Two different definitions for sinc ?

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Discussion Overview

The discussion revolves around the different definitions of the sinc function, specifically the variations of sinc(t) as defined by sin(pi t)/pi t and sin(t)/t. Participants explore the implications of these definitions in the context of Fourier transforms and signal processing.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that different texts use different definitions of the sinc function, leading to varying results in calculations, such as Fourier transforms of rect(t/tow).
  • One participant mentions that the definition sin(pi t)/pi t is commonly used in digital signal processing and information theory, while sin(t)/t is more prevalent in mathematics.
  • Another participant confirms that the first definition eliminates the pi from the period, resulting in "nice" numbers on the x-axis, although acknowledging that sinc is not periodic.
  • There is a discussion about the non-equivalence of the two definitions, as they yield different zeros: the first at integer values and the second at multiples of pi.
  • Participants agree that the sinc function in mathematics differs from its counterpart in signal processing, with one suggesting that the distinction is particularly relevant in analog versus digital signal processing contexts.
  • One participant draws an analogy to the Fourier transform, noting that definitions can vary between signal processing and physics based on normalization factors.

Areas of Agreement / Disagreement

Participants generally agree that the definitions of the sinc function can lead to different interpretations and results, indicating a lack of consensus on a single definition applicable across contexts.

Contextual Notes

Participants highlight that the ambiguity in definitions can lead to confusion, similar to other mathematical functions like the logarithm, where context is crucial for interpretation.

reddvoid
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i've seen in some texts they use
sin(pi t)/pi t = sinc(t)
and in some they've used just
sin(t)/t = sinc(t)
each gives different answer
for example
if i want to find FT of rect(t/tow)
using former one gives
sinc(w tow/2 pi)
and if i use former one i get
sinc (w tow / 2)

so how to know which one to use ??
Thnx. . . .
 
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Check how the text defines the sinc.
If it is a proper text, it should define it.

According to wiki, your first variant is used in digital signal processing and information theory, while the second is used in mathematics.
 
I can confirm I Like Serena's post. First is the definition of sinc function in signal processing. I think we like that because sin(pi*t)/pi*t effectively eliminates the pi from the period. And you get "nice" numbers on the x-axis.

Although sinc isn't periodic, but sin is.
 
but these two are not equivalent right.
i mean first one goes to zero @ 1,2,3,4. . . .
And second one goes to zero at pi, 2pi,3pi. . . . .
So
sinc function in math is different from sinc function in signal processing ?
 
Correct.

This is not the only function that is ambiguous.
Consider the log function, which can either be the natural log or the 10-log.
Often this is not even specified, so you are supposed to deduce it from the context.
 
reddvoid said:
So
sinc function in math is different from sinc function in signal processing ?

It would be better to say that the sinc function in analog signal processing theory is (sometimes) different from the sinc function in digital signal processing.

In DSP the "obvious" way to number the sampled data points is 0, 1, 2, 3, etc, not 0, π, 2π, 3π, etc.
 
It is analogous to defining the Fourier transform, which differs in signal processing and physics by details like normalization by 1, 1/2π or sqrt(1/2π).
 

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