# US 5,349,549 A · Forward transform processing apparatus and inverse processing apparatus for modified discrete cosine transforms, and method of performing spectral and temporal analyses including simplified forward and inverse orthogonal transform processing

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- Number: US 5,349,549 A
- Document ID: US5349549A
- Status: Granted
- Kind: A
- Application number: 07/950,945
- Filed: 1992-09-24
- Granted: 1994-09-20
- Earliest claimed: 1991-09-30
- Assignees: Sony Corporation
- Inventors: Kyoya Tsutsui
- Web page: https://patentgrep.com/patent/US5349549A

## CPC

- G06F 17/147: {Discrete orthonormal transforms, e.g. discrete cosine transform, discrete sine transform, and variations therefrom, e.g. modified discrete cosine transform, integer transforms approximating the discrete cosine transform (G06F17/145 takes precedence)}
- G06F 17/142: {Fast Fourier transforms, e.g. using a Cooley-Tukey type algorithm}
- G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N)

## Abstract

A method for performing a simplified digital spectral analysis on a block of N samples representing an information signal in the time domain, in which the block of N samples is multiplied by a forward transform window to provide a block of N processed samples. The block of processed samples is digitally preprocessed to provide a block of N/4 complex numbers, which is orthogonally transformed, using digital processing equivalent of a Fast Fourier Transform, to provide a block of N/4 transformed complex numbers. The block of transformed complex numbers is then digitally post-processed to provide a block of N/2 spectral coefficients representing the information signal in the frequency domain. In a method of performing a simplified digital temporal analysis on a block of N/2 spectral coefficients representing the information signal in the frequency domain, the block of spectral coefficients is digitally pre-processed to provide a block of N/4 frequency domain complex numbers, which is orthogonally transformed into a block of N/4 time domain complex numbers by digital processing equivalent to a Fast Fourier Transform. The block of N/4 time domain complex numbers is digitally post processed to provide a block of N/2 processed time domain samples, which is re-arrayed and multiplied by an inverse transform window to provide a block of N samples representing the information signal in the time domain.

## Claims

### Claim 1

A forward transform processing apparatus for carrying out a modified discrete cosine transform on a digital input signal, the digital input signal comprising plural samples in the time domain, the apparatus comprising:
(1) means for dividing the digital input signal blocks of N samples, where N is a positive multiple of four;
(2) a multiplier, the multiplier multiplying each block of N samples by a forward transform window to provide a block of N forward transform window processed samples; and
(3) transform means for transforming the block of N forward transform window processed samples to provide a block of N/2 spectral coefficients, the transform means including:
(a) a preprocessing means for preprocessing the block of N forward transform window processed samples to provide a block of N/4 complex numbers,
(b) an FFT means for performing processing equivalent to a Fast Fourier Transform on the block of N/4 complex numbers to provide a transformed block of N/4 complex numbers; and
(c) a postprocessing means for post-processing the transformed block of complex numbers to provide the block of N/2 spectral coefficients.

