डिस्क्रीट टाइम फुरिअर ट्रान्सफार्म
गणित में डिस्क्रीट टाइम फुरिअर ट्रान्सफार्म या डीटीएफटी (discrete-time Fourier transform or DTFT), फुरिअर विश्लेषण के कई रुपों में से एक रूप है। यह अनन्त तक परिभाषित किसी अनावर्ती (नॉन्-पेरिऑडिक) डिस्क्रीट-टाइम सेक्वेंस को रूपानतरित करता है। इसे यह भी कहते हैं कि समय-डोमेन का आंकड़ा आवृत्ति-डोमेन में बदल गया। डीटीएफटी द्वारा प्राप्त आवृत्ति-डोमेन का आंकड़ा सतत (कांटिन्युअस) एवं आवर्ती होता है।
डीटीएफटी की परिभाषा
[संपादित करें]यदि कोई वास्तविक (real) या समिश्र (complex) संख्याओं का समुच्चय : (पूर्णांक), दिया हो तो का डीटीएफटी प्रायः इस प्रकार व्यक्त किया जाता है:
![{\displaystyle x[n],\;n\in \mathbb {Z} }](https://wikimedia.org/api/rest_v1/media/math/render/svg/83b783e3bb4f41250a3bce26399b1885f0e2c975)
![{\displaystyle x[n]\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b575dce543a2cf10a5a3e108204b928c2c9aaa54)
![{\displaystyle X(\omega )=\sum _{n=-\infty }^{\infty }x[n]\,e^{-i\omega n}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b2c9ad19546738b0bdd7606437609877a442b1ce)
व्युत्क्रम रुपान्तर (Inverse transform)
[संपादित करें]निम्नलिखित रुपान्तर करने पर डिस्क्रीट-टाइम सेक्वेंस फिर से प्राप्त हो जायेगा:
The integrals span one full period of the DTFT, which means that the x[n] samples are also the coefficients of a Fourier series expansion of the DTFT. Infinite limits of integration change the transform into a continuous-time Fourier transform [inverse], which produces a sequence of Dirac impulses. That is:
![{\displaystyle {\begin{aligned}\int _{-\infty }^{\infty }X_{T}(f)\cdot e^{i2\pi ft}\,df&=\int _{-\infty }^{\infty }\left(T\sum _{n=-\infty }^{\infty }x(nT)\ e^{-i2\pi fTn}\right)\cdot e^{i2\pi ft}\,df\\&=\sum _{n=-\infty }^{\infty }T\cdot x(nT)\int _{-\infty }^{\infty }e^{-i2\pi fTn}\cdot e^{i2\pi ft}\,df\\&=\sum _{n=-\infty }^{\infty }x[n]\cdot \delta (t-nT).\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/decfa7832a2d257a08b027c3605593b3e80ab57f)
डीटीएफटी की सूची
[संपादित करें]नीचे कुछ मानक डिस्क्रीट टाइम सेक्वेंस एवं उनके डीटीएफटी रुपानतर दिये हुए हैं। इसमें प्रयुक्त प्रतीकों का अर्थ निम्नवत है:
- is an integer representing the discrete-time domain (in samples)
- is a real number in , representing continuous angular frequency (in radians per sample).
- The remainder of the transform is defined by:
- is the discrete-time unit step function
- is the normalized sinc function
- is the Dirac delta function
- is the Kronecker delta
- is the rectangle function for arbitrary real-valued t:
![{\displaystyle u[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8d2911183bef1ad935d3bc8b5c1be97bac439b7f)
![{\displaystyle \delta [n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1553d02b03c2a79a43f7862ebfb8352705b0b87c)
![{\displaystyle \mathrm {rect} (t)=\sqcap (t)={\begin{cases}0&{\mbox{if }}|t|>{\frac {1}{2}}\\[3pt]{\frac {1}{2}}&{\mbox{if }}|t|={\frac {1}{2}}\\[3pt]1&{\mbox{if }}|t|<{\frac {1}{2}}\end{cases}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c3585cde90bc1dfbce7b14531690022ad0a7b3a6)
- is the triangle function for arbitrary real-valued t:
| Time domain |
Frequency domain |
Remarks |
|---|---|---|
| integer M | ||
| integer M | ||
| real number a | ||
| real number a | ||
| real number a | ||
| integer M | ||
| real number a | ||
| real number W | ||
| real numbers W, a | ||
| it works as a differentiator filter | ||
| real numbers W, a | ||
| Hilbert transform | ||
 |
real numbers A, B complex C |
![{\displaystyle \delta [n-M]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2dd610c46bdeb2682e719b6a0b445c1cf1893639)
![{\displaystyle \sum _{m=-\infty }^{\infty }\delta [n-Mm]\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d3ce55d69101c8a851f1b8f09c0492852e8e9e49)
![{\displaystyle \pi \left[\delta (\omega -a)+\delta (\omega +a)\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b949fed13f25129a9f8634cb4c0aa34354b0320a)
![{\displaystyle {\frac {\pi }{i}}\left[\delta (\omega -a)-\delta (\omega +a)\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7b48e8676730fb37bcadaea9b1bb342d107cf2f9)
