getLatency

Latency of FFT calculation

Syntax

Y = getLatency(hdlfft)

Y = getLatency(hdlfft,N)

Y = getLatency(hdlfft,N,V)

Description

Y = getLatency(hdlfft) returns the number of cycles, Y, that the object takes to calculate the FFT of an input frame. The latency depends on the input vector size and the FFT length. If the object is configured to accept FFT length from an input argument, the getLatency function uses the value of FFTLengthMax.

Y = getLatency(hdlfft,N) returns the number of cycles that an object would take to calculate the FFT of an input frame, if it had FFT length N, and scalar input. This function does not change the properties of the hdlfft.

Y = getLatency(hdlfft,N,V) returns the number of cycles that an object would take to calculate the FFT of an input frame, if it had FFT length N, and vector input of size V. This function does not change the properties of hdlfft. This syntax is only valid when Architecture is set to 'Streaming Radix 2^2' and FFTLengthSource is set to 'Property'.

Examples

Explore Latency of HDL FFT Object

The latency of the object varies with the FFT length and the vector size. Use the getLatency function to find the latency of a particular configuration. The latency is the number of cycles between the first valid input and the first valid output, assuming that the input is contiguous.

Create a new dsphdl.FFT object and request the latency.

hdlfft = dsphdl.FFT(FFTLength=512);
L512 = getLatency(hdlfft)

L512 = 
599

Request hypothetical latency information about a similar object with a different FFT length. The properties of the original object do not change.

L256 = getLatency(hdlfft,256)

L256 = 
329

N = hdlfft.FFTLength

N = 
512

Request hypothetical latency information of a similar object that accepts eight-sample vector input.

L256v8 = getLatency(hdlfft,256,8)

L256v8 = 
93

Enable scaling at each stage of the FFT. The latency does not change.

hdlfft.Normalize = true;
L512n = getLatency(hdlfft)

L512n = 
599

Request the same output order as the input order. The latency increases because the object must collect the output before reordering.

hdlfft.BitReversedOutput = false;
L512r = getLatency(hdlfft)

L512r = 
1078

Input Arguments

hdlfft — FFT System object™

dsphdl.FFT | dsphdl.IFFT

FFT System object. See dsphdl.IFFT or dsphdl.FFT.

N — FFT length

integer power of 2 from 2² to 2¹⁶

FFT length, specified as an integer power of 2 from 2² to 2¹⁶. Use this argument to request the latency of an object similar to hdlfft, but with FFT length N.

V — Vector size

power of 2 from 1 to 64

Vector size, specified as a power of 2 from 1 to 64. The vector size cannot be greater than the FFT length. Use this argument to request the latency of an object similar to hdlfft, but with V-sample vector input. When you do not specify this argument, the function assumes scalar input.

Output Arguments

Y — Cycles of latency

integer

Cycles of latency that the object takes to calculate the FFT of an input frame, returned as an integer. The latency is the number of cycles between the first valid input and the first valid output, assuming the input is contiguous. Each call to the object simulates one cycle.

Version History

Introduced in R2014a