Overview

Table driven interpolation module. Supports linear and spline modes

Discussion

This module performs table lookup using unevenly spaced X and Y values. The module supports linear and pchip interpolation. Internally the interpolation is performed using a piece wise polynomial approximation. With linear interpolation a first order polynomial is used; for pchip a 4th order polynomial is used. PChip is similar to a spline interpolation but it avoids the overshoot and undershoot that plagues splines.

The input to the module serves as the X value in the lookup and the output equals the Y value derived from doing the interpolation. The module is configured by setting the internal matrix .XY. The first row of .XY represents X values and the second row Y values.

At instantiation time you specify the total number of points (MAXPOINTS) in the table. MAXPOINTS determines the amount of memory allocated to the table. Then at run time, you can change the number of points actively used by the table (.numPoints). .numPoints must be less than or equal to MAXPOINTS.

The variable .order specifes either linear (=2) or cubic (=4) interpolation.

The module also provides a custom inspector for easy configuration from the Audio Weaver Designer GUI. The GUI translates the XY table into a piece wise polynomial segments. The matrix .polyCoeffs contains 4 x (MAXPOINT-1) values. Each column represents the coefficients for each polynomial. Row 1 is the X^3 coefficent, row 2 the X^2 coefficient, row 3 the X coefficient and row 4 the constant term.

If the input x falls outside of the range of values in the XY table then the input is clipped to the allowable range.

The X values in the table have to monotonically increasing. That is, x0 < x1 < x2 < ... < xN. If this condition is not obeyed, then the module's prebuild function will generate an error.

Type Definition

typedef struct _ModuleTableInterp
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    INT32 maxPoints;                              // Maximum number of values in the lookup table. The total table size is [maxPoints 2].
    INT32 numPoints;                              // Current number of interpolation values in use.
    INT32 order;                                  // Order of the interpolation. This can be either 2 (for linear) or 4 (for pchip).
    FLOAT32* XY;                                  // Lookup table. The first row is the X values and the second row is the Y values.
    FLOAT32* polyCoeffs;                          // Interpolation coefficients returned by the grid control.
} ModuleTableInterpClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
maxPoints	int	const	0	8	4:1:1000
numPoints	int	parameter	0	4	4:1:8
order	int	parameter	0	2	2:2:4
XY	float*	parameter	0	[2 x 8]	Unrestricted
polyCoeffs	float*	state	0	[4 x 7]	Unrestricted

Pins

Input Pins

Name: in

Description: audio input

Data type: float

Channel range: Unrestricted

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output

Data type: float

MATLAB Usage

File Name: table_interp_module.m

 M=table_interp_module(NAME, MAXPOINTS)
 This Audio Weaver module performs interpolation using a lookup table together
 with a configurable interpolation order. The table contains (X,Y) value pairs
 and can be unevenly spaced.
 Arguments:
    NAME - name of the module.
    MAXPOINTS - maximum number of points allocated to the lookup table. This
         is set at design time and has a minimum value of 4. At run
         time, you can change the number of values in the lookup table
         from 4 to MAXPOINTS.
 The module can be configured to perform linear or pchip interpolation.