# "Digital Coax sounds better then Toslink"



## t3sn4f2 (Jan 3, 2007)

*MYTH* *(EDIT: on hold as of 7/2/11, see post #2)*

From an earlier post I made......

I've found that to be completely untrue. Toslink and coax are not bit identical in the analog realm but the difference lay as blips and ticks that are never more then ~90dB below the fundamental sample. And I have no way of testing this but those ticks could very well be a result of the analog process needed to verify the myth and not an actually difference between the digital transmission formats.

Here's how I did it. 

I take the coax and Toslink output from any old CD player and run them into the corresponding digital inputs of my soundcard. I then route that digital stream, one at a time, out to the DAC of the soundcard. Then I take the analog out and plug it right back into the soundcard's ADC. Then I setup up the PC to record that analog input in 24bit/44.1kHz.

I included the analog domain because that is the only way to bring jitter into the picture, which is what digital degradation is and what this topic is about. If I simply record and compare the digital streams they would always be identical since all that is happening is that the bits are flowing in with the clock encoded on them and the timing (ie determining factor for jitter) never comes into play. (see Lycan's jitter thread for more details).

K, now that I have recorded a complete track through the same player from both digital output forms, I take those two files and open them up in Audacity sound editor. Which adds extra bits for better precision when manipulating and editing (32bit from 24bit in the file). Here's an example:










I then set the cursor snap to to samples and go to the beginning of the track and zoom in to where I can start to see a similarity pattern. (You'll have to ruff cut away before this level of magnification since recording times don't start the same and we are in the millisecond range after all). Once I find a clear similar point (red arrows), 










I cut away and zoom in further till individual sample points become visible.










I then click on a peak for instance and count the sample at lays at (bottom of the screen red arrow below. Then I cut away the appropriate amount from the begin of each track so that the peak sample I choose now lays at the first sample marker.










You can see now that even 7 million samples into the song (2:50), the samples still are lined up. 










Final step is to use the invert track function which inverts all the sample values in that track.










Then you can go and play both tracks over each other and any difference will come out as anything but silence. I've performed this on two difference cheap players and my soundcard's digit out outputs and the results are the same. the tick and pops sound unique in each case but they are always at that inaudible level of ~90dB. As I pointed out earlier, this isn't a constant noise regardless of whether there is a signal or not. If the track is silent then the noise is -90db below that low noise floor.

These noise differences can even be a result of all the other variables in this test, such as random DAC/ADC/cable analog noise. So the two digital outputs could be for all intents and purposes audibly identical. 

You can try it out for yourself to check any device you are suspect of. You'll just need a decent soundcard that has digital I/O's + ADC/DAC stages that cancel out like above so you can have a _silent_ baseline to measure other devices with.

It's worth noting that any error on my part would only serve to go against what I'm saying here. As it would result in some difference that would become and audible sound. And the results were consistent over the length of the track not just a few seconds sample.

This by the way, for anyone else reading this, is a GREAT way to show how interconnects and power supplies affect component sounds. Just keep everything the same and change out the cables only.


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## t3sn4f2 (Jan 3, 2007)

I'm having trouble recreating the same results on my Pure i-20 digital iDevice dock. 

So for now all that is definite is that coax and Toslink _can_ sound identical to one another. Even on budget devices.


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## rain27 (Jan 15, 2009)

t3sn4f2 said:


> I'm having trouble recreating the same results on my Pure i-20 digital iDevice dock.
> 
> So for now all that is definite is that coax and Toslink _can_ sound identical to one another. Even on budget devices.


Odd...let us know if you find out why or what went awry.


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## nsaspook (May 19, 2009)

It's all EM field photons just at different frequencies. Thz on the fiber, Mhz on the wire. The information transmission media means nothing at these low bit counts. The encoder/decoder electronics could easily corrupt the signals over a long signal path but at the lengths of a normal consumer system even the cheapest junk works almost prefectly.


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## 24th-Alchemist (Jun 16, 2011)

I like the spirit of this thread but I have some comments.



