SPDIF Optical Cables, can they perform different?

[antcollinet]

The maximum frequency of the light pulses that aren't smeared in such a way that one cannot retrieve the original light pulses any more basically.
Of course this is dependent on the construction of the light guide (it is all about breaking indexes), length and used light color (wavelength).

Then you need to have a driver than can switch a LED, VCSEL, FP or other type of laser on and off in a quick enough and a detector that is fast enough (followed by an amplifier with enough bandwidth)

What physical mechanism causes the smearing. I've searched breaking indexes, but found nothing.

The only thing I can think of would be different path lengths through the fibre - eg some light reflecting multiple times at smaller angles along the lenght/sides of the fibre, and others going straight or fewer reflections at wider angles.

Though at the speed of light I'm struggling to believe this can be significant compared with the pulse time.

 

[LTig]

Given the other comments upthread about bandwidth what does bandwidth mean in terms of optical cables/fibres?

The limiting factors are modal dispersion of the cable (which limits its length) and the bandwidth of the electronics inside transmitter and receiver.

 

[LTig]

What physical mechanism causes the smearing. I've searched breaking indexes, but found nothing.

The only thing I can think of would be different path lengths through the fibre - eg some light reflecting multiple times at smaller angles along the lenght/sides of the fibre, and others going straight or fewer reflections at wider angles.

Though at the speed of light I'm struggling to believe this can be significant compared with the pulse time.

Your thinking is correct. See the difference between multi-mode and mono-mode fibers at Wikipedia.

 

[LTig]

Which is that physical fundamental difference you are referring to? As I see it, an optical signal and an electrical signal are both electromagnetic waves, so in terms of their capacity of transmitting information using its frequency (energy/power is a different matter), they behave fundamentally in the same way. The difference is given buy the medium: a conductor is much more complex and has much more side effects that a "transparent" medium for infrared EM, in which as far as I know the main problem is dispersion of light. Is there something I'm missing?

No.

 

[xaviescacs]

Though at the speed of light I'm struggling to believe this can be significant compared with the pulse time.

You can do some toy maths to convince yourself. Very loosely, 1 GHz means a pulse every nanosecond 1^-9 seconds. Light travels 0.3 meters (ideally) in that time. In a 1 km fiber, an accumulated difference in traveled distance of 0.03 % would make two signals indistinguishable at the end.

 

[antcollinet]

Your thinking is correct. See the difference between multi-mode and mono-mode fibers at Wikipedia.

Thanks - another interesting diversion.

But multi mode fibres seem to become bandwidth limited at fairly long cable lengths (one example give is 20MHz at 1km) The spreading effect being insignificant compared to pulse width at short cable lengths.

Which means these bandwith limitations are not really going to exist for typical Toslink cables of 1 or 2 metres, at least not at frequencies that matter for audio.

 

[antcollinet]

You can do some toy maths to convince yourself. Very loosely, 1 GHz means a pulse every nanosecond 1^-9 seconds. Light travels 0.3 meters (ideally) in that time. In a 1 km fiber, an accumulated difference in traveled distance of 0.03 % would make two signals indistinguishable at the end.

Yep - did that. 3mb/s somewhat slower than 1GHz. And toslink cables rarely go to 1km.


EDIT - though 125mb/s gets closer to 1GHz.

 

[xaviescacs]

Which means these bandwith limitations are not really going to exist for typical Toslink cables of 1 or 2 metres, at least not at frequencies that matter for audio.

Yep - did that. 3mb/s somewhat slower than 1GHz. And toslink cables rarely go to 1km.

Right! This is why intuitively, having no idea of engineering , I trust Toslink connections, and actually prefer them for computers or televisions, again, intuitively, because they don't connect electrically the TV with the DAC, which is sensible in my opinion.

 

[LTig]

Thanks - another interesting diversion.

But multi mode fibres seem to become bandwidth limited at fairly long cable lengths (one example give is 20MHz at 1km) The spreading effect being insignificant compared to pulse width at short cable lengths.

Which means these bandwith limitations are not really going to exist for typical Toslink cables of 1 or 2 metres, at least not at frequencies that matter for audio.

Correct. The limiting factor for most Toslink cables is signal loss (see above posting of @solderdude).

 

[eyes-on-you]

Not each but there are differences between POF cable (Plastic Optical Fibre) and bundled glass optical fibers.
Also in POF cables there are differences in damping/km.