### Claim 2 (depends on 1)

The forward transform processing apparatus of claim 1, where:
(1) the block of N forward transform window processed samples includes an n-th sample, a (n+3N/4)-th sample, the (n+3N/4)-th sample having a signal, and a (n-N/4)-th sample, wherein n is an integer between 0 and (N-1); and
(2) the preprocessing means includes:
(a) a first deriving means for deriving, from the block of N forward transform window processed samples, a block of first intermediate samples, the block of first intermediate samples including an n-th a (N-1-2n)-th, and a 2n-th first intermediate sample, the first deriving means deriving the block of first intermediate samples by:
(i) reversing the sign of the (n+3N/4)-th sample to provide a sign-reversed (N+3N/3)-th sample, and providing the sign-reversed (n+3N/4)-th sample as the n-th first intermediate sample, for each integral value of n from 0 to N/4-1, and
(ii) providing the (n-N/4)-th sample as the nth first intermediate sample, for each integral value of n from N/4 to N-1,
(b) a second deriving means for deriving, from the block of first intermediate samples, a block of second intermediate samples, the block of second intermediate samples including an n-th, a 2n-th, and a (2n+1)-th second intermediate sample, the second deriving means deriving the block of second intermediate samples by subtracting, for each integral value of n from 0 to N/2-1, the (N-1-2N)-th first intermediate sample from the 2n-th first intermediate sample to provide the n-th second intermediate sample;
(c) a third deriving means for deriving, from the block of second intermediate samples, the block of N/4 complex numbers, the complex numbers in the block of N/4 complex numbers each having a real part and an imaginary part, and including an n-th complex number, the third deriving means deriving the block of N/4 complex numbers by:
(i) selecting the 2n-th second intermediate sample as a real part of an n-th intermediate complex number,
(ii) selecting the (2n+1)-th second intermediate sample as an imaginary part of the n-th intermediate complex number, and
(iii) multiplying the n-th intermediate complex number by an exponential of (-4.pi.jn/N) to provide the n-th complex number in the block of N/4 complex numbers,
for each integral value of n from 0 to N/4-1.

### Claim 3 (depends on 1)

The forward transform processing apparatus of claim 1, wherein:
(1) the block of N/2 spectral coefficients includes a k-th spectral coefficient, where k is an integer between 0 and N/2-1, and
(2) the post-processing means includes:
(a) a means for expanding the transformed block of N/4 complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the block of 3N/4 complex numbers including a k-th complex number and a (N/4-1-k)-th complex number,
(b) a means for deriving a (N/4-1-k)-th complex conjugate from the (N/4-1-k)-th complex number,
(c) a means for:
(i) summing the (N/4-1-k)-th complex conjugate and the k-th complex number to provide a sum, and
(ii) dividing the sum by two to provide a k-th third intermediate complex number,
(d) a means for
(i) subtracting the (N/4-1-k)-th complex conjugate from the k-th complex number to provide a difference,
(ii) dividing the difference by 2j to provide a quotient, where j is the square root of minus one, and
(iii) multiplying the quotient by the exponential of (-2.pi./(2k+1)/N) to provide a k-th fourth intermediate complex number, and
(e) a means for
(i) summing the k-th third intermediate complex number and the k-th fourth intermediate complex number to provide a k-th fifth intermediate complex number, and
(ii) multiplying the k-th fifth intermediate complex number of an exponential of (-.pi.j(2k+1)(2N)) to provide a k-th sixth intermediate complex number having a real part and an imaginary part, and
(iii) providing the real part of the k-th sixth intermediate complex number sum as the k-th spectral coefficient.

### Claim 4 (depends on 1)

The forward transform processing apparatus of claim 1, wherein:
(1) the block of N/2 spectral coefficients includes a k-th spectral coefficient, where k is an integer between 0 and N/2-1, and
(2) the post-processing means includes:
(a) a means for expanding the transformed block of N/4 complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the complex numbers in the block of 3N/4 complex numbers having a real part and an imaginary part, and including a k-th complex number and a (N/4-1-k)-th complex number;
(b) a means for determining
(i) a sum, the sum being one half of a sum of the cosine of (2.pi.)(2k+1)/4N) and sine of (10.pi.)(2k+1)/4N),
(ii) a first difference, the first difference being one half of a difference between the cosine of (2.pi.(2k+1)/4N) and the sine of (10.pi.(2k+1)/4N), and
(iii) a second difference, the second difference being one half of a difference between and sine of (10.pi.(2k+1)/4N) and the cosine of (2.pi.(2k+1)/4N);
(c) a means for multiplying:
(i) the real part of the k-th complex number by the first difference to provide a first intermediate number,
(ii) the real part of the (N/4-1-k)-th complex number by the sum, to provide a second intermediate number,
(iii) the imaginary part of the k-th complex number by the sum to provide a third intermediate number, and
(iv) the imaginary part of the (N/4-1-k)-th complex number by the second difference to provide a fourth intermediate number; and
(d) means for summing the first, the second, the third, and the fourth intermediate numbers to provide the k-th spectral coefficient.

