S/PDIF coax cables

andrew s

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On a neverending thread on mains cables @Fourlegs and @tuga got briefly side tracked on to S/PDIF and rf noise . While surfing around on this I came upon a post by John Siau of Benchmark. He was discussing not rf interference but jitter induced by miss matches in impedance between the sender and the cable and or the cable an the receiver and the signal reflections this causes.

It seem to me to illustrate a  number of points often discussed on here. Cables can make a difference. The  quality of the sending and receiving device can make a difference.  As the impedance mismatch can impact jitter which may therefore be audible depending on the DAC and at least in this instance there is a credible scientific explanation. Anyway that's my conclusion.


"One correction to the original post:
The reflected signal can be either polarity. If the termination impedance is higher than the cable impedance, the reflection will be non-inverted. If the termination impedance is lower than the cable impedance, the reflection will be inverted.

Damage is done to the signal at the receiver after the signal has made 3 transits through the cable. The magnitude and polarity of the first reflection is determined by the impedance matching of the termination at the receiver. This first reflection will be reflected again if the drive impedance is mismatched to the cable. The magnitude and polarity of this reflection is determined by the matching of the source impedance to that of the cable. Each successive reflection is smaller than the previous reflection.

A perfect termination at the transmitter or the receiver, will eliminate 3rd transit reflections.

The 1.5 meter cable is long enough to allow completion of the rise or fall time before the third reflection reaches the receiver. A 1.5 m cable will have a transit time of 5.95 ns to 9.35 ns depending upon the velocity factor of coax used. The transit time through 1.5 meters of air would be about 5 ns (velocity factor of about 1). The velocity factor is determined by the insulating material used in the coax. Multiply the transit times by 3 to calculate the arrival time of the third transit. Multiply the transit time by 2 to arrive at the timing of the third transit relative to the original signal reaching the receiver. The first transit arrives at the receiver 5.95 to 9.35 ns after the original signal is generated. The third transit arrives 17.9 ns to 28 ns after the original signal is generated. The difference in arrival time is 2 transits which is 11.9 ns to 18.7 ns respectively. If the rise time of the original signal is less than 11.9 ns the reflections will arrive after the transition has completed and the reflections will have no impact on the timing of the data transition (assuming the use of the cable with the higher velocity factor). RG59 is commonly used for digital audio, and its lower velocity factor gives us 18.7 ns between the original signal and the reflected signal (at a cable length of 1.5 m). Slower cables give us more margin at 1.5 meters, but things start to get complicated when we look at longer cables.

As the cable gets longer, two things happen:

1: The rise times of the transitions will get longer due to high-frequency roll off. This makes clock recovery more difficult.
2: If 2 transits through the cable is equal to the period of data transitions, the third transit will arrive in sync with the next data transition. In AES or SPDIF digital audio streams, data transitions happen 128 times per sample. 128 x 44,100 = 5.6448 MHz. This means that there is a 177 ns spacing between data edges at a sample rate of 44.1 kHz. If the two-transit time is equal to 177 ns, the terminations need to be reasonably good. Using RG59, this unfortunate alignment happens when the cable length is 9.5 m long. Using the fastest coax, this problem would occur at a length of 14.9 meters. Divide these lengths by two for 88.2 kHz audio, and divide by two again for 176.4 kHz audio.

At 192 kHz, using the slowest coax, the reflections will align at a cable length of 2.2 meters. If we consider the fact that transitions have a rise time, interference will begin at shorter cable lengths. This interference will begin if the cable length is much over 1.5 meters long.

Bottom line: You can get away with really bad terminations (or no terminations at all) if your cable is 1.5 meters long. There is very little margin in either direction (longer or shorter). For this reason, cable length is a bad way to solve the problem. Nevertheless, a 1.5 meter cable can be used as a band-aid on a poorly designed system.

The best way to solve the problem is to use cables that have the correct impedance. The equipment must also have reasonably good terminations on either the transmitter or the receiver (preferably both). The receiving device should also have a good clock recovery system that provides jitter immunity. If these conditions are satisfied,  the cable length is not critical. 



