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I have found in the literature a "time-variant signal" term , but I could not understand what that's mean. I know that there is "time-variant systems", but this is the first time, which I see a time-variant signal term. Any one has an idea what this term means ?

Some suggests that this term is related to non-stationary signal, which it's spectrum changes over time. Is this right?

P.S: I have added the EEG tag because this term, is used widely to describe the EEG signals

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    $\begingroup$ What do you mean by "time-variant" signal? Do you mean "non-stationary"? $\endgroup$ – Matt L. Jan 4 '15 at 10:37
  • $\begingroup$ @MattL. if you take a look for this paper , they said : "We use SOBI instead of other PCA and ICA algorithms because SOBI takes account of the time-variant nature of our signal " , I don't think that they mean "non-stationary " signal. I know there is time-variant systems , but this is the first time I haired by time-variant signal. maybe I have asked the wrong question , maybe my first question is "what is time-variant signal ? " $\endgroup$ – Learner Jan 4 '15 at 11:31
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    $\begingroup$ I don't have access to the full paper, but the abstract refers to "non-stationary data". Could it be that by "time-variant nature" they do refer to the signal's non-stationarity after all? This is the only explanation that makes sense to me because "time-variant signal" is no commonly used term, as far as I know. $\endgroup$ – Matt L. Jan 4 '15 at 11:43
  • $\begingroup$ We talked about this before, but you should really try to work out a question before posting it. Folks put time and effort into deciphering what you mean. Not having fully mastered the English language is one thing, but you can try to carefully think about WHAT do I want to ask? Then the how, and only then you should commence with posting it. Don't get me wrong here though - I love your questions! I just sometimes don't understand them =) So: are you wondering what a time-(in)variant signal is and how that relates to the EEG? $\endgroup$ – AliceD Jan 5 '15 at 13:16
  • $\begingroup$ @ChrisStronks ,well , I will try hardly to take your advices when asking new questions in the future. And yes , this is what I'm asking for. $\endgroup$ – Learner Jan 6 '15 at 20:08
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As I've mentioned in a comment, time variant signal is a term which is not commonly used in signal processing, and I also couldn't find a clear definition. After browsing a few texts which use the term (usually written by people not specialized in signal processing), I've come to the conclusion that by time variant signals they actually mean non-stationary signals. The latter being a clearly defined term (check the wikipedia page on stationarity). In short, it means that the signal's properties, such as its mean and variance, change over time. And this is indeed the case with EEG signals, just as it is the case with speech signals.

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The term is often used for systems, not signals. However, a natural definition would be any signal $x[n]$ which is dependent on $n$ is a time-varying signal. For exaple, the singal $x[n] = 2$ is not time-variant.

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