demod (Signal Processing Toolbox)

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')
[x1,x2] = demod(y,Fc,Fs,'qam')

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.)


`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(2piFct);` `[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(2piFct);` `[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(2piFct);` `[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.
`ptm`	Pulse-time demodulation. Finds the pulse times of a pulse-time 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(2piFct);` `x2 = y.sin(2piFct);` `[b,a] = butter(5,Fc*2/Fs);` `x1 = filtfilt(b,a,x1);` `x2 = filtfilt(b,a,x2);`

The default method is 'am'. Except for the 'ptm' and 'pwm' cases, 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.

`modulate`	Modulation for communications simulation.
`vco`	Voltage controlled oscillator.