POF cable has a bandwidth of 5MHz/km. This means a 100m cable has a Bandwidth of 50MHz and a 10m cable 500MHz.
The problem is not bandwidth but attenuation.
POF cable has an attenuation of about 150dB/km so runs of over 10m means there is already a lot of attenuation.
There are some POF cables that do better.
The more attenuation there is the harder it becomes to get an not to noisy signal out again.
As TOSLINK receivers commonly are used for asynchronic transmission protocols doesn't help either.

Glass bundled fibers have much higher bandwidths and a lot less attenuation/km.

So yes, glass fiber is much better than POF and there are differences in POF cables.
Bandwidth is not the problem though... attenuation is.
The biggest culprit in the FO transmission, however, is the receiver bandwidth and sensitivity as well as speed of its comparator and internal photo diode and pre-amp.

Supra ZAC supports bandwith up to 32 bit / 384 kHz at 20 meters. It’s POF type.

I don’t see any glass toslink support this rates at 20 meter. How it’s possible if glass toslinks are better than POF’s?

 

[solderdude]

The only thing I can think of would be different path lengths through the fibre - eg some light reflecting multiple times at smaller angles along the lenght/sides of the fibre, and others going straight or fewer reflections at wider angles.

That is the exact cause, It is why Single Mode fibers can reach much higher bandwidths and why so called graded index Multi Mode fibers also can reach higher bandwidths than the so called step index fibers.
The wider the optical path is the more different 'modes' the light can take.
Light bouncing via 'walls' of the fiber simply takes longer to arrive than when it goes straight.

Imagine a gutter less bowling alley.
You throw 10 balls at the same time with the exact same speed. The one going in a straight line will arrive first. The one that is pointed towards the sides will take the longest time to get there and the ones with directions in between these extremes will arrive between the straight and most often bouncing ones.
This is what happens in a step index cable.

When you have a graded index one you can use the same analogy but the center of the bowling alley has some sand on it which makes the straight ball slow down a bit. The more you come to the side of the alley the more slippery it gets.
In this case the bouncing ball will go relatively faster to the straight ball.
This way the ball taking the longer route will arrive at about the same time as the one going through the middle that is slowed down.

With graded index multimode cables a similar track is made by using a varying breaking index glass fiber.

 

[solderdude]

Supra ZAC supports bandwith up to 32 bit / 384 kHz at 20 meters. It’s POF type.

I don’t see any glass toslink support this rates at 20 meter. How it’s possible if glass toslinks are better than POF’s?

They are using graded index with a narrower center. There are many different types of plastic with different attenuation and breaking indexes.
You will need receivers that can do at least 25Mbs for that.
When you happen to have a receiver that can only do 10 Mbs or 15Mbs you won't be reaching those bitrates regardless of the used cable.

For synchronic transmissions you can find TOSLINK receivers that can reach 150Mbs up to 30m.
POF can be used for the Gbps links but requires special receivers for that.

Bundled glass will have no problems. Specs of these cables are many many factors higher than the best POF there is. The limit of bundled glass fiber connections is only bound by the receiver bandwidth. These cables will greatly exceed any TOSLINK receiver bandwidth and can reach longer lengths as well.

 

[antcollinet]

Thanks for the discussion folks. I love it when I come here and learn stuff.

 

[xaviescacs]

The comparison between EM waves in both conductors and OF I made earlier requires some clarification to avoid this "In the end is all the same" effect, which is potentially pernicious. What follows is just the scratch of it, but a fun attempts to describe both phenomenons.

In a conductor, the electrons in the conduction bands can get excited and act as a medium for propagating EM waves, since they have that "freedom" of absorbing and emitting energy, and therefore transmitting it. Unlike in the majority of many particle systems, we can in fact solve the Schrodinger equation of a conductor because the periodicity of the material (the potential in the hamiltonian is a periodic function), and that leads to the equations of bands for each crystal (conductor), that is, the quantum energy states where electrons can be. The quantum leaps of electrons between these states in the conduction bands transmit the energy along the material.

In light transmission, a transparent medium for a given wavelength is a material that absorbs and emits all photons that have this energy. The basics of the quantum process are similar: an electron gets excited by a photon and when it decays, it emits the same energy again in the same direction. However, this process is not ideal since no material is ideal and differential effects on the frequency\wavelength will create dispersion. There will be also some absorption, meaning that the material will sometimes absorb a photon without emitting another one, and scattering, both of which will cause loss of energy (attenuation).

