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.)

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);

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(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.
`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(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'. 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

modulate, vco, fskdemod, genqamdemod, mskdemod, pamdemod, pmdemod, qamdemod

deconv dfilt