I understand about a digital signal, buffers, and the whole lot. But its still being sent via electricity, which is why I wonder how its so greatly unaffected by interference side from the use of buffers. Does it not follow the same laws and principles an analog does? If so, thats quite a feat.
An analog signal is sent in an AC waveform, does a digital signal use AC as well?
Yes, a digital signal, in the end, is an AC waveform of some sort. It's susceptible to interference as much as an analog signal, but it can handle such interference at the "other end". A very simple case of handling it at the "other end" is the use of balanced signaling. Since the interference has affected both the positive and negative signals in the balanced cable, it can be subtracted out when it reaches its destination. Using a very, very different mechanism, many digital signaling schemes can also remove errors during demodulation.
I noted in an earlier response that you said that the cables are "called XLR cables", in a correction of TLS Guy's post. I would agree that you
may call them XLR cables, but I would also suggest that they are also balanced cables. I would consider XLR cables to be a subset of balanced cables. A balanced cable doesn't have to have XLR connectors on its ends; it could also have 1/4" TRS connectors, or bare wire for that matter. Anyway, just wanted to note that.
Interferences in an analog AC waveform result in pulling, stretching, squeezing, ect. of the AC waveform, which obscures the original waveform along its path down the wire from A to B, thats what really going on is it not?
I have to agree with TLS guy that the AC waveform won't be pulled, stretched or squeezed. It's not elastic in the sense that if its affected by some outside interference it will somehow maintain its original form in a general sense. A noise spike would look just like a spike if you were to view it on an oscilloscope. It wouldn't have nice, rounded edges. Although the terminology is arguable, I think you have the right idea. An analog signal can be directly modified by other electrical sources.
You initially asked what causes such interference, and I'm not sure you were directly answered. I'll try to give one example here. In general, electrical signals general electro-magnetic radiation (or electro-magnetic interference (EMI)). Look outside at the wires carrying electricity to and from the homes in your area, and imagine there's a slight glow that surrounds the cable itself. It's invisible, but it's there, and it can be measured with various tools.
Now, when a electo-magnetic force exists, it can generate a current in a wire. When a current is generated, a new AC signal exists in that "wire"! Some motors work this way, I believe my toothbrush charger works this way, etc.
A similar radiation exists in the 120V AC cabling in your home. Now consider the signal that you are sending down a single-ended (i.e., unbalanced and analog) cable in your audio system. It's tiny compared to the 120V signals that power everything in your home. The low-level audio signal is probably on the order of 200 mV. If you place a 120 VAC cable adjacent and parallel to your audio signal, it's very possible that the EMI generated by the 120 V line will induce a current in the audio line. When that happens, you might get a 60Hz hum, since that's the frequency at which your home AC operates. That's just one basic example.
With that said, the reason why a digital signal is not affected as greatly would be because the wave form having to only represent 1's and 0's. If you squeeze a 1 it still looks like a 1, as well if you stretch a 0 it still looks like a 0. Rather than a waveform thats supposed to be directly proportionate to a sound wave, if that gets obscured in any little way its audible as distortion. Is that a crude idea of whats going on?
Digital modulation schemes don't normally just send a 1 or a 0 in the final waveform. If it existed, I would imagine that it would be the very first modulation scheme, and it's prone to problems with sampling, error recovery and EMI radiation due to potential repeating characters. I don't know exactly what modulation scheme is used for audio signals such as S/PDIF, but I would suspect that it's somewhat advanced and uses some type of error detection and correction.
Interesting, the concept of squeezing and stretching a signal applies more aptly in the digital world. Bits encoded for transmission can have effects on the signal level of adjacent bits. If you look at this on an oscilloscope, it definitely can look squished in the picture! But that's a different story... Unless there's something wrong with the setup or the system, the demodulator at the other end will be able to recover the digital data, regardless of how it looks (unless it's utterly destroyed by noise, which is indeed a possibility).
With audio, does the data being sent have headers in them for the buffers to trace back missing pieces?
I don't know specifically about audio schemes, but that type of thing is definitely a possibility in digital signaling (though I usually think of that data coming
after the audio data, so I wouldn't think of it as a header).
Why not run DAC's and mini amps in every speaker? That makes sense to me, and I claim to be far from genius.
Some do; see Meridian. More frequently, speakers have amplifiers in them, but not DACs (more in the pro audio field). The reasoning
not to do that is that the end user is then limited in flexibility in choosing their own amplifier or DAC. It also forces the user to digitize the signal at some point, which may be undesirable for some folks (e.g., someone playing a vinyl LP may not want anything digital in the audio path). Also, most consumer grade components are not set up to output a digital signal that could be sent to the speakers. Finally, there's just never been much demand from the consumer for this. There's no doubt that it's technically possible, but it's just never taken off.
Now, on to our other topic: getting an education is a great idea, and it sounds like you have some plains laid out. That's great and stick with it. At the same time, I think this type of forum is a GREAT place to ask questions! These are in-depth technical questions that are difficult to understand, and very difficult to explain through a forum like this. At the same time, it's a starting point and will encourage one to start to understand the issues, to ask more questions, and to check out other avenues of learning. I encourage you to continue to ask the questions, and hopefully you'll get some meaningful information in return.
Good luck!
