Optical Cable

[obscbyclouds]

There is the fact that coax cables can cause ground loops (i.e. 60hz hum). I had that problem with my dvd player and switched to optical. Optical cables cannot cause ground loops since it is not an electrical connection.

 

[j_garcia]

Ground loop isn't caused by the cable, it just transmits the problem from one piece of ungrounded equipment to your receiver. There are a few ways to easily fix this, but the use of an optical cable for this particular instance seems like an easy fix to remove the hum from the receiver. This breaks the connection between them, but did not remove the source of the hum at the DVD player.

 

[markw]

Good point.

Ground loop isn't caused by the cable, it just transmits the problem from one piece of ungrounded equipment to your receiver. There are a few ways to easily fix this, but the use of an optical cable for this particular instance seems like an easy fix to remove the hum from the receiver. This breaks the connection between them, but did not remove the source of the hum at the DVD player.

It can be a band-aid to cover up the symptoms of a pre-existing problem. Mind you, it doesn't fix the cause of the problem, only hide it.

 

[Nick250]

Not stupid, just wishfull. The brain *thinks* it is hearing/seeing a difference and therefore you do. If you spend a lot of money on something you want to notice a difference. If there is none, your brain makes one.

Once I *thought* I heard a difference when I put larger guage wire on my center speaker in an attempt to alieviate sibilance (at the recommendation of someont). I later figured out that it didn't do squat but at the time I sure thought it did. I didn't spend any money on it either.

The mind is a funny thing. As I mentioned in another thread my car always feels like it runs smoother, handles better and is faster after I have it washed.

 

[obscbyclouds]

Ground loop isn't caused by the cable, it just transmits the problem from one piece of ungrounded equipment to your receiver. There are a few ways to easily fix this, but the use of an optical cable for this particular instance seems like an easy fix to remove the hum from the receiver. This breaks the connection between them, but did not remove the source of the hum at the DVD player.

I was under the impression that this probably isn't dangerous. I live in an apartment, not sure if they'd rewire the outlets for me. Is this something i should further investigate?

 

[mtrycrafts]

I have tried both. With my Denon 3805 and 2910, i found the optical considerably better as the surround was wider compared to more forward with coaxial. .

For this to happen, the optical cable needs to unscramble the digital signals and alter the right data stream, in this case the surrounds. You think this is possible???

 

[tbewick]

It is probably worth spending money getting a decent quality optical cable. Not all signals sent down optical cables are digitally coded. An optical cable can, just like any other type of cable, act as a kind of filter. In poor optical cables, apparently this can (noticeably) reduce signal bandwidth (Sound on Sound magazine).

 

[markw]

Not all signals sent down optical cables are digitally coded. An optical cable can, just like any other type of cable, act as a kind of filter.

I'm curious about this. Can you elaborate on what other types of signals they can pass or filter? ...and how it can filter them?

 

[tbewick]

Hi markw,

Optical cables can be used to send analogue signals - just like an RCA cable. I used to think that they were used for digitally coded signals exclusively, but apparently this isn't the case. Output from a DVD player would normally be DTS, Dolby Digital, or audio CD data, these all being digitally coded.

The fact that cables act as a kind of filter is, if I remember correctly, to do with the fact that no cable has infinite bandwidth. I read this a while back in 'An Introduction to Digital Audio', by John Watkinson, and I have forgotten the exact technical explanation. Of course, all cables will technically perform differently. The author gave more of an explanation for electrical signals than for optical signals. The technical explanations are the things talked about by cable manufacturers - the dielectric, shielding, etc. I'm sure no one disagrees that these are things you have to consider if you were to make a cable. Personally I make do with fairly cheap cables (RCA, toslink) apart from with video cables, where the cheapest cables usually perform poorly.

If using optical cable for digitally coded data, then the cable will still distort/filter the analogue signal, but the DAC receiving the digitally coded signal should be robust enough to reject such distortion.

If using the optical cable as a connection for output to, for example, a minidisc deck, then the data may be sent as a non-digitally coded signal, (e.g. FM radio recording through an optical out on an A/V receiver to a minidisc deck). The distortion of the signal by the optical cable will not be rejected because the signal is not digitally coded. Therefore the properties of the optical cable will be important, just as it is in a standard RCA audio cable. The optical cable has the advantage over an electrical cable of not picking up electrical noise.

 

[tbewick]

I looked this up on Wikipedia - apologies - I seem to be mistaken about the analogue over optical cable. I must have been thinking of something else. It's only used for digital data.

