# US 2019/0172471 A1 · Time domain aliasing reduction for non-uniform filterbanks which use spectral analysis followed by partial synthesis

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- Number: US 2019/0172471 A1
- Document ID: US20190172471A1
- Status: Published application
- Kind: A1
- Application number: 16/252,069
- Filed: 2019-01-18
- Published: 2019-06-06
- Earliest claimed: 2016-07-29
- Assignees: Fraunhofer-Gesellschaft zur Foerderung der angewandten Forschung e.V.
- Applicants: Fraunhofer-Gesellschaft zur Foerderung der angewandten Forschung e.V.
- Inventors: Nils WERNER; Bernd EDLER
- Web page: https://patentgrep.com/patent/US20190172471A1

## CPC

- G10L 19/0204: {using subband decomposition}
- 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)}
- G10L 19/0212: {using orthogonal transformation}
- G10L 19/022: Blocking, i.e. grouping of samples in time; Choice of analysis windows; Overlap factoring
- G10L: SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
- G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N)

## Abstract

An audio processor for processing an audio signal to acquire a subband representation thereof includes a cascaded lapped critically sampled transform stage and a time domain aliasing reduction stage, the former being configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal. The latter is configured to perform a weighted combination of two corresponding sets of subband samples, which are acquired on the basis of the first and second blocks of samples of the audio signal, respectively, to acquire an aliasing reduced subband representation of the audio signal.

## Claims

### Claim 1

An audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage comprises a first lapped critically sampled transform stage configured to perform lapped critically sampled transforms on a first block of samples and a second block of samples of the at least two partially overlapping blocks of samples of the audio signal, to acquire a first set of bins for the first block of samples and a second set of bins for the second block of samples;
wherein the cascaded lapped critically sampled transform stage further comprises a second lapped critically sampled transform stage configured to perform a lapped critically sampled transform on a segment of the first set of bins and to perform a lapped critically sampled transform on a segment of the second set of bins, each segment being associated with a subband of the audio signal, to acquire a set of subband samples for the first set of bins and a set of subband samples for the second set of bins;
wherein a first set of subband samples is a result of a first lapped critically sampled transform on the basis of the first segment of the first set of bins, wherein a second set of subband samples is a result of a second lapped critically sampled transform on the basis of the second segment of the first set of bins, wherein a third set of subband samples is a result of a third lapped critically sampled transform on the basis of the first segment of the second set of bins, wherein a fourth set of subband samples is a result of a fourth lapped critically sampled transform on the basis of the second segment of the second set of bins; and
wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the first set of subband samples and the third set of subband samples, to acquire a first aliasing reduced subband representation of the audio signal, wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the second set of subband samples and the fourth set of subband samples, to acquire a second aliasing reduced subband representation of the audio signal.

### Claim 2 (depends on 1)

The audio processor according to claim 1, wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples using at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the second block of samples using the at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein the at least two window functions comprise different window widths.

### Claim 3 (depends on 1)

The audio processor according to claim 1, wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples using at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the second block of samples using the at least two window functions, and to acquire at least two sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein filter slopes of the window functions corresponding to adjacent sets of subband samples are symmetric.

### Claim 4 (depends on 1)

The audio processor according to claim 1, wherein the cascaded lapped critically sampled transform stage is configured to segment the samples of the audio signal into the first block of samples and the second block of samples using a first window function;
wherein the lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples and a set of bins acquired on the basis of the second block of samples using a second window function, to acquire the corresponding subband samples; and
wherein the first window function and the second window function comprise different window width.

### Claim 5 (depends on 1)

The audio processor according to claim 1, wherein the cascaded lapped critically sampled transform stage is configured to segment the samples of the audio signal into the first block of samples and the second block of samples using a first window function;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples and a set of bins acquired on the basis of the second block of samples using a second window function, to acquire the corresponding subband samples; and
wherein a window width of the first window function and a window width of the second window function are different from each other, wherein the window width of the first window function and the window width of the second window function differ from each other by a factor different from a power of two.