 
Regards Andrew 

 
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Nopiano

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If I’m not mistaken, Max Townshend uses transmission line theory for his speaker cables too.  But this was pretty much dismissed when there was a video of his posted a few months ago.  However, his speaker wires do seem to have a particular sound, which imho is exactly what they shouldn’t have.  

Otherwise I’m unsure how much is interchangeable between analogue and digital cables in terms of their audibility.  Unfortunately, the old ‘shortest piece of wire’ idea doesn’t seem to translate so well in the digital domain. 

 

Shadders

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Hi,

A lot of the issues are discussed at the following wiki :

https://en.wikipedia.org/wiki/Signal_reflection

What you have described in the main of the text is that impedance mismatch causes reflections as per the wiki :

"Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable cause impedance mismatches and non-linear changes in the cable characteristics."

Do you agree that if there is no impedance mismatch, that reflections are near non-existent whatever the cable length ?

Regards,

Shadders.

 
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andrew s

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Hi,

A lot of the issues are discussed at the following wiki :

https://en.wikipedia.org/wiki/Signal_reflection

What you have described in the main of the text is that impedance mismatch causes reflections as per the wiki :

"Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable cause impedance mismatches and non-linear changes in the cable characteristics."

Do you agree that if there is no impedance mismatch, that reflections are near non-existent whatever the cable length ?

Regards,

Shadders.
Yes that's also what JS says. However,  he points out  another issues with longer cables I.e. increase rise time. Regards Andrew 

 

t1no

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Do you agree that if there is no impedance mismatch, that reflections are near non-existent whatever the cable length ?
That's the ideal scenario. Hard to achieve across a wide frequency band. A Smith Chart for a particular lenth of cable across the frequency range of interest will ideally centre around the characteristic impedance.

 
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Shadders

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Yes that's also what JS says. However,  he points out  another issues with longer cables I.e. increase rise time. Regards Andrew 
Hi,

Not sure if there is a formatting problem of the text, but are the rise times issue specific to an unmatched cable and transmitter/receiver ?

Regards,

Shadders.

 
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andrew s

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If I’m not mistaken, Max Townshend uses transmission line theory for his speaker cables too.  But this was pretty much dismissed when there was a video of his posted a few months ago.  However, his speaker wires do seem to have a particular sound, which imho is exactly what they shouldn’t have.  

Otherwise I’m unsure how much is interchangeable between analogue and digital cables in terms of their audibility.  Unfortunately, the old ‘shortest piece of wire’ idea doesn’t seem to translate so well in the digital domain. 
The cases are different as the frequency of digital signals means the cables act as transmission lines while at audio frequencies the cables don't behave in the same way and normally you would not model cables as transmission line but use a lumped parameter LRC model.

Roughly when the wavelength of a signal is comparable to the size of a cables cross section you need a transmission line model when large a simpler LRC model is accurate enough.

Regards Andrew 

 
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andrew s

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Hi,

Not sure if there is a formatting problem of the text, but are the rise times issue specific to an unmatched cable and transmitter/receiver ?

Regards,

Shadders.
My reading of it was that the increasing length created a high frequency roll off which increased the rise time. Point 1: in the quote.  Regards Andrew 

 
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Shadders

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My reading of it was that the increasing length created a high frequency roll off which increased the rise time. Point 1) in the quote.  Regards Andrew 
Hi,

My interpretation of high frequency transmission is that the coaxial cable will have a bandwidth and characteristic impedance, and as long as the transmitting and receiving impedance matched the characteristic impedance, then there will be no high frequency roll off based on the length of the cable, but there will be a time delay based on the velocity of the cable.

No cable is perfect, and hence there will be effects induced by length, and there are limits.

Regards,

Shadders.

 

Lurch

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On a neverending thread on mains cables @Fourlegs and @tuga got briefly side tracked on to S/PDIF and rf noise . While surfing around on this I came upon a post by John Siau of Benchmark. He was discussing not rf interference but jitter induced by miss matches in impedance between the sender and the cable and or the cable an the receiver and the signal reflections this causes.

