Coax is digital but, why is it often called analog? - Page 2 - AVS Forum | Home Theater Discussions And Reviews
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post #31 of 51 Old 11-10-2019, 03:11 AM
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Originally Posted by Bigus View Post
Um, no. Why do you think a digital signal is converted to analog before being send down a coax cable using spdif signaling? It is a square(ish) wave signal, on or off, 1 or 0, the same as a computer IC.
The signal through coax isn't a square wave. It's a sine wave. it's converted to analog and then converted back to digital.
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post #32 of 51 Old 11-10-2019, 07:21 AM
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Originally Posted by KidHorn View Post
The signal through coax isn't a square wave. It's a sine wave. it's converted to analog and then converted back to digital.
No. It isn't an analog signal in any traditional sense. Spdif uses biphase mark code signaling which is more or less the same as used in 10base2 LAN over coax.

Why would a stand alone DAC have a digital coax input? To convert the ”analog" signal to digital, then back to analog again? That's nonsensical.
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post #33 of 51 Old 11-10-2019, 05:03 PM
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Originally Posted by KidHorn View Post
The signal through coax isn't a square wave. It's a sine wave. it's converted to analog and then converted back to digital.
KidHorn, why do say stuff like this. You do understand that the cable does not determine whether the signal it carries is digital or analog, do you not? If so, then please don't make statements like that cause it just causes confusion.

If not, then know that a cable is just a completely dumb, passive component that carries a signal from point A to point B, whether it's digital, analog or plain old DC. It's the electronics that it's connected to that determine what kind of signal it is. The design specs of the cable will determine just how well the signal is carried & thus it's important to use a cable type that is appropriate.

To directly address your above statement; a coax cable will carry a square wave or a sine wave, or any other wave pattern. It doesn't know any better, just carries whatever it's given.
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post #34 of 51 Old 11-11-2019, 03:08 PM
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Originally Posted by frankwp View Post
*snip*
To directly address your above statement; a coax cable will carry a square wave or a sine wave, or any other wave pattern. It doesn't know any better, just carries whatever it's given.
True, it will carry "whatever it's given" so long as "whatever" it is can be transmitted through a cable having its given characteristics. If it can't, the signal will be deformed, possibly enough such that it can't be received correctly. Obviously this becomes a much bigger issue when something on the rx side is trying to recover clock and figure out what's a digital 1 vs. 0 than something that just takes the input and amplifies it (e.g., a S-P/DIF link vs. an RCA connected to a power-amp).

That said, the whole thing is a "system" -- the transmitter, receiver, and cable characteristics all have to "align". The higher frequency signal that needs to be transmitted the more "rigorously" the system must be controlled (which is why, e.g., Ethernet cables have "rigid" specs that analog audio cables really don't need). For analog audio use, our ears govern how accurate this system needs to be (and, ours ears aren't the most accurate thing in the world). For digital, it's a bit more cut-and-dry because either it "works" or it "doesn't" (and "doesn't" is typically quite obvious, especially if there's robust error checking). However, getting it "to work" takes more effort and requires more "control" (e.g., if you don't use a 75ohm cable for S-P/DIF then it's more likely you won't be able to recover the signal and it "won't work" because you'll hear silence, lol).

Finally, I do wonder if this used to be a larger issue in the analog world. Is it not true that tube circuits are more impacted by impedance variance? Did this require "impedance matching" (including cable) beyond what we need today? Because, now-a-days, it's pretty common, as said in this thread, for the source device to have a low driving impedance and the receiving device to have a much higher load impedance; under these circumstances it's pretty hard to find a cable that can't transmit analog audio up to 20kHz with inaudible losses. This is especially so over the short distances typical of a home environment. Even more so when you figure in the potential for high frequency hearing loss among the folks most likely able to afford exotic cables, lol.
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post #35 of 51 Old 11-11-2019, 03:19 PM
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Originally Posted by DreamWarrior View Post
True, it will carry "whatever it's given" so long as "whatever" it is can be transmitted through a cable having its given characteristics. If it can't, the signal will be deformed, possibly enough such that it can't be received correctly. Obviously this becomes a much bigger issue when something on the rx side is trying to recover clock and figure out what's a digital 1 vs. 0 than something that just takes the input and amplifies it (e.g., a S-P/DIF link vs. an RCA connected to a power-amp).

That said, the whole thing is a "system" -- the transmitter, receiver, and cable characteristics all have to "align". The higher frequency signal that needs to be transmitted the more "rigorously" the system must be controlled (which is why, e.g., Ethernet cables have "rigid" specs that analog audio cables really don't need). For analog audio use, our ears govern how accurate this system needs to be (and, ours ears aren't the most accurate thing in the world). For digital, it's a bit more cut-and-dry because either it "works" or it "doesn't" (and "doesn't" is typically quite obvious, especially if there's robust error checking). However, getting it "to work" takes more effort and requires more "control" (e.g., if you don't use a 75ohm cable for S-P/DIF then it's more likely you won't be able to recover the signal and it "won't work" because you'll hear silence, lol).