> I included the analog domain because that is the only way to bring jitter into the picture, which is what digital degradation is and what this topic is about. If I simply record and compare the digital streams they would always be identical since all that is happening is that the bits are flowing in with the clock encoded on them and the timing (ie determining factor for jitter) never comes into play.


What is quoted above hinges on the assumption that the clock is recovered from the TOSLINK or digital coax signal. But how do we know if that's the case? Does the card run if the TOSLINK is disconnected? (Presumably yes.) If so, then we know that TOSLINK is not the sole clock source, and it raises the question of whether it's ever the clock source.

For example, I have built some TOSLINK receivers and I do not use the clock recovered from the the TOSLINK signal. Instead, I use an asynchronous sample rate converter (ASRC), such as a CS8421. This stores several full word audio samples received from the TOSLINK signal and tries to find the "average" clock from the TOSLINK, based on a high-quality clock driving the ASRC. The TOSLINK signal's average sample rate is then converted to the rate of the receiver IC, which has it's own clock not derived from TOSLINK. The end-effect is a reduction in any clock jitter that might be on the TOSLOINK signal. There are also several IC's that have their own built-in ASRCs.

Another, cheaper option is just to read the TOSLINK received audio at the clock rate of the receiver, independent of the clock of the TOSLINK signal. This results in occasional "bit-slip" (a missed sample or the same sample used twice), because the send and receive clocks will never be identical.

In either case above clock recovery would not be involved. And if PLL clock recovery were involved, I have not heard of cable material as a primary factor in the quality of the recovered clock. Instead, it seems to me that the PLL implementation is the main factor. For example, see the discussion of recovered clock quality and PLL implementation by an IC manufacture here. They discuss how updating the PLL once every bit versus once every subframe affected recovered clock jitter. The point is the issue is PLL implementation, not transmission media. (Although TOSLINK versus coax still could make jitter differences I guess). 



> at the lengths of a normal consumer system even the cheapest junk works almost prefectly


I have no scientific proof of such but, for what it's worth, I use "junk" plastic TOSLINK cables and everything sounds beautiful to me. That said though, I use ASRC's and do not recover the clock from TOSLINK.

Another point about the experiment.

I would offer the suggestion of running, say, just one minute of audio several times over TOSLINK, and then doing the same using digital coax, and then comparing the variability among several TOSLINK trials with the variability among several coax tries. It could be set up like an statistical binary hypothesis test -- is the variability of TOSLINK different from coax yes / no.

Like I said though, I think it's critical to know for sure that the clock is actually recovered from the TOSLINK / coax signal.


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## S3T (Sep 21, 2008)

Toslink could sound way better than spdif and vice versa.
Depends on specific implementations of both devices sitting on the link.

Toslink gives you ground-less signal transfer, while SPDIF without proper transformers could carry the ground between devices. Ground is a great source of noises.

I have checked the data integrity of squeezebox duet, and it came out bit-imperfect on the LSB.
I have recorder the SPDIF, inverted it, aligned with an original song and mixed them to see difference.
The difference was occasional LSB change (i've got occasional triangles with a height of 1 bit on the resulting track).


Connected thru nokia's power cord as "spdif cable" to E-MU 0404 PCI.


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## t3sn4f2 (Jan 3, 2007)

nsaspook said:


> It's all EM field photons just at different frequencies. Thz on the fiber, Mhz on the wire. The information transmission media means nothing at these low bit counts. The encoder/decoder electronics could easily corrupt the signals over a long signal path but at the lengths of a normal consumer system even the cheapest junk works almost prefectly.


That's how I understand it also. What's important is that the fiber isn't bent severely (and I'm not sure that is more for transmission or for simply protecting the cable from damage). And for coax, that it has the electrical properties to carry the signal (a 75ohm specific shield cable if I'm not mistaken).


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## t3sn4f2 (Jan 3, 2007)

24th-Alchemist said:


> I like the spirit of this thread but I have some comments.
> 
> 
> 
> ...


