DEMODAM

Purpose:

Demodulates an AM series using low pass filtering.

Syntax:

DEMODAM(series, fc, bw)

series

-

A series or table, the amplitude modulated input.

fc

-

Optional. A real, the carrier frequency. If not specified, fc is estimated in the frequency domain.

bw

-

Optional. A real, the bandwidth of the input series. If not specified, defaults to fc.

Returns:

A series or array.

Example:

W1: gsin(1000, 1/1000, 2)^3

W2: gcos(length(W1), deltax(W1), 200)

W3: W1 * W2

W4: demodam(w3, 200)

 

 

W1 contains the source series to modulate. W2 contains the carrier frequency. W3 performs the amplitude modulation by multiplying the source series by the carrier. W4 performs amplitude demodulation by multiplying the AM series by the carrier and low pass filtering the result.

 

Example:

W1: integ(gnorm(10000, 1/10000)) * 10000

W2: modam(w1, 1500)

W3: demodam(w2, 1500)

W4: demodam(w2, 1500, 200)

 

W1 contains synthesized data. W2 amplitude modulates the data with a carrier frequency of 1500 Hz. Both W3 and W4 demodulate the amplitude modulated series, but W4 specifies that the input bandwidth is 200 Hz.

Remarks:

AM demodulation is performed by multiplying the input by the carrier frequency and filtering the result with a linear phase low pass filter.

 

If the carrier frequency is not specified, DEMODAM uses SINFIT3 to estimate the carrier from the FFT and analytical rectangular window interpolation. However, because slight deviations of the estimated carrier from the actual carrier can cause large output discrepancies, it is best to specify the carrier if known.

 

The input bandwidth parameter, bw, determines the cutoff frequency of the low pass filter used to demodulate the input.

 

DEMODAM requires the DADiSP/Filters Module. to construct the FIR linear phase low pass filter used to demodulate the input.

 

See MODAM to amplitude modulate a series.

 

See DEMODFM to demodulate a frequency modulated series.

See Also:

DADiSP/Filters

DEMODFM

MODAM

MODFM

SINFIT3