FORMATION OF SIGNAL CORRUPTED WITH NOISE

Function snrs(signal,noise,snr) forms an array of a signal that is corrupted with noise and signal-to-noise ratio (SNR) has a specified value snr.

Contents

• INTRODUCTION
• DESCRIPTION
• EXAMPLE

INTRODUCTION

We suppose that SNR was defined as a ratio of the root-mean-square (rms) value of amplitudes of a signal and a noise. It is supposed that value of SNR was expressed in decibels (dB). It is supposed, too, that both the signal and the noise were measured at the same points, within the same bandwidth, and across the same impedance. Thus for calculations we used the following formula:

SNR_db = 20log₁₀(A_s / A_n)

where A_s and A_n are rms values of amplitudes of a signal and a noise respectively.

DESCRIPTION

s = snrs(signal,noise,snr) -

signal is an array of input signal.

noise is an array of noise; it is considered that noise has the same sampling frequency as the sampling frequency of signal.

Both signal and noise should have floating point format.

snr is a signal-to-noise ratio, dB.

s is an array of corrupted signal.

Then, an array noi of noise that has the same length as signal is built. After repetition of noise m times, where (m-1)*numel(noise) < numel(signal) <= m*numel(noise), the first numel(signal) samples of new array form the arrray noi. If numel(signal) = numel(noise), then noi = noise. After multiplying noi by appropriate coefficient ns, a product to input signal is added. And it is an array s. s = signal + noi * ns.

If length of noise less than length of signal we can change type of noise aggression. s = snrs(signal,noise,snr,1) apply noise only to the first numel(noise) samples of signal. To this end array noise is padded with trailing zeros to length numel(signal).

[s,stnr]=snrs(...) returns stnr, which is a signal-to-noise ratio of signal and noi.

EXAMPLE

fs =1200;                     % Sampling frequency, Hz
T= .1;                        % Signal duration, s
t0=1/fs;                      % Sampling time, s
q=fs/1000;                    % Samples per ms
L = floor(T*fs);              % Length of a signal, samples
t = (0:L)*t0;                 % Time vector
x=sin(2*pi*50*t);             % 50 Hz sinusoid
randn('state',0);             % Default initial state
nx=randn([1,numel(t)]);       % Zero-mean random noise
for snr=[0 10]
  [s,stnr]=snrs(x,nx,snr);
  figure
  plot(fs*t(1:L)/q,s(1:L),'.-',fs*t(1:L)/q,x((1:L)));grid on
  title(['Signal corrupted with zero-mean random noise. SNR = ' ...
    num2str(snr)])
  xlabel('Time, ms')
  legend('Corrupted signal','Signal','Location','southoutside');
  legend boxoff
end
disp(sprintf('%s %.1f','stnr =',stnr))
fl=floor(numel(t)/4);
s=snrs(x,nx(1:fl),snr,1);
figure
plot(fs*t(1:fl+1)/q,s(1:fl+1),'.-',fs*t(fl+1:L)/q,x((fl+1:L)),'+-')
grid on
title(['Signal partly corrupted with zero-mean random noise. SNR = ' ...
  num2str(snr)])
xlabel('Time, ms')
legend('Corrupted part of a signal','Non-corrupted part of a signal', ...
  'Location','southoutside');
legend boxoff

stnr = -2.0

Published with MATLAB® 7.6