![{\displaystyle \mathrm {rect} \left[{(n-M/2) \over M}\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4fa40f5391166dc137b30a45584c1de1a33b2f8c)
![{\displaystyle {\sin[\omega (M+1)/2] \over \sin(\omega /2)}\,e^{-i\omega M/2}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/883ef01f74f9697fd3a68706785bc6e702c8f961)
![{\displaystyle \operatorname {sinc} [(a+n)]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3526652071fa8be34f6bccac88dc94260441a47f)
![{\displaystyle W\cdot \operatorname {sinc} [W(n+a)]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d4185f56193a63b426f87329e9d7c0d1ae50fced)
![{\displaystyle {\frac {W}{(n+a)}}\left\{\cos[\pi W(n+a)]-\operatorname {sinc} [W(n+a)]\right\}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/f7d4cd8e41d9e483f5d90da18cff3661d914ce9b)
![{\displaystyle {\frac {1}{\pi n^{2}}}[(-1)^{n}-1]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4114640d8ab7e22e07fe380574400fb036941530)
![{\displaystyle {\frac {C(A+B)}{2\pi }}\cdot \operatorname {sinc} \left[{\frac {A-B}{2\pi }}n\right]\cdot \operatorname {sinc} \left[{\frac {A+B}{2\pi }}n\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/38083aff1d4f22b4848dfafdc988b43142f7c472)
डीटिएफटी के गुणधर्म
[संपादित करें]This table shows the relationships between generic discrete-time Fourier transforms. We use the following notation:
- is the convolution between two signals
- is the complex conjugate of the function x[n]
- represents the correlation between x[n] and y[n].
![{\displaystyle x[n]^{*}\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e49908fe3dfbd435e478d58b4c10c47e3a609c96)
![{\displaystyle \rho _{xy}[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/358049d59c74edde52765a758324ab97be7ee3a5)
The first column provides a description of the property, the second column shows the function in the time domain, the third column shows the spectrum in the frequency domain:
| Property | Time domain | Frequency domain | Remarks |
|---|---|---|---|
| Linearity | |||
| Shift in time | integer k | ||
| Shift in frequency (modulation) | real number a | ||
| Time reversal | |||
| Time conjugation | |||
| Time reversal & conjugation | |||
| Derivative in frequency | |||
| Integral in frequency | |||
| Convolve in time | |||
| Multiply in time | |||
| Correlation |
![{\displaystyle x[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/91384637c5188ffed9b7929f145a78fb314c4141)
![{\displaystyle ax[n]+by[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1c30c8f75093b9bafb5c5bde1392348701ce3be0)
![{\displaystyle x[n-k]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e3bfc2b24164de1192ea1e17a90312a3e045911e)
![{\displaystyle x[n]e^{ian}\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a5f540caf14c269c45b6b5d32ceb555ce8841bae)
![{\displaystyle x[-n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/61499a6c6544d338cf6ac340b27ad734c4daba05)
![{\displaystyle x[-n]^{*}\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/fb208f439fc83acd99c3db8d63304b65890993c4)
![{\displaystyle {\frac {n}{i}}x[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e2eb71af662480e119b9bd16ff2e4c49151ab4b9)
![{\displaystyle {\frac {i}{n}}x[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/384ec2e6f1fb841111d00c800196000a4d406964)
![{\displaystyle x[n]*y[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5e3b1fd5ad225be66dfa022f43a3333b08fb3fb7)
![{\displaystyle x[n]\cdot y[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d3e208ac041ff192194184d3c708d7789f9c9178)
![{\displaystyle \rho _{xy}[n]=x[-n]^{*}*y[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/37d82c9624d4b16650c5652c000d0bb459f794c6)
सममिति के गुण (Symmetry Properties)
[संपादित करें]फुरिअर रुपान्तर, वास्तविक एवं काल्पनिक (real and imaginary) या सम एवं विषम (even and odd) के योग के रूप में व्यक्त की जा सकती है।
या
| Time Domain |
Frequency Domain |
|---|---|
![{\displaystyle x^{*}[n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4793a5489c4d67f5ffcc700e9ba06acd87e231c3)
![{\displaystyle x^{*}[-n]\!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/34e0bcbaa111fee79bfcf34b83f9f3b6303ddaed)
