[zilch0md]

I was just wondering if ti's possible to confirm that a coaxial cable (for SPDIF) is actually 75-Ohm. Is getting a good impedance match here important (not for analog signals, but rather SPDIF)?

 

[cute]

I was just wondering if ti's possible to confirm that a coaxial cable (for SPDIF) is actually 75-Ohm. Is getting a good impedance match here (not for analog signals, bit rather SPDIF)?

You can only confirm if manufacturer is using 75ohm BNC connectors, I was all over that with iFi Micro.  But sound is not changed if length is kept 1 meter or less.  Longer runs it would be more of a problem.  If your cable has RCA connectors, no way, they don't make RCA's 75 ohm, but you would be OK with 1 meter or less, IMO.

 

[SilverEars]

   
OT, at very high frequencies (MHzs) if the transmitter is at 75 ohms and the receiver is at 75 ohms but the cable is below or above 75 ohms, is there a reflection? Is the reflection in voltage (which is force).

The load impedance being higher or lower than the transmission line's impedance will dictate if the reflected wave is positive or negative in voltage
 

 
Which is the reflection coefficient tells you the polarity of the reflected wave from the mismatch at the boundary.  This of course is not the same mismatch that is talked about for speakers and amps.  Where amp is rated for 8ohms and speaker is 4ohms nominal.  

 

[wuwhere]

  The load impedance being higher or lower than the transmission line's impedance will dictate if the reflected wave is positive or negative in voltage
 

 
Which is the reflection coefficient tells you the polarity of the reflected wave from the mismatch at the boundary.  This of course is not the same mismatch that is talked about for speakers and amps.  Where amp is rated for 8ohms and speaker is 4ohms nominal.  

 
But that reflection is continuous back and forth.

 

[SilverEars]

   
But that reflection is continuous back and forth.

Yes, the wave can reflect back and forth inside the transmission line since there is mismatch on both sides and eventually die out.

 

[camusmuse]

  Yes, the wave can reflect back and forth inside the transmission line since there is mismatch on both sides and eventually die out.

This means the wave reflection bounces around until it loses significant power and is no longer audible?  Does that detract from the original signal, or are they added "artifacts"?   Is there some sort of resonant frequency that can be calculated and avoided?  I would think if one wave is bouncing around a cable, then there would be many, many more waves bouncing around while playing music.  Do they affect the whole frequency spectrum?  If so, which part would be best corrected, if any of it can be corrected at all?
 
What if there is only a mismatch on one side?  What would be the ideal impedance of the cable to avoid reflections?  

 

[SilverEars]

 
This means the wave reflection bounces around until it loses significant power and is no longer audible?  Does that detract from the original signal, or are they added "artifacts"?   Is there some sort of resonant frequency that can be calculated and avoided?  I would think if one wave is bouncing around a cable, then there would be many, many more waves bouncing around while playing music.  Do they affect the whole frequency spectrum?  If so, which part would be best corrected, if any of it can be corrected at all?
 
What if there is only a mismatch on one side?  What would be the ideal impedance of the cable to avoid reflections?  

Note wuwhere mentions MHz when talking about reflections.  For audio, there should be no reflections since audio frequencies are very low for such affects to take place.  

 

[camusmuse]

I was curious if there were any harmonic effects, and what they might be...  Just because standing or bouncing waves are supersonic doesnt mean that they cannot have an affect in the audible spectrum.  I am very curious as to the core of your conversation and how to correct for any issues caused.  Cheers!

 

[Chris J]

You can only confirm if manufacturer is using 75ohm BNC connectors, I was all over that with iFi Micro.  But sound is not changed if length is kept 1 meter or less.  Longer runs it would be more of a problem.  If your cable has RCA connectors, no way, they don't make RCA's 75 ohm, but you would be OK with 1 meter or less, IMO.

What does cable length have to do with this?

The perfect digital cable transmission system would have 75 Ohm cable, 75 Ohm connectors, 75 Ohm source impedance at driving end and 75 Ohm termination at receiving end.

 

[cute]

 

You can only confirm if manufacturer is using 75ohm BNC connectors, I was all over that with iFi Micro.  But sound is not changed if length is kept 1 meter or less.  Longer runs it would be more of a problem.  If your cable has RCA connectors, no way, they don't make RCA's 75 ohm, but you would be OK with 1 meter or less, IMO.