Finally, I do wonder if this used to be a larger issue in the analog world. Is it not true that tube circuits are more impacted by impedance variance? Did this require "impedance matching" (including cable) beyond what we need today? Because, now-a-days, it's pretty common, as said in this thread, for the source device to have a low driving impedance and the receiving device to have a much higher load impedance; under these circumstances it's pretty hard to find a cable that can't transmit analog audio up to 20kHz with inaudible losses. This is especially so over the short distances typical of a home environment. Even more so when you figure in the potential for high frequency hearing loss among the folks most likely able to afford exotic cables, lol.
Agreed, and I made mention of importance of matching cables specs to the task in an earlier post. But what I'm trying to address here is the mistaken notion held by many that it is the cable that determines whether the signal is digital or analog. It just is not so.
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post #36 of 51 Old 11-11-2019, 03:36 PM
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Originally Posted by frankwp View Post
Agreed, and I made mention of importance of matching cables specs to the task in an earlier post. But what I'm trying to address here is the mistaken notion held by many that it is the cable that determines whether the signal is digital or analog. It just is not so.
I don't know that "anything" determines whether it is digital or analog, with the exception of the tx/rx devices -- I mean, a digital signal is just an analog signal with HF components on the "edges of the square waves". As long as those edges can be detected accurately enough by the rx side, it's good. The more frequently those edges appear (i.e., the higher the signaling rate), the harder it is for the "system" to maintain them (tx/rx and cable).

Another way to say this is that "there exists no digital signal" -- it's always analog, it has to be. Nothing in the world can go from 1 to 0 "immediately", there's always something "analog" between 1 and 0 (how fast the system can transition between 1 and 0 "cleanly" determines how much signal it can transmit reliably).
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post #37 of 51 Old 11-11-2019, 07:57 PM
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Originally Posted by DreamWarrior View Post
...it will carry "whatever it's given" so long as "whatever" it is can be transmitted through a cable having its given characteristics. If it can't, the signal will be deformed, possibly enough such that it can't be received correctly.

...For digital, it's a bit more cut-and-dry because either it "works" or it "doesn't"...

Those two statements seem a bit contradictory, and the second isn't true.

When it rains, some of the OTA digital stations I can receive get blocky/pixelated w/o completely disappearing.

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post #38 of 51 Old 11-11-2019, 11:20 PM
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Originally Posted by noah katz View Post
Those two statements seem a bit contradictory, and the second isn't true.

When it rains, some of the OTA digital stations I can receive get blocky/pixelated w/o completely disappearing.
I would consider that "not working". I also would posit the reason this happens is because there exists "robust" error correction. Thus, malformed (detected by error correction) packets are dropped. Because those packets "didn't work" you get a pixelated image (until a sync packet comes along to fix it all up).

Certainly the other option is that it's there exists no robust error correction so any signal errors make it through as "garbage".

I suppose what I was more trying to convey is that if an analog signal is some how "corrupted" (e.g., highs rolled off) then it may not "be obvious" that something is wrong -- what comes through the other end is likely "good enough" for our ears. But, that's not at all the case with digital and, a properly error corrected protocol, will typically toss bad digital data (that can't be corrected) as best it can. The results thereof tend to be "obvious" (in this case, pixelated data).

Either way, I meant no contradiction -- if it reads that way, mea culpa.
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post #39 of 51 Old 11-11-2019, 11:36 PM
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Originally Posted by DreamWarrior View Post
I suppose what I was more trying to convey is that if an analog signal is some how "corrupted" (e.g., highs rolled off) then it may not "be obvious" that something is wrong -- what comes through the other end is likely "good enough" for our ears. But, that's not at all the case with digital...

Fair enough.

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post #40 of 51 Old 11-12-2019, 05:33 PM
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I think it would be useful to better define the difference between analog and digital before claiming one is the other or vice versa.

We assign the term “analog” or “digital” to the category based on how the signal will be interpreted. I don’t think it’s correct or useful to say “all digital signals are also analog” just because the signal varies in strength. While I understand the point, it just doesn’t fit the definition of “analog”.

As already stated, it wholly depends on TX/RX devices. In an analog system, the interpreted signal is near infinitely variable at any given point in time based solely on the the strength of the signal. Whereas digital there can only be two interpreted signals at any given point in time — a “1” or a “0”. The signal level has to be merely above or below a certain threshold, how much over or under is irrelevant. I would argue that just because the signal does vary as in the edges of a square wave, that’s not sufficient to call it analog. Those differences have to affect the output in an almost infinitely variable way to be called analog, which they do not.