### Claim 5

An inverse transform processing apparatus for carrying out a inverse modified discrete cosine transform on spectral coefficients in the frequency domain to provide a digital output signal in the time domain, the apparatus comprising:
(1) means for dividing the spectral coefficients into blocks of N/2 spectral coefficients, where N is a multiple of 4;
(2) inverse transform means for inversely transforming each block of N/2 spectral coefficients to provide a block of N/2 processed samples in the time domain, the inverse transform means including:
(a) a pre-processing means for pre-processing the spectral coefficients in the block of N/2 spectral coefficients to provide a block of N/4 frequency domain complex numbers,
(b) an FFT means for performing processing equivalent to a Fast Fourier Transform on the block of N/4 frequency domain complex numbers to provide a block of N/4 time domain complex numbers, and
(c) a post-processing means for post-processing the block of N/4 time domain complex numbers to provide the block of N/2 processed samples in the time domain;
(3) a means for re-arraying the block of N/2 processed samples in the time domain to provide a block of N samples in the time domain;
(4) a multiplier, the multiplier multiplying the block of N samples in the time domain by an inverse transform window to provide a windowed block of N samples in the time domain;
(5) means for providing successive windowed blocks of N samples in the time domain as the digital output signal.

### Claim 6 (depends on 5)

The inverse transform processing apparatus of claim 5, wherein:
(1) the block of N/2 spectral coefficients includes a 2-th, and a (N-1-2k)-th spectral coefficient, each having a sing, where k is an integer between 0 and (2N-1),
(2) the block of N/4 frequency domain complex numbers includes a k-th frequency domain complex number, and
(3) the pre-processing means includes:
(a) a first deriving means for deriving, from the block of N/2 spectral coefficients, a block of first intermediate numbers, the first intermediate numbers including a k-th, a 2k-th, and a (2k+1)-th first intermediate number, the first deriving means deriving the first intermediate numbers by:
(i) selecting the 2k-th spectral coefficient as the k-th first intermediate number for each integral value of k between 0 and N/4-1, and
(ii) reversing the sign of the (N-1-2k)-th spectral coefficient to provide a sign-reversed (N-1-2k)-th spectral coefficient, and providing the sign-reversed (N-1-2k)-th spectral coefficient as the k-th first intermediate number for each integral value of k between N/4 to N/2-1, and
(b) a second deriving means for deriving, from the block of N/2 first intermediate numbers, the block of N/4 frequency domain complex numbers, the frequency domain complex numbers having a real part and an imaginary part, and including a k-th frequency domain complex number, the second deriving means deriving the block of N/4 frequency domain complex numbers by:
(i) selecting the 2k-th first intermediate number as a real part of a k-th complex number,
(ii) selecting the (2k+1)-th first intermediate number as an imaginary part of the k-th complex number, and
(iii) multiplying the k-th complex number by an exponential of (-4.pi.jk/N) to provide the k-th frequency domain complex number,
for each integral value of k between 0 and N/4-1.

### Claim 7 (depends on 5)

The inverse transform processing apparatus of claim 5, wherein:
(1) block of N/2 processed samples includes an n-th processed sample, where n is an integer between 0 and N/2-1, and
(2) the post-processing means includes:
(a) a means for expanding the block of N/4 time domain complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the block of 3N/4 complex numbers including an n-th complex number and a (N/4-1-n)-th complex number,
(b) a means for deriving a (N/4-1-n)-th complex conjugate from the (N/4-1-n)-th complex number,
(c) a means for
(i) summing the (N/4-1-n)-th complex conjugate and the n-th complex number to provide a first sum, and
(ii) dividing the first sum by two to provide an n-th third intermediate complex number,
(d) a means for
(i) subtracting the (N/4-1-n)-th complex conjugate from the n-th complex number to provide a difference,
(ii) dividing the difference by 2j to provide a quotient, where j is the square root of minus one, and
(iii) multiplying the quotient by the exponential of (2.pi.j(2n+1)/N) to provide an n-th fourth intermediate complex number, and
(e) a means for
(i) summing the n-th third intermediate complex number and the n-th fourth intermediate complex number to provide an n-th fifth intermediate complex number,
(ii) multiplying the n-th fifth intermediate complex number by an exponential of (-.pi.(2n+1)(2N)) to provide an n-th sixth intermediate complex number having a real part and an imaginary part, and
(iii) providing the real part of the n-th sixth intermediate complex number as the n-th processed sample.

