DSB-SC vs SSB-SC: Modulation Techniques Compared

This page explains the difference between DSB-SC and SSB-SC modulation types. It covers the basics of DSBSC and SSBSC and highlights the key differences between them.

DSB-SC stands for Double SideBand Suppressed Carrier, and SSB-SC stands for Single SideBand Suppressed Carrier. Both are modulation techniques used within the Amplitude Modulated (AM) frequency spectrum.

As shown in Figure 1, the AM spectrum carries Wc (Carrier), Wc-Wm (Lower Side Band), and Wc+Wm (Upper Side Band) signal components.

Fig-1 AM Spectrum

Here, Wc (the carrier) carries no information. Moreover, the carrier signal consumes a significant amount of power, often more than 50%. Therefore, it’s often desirable to suppress the carrier during transmission, transmitting only the two sidebands that contain the information. This, however, increases system complexity at the receiver.

Fig-2 DSBSC Transmitter

As shown in Figure 2, at the DSBSC transmitter, the message signal (positive and negative) is modulated using the carrier signal and added in such a way that it suppresses the carrier portion. This is referred to as a balanced modulator. The DSBSC modulator process is described in the equations below. Ring modulators and square law modulators are also commonly used for this purpose.

Equations:

• Carrier Signal, C(t) = m(t)cos(Wct)
• Information signal m(t) = Em * co(Wm*t)
• Output of modulator = Emcos(Wmt)cos(Wct) = (M*Ec/2)cos(Wc+Wm)t + (MEc/2)*cos(Wc-Wm)t

Fig-3 DSBSC Receiver

As shown in Figure 3, at the DSBSC receiver, the received DSB-SC modulated spectrum is multiplied using a local oscillator. This resulting signal is then passed through a Low Pass Filter to retrieve the original message signal. The equation is as follows:

y(t)=[m(t)cos(Wct)]cos(Wct) y(t)=m(t)(1/2)[1+cos(2Wc*t)] y(t)=(1/2)m(t)+(1/2)*m(t)cos(2Wc*t)

The information is contained in the first part, i.e., (1/2)m(t), which is low-pass filtered and extracted. Critically, the Local Oscillator must have the same frequency and phase as 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 Figure 4, the DSB-SC spectrum has both sidebands, and no carrier is present. Hence, it requires twice the bandwidth of SSBSC, as explained later. It offers the advantage of lower power consumption but necessitates more complex detection at the receiver. DSBSC modulation is commonly used in analog TV systems to transmit color information and for transmitting stereo information in FM sound broadcasts at VHF frequencies.

SSB-SC - SideBand Suppressed Carrier

Fig-5 SSB-SC spectrum

As shown in Figure 5, the SSB-SC spectrum has only one sideband – either the lower or the upper. There is no carrier in the transmission. It requires half of the bandwidth used for DSBSC transmission. An SSB filter is used to extract the desired sideband for transmission from the DSBSC signal. Half the power is needed for transmission in SSB-SC compared to DSBSC.

Please note that the terms DSBSC and DSB-SC, as well as SSBSC and SSB-SC, are synonymous.