Naive question about Analog and Digital Signals I have just started telecommunications in high school Physics. We are currently studying analog to digital conversion. I have 2 questions : 


*

*How is a "digital" signal transmitted? As far as I know, you can turn an analog signal digital by sampling and then applying a restriction with bits. But isn't this just an abstract idea? Or does an actual square wave gets formed that then gets transmitted? 


The other question depends on the answer to this one. 


*If the answer is yes, a square wave gets forms and that's actually what we transmit: if too many bits are used, that means we can pick tiny fluctuations better. But doesn't this also mean any noise along the way also shows up more? 


I am looking for answers for a beginner. 
 A: The answers to your questions above are:


*

*An analog signal is sampled, and a sequence of bits is sent that corresponds to the value of the sample.  This sequence of bits consists of zeros and ones, or if you prefer, on and off.  This is not a "restriction" of bits ... it is a signal encoding that closely represents the analog value that was sampled.  Also, it may not be relevant that the digital signal is a square wave.  For a zero bit corresponding to a value of 1 volt and a 1 bit corresponding to a value of 5 volts, the device reading the digital stream doesn't care if that signal plots as a square wave as long as it can distinguish the difference between a reading of 1 V and 5V.

*Too many bits are not used in the digital stream.  The number of bits that are used is determined by the precision that you need in the measurement.  For example, if you are sampling an analog value that can vary from 1 to 10, and you are using 1 bit to represent the sampled data, the computer reading this data sees a bit value of zero as an analog value of 1 and a bit value of one as an analog value of 10.  Obviously, this is very imprecise.  If, on the other hand, you use 4 bits to represent the analog measurement, those 4 bits can have 16 possible values.  This means that the analog range from 1-10 can be split up into 16 possible values that a computer can read, resulting is quite a bit more precision.  For most every-day applications, you would want something like 12-16 bit precision, which gives a high, but not infinite degree, of precision.  In other words, the digital signal does not perfectly represent the analog measurement, as the digital signal is not truly continuous because it is represented by a finite number of bits.
Regarding noise in the digital signal, there is no noise that is due to the number of bits that are used.  If there is noise in the analog measurement, that noise will be represented in the digital encoding of that measurement.  If you know enough about the characteristics of the analog process that you measured, you can do numerical tests on the digital signal and filter the noise out of that signal.
A: The question of what makes a signal analog or digital is a very good question.
Let's consider a square wave signal. If the signal is sampled at regular intervals of time and then decisions are taken based on comparing the value of the signal with a threshold, then the signal is digital. If the continuous value of the signal is used at every instant of time then it is analog. So the question of analog or digital depends on what is done with the signal.
For example, a stepper motor is driven continuously with a square wave signal. Then it is discrete valued and continuous time, since the signals are not sampled but have discrete values. It is not digital. Digital signals are discrete in both amplitude and time.
Radio Frequency microwaves are transmitted as Electromagnetic waves, which are continuous according to electromagnetic theory.
If the information is transmitted as it is without sampling and quantization then it is analog communication, else it is digital communication. For digital communication, you have to ensure that the signal is sampled at or above the Nyquist sampling rate for no loss of information (read the Sampling Theorem for more details on that). The continuous time signal is converted to a digital signal at the receiver side, while the digital signal is converted to a continuous time signal at the transmitter side in digital communication. This conversion process is done using modulation at the transmitter side and demodulation at the receiver side.
Regarding the concern of noise, there are clever tricks used to ensure the signal can be transmitted and received even in the presence of noise. The digital signal is compressed, and redundant bits are added to enable error detection and error correction of the corrupted bits. There is the Shannon limit, that tells us the maximum bandwidth of a channel at a particular noise level.
Noise can be filtered using digital filters.