### Claim 8 (depends on 5)

The inverse transform processing apparatus of claim 5, wherein:
(1) the block of N/2 processed samples includes an (n+N/4)-th, an (3N/4-1-n)-th, and an (n-3N/4)-th processed sample, each processed sample having a sign,
(2) the block of N samples in the time domain includes an n-th sample in the time domain, and
(3) the means for multiplying the block of N/2 processed samples in the time domain by an inverse transform window includes:
(a) a deriving means for deriving from the block of N/2 processed samples, the block of N intermediate processed samples, the block of N intermediate processed samples including an n-th intermediate processed sample, the deriving means deriving the block of N intermediate processed samples by:
(i) selecting the (n+N/4)-th processed sample as the n-th intermediate processed sample for each integral value of n from 0 to N/4-1,
(ii) reversing the sign of the (3N/4-1-n)-th processed sample to provide a sign-reversed (3N/4-1-n)-th processed sample, and providing the sign-reversed (3N/4-1-n)-th processed sample as the n-th intermediate processed sample for each integral value of n from N/4 to 3N/4-1, and
(iii) reversing the sign of the (n-3N/4)-th processed sample to provide a sign-reversed (n-3N/4)-th processed sample, and providing the sign-reversed (n-3N/4)-th processed sample as the n-th intermediate processed sample for each integral value of n from 3N/4 to N-1; and
(b) a means for multiplying the n-th intermediate processed sample by an inverse transform window for the n-th intermediate processed sample, for each integral value of n from 0 to N-1, to provide the n-th sample in the time domain.

### Claim 9

A method of performing a simplified digital spectral analysis of a block of N samples representing an information signal in the time domain, the spectral analysis providing a block of N/2 spectral coefficients representing the information signal in the frequency domain, the method comprising the steps of:
(1) multiplying the block of N samples representing the information signal in the time domain by a forward transform window to provide a block of N forward transform window processes samples;
(2) digitally preprocessing the block of N forward transform window processed samples to provide a block of N/4 complex numbers;
(3) performing digital processing equivalent to a Fast Fourier Transform on the block of N/4 complex numbers to provide a transformed block of N/4 complex numbers; and
(4) digitally post-processing the transformed block of complex numbers to provide the block of N/2 spectral coefficients representing the information signal in the frequency domain.

### Claim 10 (depends on 9)

The method of claim 9, wherein:
(1) in the step of multiplying the block of N samples, the block of N forward transform window processed samples includes an n-th sample, a (n+3N/4)-th sample, the (n+3N/4)-th sample having a sign, and a (n-N/4)-th sample, where n is an integer between 0 and (N-1); and
(2) the step of digitally preprocessing the block of N forward transform window processed samples includes the steps of:
(a) deriving, from the block of N forward transform window processed samples, a block of first intermediate samples, the block of first intermediate samples including an n-th, a (N-1-2n)-th, and a 2n-th first intermediate sample, the block of first intermediate samples being derived by steps including:
(i) reversing the sing of the (n+3N/4)-th sample to provide a sign-reversed (n+3N/4)-th sample, and providing the sign-reversed (n+3N/4)-th sample as the n-th first intermediate sample, for each integral value of n from 0 to N/4-1, and
(ii) providing the (n-3N/4)-th sample as the nth first intermediate sample, for each integral value of n from N/4 to n-1,
(b) deriving, from the block of first intermediate samples, a block of second intermediate samples, the block of second intermediate samples including an n-th, a 2n-th, and a (2n+1)-th second intermediate sample, the block of second intermediate samples being derived by steps including subtracting, for each integral value of n from 0 to N/2-1, the (N-1-2N)-th first intermediate sample from the 2n-th first intermediate sample to provide the n-th second intermediate sample,
(c) deriving, from the block of second intermediate samples, the block of N/4 complex numbers, the complex numbers in the block of N/4 complex numbers each having a real part and an imaginary part, and including an n-th complex number, the block of N/4 complex numbers being derived by steps including:
(i) selecting the 2n-th second intermediate sample as real part of the n-th intermediate complex number,
(ii) selecting the (2n+1)-th second intermediate sample as an imaginary part of the n-th intermediate complex number, and
(iii) multiplying the n-th intermediate complex number by the exponent of (-3.pi.jn/N) to provide the n-th complex number in the block of N/4 complex numbers,
for each integral value of n from 0 to N/4-1.

