---
name: "dsp"
section: "3"
platform: "linux"
package: "libfec-dev"
version: "1.0-26-gc5d935f-1+b3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/dsp/3"
---

# dsp(3)

SIMD-assisted digital signal processing primitives

## NAME

initdp, freedp, dotprod, sumsq, peakval - SIMD-assisted digital signal processing primitives

## SYNOPSIS

```
#include "fec.h"
void *initdp(signed short *coeffs,int len);
long dotprod(void *p,signed short *a);
void freedp(void *p);
unsigned long long sumsq(signed short *in,int cnt);
int peakval(signed short *b,int cnt);
```

## DESCRIPTION

These functions provide several basic primitives useful in digital signal processing (DSP), especially in modems.  The **initdp**, **dotprod** and **freedp** functions implement an integer dot product useful in correlation and filtering operations on signed 16-bit integers. **sumsq** computes the sum of the squares of an array of signed 16-bit integers, useful for measuring the energy of a signal. **peakval** returns the absolute value of the largest magitude element in the input array, useful for scaling a signal's amplitude.

Each function uses IA32 or PowerPC Altivec instructions when available; otherwise, a portable C version is used.

## USAGE

To create a FIR filter or correlator, call **initdp** with the coefficients in **coeff** and their number in **len**.  This creates the appropriate data structures and returns a handle.

To compute a dot product, pass the handle from **initdp** and the input array to **dotprod**. No length field is needed as the number of samples will be taken from the **len** parameter originally given to **initdp**. There must be at least as many samples in the input array as there were coefficients passed to **initdp**.

When the filter or correlator is no longer needed, the data structures may be freed by passing the handle to **freedp**.

The user is responsible for scaling the inputs to **initdp** and **dotprod**, as the 32-bit result from **dotprod** will silently wrap around in the event of overflow.

To compute the sum of the squares of an array of signed 16-bit integers, use sumsq. This returns a 64 bit sum.

**peakval** computes the absolute value of each 16-bit element in the input array and returns the largest.

## RETURN VALUES

**initdp** returns a handle that points to a control block, or NULL in the event of an error (such as a memory allocation failure). **sumsq** and **peakval** have no error returns.

## AUTHOR and COPYRIGHT

Phil Karn, KA9Q (karn@ka9q.net)