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discrete signals

bhuvanesh

Aug 29, 2013
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my professor said that everything in nature is on continuous time domain (continuous signal) and there is no discrete signals. i too know that do you have any idea that why discrete signal are not in nature .i am expecting geeky answers about this.Thank you
 

Laplace

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What is the smallest time interval that exists in nature? If there is no 'smallest interval' then time is continuous. Whereas for a discrete time signal, the smallest interval is the reciprocal of the sampling frequency and time is measured as integer multiples of the smallest interval.
 

Harald Kapp

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Discrete signals on the macroscopic level are a mathematical illusion, albeit a useful one for designing circuits in many cases.
In practise even the fastest electronics cannot generate really discrete signals. You will always have transitions between states or signals that are essentially continuous (analog) in time and amplitude (voltage, current, whatever).

When you dive really deep into quantum mechanics you will find that "...some physical quantities can change only in discrete amounts (Latin quanta), and not in a continuous (cf. analog) way..." (cited from the linked Wikipedia article).
 

bhuvanesh

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why do not we say that pulse of human nerve system as discrete signal?

okay then,

could some give me example for discrete signals and digital signals?
 

Harald Kapp

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why do not we say that pulse of human nerve system as discrete signal?
Because a nerve's pulse is continuous in time and amplitude, see here for an example.

could some give me example for discrete signals and digital signals?
In the macroscopic world that we deal with in eletronics, there is no real discrete signal. For the purpose of simplification and mathematical/logical treatment signals are very commonly called discrete but mainly to differentiate them from truly analog signals.

Take for example an analog-to-digital converter. You'd say that the input is analog in time and amplitude, whereas the output is a sequence which is discrete in time (controlled by tha clock of the ADC and amplitude, namely '0' and '1'.
But: the output signal is not really discrete in time. For one there is an output voltage at any time and the change in output happens with a clock edge where the edge itself is not 100% fixed in time but has typically slight variations called jitter. The output voltage is considered logic '0' if it is below a defined threshold and logic '1' if it is above a certain threshold. But depending on parameters like e.g. stability of the power supply, a logic '1' can be e.g. 4.5V or 5V or even 5,5V in a nominal 5V system. Any voltegae level in between is possible, the signal is still considered to be logic '1' and therefore in a sense discrete, but not physically.
Also no signal can change instantly between '0' and '1'. There is always a transistion including all voltages between these two states. If you use a fast enough oscilloscope, you can see these edges as ramps.

Note that this is a low level physical answer - you were expecting geeky answers, weren't you? :)

For many practical purposes you can consider digital signals as discrete, but the higher the clock rate, the more the analog nature of even digital signals becomes noticeable (cf. effects like ringing on transmission lines etc.).
 

Merlin3189

Aug 4, 2011
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How about radioactivity.
Particles are emitted or not, at random times. We can integrate this signal to provide a count, then differentiate it again to provide a rate, but the original signal is as discrete as you can get. There is no part of a disintegration and (I think) the time for it to happen is immeasurably small*. It seems the nearest thing to instantaneous that you can get, so it has either happened or not happened, no in between. (*Though I suspect, not incalculably small?)

Also disagree with the comment about nerve impulses not being digital. This is a common falacy which could apply to all digital signals. If you look at an electronic digital signal, it is not 0 or 1 (or 0V or 5V, etc) but is continuous with values at every level. The whole point about digital signals is simply that information is only attached to particular levels at particular times. Nerve impulses are "all or nothing". No information is attached to intermediate values of voltage or current. The information is purely whether a pulse has or has not occurred.
 

Harald Kapp

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How about radioactivity.
Agreed, this is the world of quantum mechanics (see post #3).

This is a common falacy which could apply to all digital signals.
See post #5:
For the purpose of simplification and mathematical/logical treatment signals are very commonly called discrete but mainly to differentiate them from truly analog signals.
.
We interpret this kind as signal as discrete (we attach information to '0' and '1' only, to use your words) and this is very convenient for analyzing the signals and doing computations with them.
 

Merlin3189

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Apologies Harald, I did not read your posts carefully enough. Having reread them I think I would agree with you and your detailed treatment.
I think maybe we agree, that discete/continuous and digital/analogue are more about the way information is encoded in a signal than the appearance or form of the signal itself?
 
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