Thanks for your incite Alchemist. Here are some more detail about the test setup.

-The soundcard used was an E-MU0404PCI which has the mandatory option of setting the card's sync to a specified internal sample rate or to external S/PDIF. Playing an external stream with internal sync kills the quality and vice versa when an external device is connected.

-I indirectly tried your suggested double pass experiments while doing the testing initially. I was trying to see if different media apps were playing exactly same as one another (as they should). I made a pass of the ipod app and of leechtunes and over the length of the entire song, there was silence to the inaudible degree I mention in the opening post. I didn't try it for the coax side since there wasn't a need in that experiment. I'll give it a try and report the coax results back when I check to see what went wrong with the initial experimentation.

-I have a feeling that it could be a software hiccup in the sound cards DSP. Something to do with switching sync sources and not resetting the source might be causing it. IIRC I had the same problem with the second device I tested like this. I had to do a very specific step by step in order to shift sync and digital cable source properly.

I'm also not too confident about the coax cable I used, which wasn't even a digital audio specific cable, it was a monster cable composite video cable. :blush: But, last test I used an even cheaper composite video cable and the results come out fine. So I don't know if that could be. I'll try everything I can with the software and syncing setup, if that doesn't change the results, I'll buy a proper cable and see.

-Here is my thought behind why I made this thread. I'm thinking that if I can show that 3 separate budget devices (1 <$200, 2 <$100) can have digital outputs that sounds the same as per my experiment above, then EVERY car audio device that is many times more expensive should. That might sound like a conclusion based on an assumption, but a reasonable one at that. Sure it might be worth testing your device on your own to make sure it works as it should, but at least we can say that the differences between Toslink and coax aren't as big and common place as people say they are. It can be seen as it sounds different because something is f'ed up and not because its a normal thing.


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## envisionelec (Dec 14, 2005)

t3sn4f2 said:


> So the two digital outputs could be for all intents and purposes audibly identical.


Thanks for not saying "all intensive purposes".


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## envisionelec (Dec 14, 2005)

24th-Alchemist said:


> I have no scientific proof of such but, for what it's worth, I use "junk" plastic TOSLINK cables and everything sounds beautiful to me. That said though, I use ASRC's and do not recover the clock from TOSLINK.


Toslink/coax is a connector type. SPDIF is the transmission specification.

It's not the cable material that makes the difference so much as the correct termination (impedance) matching. All things equal, terminating a coaxial cable is more reliable than dealing with reconstructing an optically transmitted signal over a fiber of questionable quality.

I can't think of any high quality DAC chipset that recovers the bit clock from SPDIF to be used as the sample clock. Maybe in low-end consumer grade stuff...


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## t3sn4f2 (Jan 3, 2007)

envisionelec said:


> Toslink/coax is a connector type. SPDIF is the transmission specification.
> 
> It's not the cable material that makes the difference so much as the correct termination (impedance) matching. All things equal, terminating a coaxial cable is more reliable than dealing with reconstructing an optically transmitted signal over a fiber of questionable quality.
> 
> *I can't think of any high quality DAC chipset that recovers the bit clock from SPDIF to be used as the sample clock.* Maybe in low-end consumer grade stuff...


Is that what is being done when my sound card is set to external sync or is my understanding of this completely off?


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## S3T (Sep 21, 2008)

envisionelec,
What can you propose as the replacement of masterclock recovering from regular (non-back synced) SPDIF?
Which keeps data integrity?

None...
WM8804/5 is as close as you can get in a single chip. It still recovers the clock from the source, passing it thru FIFO with PLL, decreasing the jitter passband frequency down to 100hz per ad-sheet.


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## envisionelec (Dec 14, 2005)

S3T said:


> envisionelec,
> What can you propose as the replacement of masterclock recovering from regular (non-back synced) SPDIF?
> Which keeps data integrity?
> 
> ...


I probably should be more clear. The uncorrected bit clock cannot be relied upon to provide a low-jitter recovery. Thus, PLL "steering" is applied against a stable, local clock. The best results still come from MCLK injection from the transport - but that requires another cable. 