 

[markw]

And, this applies to the link between a DVD and a DAC exactly how?

The signal originates in a digital state, is passed in a digital state and, at the DAC is treated as a digital signal to be converted to analog.

Are you saying the cable "knows" what's a good digital signal and what's not and will pass only the good signal?

At worst it would be a steady on or off of the light source. Even if a totally bogus "signal" somehow made it into and through the optical cable, it would still be interpreted as a digital signal bu the DAC.

If it was of a duration long enough to exceed the buffer in the DAC, then some sort of sonic anomaly would occur, the same as if a bogus signal were passed via coax.

 

[MDS]

I looked this up on Wikipedia - apologies - I seem to be mistaken about the analogue over optical cable. I must have been thinking of something else. It's only used for digital data.

Mistaken for the case of optical, but not for coax. Any time you transmit digital data using an electrical connection, the signal is analog. The zeros and ones are modulated using any number of schemes, the most common of which is NRZ (Non Return to Zero) Look that up on Wikipedia.

 

[Tritonman]

I use a coat hanger.

oops...did i say that out loud?

 

[rumble]

I use a coat hanger.

oops...did i say that out loud?

That's ok, I use a special $500 coat hanger....

 

[mtrycrafts]

That's ok, I use a special $500 coat hanger....

You are so lucky to have that I could only modify one from the closet, free

 

[tbewick]

Hi markw

The quality of the cable will have some bearing on the amount of distortion added to the digitally coded signal. Remember that the signal itself is not digital, it is an analogue signal that has been digitally coded, and can be distorted like any other analogue signal. The fact that it's digitally coded should make the original information more resistant to the added distortion/noise.

The degree to which the digital system is effective in rejecting added distortion from the cable is shown in the way you can use a coathanger as a cable in S/PDIF and still get good results. I still think though that buying a decent quality optical cable is a good idea. The cheapest cables can sometimes be of very poor build quality.

As to your description of how the system works, I think it is maybe too simplistic. If things were that simple and distortion did not occur (thanks to signal quality etc.), there would be no need for error correction, phase-locked loops, self-clocking coded signals etc. in modern digital audio systems. It is to the credit of the channel technology and the coding system used in Toslink and S/PDIF that such distortion can be rejected.

 

[markw]

Actually, I beleieve that the optical cable is pure digital. It's the coax that rides on an analog carrier.

How far back do you want to take the signal? Yesm, it ws analog when the artists createdit but at some point early on it goes digital and eventually winds up on an aluminium disc. Said disc is then put in a player and read buy a laser and the read pits are shaped, smoothed and massaged so they are in ptetty good shape bbefore hitting that LED that sends the signal down the cable. And, when they arrive at the other end and go through the opto-coupler they again go through an error correction process. Any contribution by the optical cable is rendered useless.

but, I agree that a toslink (and coax as well) cable should have good, solid connectors since these are the bain of that media and it's worth paying a little more for solid construction. ...but there's a limit.

 

[tbewick]

I'm only making this point as it was something that the author of that book I referred to earlier stressed. The signal will be degraded by the cable and this would surely be true of optical cables as well. I'm not sure about the specifics of optical cable, but there will be physical and practical limitations as to the bandwidth and the distance you can send the data. This are issues with the how good the channel (Toslink) is, and naturally will be affected by the properties of the cable.

I understand what you are saying, that the LED is on/off, therefore it is digital. In the design of the Toslink though, there will probably be a light intensity threshold when the receiver decides whether the signal is on or off. The quality of the signal will, I'm guessing, fall off with distance, and therefore the 'sharpness' and quality of the signal will decline. This is surely a result of the analogue medium, where the glass can have an essentially infinite number of light intensities. I believe that this is in contrast to an electronic integrated circuit, where the 'signal' will be kept in good condition by reclocking.

I understand that this difference does seem very trivial, but presumably it is an important consideration in the design of channels, like Toslink, S/PDIF, USB etc.

 

[Hi Ho]

Over short runs, such "defects" introduced by a cable will be completely inaudible. Unless you are running 100+ feet of optical cable I'll bet there will be no audible difference between different "grades" of optical cables.

 

[AudioSeer]

I always pick coaxial because I see no need for the signal to go through the additional optical to digital conversion and add additional jitter in the process.

As long as you are using a good quality coax, RFI is a nonissue. Even though the difference is probably not audible, why pick the technology that is inferior on paper when the cost is the same.