### Claim 6 (depends on 1)

The audio processor according to claim 1, wherein the time domain aliasing reduction stage is configured to perform the weighted combination of two corresponding sets of subband samples according to the following equation
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to acquire the aliasing reduced subband representation of the audio signal, wherein yv,i(m) is a first aliasing reduced subband representation of the audio signal, yv,i-1(N−1−m) is a second aliasing reduced subband representation of the audio signal, ŷv,i(m) is a set of subband samples on the basis of the second block of samples of the audio signal, ŷv,i-1(N−1−m) is a set of subband samples on the basis of the first block of samples of the audio signal, and av(m), bv(m), cv(m) and dv(m) are time domain aliasing reduction coefficients.

### Claim 7

An audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples using at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the second block of samples using the at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein the at least two window functions comprise different window width

### Claim 8

An audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples using at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the second block of samples using the at least two window functions, and to acquire at least two sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein filter slopes of the window functions corresponding to adjacent sets of subband samples are symmetric.

### Claim 9

An audio processor for processing a subband representation of an audio signal to acquire the audio signal, the audio processor comprising:
an inverse time domain aliasing reduction stage configured to perform a weighted combination of two corresponding aliasing reduced subband representations of the audio signal, to acquire an aliased subband representation, wherein the aliased subband representation is a set of subband samples; and
a cascaded inverse lapped critically sampled transform stage configured to perform a cascaded inverse lapped critically sampled transform on the set of subband samples, to acquire a set of samples associated with a block of samples of the audio signal;
wherein the cascaded inverse lapped critically sampled transform stage comprises a first inverse lapped critically sampled transform stage configured to perform an inverse lapped critically sampled transform on the set of subband samples, to acquire a set of bins associated with a given subband of the audio signal; and
a first overlap and add stage configured to perform a concatenation of sets of bins associated with a plurality of subbands of the audio signal, which comprises a weighted combination of the set of bins associated with the given subband of the audio signal with a set of bins associated with another subband of the audio signal, to acquire a set) of bins associated with a block of samples of the audio signal.

### Claim 10 (depends on 9)

The audio processor according to claim 9, wherein the cascaded inverse lapped critically sampled transform stage comprises a second inverse lapped critically sampled transform stage configured to perform an inverse lapped critically sampled transform on the set of bins associated with the block of samples of the audio signal, to acquire a set of samples associated with the block of samples of the audio signal.

### Claim 11 (depends on 10)

The audio processor according to claim 10, wherein the cascaded inverse lapped critically sampled transform stage comprises a second overlap and add stage configured to overlap and add the set of samples associated with the block of samples of the audio signal and another set of samples associated with another block of samples of the audio signal, the block of samples and the another block of samples of the audio signal partially overlapping, to acquire the audio signal.

### Claim 12 (depends on 9)

The audio processor according to claim 9, wherein the inverse time domain aliasing reduction stage is configured to perform the weighted combination of the two corresponding aliasing reduced subband representations of the audio signal based on the following equation
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to acquire the aliased subband representation, wherein yv,i(m) is a first aliasing reduced subband representation of the audio signal, yv,i-1(N−1−m) is a second aliasing reduced subband representation of the audio signal, ŷv,i(m) is a set of subband samples on the basis of the second block of samples of the audio signal, ŷv,i-1(N−1−m) is a set of subband samples on the basis of the first block of samples of the audio signal, and av(m), bv(m), cv(m) and dv(m) are time domain aliasing reduction coefficients.