Yep, I am just finishing a high end design around the WM8805/8741 chipset for a US-based company. Really nice chips and very easy to use.


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## 24th-Alchemist (Jun 16, 2011)

> It's not the cable material that makes the difference so much as the correct termination (impedance) matching. All things equal, terminating a coaxial cable is more reliable than dealing with reconstructing an optically transmitted signal over a fiber of questionable quality.


With respect to the second sentence, agreed -- but the coax to optical comparison hinges on what constitutes a "fiber of questionable quality". Toshiba, a manufacturer of the TOSLINK brand of optical interconnects, specifies the performance of their optical transmitters and receivers using what they call "all plastic fiber 980 / 1000 um" -- in other words, I think they're talking cheap plastic (see for example here.) Unless a plastic link is severely bent I think cheap plastic does *not* constitute an interconnect of "questionable quality".

In my experience, optical cable, as well as optical transmitters and receivers, are easier to use than coax. Proper coax requires at least one transformer to avoid ground issues among other things (see pp. 38 here and also for transformers here.) Transformers, proper impedance matching, their space footprint, and the associated "voodoo" that I have encountered with limited high-freq transmission experience leads me to conclude that optical is much easier.

I think a lot of this is moot though because if either coax or optical "fails", they fail in terms of bit errors that will result in obvious sound degradation b/c the bit errors will not occur solely in the LSB.

It seems that if there were "fine" sound differences between optical / coax, those differences would need to result from PLL clock reconstruction differences. But in such cases I don't think that expensive exotic cable that somehow effects PLL performance is going to be the fix; instead I think the fix is improving the PLL. Moreover, notice that -- at least in the cases I'm aware of -- the recovered "clock" often exceeds the bit-rate of the S/PDIF transmission (e.g. 24-bit S/PDIF at 48 kHz is 6.114 MHz data rate but "recovered" clock is often 12.228 MHz to drive the DACs). So even if there were perfect, no jitter, no pulse-width distortion S/PDIF transmission and reception, the receiver PLL would still be relied upon to keep the jitter down in the 12.228 MHz clock. (As I've stated above, there is a nice introduction to the importance of the PLL by a S/PDIF receiver manufacturer here.)

So obviously I'm biased here. I can't see how any reasonable cable -- coax or optical -- will mater for S/PDIF clock recovery.



> I'm also not too confident about the coax cable I used, which wasn't even a digital audio specific cable, it was a monster cable composite video cable.


As appears to have been said by others also, if it's 75-ohm impedance, it should be fine. I hope however _t3sn4f2_ you will remain impartial and unbiased and double check these things.



> I can't think of any high quality DAC chipset that recovers the bit clock from SPDIF to be used as the sample clock. Maybe in low-end consumer grade stuff





> Is that what is being done when my sound card is set to external sync or is my understanding of this completely off?


I'll bet your card is somehow deriving its clock from the received S/PDIF signal b/c you mentioned that things sound bad if you don't externally sync. But hopefully your card implements a "low-tech" poor quality clock recovery solution b/c then you can test whether coax versus optical and cable types make any difference. If on the other hand the card has a great clock recovery implementation then it may overcome any and all cable differences so that they can't be detected.

I'm biased and we can discuss hypotheticals ad-infinitum but _t3sn4f2_ I admire your seemingly unbiased approach to all this, and measurements IMO are far more valuable than talk so I hope you continue these experiments and keep everyone informed about what you find.


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## envisionelec (Dec 14, 2005)

24th-Alchemist said:


> With respect to the second sentence, agreed -- but the coax to optical comparison hinges on what constitutes a "fiber of questionable quality". Toshiba, a manufacturer of the TOSLINK brand of optical interconnects, specifies the performance of their optical transmitters and receivers using what they call "all plastic fiber 980 / 1000 um" -- in other words, I think they're talking cheap plastic (see for example here.) Unless a plastic link is severely bent I think cheap plastic does *not* constitute an interconnect of "questionable quality".


