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All other things being equal, if there's an option for connecting two components using RCA cabling or connecting the same two components using an optical cable, is there any reason to prefer one method over the other from a pure sound quality/fidelity perspective?

I've seen similar questions raised elsewhere, but they usually just end up with someone stating that a digital connection will get you more channels/speakers and is therefore "better"; however that's not really the question. The question is more about which, if any, preserves a more accurate signal (as measured by the difference between the played/recorded sound on device 2 and the original source waveform generated/sent from device 1) between two devices.

If there are caveats that require assumptions to be made, then please assume something along the lines of:

  • The source audio is stored digitally (as MP3, FLAC, etc.)
  • The distance between devices is < 50 feet
  • Both devices are of reasonable and approximately comparable quality
  • The source device has RCA and optical outputs, and the receiving device has RCA and optical inputs

Is there a clear and general "best" option for connecting the devices, is it basically the same either way, or does the answer depend entirely upon the specific devices being used?

migrated from music.stackexchange.com Jun 24 '18 at 19:35

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  • This site is more about creating music and not so much about listening for enjoyment. It’s not at all about home A/V equipment. – Todd Wilcox Jun 18 '18 at 1:55
  • But surely music creation benefits from knowing the optimal way to transmit sound between two devices? Like if the first is a digital piano and the second is your recording/editing workstation, for instance. – aroth Jun 18 '18 at 2:03
  • The source audio is stored digitally (as MP3, FLAC, etc.)” Seems like the first actually isn’t a digital piano. Also, why not just try it both ways and see for yourself? No one is going to be able to tell what will sound best in your particular situation. There isn’t actually a way to answer which signal is more “accurate” without clarifying that word a lot. – Todd Wilcox Jun 18 '18 at 4:16
  • But those are just clarifying assumptions to be used if necessary; although a digital piano would necessarily store its source/sample data digitally...so it's just a question of the file/encoding format, which I have no idea about (possibly WAV?) though presume doesn't make much difference in practice? I've clarified the point about 'accuracy'. I mean the minimal amount of acoustic 'noise', measured as the difference between how the signal renders on the second device compared to what was actually sent from the first (or what would be sent from the first, if going analog at device 1). – aroth Jun 18 '18 at 4:52
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    Sony appears to favour optical, which will transfer all info., although I've only come across short optical 'cables'. Longer wires, as in phono (RCA) to phono will have an optimum length before degradation starts, and it will also depend on quality of copper involved. (My solicitor said that in court as well...) – Tim Jun 18 '18 at 6:58
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If your receiving device is going to convert the sound to digital again internally, then you should connect TO it digitally if you have the option. There's generally nothing to be gained from converting to analogue and back to digital again; most likely, you'll only lose quality that way. (I'm assuming here that the digital connection is working within its design tolerances and is transferring the digital values 100% accurately, which should be the case most of the time - though it is possible to have issues with deficient cables, incompatible clocking, etc.)

If you're near the end of your signal chain and you're wondering at what point you should finally go out to analogue - that will depend on the quality of your DACs and your cabling*. In some cases it might be better to run an analogue connection from an excellent DAC to an amplifier, than to go to a digital in on the amplifier IF the amplifier has a so-so DAC (and if it isn't going to convert your analogue signal to digital internally anyway).

* and of course, 'good quality' does not necessarily mean 'expensive'...

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With consumer or prosumer equipment it is difficult to tell. The main reason is that they seldom tell the full story when it comes to specifications so you need to try.

If the source is mp3 - any will do. If you have higher quality source, it might depend.

  • RCA on analog is good, but it might have trouble with induced noise. One source may be a ground loop. A ground loop is when grounds on the different equipments are connected so that a loop is created. This works, generally in a very small way, as a transformer secondary on the mains frequency. Digital is much less sensitive to ground loops.

  • Digital might be better, but notice the might. If you start with an mp3, there will be quite a bit of conversion going on. Generally digital signals between equipment is an uncompressed bit stream at 48kHz or sometimes 44.1 kHz. But the mp3 is neither of those frequencys so it will have to be digitally converted before beeing sent. Some boxes does this well, some less. Your box might be very good at converting to analog and lousy at converting digital.

In general though, unless you have golden ears (say, audiphile) there will be no difference in sound quality.

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    " If you start with an mp3, there will be quite a bit of conversion going on" - yes, but all that decoding and conversion would have to be done even if sending over analogue (you have to decode the MP3 before it can go through the DAC to get it into an analogue state), so that's not a disadvantage of using a digital out. – topo Reinstate Monica Jun 18 '18 at 20:35
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Depends how MUCH less than 50' the connection is. 50' is too far for TOSlink, which is probably what you mean by 'optical cable' on domestic equipment. (Can you even buy a 50' TOSlink cable?) But it is also too far for the unbalanced line-level signal from a RCA jack. As you bundle MP3 and FLAC together as source, I guess you aren't really interested in accuracy. However it would be nice to not pick up hum or other interference, right?

If the distance is sufficiently less than 50', within the length of available optical cables, use optical. If not, you'll have to use the RCA cables. If it sounds good, fine. If you get hum or RF rubbish, look for a better system.

If this is between two adjacent items of equipment, in a domestic situation, either method will be fine, and I very much doubt you'll hear any difference.

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