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  4. Analog amplitude modulation and demodulation Part 2

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Introduction

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This Java applet allows you to experience the basics of DSB-SC and SSB-SC modulation and demodulation visually.
It’s interesting to look at the spectrogram and try changing the various parameters. You can understand the background to the characteristics of the signal.

To run the applet, click the picture at the bottom of this page.

*Java Runtime Environment version 5.0 or higher is required to run the applet.
Download: http://www.java.com/en/download/index.jsp

An outline of DSB-SC/SSB-SC modulation and demodulation

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DSB-SC and SSB-SC are amplitude modulation.
DSB-SC is double sideband suppressed carrier modulation and SSB-SC is single sideband suppressed carrier modulation.
When amplitude modulation is carried out, the modulating signal spectrum is shifted to the carrier frequency band. DSB-SC is so named because the modulating signal extends symmetrically on both sides of the carrier wave. SSB-SC is DSB-SC with only one side band.

There is some confusion surrounding the various systems of amplitude modulation. For example, some people refer to AM as DSW, while others refer to DSB-SC as DSB.
AM modulation is used as a generic term for amplitude modulation, but it includes a number of types, so it’s necessary to pay careful attention when discussing them.
The types can be summarized as follows.

Double sideband with carrier: DSB-WC(AM), referred to simply as DSW
Single sideband with carrier: SSB-WC
Double sideband suppressed carrier: DSB-SC
Single sideband suppressed carrier: SSB-SC

WC (With Carrier) means that the modulated wave includes a carrier wave component, and SC (Suppressed Carrier) means that there is no carrier wave component. 

The theory of DSB-SC/SSB-SC modulation and demodulation

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◆Double sideband suppressed carrier modulation
At the beginning of the explanation of amplitude modulation, we explained the AM radio system, but the term for amplitude in the theoretical expression was complex. If the amplitude of the carrier wave is simply changed and mathematised, it is as follows. This modulation method is called DSB-SC (double sideband suppressed carrier modulation). As the name suggests, there’s no wave carrier in the modulated wave.

eq039.gif

eq010.gif
With DSB-SC, the amplitude of carrier wave Ac is shifted proportionally to the modulating signal m(t).

When the modulating signal m(t) is a single sine wave, it’s as follows.

eq009.gif

eq005.gif

If the initial phase ΦC of the carrier wave is 0, and Kdsb-sc*Am is modulation factor m, the result is as follows.
eq017.gif
The first equation shows a spectrum where only the modulation signal frequency Fm is separate on either side of the carrier frequency Fc. However note that with DSB-SC, there’s no carrier wave. For this reason, DSB-SC is considered to have good electrical efficiency. However, synchronous detection is necessary on the receiving end, which involves a system with advanced technology and the associated costs.

s_am03.gif

◆Single sideband suppressed carrier modulation (SSB-SC)
The modulation method where only a single band of double sideband suppressed carrier modulation is transmitted is known simply as SSB. It offers even better electrical efficiency and frequency band efficiency than DSB. In addition, with digital modulation, modulation is performed at relatively low frequency, and this method is also used to up-convert the signal to a radio frequency. SSB-SC modulation can be achieved with various systems, but in terms of hardware, USB or LSB can be used as a filter. As with DSB-SC, the modulating signal spectrum is shifted directly to the carrier frequency band without loss.

s_am02.gif

Actual DSB-SC/SSB-SC modulation and demodulation

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◆Modulation
We explained the theory of amplitude modulation above, but what sort of circuit is actually used?
The symbol for amplitude modulation is a multiplier, but actual circuits use the nonlinearity of transistors or diodes, or switching operations.
For example, if a modulating signal is input to a circuit using non-linear amplification, several higher harmonics are generated in the output due to the amplification characteristics of the circuit. This higher harmonic contains a sum (or difference) component of the information signal frequency and carrier frequency, and if a suitable filter is used to eliminate the unwanted components, the correct amplitude modulation waveform can be obtained.

◆Demodulation
Demodulation (detection) methods for amplitude modulation on the receiving side include synchronous detection and asynchronous detection. Synchronous detection demodulates the received signal by multiplying it with a carrier frequency which has the same frequency and phase as the transmission carrier wave. Asynchronous detection includes envelope detection and rectification detection. With asynchronous detection, the information signal m(t) must be incorporated in the envelope of the receiving waveform.
With a modulation factor of 100% or more, demodulation is not possible with asynchronous detection, but with synchronous detection, correct demodulation is possible. However, synchronous detection requires complex, costly circuits.

With DSB-SC modulation, the amplitude of the carrier wave is shifted proportionally to the amplitude of the modulating signal using a modulator. A transistor or diode can be used as a modulator. For example, if the carrier wave and modulating signal are input with the transistor operating at grade C, a signal combining the DSB wave and harmonics is output. If the DSB wave is extracted with a resonance circuit, it results in an amplitude modulated wave including the carrier wave.
In addition, if the carrier wave and modulating signal are input to a DBM (double-balanced modulator) and the nonlinearity of the DBM is used for modulation, a DSB-SC (double sideband suppressed carrier) amplitude modulation wave without a carrier wave can be obtained. AM radio uses a DSB wave with a carrier wave for incorporating the modulating signal in the direct current and asynchronous detection using envelope detection at the receiver.
The DSB-SC modulated wave does not include a modulation signal frequency component, as you can see by looking at the spectrum. Whether to use a carrier wave or not is determined by the demodulation method at the receiver.

DSB-SC demodulation requires synchronous detection. The local frequency generated from the receiving wave is multiplied by the receiving wave, but this operation is modulation itself, therefore the spectrum shifts to a frequency double the local frequency and a zero frequency. The zero frequency spectrum is the transmitted modulating signal.
With synchronous detection as shown in this applet, the modulated wave (receiving wave) is multiplied with the carrier frequency which is exactly the same frequency and phase as the transmission carrier wave. 

DSB-SC/SSB-SC modulation applet

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A DSB/SSB modulation and demodulation applet
Click the “Explain” button at the bottom right of the applet for how to use it.

DSB_00.gif

Click the image: DSB/SSB modulation and demodulation applet

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