MT Modulate AM (VSB)

Performs vestigial sideband (VSB) amplitude modulation. This node performs signal processing on data and operates independently of hardware settings. Use this node in both simulated and hardware-equipped applications.

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Inputs/Outputs

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message signal

Message signal used to modulate the carrier wave.

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modulation index

Desired modulation index of the AM modulated waveform parameter. A value of 0.7, for example, corresponds to a modulation index of 70%. A value greater than unity may cause overmodulation and distortion. The node ignores this parameter if you set the suppress carrier? parameter to TRUE.

Default value: 1.0

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VSB FIR filter design

The vestigial filter design parameters corresponding to the desired VSB spectrum. The vestigial filter is generated when reset? is set to TRUE, and the filter is applied to the input message signal. Default values in this cluster correspond to NTSC analog picture carrier specifications.

Note The normalized frequencies are specified with respect to a 1 S/s sample rate.
Tip Changes to the FIR filter design take effect when reset? is set to TRUE.
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#taps

Number of taps used to generate the VSB filter. For error-free operation, specify a filter length of 85 taps or greater.

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lower PB

The normalized lower passband frequency, in Hertz (Hz), of the VSB filter spectrum, relative to the carrier frequency.

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upper PB

The normalized upper passband frequency, in Hz, of the VSB filter spectrum, relative to the carrier frequency.

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lower SB

The normalized lower stopband frequency, in Hz, of the VSB filter spectrum, relative to the carrier frequency.

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upper SB

The normalized upper stopband frequency, in Hz, of the VSB filter spectrum, relative to the carrier frequency.

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peak message amplitude

Peak message amplitude of the message signal parameter. The node uses this value to scale or normalize the input message signal prior to modulation. This value must be positive.

Default value: 1.0

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error in

Error conditions that occur before this node runs.

The node responds to this input according to standard error behavior.

Standard Error Behavior

Default value: No error

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pilot tone

Parameters for specifying pilot tone generation. The pilot tone is added to the Hilbert-filtered complex baseband signal to generate the modulated waveform. The pilot tone provides a coherent phase reference during demodulation for performing carrier phase and carrier frequency offset correction. Set the amplitude parameter to 0 to disable addition of the pilot tone.

Tip Adding a pilot tone is optional when suppress carrier? is set to FALSE. In this case, the presence of the DC component in the AM signal (corresponding to the pure carrier tone) provides a coherent reference for phase and frequency offset correction, therefore the pilot tone is not needed.
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frequency

Frequency of the complex exponential tone that is added to the complex baseband signal. Ensure that the absolute value of this tone frequency is less than the Nyquist frequency 1/(2 * dt).

Note When specifying pilot tone parameters, ensure that the pilot tone is added to a portion of the spectrum where the message component of the signal is absent. Conflict between the pilot tone and message signals can cause a residual carrier phase and frequency offset error after demodulation. For example, if you specify a negative pilot tone frequency, specify an upper sideband in the sideband parameter.
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amplitude

Amplitude of the complex exponential tone that is added to the complex analytic baseband signal.

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suppress carrier?

A Boolean that determines whether to suppress the carrier of the modulated signal.

TRUE The DC component corresponding to the carrier tone in the amplitude-modulated baseband signal is suppressed.
FALSE The output amplitude-modulated baseband signal consists of a DC component corresponding to the carrier tone.

Default value: FALSE

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reset?

A Boolean that determines whether the node continues AM-VSB modulation using internal states from previous iterations.

TRUE The node restarts AM-VSB modulation, redesigns the vestigial filter, and clears internal filter states on each call.
FALSE The node does not change the vestigial filter design across multiple calls, and sets the internal filter state to ensure continuous filter operation.

Default value: TRUE

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AM modulated waveform

The baseband AM vestigial sideband modulated signal data and its complex envelope.

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t0

The trigger (start) time of the Y array.

Default value: 0.0

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dt

Time interval between data points in the Y array.

Default value: 1.0

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Y

The complex-valued signal-only baseband modulated waveform. The real and imaginary parts of this complex data array correspond to the in-phase (I) and quadrature-phase (Q) data, respectively.

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error out

Error information.

The node produces this output according to standard error behavior.

Standard Error Behavior

Adjusting the Carrier Phase

initial phase MT Upconvert Baseband initial phase

Avoiding Distortion

message signal

Prescaling Behavior

This node prescales the message signal parameter by the scalar peak amplitude prior to modulation. The default value of 1.0 for peak message amplitude indicates that no prescaling occurs.

Vestigial Sideband (VSB) Amplitude Modulation

VSB amplitude modulation involves retaining a portion of the unwanted sideband (called the vestige) in the message spectrum prior to transmission, rather than completely eliminating it as in single-sideband modulation. The following figure illustrates the series of algorithms applied to the incoming signal m(t). The node applies the AM-DSB algorithms and then applies the Vestigial filter to remove the vestige.

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The output amplitude-modulated signal is represented by the following equations, depending on the setting of the suppress carrier? parameter:

When suppress carrier? is set to FALSE, the output amplitude-modulated signal is represented by the following formula:

s D S B ( t ) = 1 + k × m ( t ) max ⁡ | m ( t ) | s D S B ( t ) = 1 + k × m ( t ) max ⁡ | m ( t ) |
s V S B ( t ) = s D S B ( t ) ( h i ( t ) + j × h q ( t ) ) + A p i l o t e j ω p i l o t t s V S B ( t ) = s D S B ( t ) ( h i ( t ) + j × h q ( t ) ) + A p i l o t e j ω p i l o t t

where

m(t) represents the message signal data

S DSB (t) represents the double sideband AM modulated signal (unsuppressed carrier)

S VSB (t) represents the vestigial sideband AM modulated signal (unsuppressed carrier)

k represents the modulation index

A pilot represents the amplitude of the pilot tone

ω pilot represents the frequency of the pilot tone

h i represents the real component of the vestigial filter

h q represents the imaginary component of the vestigial filter

When suppress carrier? is set to TRUE, the output amplitude-modulated signal is represented by the following formula:

s D S B S C ( t ) = m ( t ) max ⁡ | m ( t ) | s D S B S C ( t ) = m ( t ) max ⁡ | m ( t ) |
s V S B S C ( t ) = s D S B S C ( t ) × ( h i + j × h q ( t ) ) + A p i l o t e j ω p i l o t t s V S B S C ( t ) = s D S B S C ( t ) × ( h i + j × h q ( t ) ) + A p i l o t e j ω p i l o t t

where

m(t) represents the message signal data

S DSBSC (t) represents the double sideband AM modulated signal (unsuppressed carrier)

S VSBSC (t) represents the vestigial sideband AM modulated signal (unsuppressed carrier)

k represents the modulation index

A pilot represents the amplitude of the pilot tone

ω pilot represents the frequency of the pilot tone

h i represents the real component of the Hilbert filter

h q represents the imaginary component of the Hilbert filter