am or amdsb-sc — 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..

y=cos(2*pi*fc*t + opt*cumsum(x))

cumsum is a rectangular approximation of the integral of x. modulate uses opt as the constant of frequency modulation. If you do not specify the opt parameter, modulate uses a default of opt = (fc/fs)*2*pi/(max(max(x))) so the maximum frequency excursion from fc is fc Hz.

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.

y=cos(2*pi*fc*t + opt*x)

modulate uses opt as the constant of phase modulation. If you do not specify the opt parameter, modulate uses a default of opt = pi/(max(max(x))) so the maximum phase excursion is π radians.

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. modulate(x,fc,fs,'pwm','centered') yields pulses centered at the beginning of each period. The length of y is length(x)*fs/fc.

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.

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 input argument opt must be the same size as y.