Home Digital Drive

Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

RE: excessive length

I am an RF engineer - and to really understand what's going on you would need to use a Time domain reflectometer - you put a pulse down the cable and you will see every reflection as the signal goes down the line. In my experience the largest reflections will come from the connectors and adapters, less so cable bends unless they are excessively tight. And based upon empirical experience of sound, this makes sense.

I have not seen normal cable bends creating more reflections - and with an eye diagram (a way of demodulating the digital on a scope and measure timing errors, clock errors, etc.) you see any small effect getting swamped out entirely by connector, clock and other effects. However if you violate the manufacturer's "minimum bend radius" you will see problems form that bend. And since the cable may be damaged, it may never come back once it has gone past that radius. A cable won't generally naturally lay in that sort of radius - you usually have to force it - and if you do the cable pinches, and the geometry deforms - sometimes permanently. If you don't know what it is, normal use will be fine - just don't force the cable into a tight bend or routing and you shoudl be 100% fine 100% of the time.

Also ...

A good impedance match is not always reflection free(*) - the design of the connectors and the method of attachment will influence this. If you could have a reflection free 75 Ohm cable - then the musings on the right length would be a non issue as it would not matter. BNC connections are generally pretty good if specified into the upper UHF to low microwave ranges. RCA connectors won't be good - even "true 75 Ohms" connectors if hooked to a typical SPDIF input won't be good.

SO my rules for coaxial RF cables for SPDIF:
1. Connectors - BNC first and specified to 1-6GHz somewhere.
2. Cable type - Specified to Microwave range (3-10GHz) with high coverage shielding. Skinny with a very narrow minimum bend radius.
3. CRIMP connections before solder. Passive Intermodulation is the only working theory for this - since multiple solder connections can be poor for this - and a good solid crimp may be a better interface.
5. Adapt from BNC to RCA - and if you have "True 75 Ohm" adapters it should work good. But don't sweat it too much - since the RCA connector itself may be a source of issues.


I use a Squeezebox into a Berkeley audio DAC right now - I have a homebrew cable at about 1.7m, and I also have a "Black Cat Silver Star" which is 1.23m - both sound great. I had an Audioquest cable that was "ok" but overall somewhat crummy and expensive. AQ I have egenrally liked for analog - but not SPDIF!

However the RCA on the SB end of it doesn't make an intimately tight connection. I haven't noticed a large effect in poor sound with one that is tighter (but not 75 Ohms) and looser (and is true 75 Ohms)- but I suspect that if I were to correct this with a hardware mod to the SB, I am sure there will be a marginal improvement.

The SB is a great source - but the source itself isn't perfect - and there will be power supply noise, etc, that once you have a cable you are happy with that you may want to address before fussing with more cables. (I also have a Sonos - and use it for a casual whole house audio. In addition to not being bit perfect, it is very jittery - the SB is head and shoulders better than that)

(*) It is possible to have a perfect match hand still have large reflections - in many connectors they use various size and phase of reflections from various parts to create an overall match when they all ring back and forth.
====
"You are precisely as big as what you love and precisely as small as what you allow to annoy you." ~ R A Wilson



Edits: 08/26/12 08/26/12

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  • RE: excessive length - Bromo33333 08:33:41 08/26/12 (0)

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