Broadband broken by old TV (say BT)...

[ppppenguin]

It's 100% strictly true. On many occasions we can treat a particular signal as digital but the real world will come back and bite if you do this inappropriately.

Try sending a digital signal with a very fast rise time over a long PCB track. What arrives at the other end will be a mess of reflections unless you have taken suitable precautions. That signal in the real world is analogue. It can be attenuated, reflected, distorted.

[Crackle]

I think you may be confusing the medium used (electromagnetic, electron movement, or even pulsed water) of relaying the information (data) and the actual information (data) which the medium caries. The digital signal will be meaningless if you view it as an analogue signal. It only becomes meaningful when it is viewed as a digital signal and processed accordingly.
Mike

[Mark Hennessy]

Mike, you're not alone there. I spend a lot of time getting people to think about the analogue reality of digital signals. In practical scenarios, it's usually analogue problems that cause digital signals to fail - anything from the wrong cable type through to interference. In effect, a digital "message" is carried on the analogue medium, and therefore the analogue medium has to be sufficiently good to allow the message to be decoded.

For example, one way to carry digital audio is a standard called AES3. This has much in common with the domestic "S/P-DIF" connections that you might have seen on the rear of some CD and DVD players. This particular standard is widely used, and is very reliable, but when it fails, it's almost always because of analogue problems like HF loss in very long wires, or a mis-connection that introduces unterminated stubs, which in term cause reflections and hence distortion of the analogue waveform that can prevent the receiver from being able to decide if the analogue voltage it gets is supposed to represent a logic 0 or a logic 1.

In practice, these "physical layer" problems dominate; it's very rare to have logic problem with the data carried on the interface. There is dedicated test equipment that looks at the data carried on the interface, but most of the time, an oscilloscope is what you need.

ADSL or VDSL were mentioned earlier. Previously I've detailed how I tripled my ADSL speed by attending to the analogue problems with the phone wiring in my house - eliminating unterminated stubs and ensuring the correct characteristic impedance of the wiring did that. A digital signal needs to sit on a solid analogue foundation

Even a CD is analogue. Ever looked at an eye waveform? It comes about because there are 2 amounts of reflectivity on a disc. The exact amounts vary from disc to disc, and of course, recordable CDs have rather less than a commercial pressing, but it's an analogue signal that comes back from the laser pickup, and the front end of the processing circuitry has to decide whether the analogue voltage is representing a logic 0 or logic 1.

The business about taking care of the analogue side of digital systems is usually called "signal integrity": https://en.wikipedia.org/wiki/Signal_integrity

[Murphyv310]

Hi.
Indeed many get this wrong and the illustration of the eye pattern off the laser is good. In fact it's called the RF output from the first stage of amplification usually in a chip. The RF test point is one of the most useful in a CD player.
Coming back to what I said about BTs cables is again analogue and the lack of screening is an analogue problem yet again.
Good posts Jeffrey and Mark, thanks for explaining as you say so many get this wrong.

[BusyBee]

Strictly speaking there are no such things as digital signals.  They are all analogue. In many cases you can treat signals as digital but sometimes you can't.  XDSL carried on copper pairs is very definitely analogue.

I am not sure if you were referring to my mention of 'digital signal' here but yes, I realise what you mean and do understand the later comments. By digital signal I meant in effect the digital representation of the analogue signal as it is received, which is perhaps the essence of later comments. This particular intercom system implementation did have guidance and the outside door intercom unit itself did come with a length of cable, the instuctions stating that under no circumstances should the cable be extended which probably took into account impedances and reflections etc.

From the top level / system view though many signals are digital. For instance FSK is obviously analogue in implementation but digital in design.

Tracy

[Crackle]

It is the carrier that is in most cases analogue, that is just the medium the signal is relayed in. It is the information in the signal that is digital. It depends on what you view as the signal. To me the signal is the information that is being relayed. Like semaphore flags which are are purely analogue but the information they relay is the signal.

Mike

[Nick]

We seem to be in a circular argument about semantics...

Using the ISO seven layer model, the lowest level is layer 1 and called the Physical Layer and layer 2 is the Transport Layer.

In this case, the Transport Layer is fully digital. The Physical Layer specifics the mechanism that carries the digital information from the Transport Layer between nodes and is typically analogue, e.g. radio, modulated laser, voltages on wires etc.

So, the analogue vs. digital argument is really about whether you view the stack from the bottom up, in which case it's analogue, or from the top down, in which case it's digital.

i.e. you can have a digital signal, but any real world implementation that transmits that signal between nodes will involve a translation to and from an analogue equivalent.

[Crackle]

We seem to be in a circular argument about semantics...

This is what happens when someone makes a broad misleading, inaccurate and incomplete assertion that there are no such things as digital signals, and then maintains their assertion is 100% true.
They need to be corrected.

Sorry Geoffrey but I found your statement misleading in the extreme.

Mike

[ppppenguin]

It is clear from my postings that I am referring to physical signals. As is Mark in post #13. At the time I made my previous posts I was away from home and posting on my phone. This made it harder to give greater detail and explanation which Mark was more readily able to do.

And my name is Jeffrey, which is clearly displayed on every post.

[ppppenguin]

I'm trying to find an articleI remember where one of the great digital designers, possibly Bob Pease or Jim WIlliams, said something like "all digital signals are analogue signals". In the meantime this article by Bob Pease is well worth reading:
https://www.electronicdesign.com/technol...uff-anyhow