### Claim 11 (depends on 9)

The method of claim 9, wherein:
(1) in the step of post-processing the transformed block of complex numbers, the block of N/2 spectral coefficients includes a k-th spectral coefficient, where k is an integer between 0 and N/2-1, and
(2) the step of post-processing the transformed block of complex numbers includes the steps of:
(a) expanding the transformed block of N/4 complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the block of 3N/4 complex numbers including a k-th complex number and a (N/4-1-k)-th complex number,
(b) deriving a (N/4-1-k)-th complex conjugate from the (N/4-1-k)-th complex number,
(c) deriving the k-th third intermediate complex number by
(i) summing the (N/4-1-k)-th complex conjugate and the k-th complex number to provide a sum, and
(ii) dividing the sum by two to provide the k-th third intermediate complex number,
(d) deriving a k-th fourth intermediate complex number by
(i) subtracting the (N/4-1-k)-th complex conjugate from the k-th complex number to provide a difference,
(ii) dividing the difference by 2j to provide a quotient, where j is the square root of minus one, and
(iii) multiplying the quotient by the exponential of (-2.pi.j(2k+1)/N) to provide the k-th fourth intermediate complex number,
(e) deriving the k-th spectral coefficient by
(i) summing the k-th third intermediate complex number and the k-th fourth intermediate complex number to provide a k-th fifth intermediate complex number,
(ii) multiplying the k-th fifth intermediate complex number by an exponential of (-.pi.j(2k+1)(2N)) to provide a k-th sixth intermediate complex number having a real part and an imaginary part, and
(iii) providing the real part of the k-th sixth intermediate complex number sum as the k-th spectral coefficient.

### Claim 12 (depends on 9)

The method of claim 9, wherein:
(1) in step of post-processing the transformed block of complex numbers, the block of N/2 spectral coefficients includes a k-th spectral coefficient, where k is an integer between 0 and N/2-1, and
(2) step of post-processing the transformed block of complex numbers includes the steps of:
(a) expanding the transformed block of N/4 complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the complex numbers in the block of 3N/4 complex numbers having a real part and an imaginary part, and including a k-th complex number and a (N/4-1-k)-th complex number;
(b) determining:
(i) a sum, the sum being one half of a sum of the cosine of (2.pi.)(2k+1)/4N) and sine of (10.pi.)(2k+1)/4N),
(ii) a first difference, the first difference being one half of a difference between the cosine of (2.pi.)(2k+1)/4N) and the sine of (10.pi.)(2k+1)/4N), and
(iii) a second difference, the second difference being one half of a difference between and sine of (10.pi.)(2k+1)/4N) and the cosine of (2.pi.)(2k+1)/4N);
(c) multiplying:
(i) the real part of the k-th complex number by the first difference to provide a first intermediate number,
(ii) the real part of the (N/4-1-k)-th complex number by the sum, to provide a second intermediate number,
(iii) the imaginary part of the k-th complex number by the sum to provide a third intermediate number, and
(iv) the imaginary part of the (N/4-1-k)-th complex number by the second difference to provide a fourth intermediate number; and
(d) summing the first, the second, the third, and the fourth intermediate numbers to provide the k-th spectral coefficient.

