FIR Interpolation (DSP Blockset)

Upsample and filter an input signal.

Library

Multirate Filters, in Filtering

Description

The FIR Interpolation block resamples the input at an integer rate L times faster than the input sample rate, where L is specified by the Interpolation factor parameter. This process consists of two steps:

The block upsamples the input to a higher rate by inserting L-1 zeros between samples.
The block filters the upsampled data with an FIR filter.

The FIR Interpolation block implements the upsampling and FIR filtering steps together using a polyphase filter structure, which is more efficient than straightforward upsample-then-filter algorithms.

In practice, the filter specified by the FIR filter coefficients vector (in descending powers of z) should be a lowpass FIR with a normalized cutoff frequency no greater than 1/L. The coefficients are scaled by L.

The Frame-based inputs parameter allows you to choose between sample-based and frame-based operation.

Sample-Based Operation

When the check box is not selected (default), the block assumes that the input is a 1-by-N sample vector or M-by-N sample matrix. Each of the N vector elements (or M*N matrix elements) is treated as an independent channel, and the block interpolates each channel over time.

Frame-Based Operation

When the Frame-based inputs check box is selected, the block assumes that the input is an M-by-N frame matrix. Each of the N frames in the matrix contains M sequential time samples from an independent signal. The illustration below shows a 6-by-4 matrix input:

The Number of channels parameter specifies the number of independent channels (columns, N) in the matrix, and the block interpolates each channel independently over time. Frame-based operation provides substantial increases in throughput rates, at the expense of greater model latency.

In frame-based operation, the Framing parameter determines how the block adjusts the rate at the output. There are two available options:

Maintain input frame rate

The block generates the output at the faster (interpolated) rate by using a proportionally larger frame size than the input. For interpolation by a factor of L, the output frame size is L times larger than the input frame size, but the input and output frame rates are equal.

The example below shows a single-channel input of frame size 16 being upsampled by a factor of 4 to a frame size of 64. The block's input and output frame rates are identical.

Maintain input frame size

The block generates the output at the faster (interpolated) rate by using a proportionally shorter frame period at the output port than at the input port. For interpolation by a factor of L, the output frame period is L times shorter than the input frame period, but the input and output frame sizes are equal.

The example below shows a single-channel input (frame size = 64) with a frame period of 1 second being upsampled by a factor of 4 to a frame period of 0.25 seconds. The input and output frame sizes are identical.

Note
If you expect to generate code for the FIR Interpolation block using the Real-Time Workshop, you should ensure that inputs are contiguous in memory. See the Contiguous Copy block for more information.

Dialog Box

FIR filter coefficients: The FIR filter coefficients, in descending powers of z.
Interpolation factor: The integer factor, L, by which to increase the sample rate of the input sequence.
Frame-based inputs: Selects frame-based operation.
Number of channels: For frame-based operation, the number of columns (channels) in the input matrix, N.
Framing: For frame-based operation, the method by which to implement the interpolation: increase the output frame rate, or increase the output frame size.