Topic: What makes a cable digital?

[Geoff G]

I'm looking for a technical response here.

What makes a cable a digital cable versus an analog cable?  An optical cable makes sense as a digital cable, but a digital coax cable and an RCA interconnect both have a wire in the middle that carries frequency from one place to another.

Let's look at one piece of gear as an example (you could use any piece):

A V-3.  The V-3 has XLR out, AES out, and coax out.  What makes any of these analog versus digital?  I though XLR out was analog, and S/PDIF out was digital.  What is AES out, and what makes that different from XLR?

My first guess is resistance, but I'd love to hear some technical talk put on these.

Geoff

[taper420]

We need to distinguish between two things here.... there is the interface, and the format. One is not neccesarily the other. First of all a cable is a cable....it carries electrical impulses...some carry it better than others, but they all essentially do the same thing. That is except optical which carries optical impulses. An optical will always carry digital information...the lights either on or off. But the language that it uses can be different depending on the format. ADAT lighpipe, as well as surround sound and spidf are all formats that can be transmited over an optical line. But the box you're plugging it into must speak the same lanuage or they won't understand each other. Just cause the cable fits doesn't mean it will work. Although allot of professional equipment that has the toslink(optical) interface will allow you to change the format it reads (at least between adat and spdif). Now on to the wire cables. You probably already know how the analog xlr works, it carries an electric current which is analogous to the audio it is receiving. Louder sounds cause the diaphram of your mic to create a higher voltage, softer sounds create a weaker voltage. The current corelates to this exactly. Some cables do a better job of getting the original source without letting extra noise in, such as balanced cables versus unbalanced cables. Unbalanced cables are the consumer connects such as rca. Both types of cable are also capable of carrying a digital signal. The way this works is nothing like with analog. The way it works (or trys to) is that the voltage is either on or off.. In reality this isn't the case, but your device marks a threshold and anything above a certain threshold is on and under is off. This is kinda like if you were sitting at a piano and everytime someone tapped on your sholder you played a c note. And then everytime they tapped twice you played a d, and three times an e and so on...this is much different from analog which would be if someone was humming the actual note and then you played it by ear. So anyway this digital signal can be passed along on consumer unbalanced rca connectors which is more suseptable to noise. The problem with digital is you either have the signal or you dont so if theres sufficient noise to disrupt the signal you wont hear increased noise, you simply won't hear anything, or it might sound like when a cell phone s going out of range. Therefor the optical interface provides the benefit of not allowing any electrical noise in but may be prone to optical anomalies. The AES format you refer to is a professional digital format that uses balanced connectors to ensure a more reliable (and locking) connection. So though the interface is the same as your analog xlr, the format is in no way compatable. So the answer to your original question what's the difference between an analog and digital cable...if its a wire cable...not much...if it's labeled for digital it just means it's a thicker cable that can handle the higher bandwidths that the format needs...any digital wire cable can be used for analog, but some cheaper or thinner  cables labeled only for analog may produce errors in a digital system.  Hope that answers your question.

[live2496]

Some wire is better suited to carrying higher frequencies than others. SPDIF electrical cables use a solid wire copper conductor that is better suited for digital signals. It has the correct physical properties to carry the digital information and is very similar if not identical to cable tv wire.

AES/EBU uses a balanced cable for the digital connection. It has the same physical connector as does a balanced XLR cable. However the physical properties of the wire are different. It can handle much higher frequencies.











 

[Scooter]

SPDIF cable is 75ohms, and AES/EBU cable is 110ohms.  Most video cable (the reg coax stuff, as well as the more flexible varieties) is acceptable for SPDIF usage.  Obviously the optical cables have no relation to analog cabling.  I believe that SPDIF uses .5v and AES uses 5v as far as the voltage is concerned.  There is another standard called AES3id(?) that is AES over SPDIF cabling but using BNC connectors (i think).

[Geoff G]

Thanks for the info so far.

So, with regard to XLR cable versus an AES/EBU cable, is the voltage what makes them different?  Is an AES/EBU-out on an ADC considered a digital out? 

Look at a V-3, again.  The back has XLR out, AES/EBU out and coax out (I think).  Are the AES/EBU and coax considered digital, and the XLR analog?

[live2496]

Here is a pretty good reference on the subject...
http://www.hinton-instruments.co.uk/audio/digiprimer.html

[live2496]

Thanks for the info so far.

So, with regard to XLR cable versus an AES/EBU cable, is the voltage what makes them different?  Is an AES/EBU-out on an ADC considered a digital out? 

Look at a V-3, again.  The back has XLR out, AES/EBU out and coax out (I think).  Are the AES/EBU and coax considered digital, and the XLR analog?

RE: AES/EBU
Yes this is a digital out.
Not just the voltage is different. But the data format is slightly different than SPDIF. No SCMS for one thing.

From looking at a picture of a V3, the XLR's on the right are labelled as analog. Two are inputs and two are outputs.

[Scooter]

Thanks for the info so far.

So, with regard to XLR cable versus an AES/EBU cable, is the voltage what makes them different?  Is an AES/EBU-out on an ADC considered a digital out? 

Look at a V-3, again.  The back has XLR out, AES/EBU out and coax out (I think).  Are the AES/EBU and coax considered digital, and the XLR analog?

There are as far as I know not a single digital cable in the world. Every cable transmits analog signals.

When the signal is moving on the cable it is analog. Inside the sender and inside the receiver it is digital. So obviously there is some kind of translation going on, from a digital signal inside the box to an analog signal on the cable, and then to a digital signal in the next box.

Please clarify this.  You are saying that

1. there are no digital cables??  yes, some digi cables can also be used for analog signals, but what abaout optical cables??  they cannot transmit analog signals AFAIK.
2. when the signal is moving along the cables it's analog??  so the cable is performing A to D then D to A??  i think not.

not trying to be contrary here, just looking for clarification

Scott

[ghellquist]

Sorry if I was dense. I might have made things unnecessarily complicated.

Digital  - a signal having digital signal levels. Most often we think of digital as having two different values, either 0 or 1.
Analog - something having any level (within a range of course).

When you transmit a digital electrical signal over a physical cable it migh swing betwen, say -4.5V and +4.5V. It will over time have any of the values in between. If nothing else the transition from one value to the other does take some time. So the cable is analog, that is transmitting any electrical value. But the values are interpreted as a digital signal. Not by the cable, which could not care less, but by the receiving end. In order to have a bit of margin it might be set to regard anything above +1.5V as a digital 0 and anything below -1.5V as a digital 1. All of this is described in the "protocol". Protocols are often published as books or papers and typically named things. Examples of protocols are RS232, AES/EBU, SPDIF, Ethernet, BlueTooth and so on. Designing protocols is a highly specialized technical skill, very few people has done that. A typical protrocol describes cables, contacts, electrical signal, limits on length and voltages as well as how the digital bits going over the cable are to be interpreted.

An optical cable transmits light. It could have just about any wavelength (= color) and any strength from pitch black over moon-light to full sun. The cable really dose not care. In that respect the cable is analog. But when we use it for SPDIF over TOSLINK we elect to use it to transfer a digital signal. The sender and receiver decides on how to interpret quite dark and rather light as a digital signal passing over the cable. So we use the cable for a digital signal.

There is no AD conversion taking place, at least not in the meaning we as audio people generally use the world. But there sure is specialized circuits connecting between the computer world inside each box and the outside real world where the cable transmits electrical or optical impulses.

Hope this helps, otherwise I guess I turned a bit too technical / philosophical. Guess my back ground in designing electronic circuits as well as computer protocols comes in the way at times.

Gunnar

[Scooter]

i understand ya now