### Claim 13

A method of performing a simplified digital temporal analysis of a block of N/2 spectral coefficients representing an information signal in the frequency domain, the temporal analysis providing a block of N samples representing the information signal in the time domain, the method comprising the steps of:
(1) digitally pre-processing the block of N/2 spectral coefficients representing the information signal in the frequency domain to provide a block of N/4 frequency domain complex numbers, where N is a multiple of 4;
(2) performing digital processing equivalent to a Fast Fourier Transform on the block of N/4 frequency domain complex numbers to provide a block of N/4 time domain complex numbers;
(3) digitally post-processing the block of N/4 time domain complex numbers to provide a block of N/2 processed samples in the time domain; and
(4) digitally re-arraying and multiplying the block of N/2 processed samples in the time domain by an inverse transform window to provide the block of N samples representing the information signal in the time domain.

### Claim 14 (depends on 13)

The method of claim 13, wherein:
(1) the block of N/2 spectral coefficients includes a 2k-th, and a (N-1-2k)-th spectral coefficient, each having a sign,
(2) the block of N/4 frequency domain complex numbers includes a k-th frequency domain complex number, and
(3) the step of pre-processing the spectral coefficients in the block of N/2 spectral coefficients to provide a block of N/4 frequency domain complex numbers includes the steps of:
(a) deriving, from the block of N/2 spectral coefficients, a block of first intermediate numbers, the first intermediate numbers including a k-th, a 2k-th, and a (2k+1)-th first intermediate number, the first intermediate numbers being derived by steps including:
(i) selecting the 2-th spectral coefficient as the k-th first intermediate number for each integral value of k between 0 and N/4-1, and
(ii) reversing the sign of the (N-1-2k)-th spectral coefficient to provide a sign-reversed (N-1-2k)-th spectral coefficient, and providing the sign-reversed (N-1-2k)-th spectral coefficient as the k-th first intermediate number for each integral value of k between N/4 to N/2-1,
(b) deriving, from the block of N/2 first intermediate numbers, the block of N/4 frequency domain complex numbers, the frequency domain complex numbers having a real part and an imaginary part, and including a k-th frequency domain complex number, the block of frequency domain complex numbers being derived by steps including:
(i) selecting the 2k-th first intermediate number as a real part of the k-th complex number, and
(ii) selecting the (2k+1)-th first intermediate number as an imaginary part of the k-th frequency domain complex number, and
(iii) multiplying the k-th complex number by an exponential of (-4.pi.jk/N) to provide the k-th frequency domain complex number,
for each integral value of k from 0 to N/4-1.

### Claim 15 (depends on 13)

The method of claim 13, wherein:
(1) the block of N/2 processed samples include an n-th processed sample, where n is an integer between 0 and N-1, and
(2) the step of post-processing the block of N/4 time domain complex numbers includes the steps of:
(a) expanding the block of N/4 time domain complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the block of 3N/4 complex numbers including an n-th complex number and a (N/4-1-n)-th complex number,
(b) deriving a (N/4-1-n)-th complex conjugate from the (N/4-1-n)-th complex number,
(c) summing the (N/4-1-n)-th complex conjugate and the n-th complex number to provide a first sum, and dividing the first sum by two to provide an n-th third intermediate complex number,
(d) deriving the n-th fourth intermediate complex number by
(i) subtracting the (N/4-1-n)-th complex conjugate and the n-th complex number to provide a difference,
(ii) dividing the difference by 2j to provide a quotient, where j is the square root of minus one, and
(iii) multiplying the quotient by an exponential of (-2.pi.j(2k+1)/N) to provide the n-th fourth intermediate complex number, and
(e) deriving a processed sample by
(i) summing the n-th third intermediate complex number and the n-th fourth intermediate complex number to provide an n-th fifth intermediate complex number,
(ii) multiplying the n-th fifth intermediate complex number by an exponential of (-.pi.j(2k+1)(2N)) to provide an n-th sixth intermediate complex number having a real part and an imaginary part, and
(iii) providing the real part of the n-th sixth intermediate complex number sum as the n-th processed sample.