### Claim 13

An audio encoder, comprising:
an audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage comprises a first lapped critically sampled transform stage configured to perform lapped critically sampled transforms on a first block of samples and a second block of samples of the at least two partially overlapping blocks of samples of the audio signal, to acquire a first set of bins for the first block of samples and a second set of bins for the second block of samples;
wherein the cascaded lapped critically sampled transform stage further comprises a second lapped critically sampled transform stage configured to perform a lapped critically sampled transform on a segment of the first set of bins and to perform a lapped critically sampled transform on a segment of the second set of bins, each segment being associated with a subband of the audio signal, to acquire a set of subband samples for the first set of bins and a set of subband samples for the second set of bins;
wherein a first set of subband samples is a result of a first lapped critically sampled transform on the basis of the first segment of the first set of bins, wherein a second set of subband samples is a result of a second lapped critically sampled transform on the basis of the second segment of the first set of bins, wherein a third set of subband samples is a result of a third lapped critically sampled transform on the basis of the first segment of the second set of bins, wherein a fourth set of subband samples is a result of a fourth lapped critically sampled transform on the basis of the second segment of the second set of bins; and
wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the first set of subband samples and the third set of subband samples, to acquire a first aliasing reduced subband representation of the audio signal, wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the second set of subband samples and the fourth set of subband samples, to acquire a second aliasing reduced subband representation of the audio signal,
an encoder configured to encode the aliasing reduced subband representation of the audio signal, to acquire an encoded aliasing reduced subband representation of the audio signal; and
a bitstream former configured to form a bitstream from the encoded aliasing reduced subband representation of the audio signal.

### Claim 14

An audio encoder, comprising:
an audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples using at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the second block of samples using the at least two window functions, and to acquire at least two sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein filter slopes of the window functions corresponding to adjacent sets of subband samples are symmetric,
an encoder configured to encode the aliasing reduced subband representation of the audio signal, to acquire an encoded aliasing reduced subband representation of the audio signal; and
a bitstream former configured to form a bitstream from the encoded aliasing reduced subband representation of the audio signal.

### Claim 15 (depends on 9)

An audio decoder, comprising;
a bitstream parser configured to parse the bitstream, to acquire the encoded aliasing reduced subband representation;
a decoder configured to decode the encoded aliasing reduced subband representation, to acquire the aliasing reduced subband representation of the audio signal; and
an audio processor according to claim 9.

### Claim 16

An audio analyzer, comprising:
an audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage comprises a first lapped critically sampled transform stage configured to perform lapped critically sampled transforms on a first block of samples and a second block of samples of the at least two partially overlapping blocks of samples of the audio signal, to acquire a first set of bins for the first block of samples and a second set of bins for the second block of samples;
wherein the cascaded lapped critically sampled transform stage further comprises a second lapped critically sampled transform stage configured to perform a lapped critically sampled transform on a segment of the first set of bins and to perform a lapped critically sampled transform on a segment of the second set of bins, each segment being associated with a subband of the audio signal, to acquire a set of subband samples for the first set of bins and a set of subband samples for the second set of bins;
wherein a first set of subband samples is a result of a first lapped critically sampled transform on the basis of the first segment of the first set of bins, wherein a second set of subband samples is a result of a second lapped critically sampled transform on the basis of the second segment of the first set of bins, wherein a third set of subband samples is a result of a third lapped critically sampled transform on the basis of the first segment of the second set of bins, wherein a fourth set of subband samples is a result of a fourth lapped critically sampled transform on the basis of the second segment of the second set of bins; and
wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the first set of subband samples and the third set of subband samples, to acquire a first aliasing reduced subband representation of the audio signal, wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the second set of subband samples and the fourth set of subband samples, to acquire a second aliasing reduced subband representation of the audio signal; and
an information extractor, configured to analyze the aliasing reduced subband representation, to provide an information describing the audio signal.

### Claim 17

An audio analyzer, comprising:
an audio processor for processing an audio signal to acquire a subband representation of the audio signal, the audio processor comprising:
a cascaded lapped critically sampled transform stage configured to perform a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
a time domain aliasing reduction stage configured to perform a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the first block of samples using at least two window functions, and to acquire at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein the cascaded lapped critically sampled transform stage is configured to segment a set of bins acquired on the basis of the second block of samples using the at least two window functions, and to acquire at least two sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein filter slopes of the window functions corresponding to adjacent sets of subband samples are symmetric; and
an information extractor, configured to analyze the aliasing reduced subband representation, to provide an information describing the audio signal.