It seem to me to illustrate a  number of points often discussed on here. Cables can make a difference. The  quality of the sending and receiving device can make a difference.  As the impedance mismatch can impact jitter which may therefore be audible depending on the DAC and at least in this instance there is a credible scientific explanation. Anyway that's my conclusion.


"One correction to the original post:
The reflected signal can be either polarity. If the termination impedance is higher than the cable impedance, the reflection will be non-inverted. If the termination impedance is lower than the cable impedance, the reflection will be inverted.

Damage is done to the signal at the receiver after the signal has made 3 transits through the cable. The magnitude and polarity of the first reflection is determined by the impedance matching of the termination at the receiver. This first reflection will be reflected again if the drive impedance is mismatched to the cable. The magnitude and polarity of this reflection is determined by the matching of the source impedance to that of the cable. Each successive reflection is smaller than the previous reflection.

A perfect termination at the transmitter or the receiver, will eliminate 3rd transit reflections.

The 1.5 meter cable is long enough to allow completion of the rise or fall time before the third reflection reaches the receiver. A 1.5 m cable will have a transit time of 5.95 ns to 9.35 ns depending upon the velocity factor of coax used. The transit time through 1.5 meters of air would be about 5 ns (velocity factor of about 1). The velocity factor is determined by the insulating material used in the coax. Multiply the transit times by 3 to calculate the arrival time of the third transit. Multiply the transit time by 2 to arrive at the timing of the third transit relative to the original signal reaching the receiver. The first transit arrives at the receiver 5.95 to 9.35 ns after the original signal is generated. The third transit arrives 17.9 ns to 28 ns after the original signal is generated. The difference in arrival time is 2 transits which is 11.9 ns to 18.7 ns respectively. If the rise time of the original signal is less than 11.9 ns the reflections will arrive after the transition has completed and the reflections will have no impact on the timing of the data transition (assuming the use of the cable with the higher velocity factor). RG59 is commonly used for digital audio, and its lower velocity factor gives us 18.7 ns between the original signal and the reflected signal (at a cable length of 1.5 m). Slower cables give us more margin at 1.5 meters, but things start to get complicated when we look at longer cables.

As the cable gets longer, two things happen:

1: The rise times of the transitions will get longer due to high-frequency roll off. This makes clock recovery more difficult.
2: If 2 transits through the cable is equal to the period of data transitions, the third transit will arrive in sync with the next data transition. In AES or SPDIF digital audio streams, data transitions happen 128 times per sample. 128 x 44,100 = 5.6448 MHz. This means that there is a 177 ns spacing between data edges at a sample rate of 44.1 kHz. If the two-transit time is equal to 177 ns, the terminations need to be reasonably good. Using RG59, this unfortunate alignment happens when the cable length is 9.5 m long. Using the fastest coax, this problem would occur at a length of 14.9 meters. Divide these lengths by two for 88.2 kHz audio, and divide by two again for 176.4 kHz audio.

At 192 kHz, using the slowest coax, the reflections will align at a cable length of 2.2 meters. If we consider the fact that transitions have a rise time, interference will begin at shorter cable lengths. This interference will begin if the cable length is much over 1.5 meters long.

Bottom line: You can get away with really bad terminations (or no terminations at all) if your cable is 1.5 meters long. There is very little margin in either direction (longer or shorter). For this reason, cable length is a bad way to solve the problem. Nevertheless, a 1.5 meter cable can be used as a band-aid on a poorly designed system.

The best way to solve the problem is to use cables that have the correct impedance. The equipment must also have reasonably good terminations on either the transmitter or the receiver (preferably both). The receiving device should also have a good clock recovery system that provides jitter immunity. If these conditions are satisfied,  the cable length is not critical. 


 
Regards Andrew 
Well, it appears I CAN read Martian. But as always I'm buggered if i can understand it. 