I also agree a cable cannot be digital or analog, but as mentioned can be better designed to suit one or the other, with digital having more of a hard cut-off when the signal degrades. This is even true with optical cables (albeit I know of no analog uses for an optical audio cable). TX/RX devices could be designed to transmit an analog signal over optical cables by varying amplitude (brightness) and frequency (color), just like an electrical signal through copper. But even though not technically correct, I see utility in being able to call an optical cable “digital”. It’s just easier for the layman to understand rather than going into a long discussion (as in this post) that cables aren’t “digital” or “analog” when you are discussing different types of interconnects.
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post #41 of 51 Old 11-12-2019, 07:04 PM
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From the perspective of a data converter (ADC/DAC) designer and high-speed interface test guy (RF/mW/mmW, GHz stuff, not really audio, though the systems must perform all the way to DC) here are the very basic "introductory" definitions I use (e.g. when giving a college lecture):

  1. Analog = continuous in time and amplitude;
  2. Sampled analog = continuous in time or amplitude, other parameter quantized (sampled); and,
  3. Digital = quantized (sampled) in time and amplitude.

I have worked with circuits in all these areas. In this context virtually any signal passing through a coax is an analog signal. (And yes, even optical, especially for multi-mode/multi-wavelength systems, and systems that modulate the light amplitude, frequency, and/or phase in a manner similar to conventional amplitude or phase modulation). Conversion to digital happens at the receiving end where it is limited (amplitude quantized) and latched (quantizing time) to create discrete digital values (1 or 0 for most audio systems, with a specific quantized time (unit interval, UI); more levels for other protocols e.g. PAM4 or QUAMnnn).

These terms are fairly loosely tossed around even among engineers, at least IME.

FWIWFM - Don
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post #42 of 51 Old 11-12-2019, 09:04 PM
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So in the audio spectrum, an ADC outputs a digital signal that is converted to analog by the TX, converted back to digital by the RX, then converted back to analog by the DAC? I think that is what many of us are saying by 'yeah, all digital signals are analog at some level' but it isn't true in a big picture sense. Plug that coax carrying spdif signal into an analog input, crank the volume, and tell me how sweet that music is!
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post #43 of 51 Old 11-12-2019, 09:42 PM
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The "digital" signal on the coax is not in the audio spectrum and is not an analog audio signal. As I said, engineering definitions, which are usually nothing like "audiophile" definitions. And I don't know what "the big picture sense" is here; the thread appears to be addressing details. Although as has been pointed out many times the coax doesn't know or care.

In any event, finally finished work stuff for the night, enjoy the rest of it. - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

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post #44 of 51 Old 11-13-2019, 03:32 AM
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see the OP was right to have this thread, he wasn't the only one "confused". Good discussion gents!

De sagittis Hungarorum libera nos, Domine!

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post #45 of 51 Old 11-13-2019, 08:59 AM
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post #46 of 51 Old 11-13-2019, 09:05 AM - Thread Starter
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Quote:
Originally Posted by KidHorn View Post
There's no such thing as a digital coax cable. They all carry analog. A digital signal is converted to analog before being sent down the cable and then sampled back to digital on the receiving end. Some may claim to be better shielded at whatever the carrier frequency is, but, the carrier frequency can vary depending on application so I would think any claim is dubious.
DING DING DING! This is the right answer (I kind of figured it out a bit after I posted it, a term good for engineers, bad for consumer cable user experience…)
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post #47 of 51 Old 11-13-2019, 11:20 AM
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Originally Posted by thehun View Post
see the OP was right to have this thread, he wasn't the only one "confused". Good discussion gents!
Based on the OP's comment (post 46), confusion still reigns.
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post #48 of 51 Old 11-13-2019, 12:38 PM
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*snip* Plug that coax carrying spdif signal into an analog input, crank the volume, and tell me how sweet that music is!
I remember a long time ago I used to have a Commodore 64 and it had a tape drive accessory called the Datasette. I could store all my BASIC programs on a standard "audio" cassette.

That was the setup to say -- A better analogy is to plug a "Datasette" (containing "digital" information) into a normal audio deck and play it back.

There you go -- "digital" data stored on an classically "analog" medium: tape. Which, by the way, for the same reason as cables, couldn't care less what "data" is stored on it. The medium (as a cable) simply limits the bitrate.
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post #49 of 51 Old 11-13-2019, 12:46 PM
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I used to play Buck Rogers on a colecovision Adam computer off of tape drive. Seems comical now thinking about itnpausijg in the middle of a game to rewind and pick up play a minute later!
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post #50 of 51 Old 11-13-2019, 03:41 PM
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Originally Posted by bobknavs View Post
Based on the OP's comment (post 46), confusion still reigns.

Though perhaps only in his and KidHorn's minds.

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post #51 of 51 Old 11-15-2019, 10:17 AM - Thread Starter
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The DOCSIS cable data standard is essentially Wi-Fi by wire. Just saying…
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