### Claim 18

A method for processing an audio signal to acquire a subband representation of the audio signal, the method comprising:
performing a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
performing a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein performing the cascaded lapped critically sampled transform comprises performing lapped critically sampled transforms on a first block of samples and a second block of samples of the at least two partially overlapping blocks of samples of the audio signal, to acquire a first set of bins for the first block of samples and a second set of bins for the second block of samples;
wherein performing the cascaded lapped critically sampled transform comprises performing a lapped critically sampled transform on a segment of the first set of bins and performing a lapped critically sampled transform on a segment of the second set of bins, each segment being associated with a subband of the audio signal, to acquire a set of subband samples for the first set of bins and a set of subband samples for the second set of bins;
wherein a first set of subband samples is a result of a first lapped critically sampled transform on the basis of the first segment of the first set of bins, wherein a second set of subband samples is a result of a second lapped critically sampled transform on the basis of the second segment of the first set of bins, wherein a third set of subband samples is a result of a third lapped critically sampled transform on the basis of the first segment of the second set of bins, wherein a fourth set of subband samples is a result of a fourth lapped critically sampled transform on the basis of the second segment of the second set of bins; and
wherein performing the weighted combination of two corresponding sets of subband samples comprises performing a weighted combination of the first set of subband samples and the third set of subband samples, to acquire a first aliasing reduced subband representation of the audio signal, wherein the time domain aliasing reduction stage is configured to perform a weighted combination of the second set of subband samples and the fourth set of subband samples, to acquire a second aliasing reduced subband representation of the audio signal.

### Claim 19

A method for processing an audio signal to acquire a subband representation of the audio signal, the method comprising:
performing a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
performing a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein performing the cascaded lapped critically sampled transform comprises segmenting a set of bins acquired on the basis of the first block of samples using at least two window functions, and acquiring at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein performing the cascaded lapped critically sampled transform comprises segmenting a set of bins acquired on the basis of the second block of samples using the at least two window functions, and acquiring at least two segmented sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein the at least two window functions comprise different window width.

### Claim 20

A method for processing an audio signal to acquire a subband representation of the audio signal, the method comprising:
performing a cascaded lapped critically sampled transform on at least two partially overlapping blocks of samples of the audio signal, to acquire a set of subband samples on the basis of a first block of samples of the audio signal, and to acquire a corresponding set of subband samples on the basis of a second block of samples of the audio signal; and
performing a weighted combination of two corresponding sets of subband samples, one acquired on the basis of the first block of samples of the audio signal and one acquired on the basis on the second block of samples of the audio signal, to acquire an aliasing reduced subband representation of the audio signal;
wherein performing the cascaded lapped critically sampled transform comprises segmenting a set of bins acquired on the basis of the first block of samples using at least two window functions, and acquiring at least two segmented sets of subband samples based on the segmented set of bins corresponding to the first block of samples;
wherein performing the cascaded lapped critically sampled transform comprises segmenting a set of bins acquired on the basis of the second block of samples using the at least two window functions, and acquiring at least two sets of subband samples based on the segmented set of bins corresponding to the second block of samples; and
wherein filter slopes of the window functions corresponding to adjacent sets of subband samples are symmetric.

### Claim 21

A method for processing a subband representation of an audio signal to acquire the audio signal, the method comprising:
performing a weighted combination of two corresponding aliasing reduced subband representations of the audio signal, to acquire an aliased subband representation, wherein the aliased subband representation is a set of subband samples; and
performing a cascaded inverse lapped critically sampled transform on the set of subband samples, to acquire a set of samples associated with a block of samples of the audio signal;
wherein performing the cascaded inverse lapped critically sampled transform comprises performing an inverse lapped critically sampled transform on the set of subband samples, to acquire a set of bins associated with a given subband of the audio signal; and
wherein performing the cascaded inverse lapped critically sampled transform comprises performing a concatenation of sets of bins associated with a plurality of subbands of the audio signal, which comprises a weighted combination of the set of bins associated with the given subband of the audio signal with a set of bins associated with another subband of the audio signal, to acquire a set of bins associated with a block of samples of the audio signal.

### Claim 22 (depends on 18, 19, 20, 21)

A non-transitory digital storage medium having a computer program stored thereon to perform the methods of claims 18 to 21, when said computer program is run by a computer.