Digital is Digital -Why this is wrong.

This is my favorite dislike "Digital is Digital" so I wanted to talk about it. I have seen this used so many times and the speakers context infers that physical 0's and 1's are shuttled down the line. A variation of that statement is "Digital is Digital, it either works or it doesn't".  Very black and white but unfortunately incorrect. Many of these statements have come about in frequency due to the confusion over HDMI cables. I will explain why these statements are incorrect and why it can cause you to purchase incorrect quality HDMI cables.



To start of, is there any truth in that statement at all where it is justified by perhaps a limited understanding of the technology and may have resulted in a statement the speaker believes is correct? the answer to that is yes. For simplicity of explanation a transmission which carries digital information can be considered a digital signal and so in this context "digital is digital".

So when is digital not digital? When you are dealing with the fundamental techniques used to represent digital information. Suppose you want to send information that is a string of 0's and 1's. Each of either a zero or a one is a single bit. Lets say we have a 4 bit string: 0 1 1 0. I send this down a wire and it will look like fig.1

fig.1

The red bands at the ends are zero's and the peaks are the 1's.
However this is like a square wave signal that has no zero crossing and it is actually analogue i.e the wave is represented by manipulating the voltages on the wire at periods known as frequency. I believe it is time to generate an internet meme equivalent of what is perceived as "Digital is Digital" and what actually happens shown in fig.2.

fig.2

The signal is affected in this way because when it is on the wire, it is an electrical signal and is susceptible to the same interference as an analogue signal simply because it too is an analogue signal. It is time to define what a digital signal is.
A digital signal is any signal that represents or encodes digital information.
As in fig.1 and fig.2 the analogue signal has positive volts and zero volts that represent zeros's and one's therefore it is a digital signal only symbolically because it represents the zero's and one's of digital infromation.

Next, is there any truth in "either it works or it doesn't"? No there isn't.
The reason digital signals are more reliable than analogue is not because they are digital, we have already cleared that up, but because of the way the signal is represented, or digital immunity. There is  a threshold on the signal where anything above it is a one, we can have the same for a zero. If there is a representation of a one passing through and it gets attacked by interference(fig.2) the receiver can still tell it is a one because the signal is still above the threshold for one. Had it been an analogue signal where some property of the signal is altered, then the original signal is lost to interference. Thus, an analogue signal is more susceptible to noise than a signal representing digital information.
That does not answer the question however of why the statement "either it works or it doesn't" is false. This is explained through what is known as error correction. Digital information can have error correction added to it and there are many forms of these such as crc16/32, parity, reed-solomon, hamming code, etc. Error correction serve to detect and correct bit errors and in some situations re-request a piece of digital information that has been detected as bad. Ultimately if errors are too great for error correction to function then it will not recover but neither will your analogue system. Most digital systems will attempt to recover from the errors and if you have digital TV that beams down through satellite to your dish you see that all the time if it rains very hard. You will see squares and so forth appearing as the system is rejecting parts of the signal it cannot correct, and the ones you see are ones it has successfully corrected and it will stop and display a message if too much of the signal is lost. A very old and very simple example that detects single bit errors is parity which comes as odd or even. Let us use our 4 bit example and apply odd parity where an additional bit is sent called the parity bit and it attempts to detect if the signal has an error.
Suppose we sent 0110, with the parity bit it becomes 10110. Lets say at the other end it arrives fine with no interference. The receiver checks that there are odd number of one's, throws away parity and keeps the data. Now suppose the second 'one' is affected and it turns to a zero. So we send 10110 but receive 10100. There is an even number of ones and so this data has an error and is rejected.

Therefore "either it works or it doesn't" is not exactly correct but how can this influence your purchase of HDMI cables? There is a popular consensus going around in the media that you dont need special expensive HDMI cables. This is correct, you do not but only if the conditions it is going to be used in is typical. This means a typical lounge with a satellite TV decoder or blu-ray player next to the television. However if you have a television for any reason several meters away you could be in trouble. If you follow the "you don't need the high quality cable" mantra you might end up buying a very long HDMI cable or several small ones connected together and it may not work or you may get a scene equivalent to satellite digital tv in bad weather. As explained before, the analogue part of the electrical signal will have losses that may not be recoverable digitally.

Next  is something I saw some "expert" mention in a TV show called FairGo here in NZ many years ago which no "expert" should say. The advise was do not get gold plated connectors because the wire inside the cable is not gold it is copper. Correct on the 'it isn't gold inside the wire' part, incorrect about not getting gold plating with that reasoning.

A note: FairGo NZ is an excellent program that is filled with correct and useful information, it is one of my favorites to watch and it is rare that they get something wrong. 

Gold plating isn't there for fancy, it is there to prevent a reaction between two dissimilar metals that can corrode the pins on the plug and the pins on your device. Expansion cards on a PC use gold for this purpose because it was discovered memory cards failed due to corrosion. Gold is a relatively unreactive metal and is ideal to use where corrosion could be a problem. Sure a non gold plated connector might work fine now, but that cable wont remain new forever and there is current passing through it even when off because of the reaction that is happening between dissimilar metals. If you have metal fillings in your mouth, chew on metal foil to see what I mean. The reality is, it is debatable whether for HDMI gold vs non gold is going to have corrosion problems but I would say where possible get gold plated. There is also an issue more recently with 3D TV's capable of receiving 1080P content in 3D mode that needs HDMI 1.4 specifications. A poor quality cable will not be able to maintain the high bandwidth needed for this transmission so you may have to invest in a decent cable for that.

My conclusion on HDMI would be this; Don't spend $100 on a cable 1 meter long in low electrical noise environment, and use your judgement when deciding if you need a expensive cable. If your budget prohibits then don't get gold plated cables because it may not cause any problems -not because it isn't gold inside, that's not the point!. On "Digital is Digital", it is not that simple and should not be used unless you are very clear that you are talking about the representation and not the signal itself. I use the term "digital signal" with peers who are versed in electronics because they are aware what the  meaning is. If you are a sales person, the best and correct way to talk to your customer would be to say "digital information" rather than "digital signal". i.e "Digital information is sent through this HDMI cable to your television".