Uh - you surely know that manufacturers do not necessarily follow a specification. And, in my terms, questionable = damaged or improperly OR poorly terminated. A lot of cables get smashed behind the audio rack and users may never know it.



24th-Alchemist said:


> In my experience, optical cable, as well as optical transmitters and receivers, are easier to use than coax. Proper coax requires at least one transformer to avoid ground issues among other things (see pp. 38 here and also for transformers here.) Transformers, proper impedance matching, their space footprint, and the associated "voodoo" that I have encountered with limited high-freq transmission experience leads me to conclude that optical is much easier.
> 
> I'm biased and we can discuss hypotheticals ad-infinitum but _t3sn4f2_ I admire your seemingly unbiased approach to all this, and measurements IMO are far more valuable than talk so I hope you continue these experiments and keep everyone informed about what you find.


You throw a lot of numbers around - and that's fine, but it's completely wasted on me. Please understand you're not talking to an audiophile, but an experienced engineer. I don't give a sh*t about hypotheticals.


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## thehatedguy (May 4, 2007)

Optical is easier because it is cheaper.


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## S3T (Sep 21, 2008)

Coax is cheaper, as you can omit the transformer, and just use a capacitor instead, without ground isolation.


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## 24th-Alchemist (Jun 16, 2011)

> Coax is cheaper, as you can omit the transformer, and just use a capacitor instead, without ground isolation.


... A dubious endeavor IMO if robust operation is the goal.

A transformer is employed not just to block DC, but rather to attenuate any common-mode interference, which may exist at frequencies other than 0.


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## oldno7brand (Mar 15, 2011)

In my Home theater I run Monster digital coax in between my Pioneer DVD and Sony ES Receiver. 

In DTS I can hear a difference between the COAX digitial and Tos-link
(3FT run)

Coax digital is warmer and more smooth IMHO......
more emotion in the sound.


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## Buff (Apr 17, 2006)

Having done numerous listening tests between different quality toslink I will share that true glass fiber toslink cables sound superior to those using plastic


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## FG79 (Jun 30, 2008)

oldno7brand said:


> In my Home theater I run Monster digital coax in between my Pioneer DVD and Sony ES Receiver.
> 
> In DTS I can hear a difference between the COAX digitial and Tos-link
> (3FT run)
> ...


This.

Look at true high end DACs in home audio....you will rarely find toslink. All of the Audio Note DACs from their $2k entry level DAC-1.1 to the $50k flagship DAC-5 are all coax. IMO they make the best DACs (and a lot of other gear at a very high level). 

For car audio, McIntosh MX-5000 head unit = coax only. Japanese spec Alpine F#1 was coax also, American spec was toslink. 

That to me is _the _argument for coax without trying to go into the details.


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## FG79 (Jun 30, 2008)

thehatedguy said:


> Optical is easier because it is cheaper.


Forgot about this one too, LOL.

The real reason why you see optical in most of the supposed high end head units now. 

Not hard to convince most consumers that "modern" fiber optic technology is way better than "obsolete" coax....much the same way transistors and digital are the future, and tubes and analog are old junk.

;-)


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## t3sn4f2 (Jan 3, 2007)

FG79 said:


> Forgot about this one too, LOL.
> 
> *The real reason why you see optical in most of the supposed high end head units now. *
> Not hard to convince most consumers that "modern" fiber optic technology is way better than "obsolete" coax....much the same way transistors and digital are the future, and tubes and analog are old junk.
> ...


There's also the reason that it can easily sound identical to coax so why not use it instead of something more expensive.


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## SoundChaser (Apr 3, 2009)

I can’t see how transmitting digital information over coax or different types of fiber would result in a difference in sound. Noise or jitter should not be a factor unless it’s great enough to corrupt the data stream; it’s either a 1 or a 0.

As for as bending fibers, I’m not sure about the ones used in audio but the ones used in communications can take extreme bends as long is it’s not 90degrees. Here at work, if I’m in need of an attenuator and there are non-around, I’ll wrap the fiber around a pen. Each loop yields approximately 2dB of attenuation.


