Lowpass Resampler

Convert signal from one sample time to another

Since R2021a

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Libraries:
Mixed-Signal Blockset / Utilities

Description

The Lowpass Resampler block converts either a fixed-step discrete or a variable-step discrete sample time at its input to a different sample time at its output. To calculate the output sample values, the block uses a lowpass filtering interpolation algorithm. The algorithm minimizes frequency aliasing at the output with respect to an output rise/fall time.

If the output rise/fall time is inherited from a fixed-step discrete input, the cutoff frequency is the Nyquist rate of the input. Otherwise, the cutoff frequency is the Nyquist rate associated with a sample interval obtained by scaling the specified 20%−80% output rise/fall time to a value for 0%−100% rise/fall time.

Examples

Define Output Samples in Lowpass Resampler

To demonstrate the variable step discrete behavior of the Lowpass Resampler block, open the model LowpassResamplerOutputSampling.

• Open Script

Ports

Input

in — Discrete time input signal

scalar

Discrete time input signal, specified as fixed-step or variable-step scalar.

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32 | Boolean

Output

out — Continuous time output signal

scalar

Continuous time output signal, returned as a fixed-step or variable-step scalar.

Data Types: single | double

Parameters

Inherit output rise/fall time — Inherit output rise/fall time from fixed-step input sample time

on (default) | off

Select to inherit the output rise/fall time from the fixed-step input sample time.

Note

In case of variable step input, there is no rise/fall time to inherit.

Output rise/fall time — 20%−80% output rise/fall time

1e-10 (default) | positive real scalar

20%−80% output rise/fall time, specified as a positive real scalar.

Dependencies

To enable this parameter, deselect the Inherit output rise/fall time parameter.

Programmatic Use

Block parameter: OutputRiseFall

Type: character vector

Values: positive real scalar

Default: 1e-10

Number of samples of delay — Number of samples of propagation delay for fixed-step operation

1 (default) | positive real scalar

Number of samples of propagation delay for fixed-step operation, specified as a positive real scalar.

If the Lowpass Resampler block inherits Output rise/fall time in fixed-step mode, the resampler conversion delay is given by NDelay·τ, where NDelay is the Number of samples of delay parameter and τ is the inherited input sample interval.

In the variable-step input mode, the resampler conversion delay is given by 0.6τ_v when NDelay equals to one, and is (NDelay-0.5)·τ_v when NDelay is greater than one. τ_v is 5/3 times the Output rise/fall time parameter.

There is a tradeoff between the delay of the conversion and the suppression of out-of-band numerical artifacts, with greater delay producing better fidelity in band and greater suppression out-of-band.

Note

If you need anti-aliasing filter rejection, set Number of samples of delay to 5 or higher.

Dependencies

To enable this parameter, deselect the Inherit output rise/fall time parameter.

Programmatic Use

Block parameter: NDelay

Type: character vector

Values: positive real scalar

Default: 1

Output sample time — Type of output sample time

Inherited (default) | Fixed step discrete | Variable step discrete

Type of the output sample time to be used by downstream blocks, specified as either fixed step discrete or variable step discrete. The output sampling rate must be higher than the input sampling rate. For more information, see Variable Step Sampling Scheme.

Programmatic Use

Block parameter: OutputTsType

Type: character vector

Values: Fixed step discrete | Variable step discrete

Default: Inherited

Samples out per rise/fall time — Number of output samples in single output rise/fall time

5 (default) | positive real scalar

Number of output samples in a single output rise/fall time, specified as a positive real scalar.

Dependencies

To enable this parameter, select the option Fixed step discrete or Variable step discrete in the Output sample time parameter.

Programmatic Use

Block parameter: OutputTsRatio

Type: character vector

Values: positive real scalar

Default: 5

Enable large buffer — Enable extra buffer samples

on (default) | off

Select to enable extra buffer samples. This option is enabled by default.

Number of extra buffer samples — Number of extra buffer samples to support sample delays

10 (default) | positive real scalar

Number of extra buffer samples needed to support the number of samples of delay, specified as a positive real scalar.

Programmatic Use

Block parameter: NBuffer

Type: character vector

Values: positive real scalar

Default: 10

More About

Frequency and Step Response

The Lowpass Resampler block interpolates the incoming discrete signal using an anti-aliasing filter. The anti-aliasing filter introduces some delay and the suppression of outputs above the specified signal bandwidth is not perfect. Introducing larger delays produces better fidelity in band and greater suppression out-of-band at the cost of more delay inserted in the signal path. In all cases, the interpolation filter is designed to produce negligible group delay distortion in-band.

In the time domain, the interpolation does introduce some ringing in the step response of the anti-aliasing filter, which could affect some digital switching applications. However, the choice of Number of samples of delay parameter of 1 and Output rise/fall time parameter greater than zero is specifically designed to produce a smooth waveform transition with no ringing.

Variable Step Sampling Scheme

In some cases, it might be necessary for the Lowpass Resampler block to define its own sample time. This is particularly true if the block receives an unspecified output sample time at its input.

If the Lowpass Resampler block receives a signal that changes its value at the input, the block generates some output samples to show the transition. The number of samples are defined by the Samples out per rise/fall time parameter.

Version History

Introduced in R2021a

R2022a: Updated in R2022a

Lowpass Resampler can define its own output sample times.

R2021a: Introduced in R2021a

Use to change the sampling time of a signal.