What does cable length have to do with this?

The perfect digital cable transmission system would have 75 Ohm cable, 75 Ohm connectors, 75 Ohm source impedance at driving end and 75 Ohm termination at receiving end.

 
Sorry, I'm not an engineer............I just made a statement based in Google searches, the shorter the better.  I do use 3 SPDIF BNC true 75 ohm cables w/RCA adapters, but I also have a RCA terminated coax cable (pure silver), 1 meter that does not sound any different from the other three.  I do hear a difference from copper vs silver wire that was used.  I won't argue, just sayin my experience.

 

[wuwhere]

  Yes, the wave can reflect back and forth inside the transmission line since there is mismatch on both sides and eventually die out.

 
It will only disappear when the transmitter stops transmitting. Some of it will be absorbed by the transmission line.

 

[camusmuse]

What does cable length have to do with this?

The perfect digital cable transmission system would have 75 Ohm cable, 75 Ohm connectors, 75 Ohm source impedance at driving end and 75 Ohm termination at receiving end.

I'm sorry Cute, are you sure that the connectors are "75 Ohm"?  Its just a connector, unless it has some sort of passive component...just passing electrons right?  RCA jacks can work just fine with the "right" 75 ohm cable, again...just connectors.  The shorter the cable the better, thats pretty much a given....but the 75 Ohm cable is designed for long runs.  Do the "digital" 75 Ohm cables use single center strand, or wound center strand; both with shielding from both foil and the ground plane. 
 
I have long been a megabuck cable skeptic.  I have used both analog "audio" cables and specific "digital cables"...granted the audio cables were "nice"; however I could not hear a difference...and neither could my customers.  I make all of my own cables now, and do know that certain materials have certain signature sounds...silver bright, copper midrange; if so inclined one could passively "EQ" with interconnects and/or speaker wires.  I have used solid silver custom made wire, and cheapo zip cord...each has their use.
 
Yes there are differences, but really not that much.  EXCEPT for those long runs....this is particularly true in prosound where balanced signals are the standard because they reject noise so well.  Why do they do that?  Both signal wires are shielded....so that brings me back to this perfect 75ohm "connection".  All it needs to be is a well shielded cable with properly attached connectors, BNC, RCA, etc.   If you really want to go to extremes you can probably use the foil as a "drain".  I used to do that with speaker connections, tie the speaker cable shielding to the "ground" on the amp(I think....its been a while since I "needed" to do it...noisy old tube amps).  It might work the same for a digital cable. 
 
I am also curious about the integrated digital cable in the mini usb plug for an Android to a portable DAC.  Is that also 75 Ohm?  How does it handle shielding? The same for any USB based DAC, do they have special (whatever)Ohm connectors, cable, source impedance, and termination???  That seems a bit improbable.  If all that was "perfect" then why doesnt the 75ohm cable system described become the industry standard(that sounds like it is already supposed to be the standard)?  
 
I am really not trying to ruffle any feathers, just my POV on cables and my lack of knowledge about digital "magic" cables.  I know that some of the voodoo is just a ridiculously high price point.  It would seem rather silly to hook up a 1/4mil$ system with zip cords.  If you have the money, spend it....but for grins...do some true A/B objective measurable comparisons....wire companies HATE those...lol.  I know it is an old old argument, but again, I am still looking for a good explanation of the important parameters of a "perfect" 75Ohm digital cable(apparently including connectors now??)  I would REALLY like to know if it will improve my laptop docking station connection to DAC, that feeds my speaker amp driving headphones.  Or would I be better off with a USB based DAC?

 

[Chris J]

This means the wave reflection bounces around until it loses significant power and is no longer audible?  Does that detract from the original signal, or are they added "artifacts"?   Is there some sort of resonant frequency that can be calculated and avoided?  I would think if one wave is bouncing around a cable, then there would be many, many more waves bouncing around while playing music.  Do they affect the whole frequency spectrum?  If so, which part would be best corrected, if any of it can be corrected at all?

What if there is only a mismatch on one side?  What would be the ideal impedance of the cable to avoid reflections?  