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## 24th-Alchemist (Jun 16, 2011)

What would be a mechanism by which coax would sound "warmer" than optical?

The way I see it, there are only two main areas for potential problems with coax and optical:
bit errors;
clock reconstruction and the associated jitter.

Are the other potential issues I'm not aware of? If so, could someone please cite some?


(1) If there is a bit error -- and the bit error is towards the most significant end (larger magnitude error) -- the audible result will be a "ticking" sound. Such errors should be obvious to a careful listener, and I think their rate of occurrence is close to never.

(2) Clock recovery "error", i.e. jitter, is more subtle, but I don't see a mechanism that would allow one type of transmission to outperform the other in terms of jitter. It seems to me that in order to understand a comparison of the two, we'd need to understand the details of the receiver and also the details of the PLL used for the recovered clock. And if an ASRC is used, it's not clear to me how coax vs optical could effect the clock in any audible way.

I'm very open to learning more about these purported differences, but until someone comes with something a little more concrete, I can only conclude that the differences exist solely in the minds of those who wish to perceive them.


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## denetnz (Jul 31, 2009)

I think there is one remarkably strong mechanism at play here - its called the placebo effect.

Do a little research on blind testing on amplifiers and you'll be shocked how strong it is. Many a "golden ears" has described vast differences between amplifier A and amplifier B - when they were actually listening to the same amp the whole time!


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## t3sn4f2 (Jan 3, 2007)

t3sn4f2 said:


> I'm having trouble recreating the same results on my Pure i-20 digital iDevice dock.
> 
> So for now all that is definite is that coax and Toslink _can_ sound identical to one another. Even on budget devices.


K, I got lucky and figured out what was wrong. You guys have seen that I edit the tracks in Audacity in order to get them set up to see if they cancel each other out. 

However, I don't record in Audacity since it does not have the option to use the pro sound driver ASIO that bypasses any processing between the recording app and the soundcard hardware. For the recording part of the experiment I then used Sonar LE, which does support ASIO. 

Now while redoing the test yesterday I realized that since ASIO lets the application take control of the soundcard hardware, I then might need to restart Sonar LE each time I made the switch from Toslink to coax in the cards DSP software. If not it might not update Sonar properly.

So I did that each time I finished a recording and it came out perfectly. I ran a comparison using an RMAA test track and that canceled out to ~90dB below the fundamental sample value. Did the same with a heavily compressed Pop track and got the same result.

So that now makes three different budget digital sources whos digital outputs sound the same.

And some more good news out of this is that no one with a Pure i-20 dock needs to worry about which digital output will be better to use. They're both the same.

Oh, and I never did get a S/PDIF specific Coax cable. I just used a cheap old composite cable in this test (as well as the other test I ran on the other two devices). But it should not matter really since composite RCA cables are rated at 75 ohms just like an S/PDIF Coax RCA cable is.


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## HisHeirness23 (Jul 28, 2009)

SoundChaser said:


> I can’t see how transmitting digital information over coax or different types of fiber would result in a difference in sound. Noise or jitter should not be a factor unless it’s great enough to corrupt the data stream; it’s either a 1 or a 0.
> 
> As for as bending fibers, I’m not sure about the ones used in audio but the ones used in communications can take extreme bends as long is it’s not 90degrees. Here at work, if I’m in need of an attenuator and there are non-around, I’ll wrap the fiber around a pen. Each loop yields approximately 2dB of attenuation.


I would imagine that the wavelength used to transmit audio information is much different than what is used in telecommunications as the laser is visible with audio. I, too, work with fiber optic communication equipment, and know what you mean. Although, we also work with bend-insensitive fiber which you can practically tie in a knot and not experience much attenuation.

I don't understand how there could be a difference in sound, unless there were bit errors with the digital transmission signal.


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## sobe3yourself (Jun 30, 2011)

ignore this, need one post to pm. didnt know classified posts dont count. sorry.


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