# conv

Convolution and polynomial multiplication of `fi` objects

## Syntax

``c = conv(a,b)``
``c = conv(a,b,shape)``

## Description

example

````c = conv(a,b)` returns the convolution of input vectors `a` and `b`, at least one of which must be a `fi` object.```

example

````c = conv(a,b,shape)` returns a subsection of the convolution, as specified by `shape`.```

## Examples

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Find the convolution of a 22-sample sequence with a 16-tap FIR filter.

`x` is a 22-sample sequence of signed values with a word length of 16 bits and a fraction length of 15 bits. `h` is the 16-tap FIR filter.

```u = (pi/4)*[1 1 1 -1 -1 -1 1 -1 -1 1 -1]; x = fi(kron(u,[1 1])); h = firls(15, [0 .1 .2 .5]*2, [1 1 0 0]);```

Because `x` is a `fi` object, you do not need to cast `h` into a `fi` object before performing the convolution operation. The `conv` function does this automatically using best-precision scaling.

Use the `conv` function to convolve the two vectors.

`y = conv(x,h);`

The operation results in a signed `fi` object `y` with a word length of 36 bits and a fraction length of 31 bits. The default `fimath` properties associated with the inputs determine the `numerictype` of the output. The output does not have a local `fimath`.

Create two `fi` vectors. Find the central part of the convolution of `a` and `b` that is the same size as `a`.

```a = fi([-1 2 3 -2 0 1 2]); b = fi([2 4 -1 1]); c = conv(a,b,'same')```
```c = 15 5 -9 7 6 7 -1 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 34 FractionLength: 25```

`c` has a length of 7. The full convolution would be of length `length(a)+length(b)-1`, which in this example would be 10.

## Input Arguments

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Input vectors, specified as either row or column vectors.

If either input is a built-in data type, `conv` casts it into a `fi` object using best-precision rules before the performing the convolution operation.

Data Types: `single` | `double` | `int8` | `int16` | `int32` | `int64` | `uint8` | `uint16` | `uint32` | `uint64` | `fi`
Complex Number Support: Yes

Subset of convolution, specified as one of these values:

• `'full'` — Returns the full convolution. This option is the default shape.

• `'same'` — Returns the central part of the convolution that is the same size as input vector `a`.

• `'valid'` — Returns only those parts of the convolution that the function computes without zero-padded edges. Using this option, the length of output vector `c` is `max(length(a)-max(0,length(b)-1),0)`.

Data Types: `char`

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### Convolution

The convolution of two vectors, `u` and `v`, represents the area of overlap under the points as `v` slides across `u`. Algebraically, convolution is the same operation as multiplying polynomials whose coefficients are the elements of `u` and `v`.

Let `m = length(u)` and `n = length(v)` . Then `w` is the vector of length `m+n-1` whose `k`th element is

`$w\left(k\right)=\sum _{j}u\left(j\right)v\left(k-j+1\right).$`

The sum is over all the values of `j` that lead to legal subscripts for `u(j)` and `v(k-j+1)`, specifically `j` `=` `max(1,k+1-n):1:min(k,m)`. When `m` `=` `n`, this gives

```w(1) = u(1)*v(1) w(2) = u(1)*v(2)+u(2)*v(1) w(3) = u(1)*v(3)+u(2)*v(2)+u(3)*v(1) ... w(n) = u(1)*v(n)+u(2)*v(n-1)+ ... +u(n)*v(1) ... w(2*n-1) = u(n)*v(n)```

## Algorithms

The `fimath` properties associated with the inputs determine the `numerictype` properties of output `fi` object `c`:

• If either `a` or `b` has a local `fimath` object, `conv` uses that `fimath` object to compute intermediate quantities and determine the `numerictype` properties of `c`.

• If neither `a` nor `b` have an attached `fimath`, `conv` uses the default `fimath` to compute intermediate quantities and determine the `numerictype` properties of `c`.

If either input is a built-in data type, `conv` casts it into a `fi` object using best-precision rules before the performing the convolution operation.

The output `fi` object `c` always uses the default `fimath`.

## Version History

Introduced in R2009b