Yes, the reflections bounce around, similar to echoes in a lively room, like a gymnasium.
It's a digital signal we are talking about, so it really comes down to whether the receiver can correctly discriminate the ones and zeroes.

Both ends should be 75 Ohms, that should already be taken care of if you have purchased a DAC with a coaxial (I.e. Copper) input and a CD player with a coaxial output.

In a 75 Ohm SPDIF system, the ideal cable characteristic impedance to avoid reflections is 75 Ohm.

 

[Chris J]

Sorry, I'm not an engineer............I just made a statement based in Google searches, the shorter the better.  I do use 3 SPDIF BNC true 75 ohm cables w/RCA adapters, but I also have a RCA terminated coax cable (pure silver), 1 meter that does not sound any different from the other three.  I do hear a difference from copper vs silver wire that was used.  I won't argue, just sayin my experience.

I guess I came across a bit confrontational, sorry!

I've heard it said that RCA connectors are not a true 75 Ohm connector, but just about all equipment uses them for digital inputs and outputs so maybe it's not as important as using a 75 Ohm cable.
Wadia claims that the BNC input sounds better than the RCA input on their 121 DAC.
I'be never heard the DAC so I can't say they are right or wrong.

 

[Chris J]

I'm sorry Cute, are you sure that the connectors are "75 Ohm"?  Its just a connector, unless it has some sort of passive component...just passing electrons right?  RCA jacks can work just fine with the "right" 75 ohm cable, again...just connectors.  The shorter the cable the better, thats pretty much a given....but the 75 Ohm cable is designed for long runs.  Do the "digital" 75 Ohm cables use single center strand, or wound center strand; both with shielding from both foil and the ground plane. 

I have long been a megabuck cable skeptic.  I have used both analog "audio" cables and specific "digital cables"...granted the audio cables were "nice"; however I could not hear a difference...and neither could my customers.  I make all of my own cables now, and do know that certain materials have certain signature sounds...silver bright, copper midrange; if so inclined one could passively "EQ" with interconnects and/or speaker wires.  I have used solid silver custom made wire, and cheapo zip cord...each has their use.

Yes there are differences, but really not that much.  EXCEPT for those long runs....this is particularly true in prosound where balanced signals are the standard because they reject noise so well.  Why do they do that?  Both signal wires are shielded....so that brings me back to this perfect 75ohm "connection".  All it needs to be is a well shielded cable with properly attached connectors, BNC, RCA, etc.   If you really want to go to extremes you can probably use the foil as a "drain".  I used to do that with speaker connections, tie the speaker cable shielding to the "ground" on the amp(I think....its been a while since I "needed" to do it...noisy old tube amps).  It might work the same for a digital cable. 

I am also curious about the integrated digital cable in the mini usb plug for an Android to a portable DAC.  Is that also 75 Ohm?  How does it handle shielding? The same for any USB based DAC, do they have special (whatever)Ohm connectors, cable, source impedance, and termination???  That seems a bit improbable.  If all that was "perfect" then why doesnt the 75ohm cable system described become the industry standard(that sounds like it is already supposed to be the standard)?  

I am really not trying to ruffle any feathers, just my POV on cables and my lack of knowledge about digital "magic" cables.  I know that some of the voodoo is just a ridiculously high price point.  It would seem rather silly to hook up a 1/4mil$ system with zip cords.  If you have the money, spend it....but for grins...do some true A/B objective measurable comparisons....wire companies HATE those...lol.  I know it is an old old argument, but again, I am still looking for a good explanation of the important parameters of a "perfect" 75Ohm digital cable(apparently including connectors now??)  I would REALLY like to know if it will improve my laptop docking station connection to DAC, that feeds my speaker amp driving headphones.  Or would I be better off with a USB based DAC?

So you think I'm cute?

They are considered 75 Ohm connectors because they have a 75 Ohm characteristic impedance, not because they have a passive component in them.

The digital cables used in audio are typically coaxial cables.

Balanced cables reject noise because the driving end is balanced, the cables are balanced, and the receiving end is differential.
It is actually the receiving end that rejects noise.
The cable is "balanced" because both the Hot and Cold conductors (I.e. Positive and Negative, or more correcting, Non-Inverting and Inverting) have the same impedance to ground.

I've never seen any measurements comparing using an RCA connector to a BNC connector in an SPDIF system so who knows?