 

[steveminnesota]

I tried to stream 384KHz with optical last week and for some reason it seems to be screwed up, making everything obviously distorted. IDK if the problem was with the motherboard or with the DAC. So, I personally still prefer USB on machines that don't CLICK CLICK CLICK all the time when switching songs on USB. I can't really hear 384KHz but why settle for 44.1 when you can see that 384 or 768 light up on the LEDs?

 

[antcollinet]

Well one reason would be that you can use an optical interface that is only working correctly at the lower frequencies - thus avoiding the inherent problems of an electrical connection (noise/hum/ground loops etc). Expecially if (like most) you are unable to hear the difference.

 

[DanielT]

My daily work is in opto electronics and work with fiber optics all day.
Not 2 cables measure the same.
Unless attenuation becomes too much and data is corrupted the transmission of data is the same though.
Just as every different electrical cable will all measure differently (far outside audible range) yet may have no (audible) effect on the transmitted waveform.

Is there even something that is exactly the same? If you see it purely theoretically, at the lowest, tiny smallest possible level, mix in time plus some cat from Schrödinger so ..? I'm not an EE but when it comes to the type of electronics that can cause audible hearing regarding HiFi and I'm sure
it has nothing to do with cats anyway, but still.

 

[Bob Olhsson]

A problem with SPDIF and AES/EBU is that they transmit an analog clock which can obviously become a problem. The first generation TOSLINK and SPDIF receiver chips generated massive amounts of clock jitter, often beyond the capabilities of the DAC chips to suppress it. TOSLINK was especially amusing because you could hear the soundstage wander about as you moved the cable. Newer receiver chips are excellent but the incompetently engineered early ones gave rise to a lot of today's digital mythology.

Digital audio has the potential of being nearly perfect but is far more dependent on design engineering and QC than most people realize. The "bits" are only the beginning.

 

[bravomail]

They are using graded index with a narrower center. There are many different types of plastic with different attenuation and breaking indexes.
You will need receivers that can do at least 25Mbs for that.
When you happen to have a receiver that can only do 10 Mbs or 15Mbs you won't be reaching those bitrates regardless of the used cable.

For synchronic transmissions you can find TOSLINK receivers that can reach 150Mbs up to 30m.
POF can be used for the Gbps links but requires special receivers for that.

These cables will greatly exceed any TOSLINK receiver bandwidth and can reach longer lengths as well.

Thx, solderdude!
I think the only problem to solve with better cables will be a distance.
I found in my testing that 24/96 is highest reliable setting I could reach in my setup. In further testing I found that I prefer 44.1kHz or 48kHz sampling rate to any artificially multiplied rate. It sounded better. Maybe it was limitation of DACs. So I think 24/48 will work for me. I could imagine you want to have longer cables - mine r pretty short at 6-10 feet (2-3 meters).

 

[Tim]

Longer, with a powerful transmit laser, and Corning glass fiber, was up to 60 miles when I last fooled with it in 2002.

The signal would becone distorted at that distance, but run it through a short length of fiber with the opposite dispersion characteristics, and it would be fixed up well enough to apply some form of Rahman Amplification and shoot it down the road (often rail tracks) to the next amplifier, or to the receiver which splits out the transmitted wavelengths (channels) and converts to an electrical signal for switching, or could be retransmitted as a fresh undistorted pulse train on a long run, like LA to Chicago.


I'm sure you're talking terrabit rates now. We were doing 32 channels and a half-terrabit.

Yup.

Seven years ago:
255Tbps: World’s fastest network could carry all of the internet’s traffic on a single fiber

255Tbps: World's fastest network could carry all of the internet's traffic on a single fiber

A joint group of researchers from the Netherlands and the US have smashed the world speed record for a fiber network, pushing 255 terabits per second down a single strand of glass fiber. This is equivalent to around 32 terabytes per second -- enough to transfer a 1GB movie in two milliseconds...

www.extremetech.com

Glass is "better" than plastic, but no need at home.

I guess you could say that glass cables are not needed at home. Not sure how much the the error correction kicks in with plastic optical, but way back when many at Stereophile folks stated that they perceived a difference for the better with glass optical cables. One can buy glass cables, 3 foot about $35. So the price is not exorbitant to compare for yourself. Has anyone here tried measuring a difference between plastic and glass?