 

rabski

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I use SPDIF cables of 1.5M for exactly this reason. In fact, apart from the CDP and the D10 converter (which I haven't got round to hacking about yet) I use BNC anyway and I always use 75 ohn characteristic impedance. Belt and braces, you could say. I would call it ideal best practice.

 
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Fourlegs

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On a neverending thread on mains cables @Fourlegs and @tuga got briefly side tracked on to S/PDIF and rf noise . While surfing around on this I came upon a post by John Siau of Benchmark. He was discussing not rf interference but jitter induced by miss matches in impedance between the sender and the cable and or the cable an the receiver and the signal reflections this causes.

It seem to me to illustrate a  number of points often discussed on here. Cables can make a difference. The  quality of the sending and receiving device can make a difference.  As the impedance mismatch can impact jitter which may therefore be audible depending on the DAC and at least in this instance there is a credible scientific explanation. Anyway that's my conclusion.


"One correction to the original post:
The reflected signal can be either polarity. If the termination impedance is higher than the cable impedance, the reflection will be non-inverted. If the termination impedance is lower than the cable impedance, the reflection will be inverted.

Damage is done to the signal at the receiver after the signal has made 3 transits through the cable. The magnitude and polarity of the first reflection is determined by the impedance matching of the termination at the receiver. This first reflection will be reflected again if the drive impedance is mismatched to the cable. The magnitude and polarity of this reflection is determined by the matching of the source impedance to that of the cable. Each successive reflection is smaller than the previous reflection.

A perfect termination at the transmitter or the receiver, will eliminate 3rd transit reflections.

The 1.5 meter cable is long enough to allow completion of the rise or fall time before the third reflection reaches the receiver. A 1.5 m cable will have a transit time of 5.95 ns to 9.35 ns depending upon the velocity factor of coax used. The transit time through 1.5 meters of air would be about 5 ns (velocity factor of about 1). The velocity factor is determined by the insulating material used in the coax. Multiply the transit times by 3 to calculate the arrival time of the third transit. Multiply the transit time by 2 to arrive at the timing of the third transit relative to the original signal reaching the receiver. The first transit arrives at the receiver 5.95 to 9.35 ns after the original signal is generated. The third transit arrives 17.9 ns to 28 ns after the original signal is generated. The difference in arrival time is 2 transits which is 11.9 ns to 18.7 ns respectively. If the rise time of the original signal is less than 11.9 ns the reflections will arrive after the transition has completed and the reflections will have no impact on the timing of the data transition (assuming the use of the cable with the higher velocity factor). RG59 is commonly used for digital audio, and its lower velocity factor gives us 18.7 ns between the original signal and the reflected signal (at a cable length of 1.5 m). Slower cables give us more margin at 1.5 meters, but things start to get complicated when we look at longer cables.

As the cable gets longer, two things happen:

1: The rise times of the transitions will get longer due to high-frequency roll off. This makes clock recovery more difficult.
2: If 2 transits through the cable is equal to the period of data transitions, the third transit will arrive in sync with the next data transition. In AES or SPDIF digital audio streams, data transitions happen 128 times per sample. 128 x 44,100 = 5.6448 MHz. This means that there is a 177 ns spacing between data edges at a sample rate of 44.1 kHz. If the two-transit time is equal to 177 ns, the terminations need to be reasonably good. Using RG59, this unfortunate alignment happens when the cable length is 9.5 m long. Using the fastest coax, this problem would occur at a length of 14.9 meters. Divide these lengths by two for 88.2 kHz audio, and divide by two again for 176.4 kHz audio.

At 192 kHz, using the slowest coax, the reflections will align at a cable length of 2.2 meters. If we consider the fact that transitions have a rise time, interference will begin at shorter cable lengths. This interference will begin if the cable length is much over 1.5 meters long.

Bottom line: You can get away with really bad terminations (or no terminations at all) if your cable is 1.5 meters long. There is very little margin in either direction (longer or shorter). For this reason, cable length is a bad way to solve the problem. Nevertheless, a 1.5 meter cable can be used as a band-aid on a poorly designed system.