### Claim 16 (depends on 13)

The method of claim 13, wherein:
(1) the block of N/2 processed samples includes an (n+N/4)-th, an (3N/4-1-n)-th, and an (n-3N/4)-th processed sample, each processed sample having a sign, where n is an integer between 0 and N-1,
(2) the block of N samples in the time domain includes an n-th sample in the time domain, and
(3) the step of multiplying the block of N/2 processed samples in the time domain by an inverse transform window includes the steps of:
(a) deriving, from the block of N/2 processed samples, a block of N intermediate processed samples, the block of N intermediate processed samples including an n-th intermediate processed sample, the block of N intermediate processed samples being derived by steps including:
(i) selecting the (n+N/4)-th processed sample selecting the (n+N/4)-th processed sample as the n-th intermediate processed sample for each integral value of n from 0 and N/4-1,
(ii) reversing the sign of the (3N/4-1-n)-th processed sample to provide a sign-reversed (3N/4-1-n)-th processed sample, and providing the sign-reversed (3N/4-1-n)-th processed sample as the n-th intermediate processed sample for each integral value of n from N/4 to 3N/4-1, and
(iii) reversing the sign of the (n-3N/4)-th processed sample to provide a sign-reversed (n-3N/4)-th processed sample, and providing the (n-3N/4)-th processed sample as the n-th intermediate processed sample for each integral value of n from 3N/4-1, and
(b) multiplying the n-th intermediate processed sample by an inverse transform window for the n-th intermediate processed sample, for each integral value of n from 0 to N-1, to provide the n-th sample in the time domain.

### Claim 17

A medium for storing a compressed digital signal derived from a digital input signal composed of plural samples in the time domain by a method comprising the steps of:
(1) dividing the digital input signal into blocks of N samples, where N is a multiple of four;
(2) multiplying each block of N samples by a forward transform window to provide a block of N forward transform window processed samples;
(3) digitally preprocessing the block of N forward transform window processed samples to provide a block of N/4 complex numbers;
(4) digitally performing processing equivalent to a Fast Fourier Transform on the block of N/4 complex numbers to provide a transformed block of N/4 complex numbers;
(5) digitally post-processing the transformed block of complex numbers to provide a block of N/2 spectral coefficients;
(6) deriving a block of the compressed digital signal from the block of N/2 spectral coefficients; and
(7) recording successive blocks of the compressed digital signal on the medium.

### Claim 18 (depends on 17)

The medium of claim 17, wherein:
(1) in the step of multiplying each block of N samples, the block of N forward transform window processed samples includes an n-th sample, a (n+3N/4)-th sample, the (n+3N/4)-th sample having a sign, and a (n-N/4)-th sample, where n is an integer between 0 and (N-1); and
(2) the step of preprocessing the block of N forward transform window processed samples includes the steps of:
(a) deriving, from the block of N forward transform window processed samples, a block of first intermediate samples, the block of first intermediate samples including an n-th a (N-1-2n)-th, and a 2n-th first intermediate sample, the block of first intermediate samples being derived by steps including:
(i) reversing the sign of the (n+2N/4)-th sample to provide a sign-reversed (n+2N/4)-th sample, and providing the sign-reversed (n+2N/4)-th sample as the n-th first intermediate sample, for each integral value of n from 0 to N/4-1, and
(ii) providing the (n-N/4)-th sample as the nth first intermediate sample, for each integral value of n from N/4 to N-1,
(b) deriving, from the block of first intermediate samples, a block of second intermediate samples, the block of second intermediate samples including an n-th, a 2n-th, and a (2n+1)-th second intermediate sample, the block of second intermediate samples being derived by steps including subtracting, for each integral value of n from 0 to N/2-1, the (N-1-2n)-th first intermediate sample from the 2n-th first intermediate sample to provide the n-th second intermediate sample,
(c) deriving, from the block of second intermediate samples, the block of N/4 complex numbers, the complex numbers in the block of N/4 complex numbers each having a real part and an imaginary part, and including an n-th complex number, the block of N/4 complex numbers being derived by steps including:
(i) selecting the 2n-th second intermediate sample as a real part of an n-th intermediate complex number,
(ii) selecting the (2n+1)-th second intermediate sample as an imaginary part of the n-th intermediate complex number, and
(iii) multiplying the n-th intermediate complex number by the exponent of (-4.pi.jn/N) to provide the n-th complex number in the block of N/4 complex numbers,
for each integral value of n from 0 to N/4-1.

