DSB-SC vs SSB-SC

DSB-SC vs SSB-SC | Difference between DSB-SC and SSB-SC

This page on DSB-SC vs SSB-SC describes difference between DSB-SC and SSB-SC modulation types. It explains basics of DSBSC and SSBSC and mentions useful difference between terms. DSB-SC stands for Double SideBand Suppressed Carrier and SSB-SC stands for Single SideBand Suppressed Carrier.

Both of these are modulation techiques used in AM(Amplitude Modulated) frequency spectrum. As shown in the figure-1 AM spectrum carries Wc(Carrier), Wc-Wm(Lower Side Band) and Wc+Wm(Upper Side Band) signal components.

Fig-1 AM Spectrum

Here Wc carries no information. Moreover carrier signal consumes a lot of power more than about 50%. Due to this facts, it has been decided to suppress the carrier during the transmission. Only the two side bands which contain the information are transmitted. But this results into system complexity at the receiver.

Fig-2 DSBSC Transmitter

As shown in the fig-2, at DSBSC transmitter, message signal (positive and negative) is modulated using carrier signal and added to suppress the carrier portion. This is referred as balanced modulator. The DSBSC modulator process is mentioned in the equations below. The modulators such as ring modulator and square law modulators are also used for the purpose.
Equations: Carrier Signal, C(t) = m(t)*cos(Wc*t)
Information signal m(t) = Em * co(Wm*t)
Output of modulator = Em*cos(Wm*t)*cos(Wc*t)
= (M*Ec/2)*cos(Wc+Wm)t + (M*Ec/2)*cos(Wc-Wm)t

Fig-3 DSBSC Receiver

As shown in the fig-3, at DSBSC receiver, received DSB-SC modulated spectrum is multiplied using local oscillator. This resulting signal is passed through the Low Pass Filter to retrieve back the message signal. The equation is as follows.
y(t)=[m(t)*cos(Wc*t)]*cos(Wc*t)
y(t)=m(t)*(1/2)[1+cos(2*Wc*t)]
y(t)=(1/2)m(t)+(1/2)*m(t)*cos(2*Wc*t)
Information is contained in the first part i.e. (1/2)m(t) which is low pass filtered and extracted.

Here Local Oscillator should have the same frequency and phase as that of the carrier frequency signal used at the transmitter. This process is known as synchronous detection.

DSB-SC - Double SideBand Suppressed Carrier

Fig-4 DSB-SC spectrum

As shown in the fig-4, DSB-SC spectrum has both the sidebands and no carrier is present. Hence it needs two times the bandwidth of SSBSC as explained later. It has advantages of having lower power consumption but needs complex detection at the receiver side.

DSBSC modulation technique is used in analogue TV systems to transmit colour information. It is also used for transmitting stereo information in FM sound broadcast at VHF frequencies.

SSB-SC - SideBand Suppressed Carrier

Fig-5 SSB-SC spectrum

As shown in the fig-5, SSB-SC spectrum has only one sidebands either lower or upper. There is no carrier in the transmission. It needs half of the bandwidth used for DSBSC transmission.

SSB filter is used to extract desired sideband for transmission from DSBSC signal. Half of the power is needed for transmission in SSB-SC in comparison to DSBSC.

Pls. note that terms DSBSC and DSB-SC as well as SSBSC and SSB-SC are synonymous to each other. After comparing DSBSC vs SSBSC and studying the difference between DSBSC and SSBSC one can also refer following comparison between various modulation types as mentioned below.