The best way to solve the problem is to use cables that have the correct impedance. The equipment must also have reasonably good terminations on either the transmitter or the receiver (preferably both). The receiving device should also have a good clock recovery system that provides jitter immunity. If these conditions are satisfied,  the cable length is not critical. 


 
Regards Andrew 
Hi, Yes I did get dragged off on that tangent. I can’t remember why, possibly because a forum member was attempting to allege that cables cannot have any filtering affect unless they incorporate electronic components.

Anyway, having now sold several hundred spdif cables my take on the length thing is that the 1.5m length is somewhere between a red herring and an old wives tale. I have listened to various lengths of spdif cable and of various construction and materials  - all without any of my ferrites - and have also discussed the issue of spdif cable length with Rob Watts because at the time he was doing the same listening experiments as me. Although Rob seems to irritate a few wammers he does know an awful lot about digital design including jitter etc and his take on the cable length was nothing to do with reflections and instead he thinks the affect of cable length on RF noise filtering to be the dominant factor to consider. The progression of spdif cable sound getting better with cable length ties in with this and where say a 2m cable sounded better than a 1m cable and also better than a 1.5m cable. Considering RF noise also correlated with what we heard when ferrites were added because one could make a 1m cable with a few ferrites sound exactly the same as a 2m length of the same cable without any ferrites.

So I say forget the 1.5m length. For convenience it is easier to use a 1m cable with a few (or ideally a lot) of correctly specified ferrites and this will sound much better than any 1.5m spdif cable. If not into ferrites one could use a longer cable of 2m or more but even so that would also sound better with the addition of ferrites to filter RF noise.

But don’t just take what I say, have a play with different lengths of different types of 75ohm spdif cable and listen to the effect on the sound quality. Try perhaps 0.5m, 1m, 1.5m, 2m and maybe 3 or 4m. See which sounds best. Then add some high frequency clip on ferrites (ideally ones with a target frequency of around 2GHz) and see what that does to the sound. 

And before anyone says it, this is not a ‘Chord thing’ or issue. Many of my spdif cables go to people connecting various CD players to all sorts of DACs or connecting quite humble streamers such as the Node 2i and various makes of DAC. Theoretical articles are quite interesting but listening for oneself is probably more useful. 

 

andrew s

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@Fourlegs thanks for your experience.  Your may well be right that rf is the issue in your tests. As pointed out if the kit has the correct 50 ohm impedance then reflections are a non issue and cable length indeed does not matter. 

In this case it may be a red herring but not an old wives tale! It was possibly more of an issue in days gone by when the impact of jitter was less well understood and mitigated. 

Regards Andrew 

 

tuga

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Hi, Yes I did get dragged off on that tangent. I can’t remember why, possibly because a forum member was attempting to allege that cables cannot have any filtering affect unless they incorporate electronic components.

Anyway, having now sold several hundred spdif cables my take on the length thing is that the 1.5m length is somewhere between a red herring and an old wives tale. I have listened to various lengths of spdif cable and of various construction and materials  - all without any of my ferrites - and have also discussed the issue of spdif cable length with Rob Watts because at the time he was doing the same listening experiments as me. Although Rob seems to irritate a few wammers he does know an awful lot about digital design including jitter etc and his take on the cable length was nothing to do with reflections and instead he thinks the affect of cable length on RF noise filtering to be the dominant factor to consider. The progression of spdif cable sound getting better with cable length ties in with this and where say a 2m cable sounded better than a 1m cable and also better than a 1.5m cable. Considering RF noise also correlated with what we heard when ferrites were added because one could make a 1m cable with a few ferrites sound exactly the same as a 2m length of the same cable without any ferrites.

So I say forget the 1.5m length. For convenience it is easier to use a 1m cable with a few (or ideally a lot) of correctly specified ferrites and this will sound much better than any 1.5m spdif cable. If not into ferrites one could use a longer cable of 2m or more but even so that would also sound better with the addition of ferrites to filter RF noise.