### Claim 19 (depends on 17)

The medium of claim 17, wherein:
(1) in the step of digitally post-processing the transformed block of complex numbers, the block of N/2 spectral coefficients includes a k-th spectral coefficient, where k is an integer between 0 and N/2-, and
(2) the step of post-processing the transformed block of complex numbers includes the steps of:
(a) expanding the transformed block of N/4 complex numbers based on periodicity to provide a block of N/2 complex numbers, the block of N/2 complex numbers including a k-th complex number and a (N/4-1-k)-th complex number,
(b) deriving a (N/4-1-k)-th complex conjugate from the (N/4-1-k)-th complex number,
(c) deriving a k-th third intermediate complex number by
(i) summing the (N/4-1-k)-th complex conjugate and the k-th complex number to provide a sum, and
(ii) dividing the sum by two to provide the k-th third intermediate complex number,
(d) deriving a k-th fourth intermediate complex number by
(i) subtracting the (N/4-1-k)-th complex conjugate from the k-th complex number to provide a difference,
(ii) dividing the difference by 2j to provide a quotient, where j is the square root of minus one, and
(iii) multiplying the quotient by the exponential of (-2.pi.j)(2k+1/N) to provide the k-th fourth intermediate complex number,
(e) deriving the k-th spectral coefficient by
(i) summing the k-th third intermediate complex number and the k-th fourth intermediate complex number to provide a k-th fifth intermediate complex number,
(ii) multiplying the k-th fifth intermediate complex number by an exponential of (-.pi.j(2k+1)(2N)) to provide a k-th sixth intermediate complex number having a real part and an imaginary part, and
(iii) providing the real part of the k-th sixth intermediate complex number sum as the k-th spectral coefficient.

### Claim 20 (depends on 17)

The medium of claim 17, wherein:
(1) in the step of digitally post-processing the transformed block of complex numbers, the block of N/2 spectral coefficients includes a k-th spectral coefficient, where k is an integer between 0 and N/2-1, and
(2) step of post-processing the transformed block of complex numbers includes the steps of:
(a) expanding the transformed block of N/4 complex numbers based on periodicity to provide a block of 3N/4 complex numbers, the complex numbers in the block of 3N/4 complex numbers having a real part and an imaginary part, and including a k-th complex number and a (N/4-1-k)-th complex number;
(b) determining:
(i) a sum, the sum being one half of a sum of the cosine of (2.pi.)(2k+1)/4N) and sine of (10.pi.)(2k+1)/4N),
(ii) a first difference, the first difference being one half of a difference between the cosine of (2.pi.)(2k+1)/4N) and the sine of (10.pi.)(2k+1)/4N), and
(iii) a second difference, the second difference being one half of a difference between and sine of (10.pi.)(2k+1)/4N) and the cosine of (2.pi.)(2k+1)/4N);
(c) multiplying:
(i) the real part of the k-th complex number by the first difference to provide a first intermediate number,
(ii) the real part of the (N/4-1-k)-th complex number by the sum, to provide a second intermediate number,
(iii) the imaginary part of the k-th complex number by the sum to provide a third intermediate number, and
(iv) the imaginary part of the (N/4-1-k)-th complex number by the second difference to provide a fourth intermediate number; and
(d) summing the first, the second, the third, and the fourth intermediate numbers to provide the k-th spectral coefficient.

### Claim 21 (depends on 17)

The medium of claim 17, wherein the medium is an optical disc.

### Claim 22 (depends on 17)

The medium of claim 17, wherein the medium comprises a semiconductor memory.

### Claim 23 (depends on 17)

The medium of claim 17, wherein the medium comprises a magnetic tape.