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demod

Demodulation for communications simulation

Syntax

x = demod(y,fc,fs,'method')
x = demod(y,fc,fs,'method',opt)
x = demod(y,fc,fs,'pwm','centered')

Description

demod performs demodulation, that is, it obtains the original signal from a modulated version of the signal. demod undoes the operation performed by modulate.

x = demod(y,fc,fs,'method') and

x = demod(y,fc,fs,'method',opt) demodulate the real carrier signal y with a carrier frequency fc and sampling frequency fs, using one of the options listed below for method. (Note that some methods accept an option, opt.)

    Note:   Use demod and modulate in the Signal Processing Toolbox™ with real-valued signals to obtain real-valued outputs. demod and modulate are not intended to accept complex-valued inputs or produce complex-valued outputs.

Method

Description

amdsb-sc

or

am

Amplitude demodulation, double sideband, suppressed carrier. Multiplies y by a sinusoid of frequency fc and applies a fifth-order Butterworth lowpass filter using filtfilt.

x = y.*cos(2*pi*fc*t);

[b,a] = butter(5,fc*2/fs);

x = filtfilt(b,a,x);

amdsb-tc

Amplitude demodulation, double sideband, transmitted carrier. Multiplies y by a sinusoid of frequency fc, and applies a fifth-order Butterworth lowpass filter using filtfilt.

x = y.*cos(2*pi*fc*t);

[b,a] = butter(5,fc*2/fs);

x = filtfilt(b,a,x);

If you specify opt, demod subtracts scalar opt from x. The default value for opt is 0.

amssb

Amplitude demodulation, single sideband. Multiplies y by a sinusoid of frequency fc and applies a fifth-order Butterworth lowpass filter using filtfilt.

x = y.*cos(2*pi*fc*t);

[b,a] = butter(5,fc*2/fs);

x = filtfilt(b,a,x);

fm

Frequency demodulation. Demodulates the FM waveform by modulating the Hilbert transform of y by a complex exponential of frequency -fc Hz and obtains the instantaneous frequency of the result.

pm

Phase demodulation. Demodulates the PM waveform by modulating the Hilbert transform of y by a complex exponential of frequency -fc Hz and obtains the instantaneous phase of the result.

ppm

Pulse-position demodulation. Finds the pulse positions of a pulse-position modulated signal y. For correct demodulation, the pulses cannot overlap. x is length length(t)*fc/fs.

pwm

Pulse-width demodulation. Finds the pulse widths of a pulse-width modulated signal y. demod returns in x a vector whose elements specify the width of each pulse in fractions of a period. The pulses in y should start at the beginning of each carrier period, that is, they should be left justified.

qam

Quadrature amplitude demodulation.

[x1,x2] = demod(y,fc,fs,'qam') multiplies y by a cosine and a sine of frequency fc and applies a fifth-order Butterworth lowpass filter using filtfilt.

x1 = y.*cos(2*pi*fc*t);

x2 = y.*sin(2*pi*fc*t);

[b,a] = butter(5,fc*2/fs);

x1 = filtfilt(b,a,x1);

x2 = filtfilt(b,a,x2);

The default method is 'am'. In all cases except 'ppm' and 'pwm', x is the same size as y.

If y is a matrix, demod demodulates its columns.

x = demod(y,fc,fs,'pwm','centered') finds the pulse widths assuming they are centered at the beginning of each period. x is length length(y)*fc/fs.

See Also

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