But don’t just take what I say, have a play with different lengths of different types of 75ohm spdif cable and listen to the effect on the sound quality. Try perhaps 0.5m, 1m, 1.5m, 2m and maybe 3 or 4m. See which sounds best. Then add some high frequency clip on ferrites (ideally ones with a target frequency of around 2GHz) and see what that does to the sound. 

And before anyone says it, this is not a ‘Chord thing’ or issue. Many of my spdif cables go to people connecting various CD players to all sorts of DACs or connecting quite humble streamers such as the Node 2i and various makes of DAC. Theoretical articles are quite interesting but listening for oneself is probably more useful. 
At the time I asked you why you thought increasing the length improved filtering and you didn’t reply.

For those of us with a more rational approach, yours and Watts’ listening experiences are of little worth without measurements. These would also help to rule out a potentially negative impact of using so many beads.

Likewise I am not sold on the “1.5m to reduce reflections” hypothesis that Rabski mentioned for that same reason, although the reasoning behind it sounds a lot more solid and the effect a lot more plausible.

 
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andrew s

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Likewise I am not sold on the “1.5m to reduce reflections” hypothesis that Arab ski mentioned for that same reason, although the reasoning behind it sounds a lot more solid and the effect a lot more plausible.
What are you not sold on? Is that the effect is not scientifically valid, it has no audible effect or something else? 

The 1.5m does not reduce reflections but if there are reflections it ensures they do not interact with detecting the initial signals leading edge or at least that's how I understood it.

Regards Andrew 

 

Cable Monkey

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At the time I asked you why you thought increasing the length improved filtering and you didn’t reply.

For those of us with a more rational approach, yours and Watts’ listening experiences are of little worth without measurements. These would also help to rule out a potentially negative impact of using so many beads.

Likewise I am not sold on the “1.5m to reduce reflections” hypothesis that Rabski mentioned for that same reason, although the reasoning behind it sounds a lot more solid and the effect a lot more plausible.
In RF theory you need three things to align for a connection to work at full efficiency. These are impedance, conductor length and oddly conductor shape. When you drop down to lower frequencies the skin effect exhibited by UHF frequencies is a lot less important but for transmission at something close to full efficiency, impedance and length still matter. This isn’t a mystery. It is simply not audio theory so gets missed when we are talking about our hobby. If you want to counter that anything up to 96k still sits within the audio frequency band (which extends to 100k for those of us who do this professionally) then consider the frequency makeup of a square wave.

What are you not sold on? Is that the effect is not scientifically valid, it has no audible effect or something else? 

The 1.5m does not reduce reflections but if there are reflections it ensures they do not interact with detecting the initial signals leading edge or at least that's how I understood it.

Regards Andrew 
If you get the length right you achieve a standing wave. Then you have no reflections.

 

tuga

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What are you not sold on? Is that the effect is not scientifically valid, it has no audible effect or something else? 

The 1.5m does not reduce reflections but if there are reflections it ensures they do not interact with detecting the initial signals leading edge or at least that's how I understood it.

Regards Andrew 
I have not seen any measurements of the effectiveness of avoiding reflections, nor is this issue mentioned by major players such as Weiss, RME, Apogee, etc.

What is the real impact of not dealing with reflections?

I have read the theory, it’s been available at Positive Feedback if I’m not mistaken for over a decade but in practice is it a real issue?

.

In any case S/PDIF is an obsolete technology that has many flaws.

 
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tuga

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If you want to counter that anything up to 96k still sits within the audio frequency band (which extends to 100k for those of us who do this professionally) then consider the frequency makeup of a square wave.
What about 352.8kHz?

Your point is indeed well taken, in S/PDIF we are discussing data transmission.

 

andrew s

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Just to be clear, I am not claiming it's a real issue today (or ever was). It just seemed to me a case where "theory" and a possible audible issue, via jitter, could be discussed within a scientific framework accessible to measurement. 

Regards Andrew 

 
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