it can result in 1.9m.

reflection being a 2-way travel, cable can be 1/2 of that length - 1m would be minimum needed. Recommended 1.5m would be safe 'enough'.

I personally stay at 6ft/2m lenghts, just because I don't know exactly the parameters of my cables or my SPDIF receiver - that 8ns 'average' might be actually 16ns on my particular receiver chip.

 

LE: If you use a too long of a cable (like 5m), the second reflection might come in the 'gated' time to affect the receiver. But the level of second reflection is so low (each reflection attenuates the signal) that it might not be detectable.

[SoNic67]

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

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

[SoNic67]

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

[SoNic67]

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.

Why did you stop there? Next phrase was very interesting:

The situation is different for a transmission line of finite length that is not

terminated in its characteristic impedance.

Yes, that's a big IF. Real life situations are not ideal. Line is not infinite. Matching impedances is not perfect. Also, the model of real lines is a distributed parameter, not the usual concentrated parameter approximation.

Cable impedance needs to match what was considered at design stage in order to minimize any tolerance influences.

 

[SoNic67]

There is no RCA 75ohm. Only BNC.

True. But you can still use a 75 ohm cable to it. The induced difference is minimal (especially when board components are adapted properly), but yes, it is. That's why some of the higher end equipment have BNC connectors.

[SoNic67]

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:

http://www.audio-hifi-shop.de/product_info.php/products_id/410

[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.

[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.